iUntfH tliilillirlHjJ iiiiSif m&fflb 4'. J J. 1 J 1 III Digitized by the Internet Archive in 2011 with funding from The Library of Congress http://www.archive.org/details/elementaryphysicOOtarr ELEMENTAKY PHYSICAL GEOGKAPHY ; T ^7^ ( Plate 1. — Frontispiece. Watkins Glen, N7¥. A post-glacial gorge in a shale rock. ELEMENTARY PHYSICAL GEOGRAPHY BY RALPH S. TARR, B.S., F.G.S.A. PROFESSOR OF DYNAMIC GEOLOGY AND PHYSICAL GEOGRAPHY AT CORNELL UNIVERSITY AUTHOR OF " ECONOMIC GEOLOGY OF THE UNITED STATES " THE MACMILLAN COMPANY LONDON: MACMILLAN & CO., Ltd. 1907 All rights reserved, T\JL SUIERARYo. CONGRESS { tfne «uuv Kfcceivea jJUft 13 1S08 7 sa no, COPY A. «! Copyright, 1895, By MACMILLAN AND CO. Set up and electrotyped October, 1895. Reprinted March, July, October, 1896; May, July, September, 1897; July, 1898; August, 1899; August, 1900 ; April, December, 1902; Novem- ber, 1903 ; April, 1905 ; October, 1907. PREFACE. For some time there have been indications that new text- books on physical geography are demanded ; and in the report of the Committee of Ten this finds definite expression. In the preparation of this book, which has been in hand for several years, there is an attempt to meet this apparent demand; but for reasons which are obvious to many, it has not seemed wise to attempt to follow the somewhat radical suggestions which were made by the majority of the geogra- phy conference of the Committee of Ten. Therefore, while the physiographic side is given more prominence than is customary in works of this kind, this book attempts to only partly meet the Committee's suggestions. In the preparation of the book, effort has been made to introduce new material, particularly in the illustrations, which are a prominent part of the book. Also, there has been an endeavor to make the book scientifically accurate, and to introduce the latest knowledge on the subjects treated. There are probably places in which this is not done, for the field is so large that much must be compilation ; and the compiler is liable to fall into error. I anticipate criticism of the order of presentation, of the relative amount of space allotted the various topics, of the viii PREFACE. omission of some subjects which are usually found in such books, and of the inclusion of some not usually discussed ; but these matters have been carefully considered, and the book is the result of a well-matured plan. In many respects it is experimental, but it is a deliberate attempt to supply a book which is certainly needed. It should not be inferred that the author is satisfied with the attempt, — he is keenly disappointed at the constant necessity of saving space and thereby weakening description and explanation. In many cases, explanations have been omitted; in others, perhaps it would have been better to have done so. It is hoped that the more advanced teachers will find it possible to accompany the text-book work with laboratory and field study, along the line suggested in the appendix. The discussion of method has been systematically eliminated from the text, and the sole effort has been to present facts and furnish information ; but if this alone is put before the pupils, the value of the study will be very slight indeed. It furnishes the main story in a connected way, and supplies certain information ; but the laboratory and field will supply applications and extensions of the principles, at the same time giving value to the study as a means of mental disci- pline. Merely to hear recitations from the book, will be the continuation of an all too prevalent habit, which in so many cases makes the science teaching in our secondary schools the weakest part of the curriculum. While the author has done much work in some of the subjects treated, particularly the ocean and the land, he would not wish to claim that much in the book is original. In reality, this book is based upon the manuscript of another and more advanced work, which is soon to be published as PREFACE. ix a handbook for teachers and for reference. Both of these represent an attempt to gather from all available sources, the kind of matter which it seemed desirable to include in such books. While in the larger work direct reference is made to the sources of information, it has not seemed desirable to do so in this case ; for the acknowledgments take much space and distract the attention, without benefiting the pupil. I have had much generous assistance in the supply of illus- trations, particularly of photographs ; and grateful general acknowledgment is made here, while special mention of the sources is made in a list in the succeeding pages. Although I have received aid from many sources, there are a few which I must mention especially. The writings of Geikie, Dutton, Powell, and Gilbert, particularly the latter, have not only given me bodies of fact, but also inspiration, as indeed they have to all who are working in physiographic geology. To the writings and teachings of Professors Shaler and Davis of Harvard University, I owe more than I could possibly acknowledge ; and to the latter I am under an added obliga- tion for his examination and kindly criticism of parts of my manuscript. While I acknowledge the debt which I owe these scientists, it must be understood that the mode of presentation is my own, and that I alone am responsible for any shortcomings which may appear. RALPH S. TARR. Ithaca, N.Y., August 30, 1895. CONTENTS. Part I. The Air. CHAPTER I. The Earth as a Planet. PAGE Form of the Earth 3 The Solar System 5 The Sun 6 The Planets 8 Asteroids 11 The Earth 11 The Moon 13 Comets, Shooting- Stars, and Meteors .15 The Stellar System 17 Symmetry of the Solar System 18 The Nebular Hypothesis 19 Verification of the Nebular Hypothesis 20 CHAPTER II. The Atmosphere. General Statement 23 Light 25 Electricity and Magnetism 29 Heat 30 Moisture 35 Pressure 39 Effect of Gravity 39 Effect of the Earth's Rotation . .39 CHAPTER III. Distribution of Temperature. General Statement 43 Effect of Atmospheric Movements 44 xi Xli CONTENTS. PAGE Influence of Oceans 45 Effect of Topography .47 Seasonal Temperature Range .48 Isothermal Charts ........... 51 Daily Temperature Curve .60 Temperature Ranges 62 Earth Temperatures 65 CHAPTER IV. General Circulation of the Atmosphere. General Statement 68 Classification of the Winds 70 Planetary or Permanent Winds 71 Trade Winds .71 Doldrum Belt 74 Anti-trade Winds 74 Horse Latitude Winds 75 Prevailing Westerlies 75 Periodical Winds 76 Seasonal Winds 76 Migrating Wind and Calm Belts 76 Monsoon Winds 77 Diurnal Winds 79 Sea and Land Breezes 79 Mountain and Valley Breezes 80 Eclipse and Tidal Breezes 82 Irregular Winds 82 Accidental Winds .......... 82 The Nature of Winds 83 CHAPTER V. Storms. Cyclonic Storms 85 Hurricanes 86 Description 86 Effects 88 Path 90 Time of Occurrence 91 Cause 91 CONTENTS. Xlll Temperate Latitude Cyclones . Resemblance to Hurricanes Differences from Hurricanes Effects Winds Anticyclones Cause Secondary Storms . Thunderstorms Tornadoes and Waterspouts PAGE 93 93 95 98 98 100 100 101 101 104 CHAPTER VI. The Moisture of the Atmosphere. Dew . 107 Frost . . . . .108 Fog 109 Haze . 110 Mist Ill Clouds Ill Rain 114 Snow 115 Hail 116 Distribution of Rainfall in the World . . . . . . . 117 Distribution of Rainfall in the United States 118 Distribution of Snowfall ......... 121 Seasonal Distribution of Rainfall ........ 122 Irregularities of Rainfall 123 CHAPTER VII. Weather and Climate. Weather 124 Tropical and Arctic 124 Temperate Latitude Weather 125 Climate 129 Tropical Climate 130 Temperate Climate .......... 130 Arctic Climate 132 Minor Variations 132 Changes in Climate 132 xiv CONTENTS. CHAPTER VIII. Geographic Distribution of Animals and Plants. PAGE General Statement 135 The Ocean 135 FreshWater 137 The Land 137 Effect of Temperature and Moisture . . . . . . 137 Plant and Animal Habits 141 Life Zones ' . 143 The Spread of Life 145 Barriers to the Spread of Life 147 Effect of Man 147 Part II. The Ocean. CHAPTER IX. Form and General Characteristics of the Ocean. Distribution of Land and Water 151 Composition of Ocean Water 151 Color and Phosphorescence 152 Exploration of the Ocean Bottom 153 Methods used in Deep-sea Explorations 153 Sounding . 153 Dredging 155 Topography of the Ocean Bottom 156 General 156 The Atlantic Ocean 158 Other Oceans 160 Topography near the Coast 160 Temperature of the Ocean Bottom 162 Light on the Ocean Bottom 163 Materials composing the Ocean Floor 164 Mechanical Sediments 164 Globigerina Ooze 164 Red Clay 165 Life in the Ocean 166 Pelagic or Surface Faunas 166 Littoral or Shore Faunas 167 Faunas of the Ocean Bottom 169 CONTENTS. XV CHAPTER X. Ocean Waves and Currents. PAGK Wind Waves 174 Earthquake Waves 178 Storm Waves 179 Ocean Surface Temperatures 179 Ocean Currents 182 Planetary Circulation . . . . . . . . 182 The System of Ocean Currents 183 Cause of Ocean Currents 185 The Gulf Stream 187 The Labrador Current 189 Effects of Ocean Currents . . . . . . . . 189 CHAPTER XI. Tides. Nature of the Tidal Wave 192 Cause of Tides 192 Effect of the Land 193 Other Causes for Variation in Tidal Height 198 Effects of Tides * . 201 Part III. The Land. CHAPTER XII. The Crust of the Earth. Interior Condition 205 Movements of the Crust 206 Disturbance of the Rocks . 207 Volcanic Action 211 Rocks of the Earth's Crust 212 Igneous Rocks 213 Metamorphic Rocks 214 Sedimentary Rocks 214 Deposition of Sedimentary Rocks 215 Consolidation of Sedimentary Rocks ....... 217 Geological Chronology 218 Age of the Earth 221 XVI CONTENTS. CHAPTER XIII. Denudation of the Land. PAGE Underground Water . 224 The Formation of Caverns 226 Springs and Artesian Wells 228 Durability of Rocks 231 Weathering 233 Agents of Erosion 238 Wind Erosion 238 Rain Erosion 239 Percolating Water 240 River Erosion 241 Ocean Erosion ........... 244 Glacial Erosion 245 Denudation 246 CHAPTER XIV. Topographic Features of the Earth's Surface. Continents and Ocean Basins 249 Physical Geography of the United States 253 Atlantic Coast Area ......... 254 The Eastern Mountains 254 The Canadian Highlands 256 The Central Plains ' . . 256 The Cordilleran Area 25 The Drainage of the Country 259 The Shore Line 26i CHAPTER XV. River Valleys. General Description 262 Development of River Valleys ........ 265 Adjustment of Streams 272 The River Divide 273 Accidents to Streams . . 275 Land Movements 276 Climatic Accidents .......... 279 Other Accidents -..2 CONTENTS. XVll CHAPTER XVI. Deltas, Floodplains, Waterfalls, and Lakes. PAGE Deltas . . 285 Floodplains 288 Waterfalls ' 294 Lakes 298 Swamps 303 CHAPTER XVII. Glaciers. Cause of Glaciers ........... 306 Alpine or Valley Glacier ......... 307 Continental Glaciers 313 Icebergs 315 Glacial Period ........... 316 Area covered by Ice ......... 316 Terminal Moraine .......... 319 Formation of Soil .......... 321 Formation of Lakes 323 Formation of Waterfalls 325 v/cHAPTER XVIII. The Coast Line. General Statement 328 Effect of Elevation . 329 Effect of Depression 329 .Effect of Sediment 330 .Effect of Waves and Currents 332 Effect of Plants 337 Effect of Animals . 340 Changes in Coast Form 343 Islands . . . . 344 Promontories 346 Lake Shores . . . . . .347 Fossil Shore Lines 348 ^CHAPTER XIX. Plateaus and Mountains. Plateaus 350 Mountains 353 Characteristics of Mountains 353 XV111 CONTENTS. PAGB The Origin of Mountains 362 Sculpturing of Mountains . 364 The Drainage of Mountains ........ 365 Destruction of Mountains 367 CHAPTER XX. Volcanoes, Earthquakes, and Getsees. Volcanoes 370 Distribution 370 Materials Erupted .371 Eruptions of Volcanoes 374 Eorm of Cone 378 Effects of Volcanic Eruptions 381 Extinct Volcanoes 381 Cause of Volcanoes 383 Earthquakes 383 Geysers and Hot Springs 386 CHAPTER XXI. The Topography of the Land. General Statement 390 Constructive Land Forms 392 By Internal Forces 392 By Agents of Denudation 393 By Animal and Plant Life 395 Effect of Rock Structure upon Topography 395 CHAPTER XXII. Man and Nature. General Statement 407 Modifying Influence of Man 407 Man and the Forest 409 Influence of Nature upon Man 412 CHAPTER XXIII. Economic Products of the Earth. Soil 420 Building Stones 420 Economic Deposits of Sedimentary Origin 422 Miscellaneous Substances 423 CONTENTS. xix PAGE Coal 423 Natural Gas and Petroleum . 425 Ore Deposits 426 Distribution of Ore Deposits 428 Mineral Wealth of the United States 429 APPENDIX I. Meteorological Instruments, Apparatus, and Methods. Thermometric Records 431 Barometric Records 432 Measurement of Wind Direction and Force 433 Measurement of Evaporation . . . 434 Measurement of Moisture in the Air 434 Study of Clouds and Sunshine 434 Measurement of Rainfall 435 Meteorological Methods and Results 435 APPENDIX II. Topographic Maps 437 APPENDIX III. Suggestions to Teachers 440 APPENDIX IV. Questions upon the Text 453 ILLUSTRATIONS. DIAGRAMS AND PHOTOGRAPHS. FIG. PAGE 1. Sphere and oblate spheroid 3 2. Land and water hemispheres 4 3. The solar system 5 4. Relative size of sun and large planets 7 5. Sun spots, 1872 8 6. Relative distances of planets from the sun 8 7. Relative size of smaller planets 9 8. Illustration of the cause of seasons 12 9. Relative size of earth and moon 14 10. Lunar craters 14 11. Comet of Donati, 1858 15 12. Orbit of comet of 1862 16 13. Andromeda nebula 17 14. Thickness of the atmosphere 23 15. Decrease in density of the atmosphere .24 16. Passage of sun's rays through the atmosphere .... 26 17. Inclination of the sun's rays 34 18. Daily change in relative humidity . . 37 19. Increase in temperature of descending air 38 20. Deflection of air currents 40 21. Decrease in diameter on different latitudes 40 22. Daily temperature curves 44 23. Irregularities of seasonal curve . . . . . . .45 24. Seasonal temperature ranges 49 25. Seasonal temperature range (New York) 51 26. Isotherms for February (northern hemisphere) .... 52 27. Daily temperature curve (summer and winter) .... 60 28. Daily temperature range for several days 61 29. Daily temperature record for several days ..... 61 xxi xxil ILL US TEA TIONS. FIG. PAGE 30. Temperature ranges, United States, 1892 62 31. Minimum temperatures, United States, 1892 63 32. Maximum temperatures, United States, 1892 64 33. Daily temperature range near and above the ground ... 66 34. General circulation of the globe 69 35. Summer monsoons, India 77 36. Winter monsoons, India 77 37. The sea breeze 78 38. The land breeze . 79 39. Effect of sea breeze on air temperature 80 40. Valley breeze 81 41. Ideal diagram of a storm 85 42. Barometric record during passage of a hurricane . .- .86 43. Diagram of hurricane winds . . . . . . .87 44. Map of a hurricane .88 45. Tracks of August hurricanes 89 46. Map of temperate latitude cyclone 94 47. Paths of low-pressure areas .95 48. Average storm tracks, 1878-1887 (Northern hemisphere) . . 96 49. Tracks of low-pressure areas .97 50. Photograph of thunderstorm . . . 102 51. Path of thunderstorm . 103 52. View of a tornado 104 53. Effect of tornado at Lawrence, Mass 105 54. Distribution of tornadoes (1794-1881) . . . . . .106 55. Valley fog in the Himalayas 110 56. The banner cloud Ill 57. Photographs of clouds 112 58. Photographs of snowflakes 115 59. Damp snowfall 116 60. Evaporation in United States 120 61. Monthly rainfall in the West .121 62. Variation in annual rainfall in the West 122 63. A cold wave .127 64. Temperature descent during cold wave 128 65. Climatic zones ........... 129 66. Near the timber line 138 67. Above the snow line, Mount St. Elias, Alaska .... 139 68. Effect of sunlight on mountain vegetation . . . . . 140 69. Arid land vegetation 141 70. Arid land vegetation, Eio Grande valley 142 ILLUSTBATIONS. xxiii FIO. PAGE 71. The tropical forest 143 72. Life zones of United States 144 73. Deep-sea sounding machine 154 74. Deep-sea trawl 155 75. Contrast between land and ocean bottom topography . . . 156 76. Cross-section of Atlantic Ocean 158 77. Temperature of the Mediterranean 163 78. Globigerina ooze 165 79. Coral reef on Australian coast 168 80. Ocean waves 174 81. Breakers on the coast 175 82. Effect of storm waves on the coast 177 83. Normal vertical descent of ocean temperatures .... 180 84. Tides near Hell Gate, N.Y 196 85. Time and height of tides at Hell Gate 196 86. The tides at Eastport, Me., September, 1893 199 87. Height of high tide, Eastport, Me., 1893 and 1894 . . .200 88. Tidal mud flat in Bay of Fundy 202 89. Tidal rise and fall, Cape Ann, Mass 203 90. Horizontal rocks in Kansas 208 91. A monoclinal fold .208 92. Anticline 208 93. Syncline . . . 209 94. Photograph of anticline, Hancock, W.Va. . . . . 209 95. Photograph of anticline near Quebec, Canada .... 209 96. Photograph of a fault in Arizona 210 97. Photograph of a fault in glacial clay, Massachusetts . . . 210 98. A dike crossing granite 212 99. Contorted limestone 214 100. Stratified shale, near Ithaca, N.Y 215 101. Section of alternating strata 216 102. Unconformity in horizontal rocks 217 103. Unconformity in inclined rocks 217 104. Photograph of fossiliferous rock 219 105. Mammoth Hot Springs, Yellowstone Park 225 106. Diagram illustrating formation of caverns 226 107. A sink hole in limestone region 227 108. Stalactites in Luray Cave 227 109. The Natural Bridge, Va 228 110. A spring on a fault plane 228 111. Hillside spring 229 xxiv ILLUSTRATIONS. FIG. PAGE 112. Photograph of an artesian well 229 113. Artesian well in monoclinal strata 230 114. Artesian well in syncline 230 115. Rock pillars in Garden of Gods, Col 231 116. The weathering of granite 233 117. Effect of roots in breaking up rocks 235 118. Talus in Rio Grande valley, N.M 236 119. The formation of residual soil 237 120. Sand dunes, Cape Ann, Mass . .238 121. Moqui pueblo, New Mexico . 239 122. Talus furnishing load to river . 240 123. Yellowstone valley, broadening by weathering .... 242 124. Boulder bed of Westfield River, Mass 243 125. Sea cliffs on volcanic island 244 126. Granite hill rounded by glacial action 245 127. Relief map of Eurasia 250 128. Section across the Atlantic and United States .... 251 129. Relief map of North America 252 130. A deep mountain valley 262 131. Stream issuing from a limestone cave 263 132. Brink of Niagara Falls 264 133. Gorge near Ithaca, N.Y 265 134. Royal Gorge, Col 265 135. Oxbow cut-off in Connecticut valley 266 136. Development of the canon 267 137. Development of the canon profile 267 138. Development of old valley 267 139. The Yellowstone, broadening by weathering 268 140. A bit of Illinois drainage 269 141. A bit of West Virginia drainage 269 142. Canon of the Colorado 270 143. A broad Alpine valley 271 144. Mountain gorge in the Alps 272 145. Diagram illustrating change in divide . ... . . . 273 146. Diagram illustrating change in divide 274 147. Diagram illustrating monoclinal shifting 274 148. Diagram illustrating sudden change in divide .... 275 149. Effect of elevation on Colorado canon 276 150. The drainage of an arid region 280 151. The Great Basin 281 152 Effect of glaciation on stream courses 282 ILL US TEA TIONS. XXV FIG. PAGE 153. Delta of the Mississippi 286 154. Mode of formation of a delta . 288 155. An alluvial fan 288 156. Floodplain among mountains 289 157. Floodplain of a great river 290 158. Meandering of the Mississippi 291 159. Meandering of the Mississippi 292 160. Meandering of the Mississippi . 292 161. Falls of the Yellowstone 293 162. Taughannock Falls, N.Y. . . 294 163. American Falls, Niagara . . . 295 164. Yosemite Falls 296 165. Falls in a gorge near Ithaca, N.Y. ....... 297 166. Diagram illustrating origin of Niagara 298 167. River valley transformed to a lake (Adirondacks) . . . 299 168. Glacial lakes in the Adirondacks 300 169. Bird's-eye view of Niagara River 301 170. Shore lines of extinct Lake Bonneville 302 171. A Florida swamp 303 172. Ray Brook, Adirondacks 304 173. An Alpine snow field 306 174. "Whitney Glacier, Mount Shasta 307 175. The Rhone glacier . . . 308 176. Crevasse in a glacier . . 309 177. Glacier, Mount Dana, Cal . 310 178. Section of a glacier 312 179. Ice cave at terminus of a glacier . . . . . . . 312 180. Forest at foot of Malaspina Glacier, Alaska 313 181. A Nunatak in Greenland . 314 182. Icebergs in the Antarctic 315 183. An iceberg in water 316 184. Glacial lakes and moraine in a mountain valley .... 317 185. Extent of the continental ice sheet in America .... 318 186. Boulder in moraine, Cape Ann, Mass 320 187. Bear-den moraine, Cape Ann, Mass 321 188. Boulder-strewn till soil in Maine 321 189. Glacial scratches on a pebble 322 190. Glacial lakes in Massachusetts . 324 191. Watkins Glen, N.Y 326 192. Sea cliff, Cape Cod, Mass . .328 193. Submerged valley on the coast of Mount Desert, Me. . . . 330 XXY1 ILLUSTRATIONS. FIG. 194. Ocean bar on the Texas coast 195. Destruction of Heligoland by the ocean 196. Lake Spit 197. Hook, Lake Michigan .... 198. Sea cave in granite rock, Cape Ann, Mass. 199. Effect of dike on form of coast, Cape Ann, Mass. 200. Pond formed by beach barrier, Cape Ann, Mass. 201. Crescent-shaped beach, Cape Ann, Mass. 202. Boulders worn from headland by waves 203. Rocky beach on exposed coast, Cape Ann, Mass. 204. Mat of seaweed between tides, Cape Ann, Mass. 205. A mangrove swamp 206. Salt marsh, Cape Ann, Mass. 207. Coral reef on the Australian coast 208. Keys on the Florida coast 209. An atoll in the Pacific . 210. Diagram illustrating origin of atolls 211. The coast of Casco Bay, Me. 212. Cliff on the shore of Lake Michigan 213. Lagoon enclosed behind lake beach 214. Plain in Pecos Valley, N.M. . 215. Plain in valley of Red River of the North 216. Taos Mountains, N.M. . 217. Plateau near Colorado River 218. Butte in New Mexico . 219. Talus slope in the Elk Mountains, Col. 220. Matterhorn, Switzerland 221. A mountain park (Baker's) . 222. Mountain gorge in the Peruvian Andes 223. Mount of the Holy Cross, Col. 224. Trail on Long's Peak, Col. . 225. Mountain ridge on the Canadian Pacific 226. Section across a mountain ridge . 227. A bit of mountain drainage . 228. Map of mountain drainage . 229. Diagram illustrating the development of a mountain 230. A mountain ridge in Colorado 231. Vesuvius in eruption, 1872 . 232. Surface of a recent lava flow 233. Lake formed by a lava dam . 234. Volcano in the Lipari Islands ILLUSTRATIONS. XXV11 FIG. 235. Disruption of Krakatoa .... 236. Vesuvius, from Pompeii .... 237. Mount Hood — an apparently extinct volcano 238. Muir's Butte, Cal., — a recent volcano 239. Fusiyama, a Japanese volcano 240. Angle of slope of volcanic cones . 241. Mounts Shasta and Shastina 242. Mato Tepee, Wyo., — a volcanic neck . 243. Diagram illustrating the earthquake wave . 244. Waves of Charleston earthquake . 245. Earthquake shock in Japan .... 246. Effect of earthquake in Japan, 1891 247. Eault line associated with Japanese earthquake of 1891 248. Crater of Oblong Geyser, Yellowstone Park 249. Old Faithful Geyser, Yellowstone Park 250. Etching of hard layer by denudation, Brazil 251. A cliff in the Yosemite .... 252. Cliffs in the loess of China . 253. A wave-worn chasm, Gloucester, Mass. 254. A rugged granite coast, Cape Ann, Mass. 255. A sloping granite coast, Cape Ann, Mass. 256. Effect of hard layers on topography 257. Signal Butte, Tex 258. Effect of tilted layers on topography 259. Form of seacoast in inclined strata 260. Form of seacoast in inclined strata 261. Ridge of hard rock, etched by denudation 262. Topography in region of folded rocks . 263. A part of the Adirondack forest . 264. Deforesting of the Adirondacks . 265. Bare rock exposed by removal of forest 266. Model of Cumberland Valley, Penn. . 267. Hachure map . . PAGE 375 376 378 379 380 380 382 383 384 384 385 386 387 388 389 396 398 399 400 401 401 402 402 403 403 403 404 405 409 410 411 437 438 XXV111 ILLUSTRATIONS. PLATES AND CHARTS. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. "Watkins Glen, New York Isotherms for the year (world) Isotherms for the year 1892 (United States) Isothermal chart for July (world) . Isothermal chart for January (world) . Isothermal chart for July (United States) Isothermal chart for January (United States) Isothermal chart of New York (year) Winds and isobars for January (world) General circulation of the Atlantic, July General circulation of the Atlantic, January Rainfall chart of the world Rainfall chart of the United States Depths of the ocean Ocean surface temperature, Atlantic Oceanic circulation Gulf Stream Co-tidal lines English Channel tides . Earth columns, New Mexico . The Bad Lands of South Dakota . Relief map of the United States Drainage areas of the United States Delaware and Chesapeake bays Drainage in glaciated region, "Wisconsin White Glacier, Alaska ... Distribution of volcanoes and ocean (world) Marble Canon, Colorado River Navajo church, Arizona . PAGE Frontispiece facing 50 54 55 56 facing facing facing 57 58 59 70 72 73 facing 117 119 161 181 facing 183 188 Jacing 194 195 232 247 facina 253 260 277 283 311 temperatures facing 370 ' . 391 . 397 ILL US TEA TIONS. XXIX ACKNOWLEDGMENT OF ILLUSTRATIONS. The following illustrations are from the sources indicated. In some cases they have been exactly reproduced, but in others they have been made more diagrammatic to suit the needs of this book. Some of the illustrations not acknowledged are from photographs or lantern slides, the source of which could not be ascertained. 1 Abbe, U. S. S. S., Annual Eeport for 1890, Fig. 56. Agassiz, Three Cruises of the Blake, Plate 15. Branner, Journal of Geology, Vol. 1, Fig. 250. Brown, C. D. (dealer in photographs, Gloucester, Mass.), Figs. 81, 89, 203, 204, and 254. Buchan, Atmospheric Circulation, Challenger Reports, Plates 2, 4, 5, and 9. Calvin, Prof. S., State Geologist of Iowa, Des Moines, — Photograph by the Survey, Fig. 131. Canadian Geological Survey, Photograph, Fig. 99. Challenger Reports, Narrative, Figs. 78, 125, and 182. Chamberlin, Third Annual Report, U. S. G. S., Fig. 185. Diller, Bulletin 79, U. S. G. S., 2 Figs. 232, 233. Dunwoody, Summary of International Meteorological Observations, Figs. 26 and 48 ; same, Professional Paper IX., U. S. S. S., Plate 13. Dutton, Second Annual Report, U. S. G. S., Figs. 137 and 149 ; same, Sixth Annual, Plate 29 ; same, Ninth Annual, Fig. 244; same, Monograph II., U. S. G. S., Fig. 136. Ferrel, Popular Treatise on the Winds, Fig. 34. Finley, U. S. S. S., Professional Paper VII., Fig. 54. Gannett, Thirteenth Annual Report U. S. G. S., Plate 22. Gardner, J. L., 2d, Boston, Mass. (Photographs by), Figs. 98, 116, 8 117, 8 120, 126, 3 186,3 187, 198, 3 199, 3 200, 201, 202, 3 206, and 255.8 Gilbert, Monograph I., U. S. G. S., Figs. 151, 154, 155, 170, 197, and 213 ; same, Fifth Annual Report U. S. G. S., Figs. 212 and 217 ; same, Annual Report Smithsonian Institution, 1890, Figs. 166 and 169 ; same, Geology of the Henry Mountains, Fig. 147. 1 TJ. S. C. S., refers to the United States Coast Survey ; U. S. G. S., to the United States Geological Survey; and U. S. S. S., to the United States Signal Service. 2 Some of these which are referred to the Geological Survey publication were made from photographs obtained from the Survey. 8 Also published by Shaler in Ninth Annual Eeport, U. S. G. S. XXX ILL US TEA TIONS. Greely, U. S. S. S., Professional Paper II. , Plates 6 and 7. Griswold, L. S., Dorchester, Mass. (Photograph by), Eig. 97. Guyot, Physical Geography, Fig. 2. Hann, Berghaus, Atlas der Meteorologie, Plate 12. Hann, Hochstetter, and Pokorny, Allgemeine Erdkunde, Pig. 77. Harvard College Astronomical Observatory Engravings, Figs. 5, 11, and 13. Harvard College Geological Department, Figs. 215 and 231 (former, photo- graph from South Dakota World's Fair Commissioner ; latter, pho- tograph by Sommer). Haynes, F. Jay, St. Paul, Minn. (Photographer), Figs. 105, 123, 139, 161, 248, 249. Hellmann, Schneekrystalle, Fig. 58. Hill, First Annual Eeport, Texas Geological Survey, Fig. 257. Hope, J. D., Photographer, Watkins, N.Y., Plate 1 and Fig. 191. Jackson Photograph Co., Denver, Col., Figs. 134, 221, 224, 237, 238, and 251. James, C. H., Photographer, Philadelphia, Pa., Eig. 108. Jukes-Browne, Handbook of Physical Geology, Fig. 195. Kent, Great Barrier Reef, Eigs. 79 and 207. Kobayashi, Earthquake Observations in Japan, Fig. 245. Koppen, Segelhandbuch fur den Atlantischen Ozean (reproduced by Davis, American Meteorological Journal, Vol. IX.), Plates 10 and 11. Lesley, Coal and its Topography, Figs. 256 and 262. Levy and Co., Paris (Dealers in Photographs), Figs. 143, 144, 175, and 220. Merriam, North American Fauna, Bulletin No. 3, U. S. Dept. of Agriculture, Eig. 68 ; same, National Geographic Magazine, Vol. VI. , 1894, Fig. 72. Mills, H. E., Annals, Harvard College Astronomical Observatory, Vol. 31, Fig. 53. Mills, H. R., Realm of Nature, Plates 16 and 27. Mississippi River Commission (Maps), Eigs. 158, 159, and 160. Mitchell, U. S. C. S., Annual Report for 1886, Eig. 85. Murray and Renard, Challenger Reports — Deep Sea Deposits, Plate 14. Nasmyth and Carpenter, The Moon, Fig. 10. Newcomb, Popular Astronomy, Eig. 12. Newell, Eleventh Census Report on Irrigation, Eigs. 61 and 62. Newton & Co., London, England (Dealers in Lantern Slides), Figs. 52, 55, 71, 106, 181, 205, 209, 234, and 339. New York State Weather Bureau, Fifth Annual Report, Plate 8 and Eig. 25 ; Figures based on the records of this bureau : 18, 28, 29, 33, 42, and 64. Notman (Photographer), Montreal, Canada, Fig. 225. Pach (Photographer), New York, N.Y., Eig. 82. Peschels (Leipoldt), Physische Erdkunde, Plates 18 and 19. Pillsbury, Annual Report, U. S. C. S. for 1890, Plate 17. ILL US TEA TIONS. XXxi Proctor Bros. (Dealers in Photographs), Gloucester, Mass., Fig. 80. Reid, National Geographic Magazine, Vol. IV., Plate 26. Richthofen, China, Fig. 252. Riggenbach (Photographs), Pigs. 50 and 57 (latter from several sources). Ritchie, J., Jr., Boston, Mass. (Photographs by), Pigs. 124 and 188. Russell, Pifth Annual Report, U. S. G. S., Pig. 177 ; same, Eighth Annual, Fig. 184 ; same, Thirteenth Annual, Figs. 67 and 180. Sella (Photographs; Chas. Pollock, Boston, Agent), Figs. 176 and 179. Shaler, Twelfth Annual Report, U. S. G. S., Figs. 107, 157, and 171. Sigsbee, U. S. C. S., Deep Sea Sounding and Dredging, Figs. 73 and 74. Smith, W. M. (Dealer in Photographs, Provincetown, Mass.), Fig. 192. Stoddard, S. R. (Photographer), Glens Falls, N.Y., Figs. 88, 167, 168, 172, 193, 263, 264, and 265. Symons, Eruption of Krakatoa, Fig. 235. Todd, Bulletin I., South Dakota Geological Survey, Fig. 112. Trotter, Lessons in the New Geography, Figs. 127 and 129. United States Coast Survey Charts, Figs. 153, 194, 208, 211, 267, and Plate 24. United States Geological Survey Photographs, Figs. 66, 94, 95, 96, 119, 122, 132, 142, 163, 174, 196, 230, 241, 242, 261, and Plate 28 ; same, Topo- graphic Maps, Figs. 150, 190, 228, and Plate 25. United States Geological Survey of the Territories (Hayden), Photographs, Figs. 69, 115, 121, 130, 156, 164, 219, 223. United States Hydrographic Bureau (Coast Pilot), Figs. 43, 44, 45. United States Signal Service and Weather Bureau, Figs. 30, 31, 32, 46, 47, 49, 60, 63, and Plate 3. Van Bebber, Lehrbuch der Meteorologie, Fig. 41 . Walcott, National Geographic Magazine, Vol. V., Fig. 109. Ward, Annals Harvard College Astronomical Observatory, Vol. 31, Fig. 51. Wild, Thalassa, Fig. 21. Willis, Thirteenth Annual Report, U. S. G. S., Figs. 92, 93, and 101. Williston, Prof. S. W., Kansas University Geological Department, Lawrence, Kansas (Photograph by), Fig. 90 and Plate 21. Part I. THE A IB. WITH AN INTRODUCTORY CHAPTER ON THE ASTRONOMICAL RELATIONS OF THE EARTH. ELEMENTARY PHYSICAL GEOGBAPHY. ?>Kc CHAPTER I. THE EARTH AS A PLANET. Form of the Earth. — The earth is a spherical body com- posed of three different portions, — a dense central mass, which is probably solid, and two envelopes, the ocean and the air. The central part has a much greater bulk than either of the other portions. In reality the form is not exactly spherical, for the diameters of a sphere should have the same length in all parts ; but in the earth the diameter at the equator is 26^ miles longer than that at the poles, where its length is 7899 miles. This flattening of the poles gives to the earth the form of an oblate spheroid instead of a true sphere (Fig. 1), While this irregularity of the earth was detected only after a series of very careful measure- ments, it is in reality the greatest on the surface of the earth; but there are other and less extensive irregularities, which are much more noticeable. These are of two kinds, — continents and mountains. The surface rises and falls in a series of great wave-like irregularities, which form the continents and ocean 3 Fig. 1. Diagram showing a section of a sphere (heavy line) , and an oblate spheroid (dotted line). PHYSICAL GEOGRAPHY. basins. On the continents, and occasionally in the oceans, the surface rises along relatively narrow lines into a series of high mountain ridges. Although these are the greatest elevations on the earth's surface, and therefore attract our attention, they are really very small irregularities when com- pared with the continents of which they usually form a small portion (Fig. 128). Considering the sea level as 0, the highest point on the earth is about 29,000 feet in elevation. Depressions of over 25,000 feet are found in several places in the ocean beds. The total range in elevation between the highest mountain, and the greatest ocean depth is about 57,000 feet. It can be readily seen how small this is in comparison with the earth as a whole, when we remember that the diameter of the earth at the equator is 41,847,192 feet. Upon a globe of ordinary size they could not be shown on true scale. Although there are points on the land whose height is greater than the deepest known parts of the ocean, the average depth of the ocean, which is about 12,000 feet, is much greater than the average height of the land, which is approximately 2500 feet (see Chap. XIV.). The greater part of the water on the earth's sur- face is accumulated in the broad hollows between the continents. The sur- face of this water mass is much greater in area than that of the land (Fig. 2), the proportion being 1 of land to 2. 6 of water (roughly 3:8). Late calculations give the area of the land as 142,000,000 square kilometers, and of the water as 368,000,000 square kilometers. The total volume of Fig. 2. Land and water hemispheres. THE EARTH AS A PLANET. 5 the water of the oceans is estimated to be 1,347,874,850 cubic kilometers. There are other smaller irregularities on the surface of the earth, and many minor peculiarities, some of which are dis- cussed in the later chapters. Surrounding the earth is a gaseous envelope, the atmosphere, which extends to an Fig. 3. The solar system, showing the relative distances from the sun, the direction of revolutions, relative size of the orbits, and the number of satellites. unknown distance, but which at a height of five or six miles from the surface is very much rarified. The Solar System. — The earth is one of several bodies which together form the solar system. They are a family of bodies bound together by the tie of gravitation and engaged in a series of movements around a central body, the sun (Fig. 3). In the solar system there are five classes of 6 PHYSICAL GEOGRAPHY. bodies. In the center is the sun, the largest of all, and the one upon which the others depend more than upon any other member. The second class of bodies is that of the planets, of which eight are known. These all revolve around the sun in orbits which are nearly circular, but not exactly so, being in reality, ellipses with the sun at one of the foci. The third class of bodies is that of the satellites, of which the moon is an example. Most of the planets have satellites, which are always much smaller than the planet about which they revolve. The earth has but one moon, but some of the planets have several. Twenty moons have already been discovered, of which all but three belong to the outer group of planets, Jupiter, Saturn, Uranus, and Neptune. A fourth group of bodies in the solar system is that of the asteroids, of which about 400 are now known. These small planets revolve about the sun in the space between the orbits of Mars and Jupiter. Aside from these members, there is a fifth group of irregular bodies, the comets and meteors, which move in a manner different from that of the other members of the solar system. The Sun. — The central and largest member of the solar system, the sun itself, unlike the planets, is so constituted that it sends out into space a form of energy which produces both light and heat. It is the source of much of the energy which finds expression upon the surface of the earth in the forms of light, heat, and life itself. This immense body is fully 92,750,000 miles distant from the earth. Since the sun is able to emit rays which produce heat, we know that it must be a hot body ; but there is as yet no means of telling what its temperature is. Owing to the way it affects the movements of the several members of the solar system, it is known that the materials composing the sun are not so dense as the solid part of the earth. It seems quite THE EARTH AS A PLANET. certain that at least a large part of the sun is in the form of gas. By means of the instrument known as the spectro- scope, we have learned much concerning the actual composi- tion of the sun. By this instrument it has been found that many of the elements known on the earth exist in the sun in a gaseous form. Since we know very little about the condition of the earth on which we live, it is hardly to be expected that our knowledge of a body so distant as the sun would be very accurate. Still the studies which have been carried on by means of the telescope have revealed the fact that there are at least three quite different parts to the sun. These are the corona, which is outermost, the chromo- sphere, and the photosphere, the latter being the densest part. It is the portion from which the light and heat are emitted ; and from its surface the diame- ter of the sun is about 860,000 miles (Fig. 4). Above the photosphere comes the chromosphere, which appears to be the true atmosphere of the sun. It consists mainly of glowing hydrogen gas ; but in its lower portions many metals, such as iron, are known to exist in the form of gas. It is in violent commotion, as if in eruption ; and the photosphere itself also presents signs of violent activity. Extending to a distance sometimes as great as 300,000 miles above the surface of the sun, is the corona, the character of which is not understood. Fig. 4. Diagram to show the relative size of the sun and the largest planets, true scale. Drawn on 8 PHYSICAL GEOGRAPHY. Certain peculiar spots known as sun spots (Fig. 5) appear upon the surface of the sun and move across its face until they disappear on the opposite side, being carried around by the rotation of the sun. Their origin H is not known, but they appear to have an influence upon the earth '- 1 . ; in at least two ways, one upon H atmospheric electricity, the other -- fi upon certain climatic features. IG ' ' _ The sun is engaged in two mo- Sun spots, 1872. ° ° tions. It rotates, as do all the larger bodies of the solar system ; but the period of rotation is not exactly known, though it is somewhere between 25 and 26^- days. Strangely enough, the period of rotation appears to vary according to the latitude. The second mo- tion is one in which the entire solar system is engaged ; but the amount and exact nature of this is not known. The sys- tem is moving through space at an unknown rate, toward the constellation Hercules. The Planets. — Mercury, the smallest of the planets, is nearest to the sun, on the average being about 35,750,000 miles from it (Fig. 6). The diameter is a little more than one-third Mars Jupiter Saturn Uranus Neptune |m-H — — h H — I » • •W. V Earth Fig. 6. Diagram to show the relative distances of the various planets from the sun. that of the earth (or 2992 miles), and it rotates on its axis in about 24 hours, while it revolves around the sun once in about 88 days. We know little concerning the condi- tions on this planet. THE EARTH AS A PLANET. Fig. 7. Diagram to show the rela- tive size of the smaller planets. The next body outside of Mercury is Venus, the most brilliant of planets. It is almost the same size as the earth, being in reality about 250 miles less in diameter (7660 miles) (Fig. 7). Some observers think that they have detected a rotation with a period of a little more than 24 hours ; but this is doubted by most astronomers. The period of revolution is considerably less than ours, or about 225 days. It appears quite certain that there is an atmosphere upon this planet, and so far as we can tell, it closely resembles ours. No satellite is known to exist. Outside of the earth, which is the next planet in the solar system, comes Mars, which next to Mercury, is the smallest of the planets, having a diameter of but little more than 4200 miles. Its time of rotation is a little over 24|- hours, and its revolution about the sun is accomplished in nearly 687 days. Its mean distance from the sun is 141,000,000 miles. The axis of Mars is inclined about 27° to the plane of its orbit, which is about 4° more than the inclination of the earth's axis. There are two tiny satellites, one less than 10 miles in diameter, the other perhaps twice that size ; and the latter is not more than 4000 miles from the surface of the planet, about which it revolves in a period of 7 h. 39 m. Jupiter, the largest of planets (Fig. 4), has a mass greater than that of all the others combined, the mean diameter being about 86,000 miles ; but the diameter at the equator is fully 5000 miles greater than that at the poles. The volume of Jupiter is about 1300 times that of the earth. On the average, the distance from the sun is about 480,000,000 10 PHYSICAL GEOGRAPHY. miles, and it takes nearly 12 years for it to make a revolu- tion about the sun. The time of rotation is a very little over 9 h. 55 m. It is evident that what we see with the telescope is not the surface of the planet, but a dense atmosphere of some form of cloud. Therefore we have no means of knowing what the actual condition of Jupiter is, though we may infer that the planet is still heated, and that the clouds which we see are the result of this heated condition. Five moons revolve about Jupiter, the most distant being 1,162,000 miles from the planet, while the nearest is only a little farther away than our moon is from us. Next beyond Jupiter is Saturn, the second largest of the solar planets. Its distance is 881,000,000 miles from the sun, around which it revolves in about 29|- years, while it rotates upon its axis in 10 h. 14 m. 1 This planet has eight moons ; but the most remarkable feature connected with it, is its surrounding group of flattened rings, whose inner diameter is 100,000 miles. The telescope has not yet defi- nitely revealed the nature of these rings. As the distance from the earth increases, our knowledge of the members of the solar system becomes less accu- rate. Hence, since its mean distance from the sun is fully 1,771,000,000 miles, Uranus is scarcely known. It revolves about the sun once in 84 years, but its period of rotation is not known. There are four satellites. Until 1846 no other large planet was known; but as a result of prediction, Neptune was discovered in that year. The discovery of this planet is one of the most remarkable proofs of the accuracy of the theory of gravitation; for it 1 It will be noticed that as the distance from the sun increases, the time required for a revolution also increases, while the period of rotation rapidly decreases. THE EABTH AS A PLANET. 11 was determined by irregularities in the movement of Uranus, that another planet must exist outside of its orbit; and after •careful calculations, the place where this planet could be found was predicted, and Neptune was discovered at a mean distance of 2,775,000,000 miles from the sun. One moon has been detected. Asteroids. — In the year 1801, a small planet known as Ceres was discovered in the space between Mars and Jupiter. Since that time about 400 other smaller bodies have been found in the same general region. In no cases have these small planets a diameter greater than 520 miles, while the smallest that have been discovered have diameters of less than 40 miles. Their movement through space is some- what irregular; and there have been many speculations con- cerning their origin, though as yet no satisfactory explana- tion has been advanced. The Earth. — While cold at the surface, we have many reasons for believing that the interior of the earth is highly heated. Proof of this is found in the facts that at the surface, volcanoes emit quantities of molten rock which come from below, and that in all deep mines and well-borings the temperature of the rocks is found to increase at a moderately uniform rate, on the average 1° for about every 50 or 60 feet of descent. If this rate of increase continues, the rocks at a depth of less than 100 miles are so hot that they would be molten under the conditions which exist at the surface. It was once believed that the interior of the earth was in a molten condition, and that the solid surface was merely a crust resting upon this liquid sphere; but many facts now lead us to the belief that the interior is at least as rigid as steel. The proof of this has been furnished by the studies of physicists and astronomers. At present we are forced to the belief, that although highly heated, the rocks in the interior 12 PHYSICAL GEOGRAPHY. of the earth are prevented from melting by the great pres- sure of the overlying layers; and by this theory we are able to satisfactorily account for all of the phenomena that Fig. 8. Diagram illustrating the cause of seasons. formerly seemed to demand the explanation of a liquid interior. The earth is engaged in a number of movements in space. It revolves around the sun in about 365.24 days, in an THE EARTH AS A PLANET. 13 orbit which is nearly a circle; but instead of being actually a circle with the sun at its center, the orbit is really an ellipse with the sun at one of the foci. Therefore, in the course of its revolution, the earth is at one time farther from the sun than in the opposite season, the distance now vary- ing between 91,000,000 and 94,000,000 miles, with an average distance of about 92,750,000 miles. During the revolution, the earth rotates about one of its diameters, which we call the axis, and this rotation occu- pies a little less than 24 hours (23 h. 56 m.), or one day. This rotation causes the familiar alternation of day and night; and if the earth's axis were at right angles to the plane of revolution, the day and night would be equal in length; but since it is inclined to this plane at an angle of 23° 27', the relative length of day and night varies from day to day. Indeed, the seasons themselves depend upon this inclination of the poles (Fig. 8); for in the course of a revolution, the pole is always pointed toward a certain part of the heavens ; and as the earth moves about the sun, the northern hemisphere alternately faces and is turned away from the sun. When turned toward the sun, the summer season is caused, and when turned away from it, the winter season results, because the solar rays then fall less vertically upon the hemisphere, and the length of the day is shorter. Between these two opposite seasons we have spring and autumn. The Moon. — This, the nearest to our earth of all the heavenly bodies, has an average distance of about 240,000 miles, and a diameter of 2160 miles (Fig. 9). Since the path of the moon about the earth is an ellipse with the earth at one of the foci, the distance varies ; but it is rarely more than 253,000 miles nor less than 227,000 miles distant. When farthest from the earth it is said to be in Apogee, 14 PHYSICAL GEOGRAPHY. Fig, 9. The relative size of earth and moon. and when nearest in Perigee ; and once in every revolution Apogee and Perigee are reached. Aside from those it makes in company with the earth, its two important movements in space are a revolution around the earth and a rotation about an axis, both of these movements occurring in the same period of time, or 29^ days. Therefore one side of the moon is never seen from the earth. Also, as a result of this condition, the length of the lunar day is 29J of our days ; and therefore at the lunar equator the sun shines steadily for nearly 15 days and is absent an equal length of time. Under these conditions the surface of the moon is warmed during the long day, and at night becomes cooled down to temperatures which are perhaps as low as - 200°. There is no atmos- phere and no ocean on the moon ; and the only change upon the surface seems to be that between con- ditions of heat and cold, and light and darkness. It emits an almost impercepti- Fig- 10 - ble amount of radiant Lunar craters, the largest being Gassendi. energy, and the light from the moon is reflected sunlight. 1 As a result of the careful telescopic study of the moon, 1 Direct light from the sun is 600,000 times as strong as that which is reflected from the moon. THE EARTH AS A PLANET. 15 astronomers have been able to map many of the details of lunar topography, with considerable accuracy, and even to measure mountain heights. While there are other striking topo- graphic features, the most notable thing about the lunar land- scape is the great number of crater-like mountains, which bear a certain resemblance to the volcanoes on the earth's surface, excepting that many of them are of immense size (Fig. 10). Comets, Shooting Stars and Meteors. — Aside from those described, which may be considered the normal members of the solar system, there are other heavenly bodies which do not appear to be regular parts of the system. The strangest of these are comets. Some 500 of these have been recorded as visible to the naked eye ; and in addition, over 200 have been detected by the aid of the telescope, some of these being millions of miles in length. When near the sun, they usually have a relatively dense head and a vaporous tail, through which stars are visible (Fig. 11). Some have regular elliptical orbits, and their time of appearance can be closely calculated; but the orbits of others are ap- parently parabolas, so that if they ever return to the solar system, it is only after long periods of time have elapsed, and after having made a journey far beyond ^ ^^ ^ the outermost limits of the solar system. Perhaps these may be mere wanderers through space, which after one visit to the solar system, depart never to return again. What they are, whence they came, whither they are going, or what relation they bear to the solar sys- tem, is still an unsolved mystery. 16 PHYSICAL GEOGRAPHY. Comets have an added interest to us, from the fact that some shooting stars and meteors seem to be remnants of comets, which at some former time have crossed the orbit of the earth. Thus the November meteorites are due to the fact that in its movement around the sun the earth en- counters particles that are left in the trail of a comet (Tem- pers) which has a period of revolution of about thirty-three years ; and the August meteors (Fig. 12) appear to have a similar origin. Meteors and shooting stars (meteors are large shooting stars) enter the earth's atmosphere at a high rate of speed, and are burned up ■* •, 0/ August Mehpre in the higher lay ers of the atmosphere, often at an eleva- tion as great as 100 miles from the sur- face of the earth. This burning is the result of fric- tion with the air, which produces a high heat, because in addition to the movement of the meteor, there is often added the motion of the earth itself, which is about 98,000 feet a second. Hence in small bodies, the burning is almost instantaneous ; but some of the larger meteors pass entirely through the atmosphere, and reach the earth's surface. A study of these rather rare meteorites, reveals to us the very interesting fact that no new element exists in them ; and therefore we may fairly conclude that the elements composing comets are the same as some of those which make up the earth's crust. In watching the heavens at night, scarcely an hour can pass without noticing shooting stars ; and since the same would probably be true of the day if we Fig. 12. Orbit of the second comet of 1862. THE EARTH AS A PLANET. 17 could then see them, we conclude that there are immense numbers of these bodies in the space through which the earth travels. The Stellar System. — Far away in space, many times farther than the sun is from us, innumerable stars are scattered. Already many thousands are known, and it is estimated that over 30,000,000 are visible with the telescope. Like the sun, they emit an energy which produces both light and heat ; and it is very probable that many, if not all, have planetary bodies revolving about them. One satellite, that belonging to Sirius, has already been dis- covered ; and some double stars are known to be re- volving about a common center of gravity. The distance between the stars, and even between the earth and the nearest stars, is im- mense, and in most cases in- calculable. If each star is a sun with accompanying planets, and if each of these suns is as far from its near- est stellar neighbors as we are from ours, the immensity and grandeur of the system transcends our imagination. The stars are arranged in a disc-like belt, the greatest diameter of which is in the direction of the Milky Way. At right angles to this there is a zone of abundant nebulce, (Fig. 13), although these strange bodies are not absent from other parts of the heavens. Some have conjectured that Fig. 13. Andromeda nebula, from a drawing. 18 PHYSICAL GEOGRAPHY. nebulae are other stellar systems, so distant from us that the individual members cannot be separated by our tele- scopes ; but the spectroscope seems to show that they are bodies of glowing gas, and this has an important bearing upon the nebular hypothesis, which we soon discuss. One very important thing concerning both stars and nebulas, is that the spectroscope has detected in them many of the elements which we find upon the earth. A question of very deep interest, is whether the stars form a great system in which the individual members are inter- related, as is the case among the members of the solar system ? Unfortunately, in the present state of science, we are unable to return a definite answer to this question. Symmetry of the Solar System. — In theorizing upon a basis of known facts we must confine ourselves to the solar system ; and it is interesting to note the wonderful symmetry of arrangement and the beautiful order which exists here. Throughout the entire system, the law of gravitation prevails and governs the movements of all the bodies, each member attracting the other in direct proportion to the product of the masses and inversely proportional to the square of the distance. The regular members of the system are all nearly spherical, and they rotate about an axis and revolve in an orbit which is nearly circular. In direction of rotation and revolution there is a marked uniformity, as there is also in the plane of revolution. All of these regularities of behavior, take place notwith- standing the fact that immense distances separate the various bodies, and that this space is practically void. We can form no accurate conception of these immense distances ; but the following quotation from Newcomb's Astronomy furnishes some idea of this : — "To give an idea of the relative distances, suppose a THE EARTH AS A PLANET. 19 voyager through the celestial spaces could travel from the sun to the outermost planet of our system in twenty-four hours. So enormous would be his velocity, that it would carry him across the Atlantic Ocean, from New York to Liverpool, in less than a tenth of a second of the clock. Starting from the sun with this velocity, he would cross the orbits of the inner planets in rapid succession, and the outer ones more slowly, until, at the end of a single day, he would reach the confines of our system, crossing the orbit of Nep- tune. But, though he passed eight planets the first day, he would pass none the next, for he would have to journey eighteen or twenty years, without diminution of speed, before he would reach the nearest star, and would then have to continue his journey as far again before he could reach another. All the planets of our system would have vanished in the distance, in the course of the first three days, and the sun would be but an insignificant star in the firmament." The sun in the center of the solar system is a true star, in many respects like the others which dot the firmament. This being the case, may we not fairly speculate as to the possibility of other worlds and systems like our own, far away in space, even to the outermost limits which can be reached by the human vision ; and if this be so, how vast is the universe, and how insignificant the small cold body of matter upon which we dwell ! The Nebular Hypothesis. — Before many facts concerning the universe were known, the philosopher Kant proposed a hypothesis to account for the origin of the solar system ; and later, Herschel and Laplace proposed an explanation which in many respects was like that of Kant. We know this explanation under the name of the nebular hypothesis. By this it is assumed that the space occupied by the members of the solar system, and probably even to a con- 20 PHYSICAL GEOGRAPHY. siderable distance beyond this, was occupied by a nebulous mass of highly heated vapor. It is one of the laws of nature that radiant energy passes from warmer to colder bodies, and that by this radiation a contraction and condensation neces- sarily follow. This nebulous mass, composed of all the ele- ments which now enter into the composition of the various members of the solar system, during the process of cooling separated into rings which were the parents of the several planets. As the mass lost heat and began to condense and contract, the materials began to accumulate about some denser part of these rings, the accumulations about these denser portions being determined by the fact that gravita- tive action was stronger there than elsewhere. As a result of this accumulation about centers, the original nebulous mass became broken up into several smaller masses of similar nature ; and by a continuation of the process other rings were thrown off, out of which the satellites were formed. Original motion about a central portion of the nebula has naturally been inherited and is now indicated by the movements of the bodies in the solar system. The cooling of these bodies is still in progress, and different members of the system have reached different stages. Verification of the Nebular Hypothesis. — While we cannot state that this theory is definitely proven, many facts point to its truth as a general explanation of the solar universe. For instance, it would account for the fact that the planets move about the sun in a common direction, and that the planes of revolution are nearly the same in the different planets (the inclination in no case being more than a few degrees) . This similarity also extends even to the satellites ; and the rotation of the bodies whose rotation has been determined has the same kind of uniformity. All of the orbits of the members of the solar system are ellipses THE EARTH AS A PLANET. 21 approaching a circle. This together with the uniform action of gravitation suggests a common origin. The fact that all the bodies regularly belonging to the solar system are nearly spherical in form is suggestive ; and this form can readily be accounted for if the bodies were once liquid. A former liquid condition is suggested by the fact that those bodies which are well known, all have a larger diameter at the equator than at the poles, although it is true that this may be explained in other ways. Then also, signs of heat are plainly seen in some of the mem- bers of the solar system ; and in the smaller bodies these signs are less apparent. Thus the sun is highly heated; Jupiter, Saturn, and other of the outer planets show signs of considerable heat ; the earth is cold at the surface, and hot in the center ; Mars, Venus, and Mercury are cold at the surface ; and the moon appears to be entirely cold. Upon the nebular hypothesis, we should expect that the density of the members of the solar system would increase from the outer bodies toward the center ; and this actually is the case, the only exceptions being the easily explained cases of Saturn and the sun. There are other reasons for believing in the nebular hypothesis. So far as we may judge from the results of spectroscopic study and from the examinations of meteorites that have fallen upon the earth, the bodies in the solar system are composed of the same elements as those which make the earth ; and this sug- gests that they have been made from the same original mass. Far away in space, beyond the solar system, we even find nebulous masses of gas which are exactly like those out of which the solar system is believed to have been made ; and in some of these nebulae the condensation into planetary bodies appears to be in progress (Fig. 13). Nearly every gradation has been found between this kind of nebula and 22 PHYSICAL GEOGRAPHY. that which is apparently one mass of glowing gas. It is not improbable that even now other worlds are in process of formation in the far distant regions of space. REFERENCE BOOKS. 1 Newcomb. — Popular Astronomy (school edition). Harper Brothers, New York. Seventh edition, 1894. 8vo. Published also in larger form. School edition, $1.30 ; larger book, $2.50. (General and quite elementary.) Lockyer. — Elementary Lessons in Astronomy. Macmillan & Co., New York. 8vo. $1.25. (General and elementary.) Chambers. — Handbook of Descriptive and Practical Astronomy. Mac- millan & Co., New York. Fourth edition, 1889. 8vo. Three volumes. Vol. I., $5.25 ; Vol. II., $5.25; Vol. III., $3.50. (Large and comprehen- sive.) Proctor and Ranyard. — Old and New Astronomy. Longmans, Green, & Co., New York, 1892. 8vo. $12.00. (Complete and well illustrated.) Young. — The Sun. International Scientific Series. Appleton & Co., New York, 1893. 12mo. $2.00. Lockyer. — The Chemistry of the Sun. Macmillan & Co., New York, 1887. 8vo. $4.50. Nasmyth and Carpenter. — The Moon. Murray, London (Scribner, New York agents), 1885. 8vo. $8.40. (Many remarkable photographs.) Neison. — The Moon. Longmans, Green, & Co., New York, 1876. 8vo. $10.00. (Well illustrated.) Lockyer. — The Meteoritic Hypothesis. Macmillan & Co., New York, 1890. 8vo. $5.25. (Suggestion of modification of the nebular hypothesis.) Scheiner (translated by Frost). — A Treatise on Astronomical Spec- troscopy. Ginn & Co., Boston, 1894. 8vo. $5.00. 1 In giving the publisher's name, the real publishing house is often not mentioned. Wherever possible American houses are given, and since some of these act as agents for European houses, the name of the agent will at times appear in the place of the English publisher. CHAPTER II. THE ATMOSPHERE. General Statement. — Outside of the solid earth, and ex- tending to a distance of several hundred miles above it, is a gaseous envelope, which we know as the atmosphere (Fig. 14). Its density decreases from the surface of the earth toward the upper portions; and at a height of five miles it is very much rarefied. That it ex- tends to this great height is shown by the fact that meteors become white hot by friction with it, even at a greater dis- tance than this from the earth. Fully one-half of the mass of the atmosphere is within four miles of the surface of the earth ; and two-thirds of it is within six miles of the surface (Fig. 15). The atmosphere is composed almost entirely of two gases, nitrogen and oxygen, in the proportion of about 79 to 21. These gases are not in chemical combination, but are mechanically mixed. Nitrogen is a very inert element, while oxygen is active in the production of many changes, and from 23 The earth with its atmospheric envel- ope, drawn to scale. 24 PHYSICAL GEOGRAPHY. this standpoint the nitrogen of the air may be considered as an adulterant of the active oxygen. In additf.on to these gases there is a comparatively small amount (about 0.03 per cent) of carbonic acid gas, the percentage varying some- what according to the location. Its percentage increases in the vicinity of volcanoes and large cities. 1 Beside these three gases there are minor and variable quan- tities of other substances ; but of these, only two, water vapor and dust particles, are of sufficient general importance for consideration here. The term "dust" includes a great C^-icS'w^'S ^y^*5Sc/^-;::V"c •■"! Fig. 15. Diagram to illustrate decrease in density of the atmosphere. :;:;,:;, variety of substances, such, for instance, as microbes, smoke particles, and true dust, which is borne into the air by the winds. It seems certain that dust is of much importance in the formation of rain and fog. Water is readily evaporated, and hence at all times there is some water vapor in the air; but the amount depends upon a variety of circumstances, chiefly the temperature of the air and the presence or absence of bodies of water. With a 1 While this book is in preparation, the discovery of a new constituent of the atmosphere is announced. This, which is called argon, may be a new element, but it is now too early to state anything definite about this sub- stance. THE ATMOSPHEBE. 26 given amount of moisture, the higher the temperature, the greater the rate of evaporation ; but even at temperatures below freezing-point small quantities of water vapor may be present. The atmosphere is of great importance in many respects. It distributes the light which comes to us from the sun. It is set in motion by the solar energy, and by this means distrib- utes heat over the earth. As a result of the effect of solar heat upon the atmosphere a great variety of phenomena, such as winds, storms, clouds, etc., are produced. These cause many changes upon the surface of the earth, and directly and indirectly the air makes the earth a place fit for habitation. Light. — We obtain light from several sources, — the sun, the stars, and the moon and planets. Light from the latter source is merely reflected sunlight, and it is small in amount. That which comes from the stars is radiated from them directly, but it also is insignificant in comparison with that received from the sun. Solar light, when it reaches the lower layers of the atmos- phere, produces the impression upon the eye which we know as white; but it has been shown that it probably has a bluish tinge before its passage through the air. According to the undulatory theory, light passes through the space between us and the sun at a very rapid rate in the form of a series of waves of ether. It is made up of many waves of different lengths, the combination of which gives white. When separated, these appear as different colors, and in the rain- bow we recognize seven primary colors with intermediate hues. The violets and blues have the shortest vibrations, and the yellows and reds the longest. As a result of the effect of the atmosphere upon these parts of white light many optical phenomena are produced. If there were no atmosphere, the earth's surface would be 26 PHYSICAL GEOGRAPHY. illuminated only where the direct rays of the sun fell. The atmosphere serves to diffuse light and to render the darkness of shadows less intense. This diffusion of light in large measure depends upon the amount of solid or liquid impuri- ties in the air. In its passage through the air, certain of the rays are diffused more readily than others by the process of selective scattering. It is those rays that have the shortest wave lengths that are thus scattered ; and hence it is that the sky is ordinarily blue. The intensity of the blue is great- est when coarse dust impurities are least abundant, as is the case when the air is clear and dry. If dust particles happen to be very abundant, even the coarser rays of yellow light may be scattered ; and under rare conditions of very smoky air the entire sky may assume a brassy color. Since the light is obliged to travel through a greater distance of ais near the time of sunset than in midday, the color of the western sky in the late afternoon is often yellow, while that of midday was a dull hazy blue (Fig. 16). Among the most beautiful of light effects in the atmos- phere is that of the sunset colors, which are due to the scat- FlG - 16 - tering of the waves Diagram to show that the sun's rays pass through a „>, ; n y. hc^ra + >> a ,i .. j. , 1 , t WiliOll lldvc bile greacer thickness of atmosphere at sunset and sun- rise than at midday. (Thickness of atmosphere smaller lengths. As greatly exaggerated). a ^^ Qf ^ ^ coarser yellows and reds come to us, the reason for the scat- tering being the fact that the light at the time of sunset and sunrise passes through a great thickness of air, and hence the THE ATMOSPHERE. 27 waves encounter a greater number of dust particles. When the atmosphere contains much dust, the morning and evening colors are often very intense, but an increase in the quantity of dust beyond a certain point tends to dull the tints. With clouds in the horizon at sunset or sunrise, these colors of red and yellow are often reflected in infinite variety of shade and tint. Other phenomena, such as the twilight arch, the glow and the afterglow, are associated with the setting of the sun. Another property of light is that of reflection, and as a result of this many interesting optical effects are produced. The light of the moon depends upon the reflection of sun- light from its surface. The earth also reflects light, and this is one of the reasons for the illumination of places that are in the shadow of the direct rays of the sun. Other places which are illuminated reflect some of their light to the parts that are in shadow. Clouds also reflect the light of the sun ; and on summer days, when great banks of clouds rise high in the air, their surfaces are brilliantly illuminated and beautiful cloud effects are produced. Another effect of reflection is the mirage, which occurs *f- when the air near the surface is warmer than the layers above it, and when the reflection from this warm air layer reaches the eye of the observer. It often gives rise to an appearance like that of a sheet of water ; and travelers in desert lands, where this phenomenon is common, are often led to think that they are actually approaching a lake. One very commonly sees such an appearance as this at the sea or lake shore when distant coasts appear to rise above the sur- face of the water. It sometimes happens that light is reflected from a warm layer which is above the observer ; and then the objects appear upside down. This " looming," as it is called, is particularly common in Arctic regions ; and 28 PHYSICAL GEOGRAPHY, the effect produced is so fantastic and wonderful that nearly all Arctic explorers describe it. The rainbow is a phenomenon which partly depends upon the reflection of sunlight ; but it is chiefly due to refraction. the result being a separation of the several components of white light into the colors of the spectrum. Each person sees a different rainbow even though two observers may stand side by side. The cause for the phenomenon is the effect of raindrops which, being denser than the air, bend and separate the rays of white light so that we see the component colored rays, just as we do when a sunbeam passes through a prism. A rainbow is often produced in the spray that rises at the base of a waterfall, and at the distance of only a few yards one may see it outlined in the spray. Another phenomenon resulting from the combined action of refraction and reflection is the ring of light or halo which often surrounds the sun or moon when their light passes through thin hazy clouds in the upper atmosphere. These clouds are composed of ice particles, which act upon the light in a manner analogous to the effect of raindrops in the production of the rainbow. Very remarkable halos are formed, particularly in Arctic regions, where the air is often filled with minute crystals of ice. Sometimes rings of light of very brilliant colors are thus produced. The interference with light resulting from the presence of water or ice in clouds often produces a ring of light immediately around the sun or moon. These are called coronas, and they are often beautifully colored, the colors being arranged in con centric rings with the red on the outside. One of the most important of the phenomena of light is that of absorption. Many bodies, such as pure air and water, allow most of the rays of light to pass through them with little change, and such bodies are called transparent. Other sub- THE ATMOSPHERE. 29 stances are only partially transparent, and we know them under the name of translucent bodies. Still others which we know as opaque do not allow any light to pass. Thus objects have a red color when they reflect a greater number of the red than of the other rays ; and other colors are produced in the same way by the absorption of different proportions of the rays. Electricity and Magnetism. — There are certain phenomena of magnetism in the earth which some believe to exercise a decided influence upon the atmosphere. The earth is a great magnet, and the region of greatest magnetic attraction is near Hudson's Bay, toward which the needle of the compass points in our hemisphere. This may be called the north magnetic pole. The magnetic condition of the earth is con- stantly changing, both in small daily variations and in annual changes, as well as in variations covering many years. Occasionally there are magnetic storms, when there is a disturbance of magnetic instruments, and when the aurora sometimes develops in wonderful complexity and weird beauty. This is some electrical effect in the thin apper atmosphere ; but our knowledge of this phenomenon is obscure. Electricity is produced in the atmosphere by various causes, and it is nearly always present ; but only rarely does it develop sufficient intensity to become visible to the eye. In thunderstorms and tornadoes, when the air is in violent commotion, there is often sufficient electricity to cause vivid discharges from one cloud to another, or to the earth. This lightning is an interesting phenomenon, but it does not appear to have an important influence in the formation of the storms, being really a result of them. The accompanying sound is often changed to a rumble by reverberation and echoes among the clouds, and between them and the earth. Often 30 PHYSICAL GEOGRAPHY. in violent thunderstorms the air is filled with a constant roar of thunder. The lightning spark or bolt is sometimes a single large spark, or it may divide and sub-divide, giving a branching type of discharge ; and many interesting irregu- larities of direction, color, and form are produced. The light from the flash moves with great velocity while the sound of the thunder travels slowly, at the rate of ordinary sound waves. The sound wave is readily worn out, and at a distance of a few miles lightning produces no perceptible sound. Seat lightning is often the result of the reflection among the clouds, or on the horizon, of lightning in some far-distant thunderstorm, perhaps en- tirely hidden behind the curvature of the earth. Heat. 1 — Aside from the heat which comes to us from the sun, we obtain a certain small but more constant supply from the other bodies of space and from the earth itself; but these are relatively unimportant. The radiant energy from the sun travels at an enormous velocity as a series of waves, which are radiated out from the sun in all directions ; and only that small portion of them is received by the earth which it happens to intercept in its passage about the sun. Some substances allow this energy to pass through them with readiness, and these are said to be diathermanous ; others absorb it ; and still others reflect the greater part of the rays that come to them. The air is comparatively diathermanous, as indeed most transparent substances are. The smooth glassy surface of water is a good illustration of a substance that reflects much of the radiant energy coming to it. On the other hand, while the earth reflects some, it absorbs a large quantity of heat ; and this is 1 The sun is emitting a form of energy which under favorable conditions be- comes heat, while under other conditions it takes the form of chemical energy These rays are therefore properly radiant energy until transformed to heat. THE ATMOSPHERE. 31 particularly true for parts of the earth which are dark in color. The rays that enter the atmosphere pass through it with little interference, because it is diathermanous ; but if there is much dust or water vapor in it, a considerable share of the rays are intercepted. Thus clouds effectually check the passage of many of the rays, and hence cloudy summer days are cool. The same effect is produced by a very hazy atmos- phere, and in the late afternoon when the solar rays pass through a great thickness of air (Fig. 16), the amount of heat that reaches the earth is very much less than that which comes to the surface at midday. Since different parts of the earth's surface behave dif- ferently toward the radiant energy, there is much varia- tion in the effect produced. This is particularly well illustrated by the very marked difference in behavior be- tween water and land. The rays that reach the water sur- face are in part reflected back into space and thus lost, so far as the earth is concerned. Much of that which remains raises the temperature of the water ; but as the specific heat of the water is high, its temperature is raised very slowly. Some is used in the evaporation of the surface layers ; and in that case the solar rays are transformed to the so-called " latent heat," 1 which does not become appar- ent until the vapor is condensed to water. Moreover, the water surface is in motion ; and this tends to distribute the heat, and thus to prevent the excessive warming of the ocean surface. Therefore for these various reasons, even at the equator the ocean surface remains relatively cool. On the other hand, land reflects very little of the radiant energy, and it is a solid body, in which neither evaporation 1 The old term is still used, though perhaps heat of vaporization would be better. 32 PHYSICAL GEOGRAPHY. nor motion is possible. The earth is distinctly not diather- manous, and the greater part of the rays which reach it are absorbed by the surface portions. Therefore during the day the ground tends to become warmed by absorption ; and this peculiarity is responsible for many of the phenomena of the atmosphere, which are later described. Pure air is very slightly warmed by the passage of the direct rays of the sun. The small amount of heat thus obtained is slightly increased by a supply received from the rays which the earth reflects ; but much more is obtained from the supply which the earth absorbs. All bodies in space are radiating a form of energy, either that which belongs to them or that which is radiated to them ; there- fore the earth is at all times emitting rays by direct radi- ation. During the daytime the amount radiated is less in quantity than that received from the sun ; but at night, when this supply is cut off, the process of radiation proceeds so far that the earth loses much of the heat which it had received. Radiation is interfered with by the presence of clouds or dust; and hence nights which are cloudy or hazy are warmer than those which are clear. By the process of conduction, all bodies which are warmed tend to transmit their energy to cooler portions. This is well illustrated when a cold iron is placed upon a warm stove. In the same way, the air in contact with the warmer earth is thus warmed by conduction ; but neither air nor earth are good conductors of heat, and if this process were unaided, the effect would be slight and confined to those lower layers of the air which were almost immediately in contact with the earth. It is a property of gases that when heated they are expanded and thus made lighter. By this means a process of convection is started which bears some analogy to the boiling of water, and the warm lower layers THE ATMOSPHERE. 33 of air rise above the surface, because the colder and denser air forces the lighter layers to ascend. The process of convection is one of the most important in meteorology ; for upon it in large measure depends the development of the winds and other features of atmospheric circulation. When air rises it expands, and in the process of expansion necessarily cools, the rate of cooling being 1.6° for every 300 feet of ascent; and descending air, as a result of compression, becomes warmed. This feature of cooling on ascension gives rise to the formation of many of the clouds and rainstorms. Thus the air is warmed, partly by the rays which come direct from the sun ; partly by those which are reflected from the earth ; partly by those emitted from the earth by the process of radiation ; but mainly by conduction from the warm earth's surface and the convectional rising of these warmed layers. Highlands are cooler than low- lands, largely because the air in these places is less dense than that nearer the sea level (Fig. 15). The presence or absence of large bodies of water very markedly modifies the effect of solar energy upon the atmosphere. As a result of these differences, the atmosphere is put in motion, winds are produced, clouds are formed, storms are started, and rains are caused. The movements of the earth in space also give rise to many variations in heat effect and atmospheric phenomena. As a result of the rotation of the earth, the greater part of its surface is lighted and warmed during a part of every twenty-four hours, and thus we have day and night. A second important movement of the earth is that of revolution, which causes the seasons (Figs. 8 and 17). Since the pole is inclined to the plane of revolution, the sun is made to appear to migrate in the heavens. During 34 PHYSICAL GEOGRAPHY. our winter, when the sun is vertical over that part of the earth which lies between the equator and the tropic oi Capricorn, the sun rises in the southern part of the heavens, and passes westward without rising high toward the zenith. Then in Arctic latitudes, the sun does not rise above the horizon ; and therefore in this region there is no alterna- tion of day and night. In the winter season, in temperate Fig. 17. Diagram to show the inclination of the sun's rays in different parts of the earth during the various seasons. Upper figure, spring and autumn ; right-hand figure, northern winter ; left-hand, northern summer. latitudes the journey of the sun across the heavens occupies a small fraction of the whole day ; and therefore in such regions the time during which the earth is receiving heat is less than the length of the night, during which almost none is received. Besides this fact of short days and long nights, the angle at which the rays reach the surface is much THE ATMOSPHERE. 35 more oblique than in the summer season ; and before reach- ing the surface they are obliged to pass through a great thickness of atmosphere. These facts make the effect of the small amount of energy that does come, less apparent in winter than in summer, when many of the rays pass from a point near the zenith through a relatively small amount of atmosphere, reaching the surface more nearly at right angles (Fig. 17). After the sun has passed north of the equator, summer comes to the northern hemisphere, while winter prevails south of the equator. Thus at any point between equatorial and Arctic regions, there are two variations in the effect of the solar rays, one a daily and the other a seasonal variation. The tempera- ture of the air over the land normally rises during the day, and falls at night ; it rises in summer, and falls in winter ; and the amount of daily rising and falling is greater in summer than in winter. There is much variation in these respects according to latitude ; and there is less change in temperature between day and night, and between seasons, at the equator than in most other latitudes ; but the amount of heat received there is greater than in other parts of the earth. The greatest range in temperature, both sea- sonal and daily, is experienced in the higher latitudes. The least heat supply is received in polar latitudes; and here there is a great range between the summer and winter tem- peratures, but slight daily ranges, because in winter the sun does not rise above the horizon, while in summer it does not set. Moisture. — When rays of radiant energy enter a water body, they are in part transformed to "latent heat," being engaged in the process of changing the liquid to a gaseous condition. By this process of evaporation much of the energy exists in a form which is not apparent as heat so 36 PHYSICAL GEOGRAPHY. long as the vapor condition lasts; but when the vapor is con- densed, this store of heat becomes apparent. Evaporation will take place even from a snow surface ; but the most favorable conditions for the production of water vapor are warm air in contact with a water surface. The capacity of the air for water vapor is limited ; and when no more can be contained it is said to be saturated. When there is little vapor in the air it is constantly capable of taking more until the limit of saturation is reached. We commonly say that dry air can absorb vapor. 1 If the amount of water upon the land is slight, the air in these places remains dry ; but naturally this cannot be the case with air over bodies of water, for there the conditions favor satu- ration. In the interior of continents, and in the upper layers of the atmosphere, there is the smallest proportion of water vapor. If the air from these places reaches the oceans, it may bring to them conditions of dryness, which, however, are soon changed to relative dampness. With the air in movement, saturation is less liable to occur than would be the case if the air were quiet. Therefore winds favor evaporation by bringing fresh supplies of air, and for the same reason they tend to prevent saturation. The capacity of air for water vapor also depends upon its temperature. A layer of air which is saturated at the tem- perature of 50° becomes relatively dry if its temperature is raised to 90°; and an air layer which is nearly saturated at 90° will be obliged to give up some of its water vapor if the temperature is lowered a number of degrees. This is a very important point in the formation of clouds, storms, and rains. The actual amount of water vapor in the air represents its 1 Strictly the air does not absorb vapor, but the water vaporizes regardless of the presence of the air. However, it is convenient to speak of the capacity of the air for water vapor, especially as the air determines the temperature. THE ATMOSPHERE. 37 absolute humidity ; but this is not a very important factor, because the same amount of vapor in air of different tem- peratures will produce very different effects. The point of greatest importance is the relative humidity, which is the percentage of water vapor actually contained in the air compared with the amount which the air at that tem- perature could contain if it were saturated. Thus the relative humidity of saturated air at a temperature of 60° is 100 per MONDAY TUESDAY WEDNESDAY THURSDAY 6 XII 6 6 XII 6 6 XII 6 6 XII 6 luu 80 60 40 20 /^ ^\l AlA" 1 I w 1 J' 1 1 ' ! | ! 1 ! SEP. II 13 14 Pig. 18. Diagram showing daily change in relative humidity as a result of the daily change in temperature at Ithaca, N.Y. cent, for at that temperature no more can be contained ; but if the temperature is raised a few degrees, the air becomes capable of containing more water vapor, and the relative humidity is then less than 100 per cent. The tem- perature at which air containing a given amount of moisture becomes saturated is known as the dew 'point, for then vapor must be condensed. After a warm and apparently dry day, dew may be formed at night merely by lowering the tempera- 38 PHYSICAL GEOGRAPHY. 1000 FT. \ Saturated. ture of the air, and thus increasing the relative humidity, with- out any change whatsoever in the absolute humidity (Fig. 18). It follows from this that there must be very marked differ- ences in the amount and effect of water vapor contained in the air. Over the oceans, the relative humidity is great, and the air nearly always near the point of saturation ; in the tropics, where the temperature is high, the absolute humidity is high, because warm air can contain much vapor ; and on mountain peaks, where the temperature is low, the amount of vapor is slight, because cold air has little capacity for water vapor. If the dry upper air descends to the earth, its absolute humidity is low ; and even if it commenced its descent in a saturated con- dition, its relative humidity decreases be- cause the temperature rises (Fig. 19) ; and if air currents move from cooler to warmer latitudes, their capacity for vapor is con- stantly increasing, because they grow con- stantly warmer and have a greater power of absorbing vapor. When they move . from warm to cooler regions ture of descending air. Starting in a their relative humidity in- saturated condition with a temperature , , . of 40° at 1000 feet, it reaches the surface creases, Decause tUeir tem- with a higher temperature and its ca- perature descends ; and pacity for vapor increased, while its . relative humidity has decreased. The when air rises Over land reverse takes place with ascent. elevations, or vertically by convection, the relative humidity is also increased, because air cools by expansion as it ascends ; and under such condi- tions the vapor is often condensed in clouds and rain. As a result of these varying conditions we get many varia- ble phenomena. Where the winds are prevailingly dry, and capacity for vapor considerably increased. THE ATMOSPHERE. 39 the relative humidity low, desert conditions result; and where moist winds rise over rapidly ascending lands, condi- tions of excessive rainfall are produced. With air prevail- ingly dry, evaporation is rapid, while in regions of great rela- tive humidity, evaporation is slow and small in amount (Fig. 60). Since water vapor contains a store of " latent heat " great stores of heat energy are transported from one latitude to another by the movements of vapor-laden air currents. Pressure. — The air, though so light and apparently almost without substance, actually has weight. At the seashore, the average weight of the air column is 15 pounds to the square inch ; but as we ascend into the air, whether in a balloon or on a mountain, the pressure of the air becomes less and less. Aside from this difference in air pressure the weight of the column of atmosphere at any single point is almost constantly changing. This is due to the fact that the air is very elastic and is subjected to a complicated series of movements. We shall be better able to understand the causes for these changes in pressure, and their effects upon the atmosphere, after we have examined in more detail the subjects of air temperatures and circulation. Effect of Gravity. — In a measure heat and gravity are in conflict in their effect upon the air. Heat is always expand- ing, some portions more than others, but gravity in trying to hold the air to the earth attracts the cooler and therefore denser parts more strongly than it does the lighter warmed portions. This starts a movement of the air, for the denser portions are drawn down to the surface and the lighter parts pushed above it. Gravity is thus a most important factor in determining the equilibrium of the atmosphere ; for its constant tendency is to restore an equilibrium which other causes are tending to destroy. Effect of the Earth's Rotation. — As the air moves in the form of winds or currents, there is a constant tendency to 40 PHYSICAL GEOGRAPHY. be deflected to one side, as a result of the effect of the earth's rotation. • This not only tends to turn the currents of air, but its influence is also felt in the ocean currents. In the southern hemisphere the currents are deflected toward the left, and in the northern hemisphere toward the right ; and we common- ly speak of the latter as the right-hand deflection (Fig. 20). By revolving an orange or a ball around an axis, one can see that the motion at the equator is much more rapid than that at the poles, revolution carries every point along a circle, but the diameter of the circle decreases toward the pole (Fig. 21). Therefore in the course of a revolution a point near the equator travels a much greater distance than one near the pole. To do this, it must go faster, since the same period of time is al- lowed for revolution in any latitude. At the equator the rate is 1521 feet a second, while near the poles the rate is greatly reduced. A body, no matter what its direction of movement (north, Fig. 20. Diagram to show how the moving currents np„ „!, are deflected from a straight line N-S 70 / \ .y \ 36°, 26°, V \ I "" I 1 f to' Fig. 21. Diagram illustrating the decrease in diametei on different latitudes. THE ATMOSPHERE. 41 east, south, or west), upon moving over the surface of a rotating sphere is constantly subjected to deflection, toward the right in the northern, and the left in the southern hemi- sphere. The amount of this deflection depends in part upon the velocity of the moving body, a rapidly moving current being less deflected than one with slow movement. It is, moreover, more decided in the higher latitudes than near the equator, reaching a minimum at the equator. So a body, such as a current of air or of water, moving southward in the northern hemisphere is turned to the right at a rate which will vary with its velocity and with the lati- tude. Hence it becomes turned toward the west. If the movement be toward the north, the right-hand deflection turns the body toward the east. In the southern hemi- sphere the turning is just opposite. That is to say, a body moving northward, or toward the equator, is turned toward the left or the west, and if moving away from the equator it is turned toward the left, or the east. Unfortunately the reason for this important principle seems incapable of clear non-mathematical expression ; at least, all such explanations known to the author are either mathe- matical, or incorrect or obscure. It therefore seems best for the present to leave the matter with a mere statement of the fact. 1 The fact, however, should be clearly understood, for upon it depends much of the matter which follows. It will be seen that the ocean currents and winds are thus deflected ; and upon this depends many peculiarities of climate in vari- ous parts of the world. 1 The teacher will find the subject fully discussed in Ferrel's Popular Treatise on the Wind, referred to on page 84. A simple experiment illus- trating the principle may be tried by means of a circular table top which can be revolved. A marble dipped in ink, and allowed to run over the revolving table, will be deflected from a straight course in a direction varying with the direction of rotation of the table. 42 PHYSICAL GEOGBAPHT. REFERENCE BOOKS. See also references at the close of Chapters III. -VII. Davis. — Elementary Meteorology. Ginn & Co., Boston, 1894. 8vo. $2.70. (Almost all points thoroughly treated in the light of the best modern knowledge.) Loomis. — Treatise on Meteorology. Harper Brothers, New York, 1870. 8vo. $1.50. Scott. — Elementary Meteorology. Scribner, New York (Agents). Fifth edition, 1890. 12mo. $1.75. Tait. — Light. Macmillan & Co., New York (Agents). Second edition, 1889. 8vo. $2.00. Capron. — Aurora. E. & E. N. Spon, New York (446 Brown St.), 1879. 4to. $17.00. Guillemin (translated by Thompson). — Electricity and Magnetism. Macmillan and Co., New York, 1891. 8vo. $8.00. (Much on atmos- pheric and terrestrial electricity and magnetism.) Maxwell. — The Theory of Heat. Longmans, Green & Co. Tenth edi- tion. (Edited by Lord Eayleigh.) 1892. 12mo. $1.50. lyndall. — Heat as a Mode op Motion. Appleton & Co., New York. Fourth edition, 1883. 12mo. $2.50. In most good books on physics, the subjects of heat, light, and electricity are well treated from the physical standpoint. The American Meteorological Journal (monthly, Ginn & Co., Boston) contains a record of the progress in the subject, and many original articles of general interest. $3.00 a volume ; eleven volumes published. The publi- cation of this magazine has now (May, 1896) been suspended ; but in the various published volumes there is much of value. CHAPTER III. DISTRIBUTION OF TEMPERATURE. General Statement. — If nothing were present to interfere with or to distribute the solar rays that come to us, we should have a very regular distribution of heat over the earth's surface. At the equator the temperature would be extremely high, much higher than at present ; in the Arctic latitudes there would be very low temperatures ; and between these two belts there would be intermediate condi- tions. In each of these belts there would be seasons, and the difference between the day and night as at present^ This theoretical distribution of the solar heat is in reality so well defined that we are able to divide the earth's surface into three great climatic zones, — the Arctic, Temperate, and Tropical belts (Fig. 65~). In each of these zones there is a regular normal variation in the temperature of the different seasons, there being a gradual rise from winter to summer which with the corre- sponding descent from summer to winter makes what we may call the seasonal range or curve (Fig. 24). By the rise of tem- perature during the day, and its fall at night, a daily curve is also produced (Figs. 22, 27-29, and 33) ; and therefore the seasonal curve is made up of a large number of daily curves (Fig. 23). Theoretically, these should all be regular, and season after season we should have an almost exact repetition of these curves. However, in reality, this is far from being 43 44 PHYSICAL GEOGRAPHY. Fahr. M 100° the case ; and the divergence from the theoretical is due to the presence of a number of disturbing influences. These are (1) the effect of atmospheric movements, (2) the in- fluence of the oceans, or the absence of such influence, (3) the effect of topography. Effect of Atmospheric Move- ments. — This subject is again referred to in the chapter on winds, and now we need only consider a few of its general features. There is a regular circula- tion of the atmosphere, and numerous other movements which we may call irregular. Certain winds blow with moderate steadiness toward the equator, where the air rises and then flows away at a considerable elevation above the earth's surface. By this means much of the heat which reaches equatorial FlG - 22 - regions is borne away and Daily temperature curves. Lowest, Arc- distributed ill other zones, tic (winter) ; second, north temperate land interior (winter) ; third, same In the north temperate lati- (summer); fourth sea coast near the tudeg th general mov ement tropics (winter) ; fifth, same (summer). ° of the atmosphere is toward the east ; and this brings to west coasts the warm air from over the oceans, while to the eastern parts of continents, air is brought from the interior regions. By means of these and other general influences of the atmospheric circulation, the temperature of the earth's surface is greatly modified. DISTRIBUTION OF TEMPERATURE. 45 Smaller movements do locally what these great movements do in a general way. Thus a storm passing across the country brings conditions of cloudiness and rain, and pro- duces winds which are sometimes warm and sometimes cold. By this means air is sometimes drawn from cold, snow- covered lands ; or it settles from the upper cold layers of air ; or it may be drawn from the equable ocean. At the seashore, during the summer, the cool sea breeze may blow and modify the heat of the hot summer day (Fig. 38). April May fi(f ttP 1,0° 30° 90 10° 0- ■100 00° 80° 70° 60° 50° h0° Fig. 23. Diagram illustrating mean seasonal rise in temperature, with daily and irregular changes superimposed. Influence of Oceans. — The ocean, and even large bodies of fresh water, are important modifiers of climate. As we have already seen, the ocean water warms very slowly, and it cools with almost equal slowness. Therefore the difference between the temperature of day and night, and summer and winter, is much less there than on the land, which warms rapidly during the summer day and cools readily at night and in winter. Over the ocean, in tropical latitudes, the 46 PHYSICAL GEOGRAPHY. temperature range throughout the year is very slight ; and in temperate latitudes, while the range is much greater than this, it is still small compared with the range on the land (Fig. 24). Therefore near the seashore, the temperatures of the summer and the day are relatively low, while the tempera- tures of winter and the night are relatively high. Even on the shores of small lakes this influence of water is noticeable. On those coasts which are reached by prevailing winds from the ocean, as on the west coast of the United States, the general temperature is high, and the climate equable. Even in a short distance the temperature difference may be very marked ; and while on the shore the effect of the ocean is plainly felt, this influence becomes very much less marked at a distance of a few miles from the coast. Another very important influence of the ocean is that caused by the fact that this body itself is in motion. Both warm and cold ocean currents move on the surface of the sea and tend to equalize the temperatures of different parts of the earth. By this circulation, lands that would other- wise be uninhabitable have their climate rendered much more equable than that of regions in lower latitudes where these conditions of oceanic circulation do not exist. One of the best illustrations of this is the difference between the climate of Western Europe and Eastern America. As a general statement it may be said, that under the present conditions of distribution of land and water, ocean and air circulation, and alternation of day and season, the general climate of the globe becomes progressively colder as the polar regions are approached ; and as we pass from the seashore toward the interior of continents, we go from regions of equable climate, to those possessing great ranges in temperature between the winter and summer, and day and night. DISTRIBUTION OF TEMPERATURE. 47 Effect of Topography. — It would be quite impossible to enter into this subject in much detail. In general, valleys are warmer than hilltops, partly because they are protected from the wind, and partly because the solar rays that fall upon the valley sides are in some degree reflected into the valley. The sides of hills, or of mountains which face toward the sun, are warmer than the north-facing sides ; and this is often very well shown in the natural distribution of plants, which rise higher on the southern side of the hill than on the northern side, where the temperature is less favorable to their existence (Fig. 68). Next to latitude, altitude is probably the most important feature in determining climate. If the elevation be suffi- cient, conditions in some respects resembling those of the Arctic climate may be found even under the equator. At a height of from 15,000 to 18,000 feet above sea level, vegetation ceases to exist, and perpetual snow covers the mountain tops. This is due to several causes, the most important of which is the fact that the air at great eleva- tions is less dense (Fig. 15), and hence cooler. Through this relatively thin layer, which is clear and free from large quantities of dust particles and water vapor, the rays that fall upon the surface are readily radiated into space. This illustration is interesting, since it shows that in the same latitude, and consequently with the same amount of solar energy, the two opposite extremes of tropical and Arctic climates may result. It brings out very strongly the fact that the mere amount of energy received does not determine the temperature of a place ; the subsequent be- havior of this is equally important. This same fact is shown by the difference between the climates of the sea- shore and the land at different places in the same latitude. Almost everywhere on the earth the influence of topog- 48 PHYSICAL GEOGRAPHY. raphy upon temperature is shown, sometimes in great differ- ences extending over wide areas, again very locally and in small amount. Mountain ranges prevent the passage of vapor-laden air into the great enclosed basins, where dry clear skies exist, and where desert conditions are conse- quently produced ; and we might find many instances, great and small, to illustrate the influence of land forms upon the distribution of temperatures. 1 Seasonal Temperature Range. — From the above, it is seen that latitude is no true indication of temperature ; for it is but one of several factors which tend to determine climate. However, it is one of the most important of the factors, and in general the temperature decreases from the equator toward the poles. Still, owing to the disturbing influence of the other factors, this decrease is not regular ; and hence the lines of equal temperature, or the isotherms, are not parallel to the lines of latitude, but often diverge very widely from them. If we examine the charts of isotherms (Plates 2, 3, and 4), we find that they are irregular, and that the irregularities vary with the season. Moreover, any given line, such for instance as the 50° isotherm, is in a different place in the opposite seasons. In other words, I the temperature of every part of the earth changes with the season ; but the change is different in amount in different places. This seasonal change may be called the temperature range or curve. If the temperature changes of any given region are plotted upon a diagram, in which both the months and the scale of degrees are shown (Fig. 24), we find that there is a gradual rise in the spring to a time after midsummer, when the temperature falls until after 1 Many of these features are illustrated in the accompanying isothermal charts. DISTBIBUTION OF TEMPERATURE. 49 midwinter. Year after year this is true, though each year will show a slight difference from those which precede and follow. Even in different regions the same is shown ; but there is much variation in the form of the seasonal curve of different places. Such a curve shows how much difference there is between seasons, and when it occurs. We find that the height to which the temperature rises Fig. 24. Seasonal temperature ranges. Constructed to have northern and southern sum- mer coincide. Hence for southern hemisphere June should read January, etc. in the curves is very variable in different parts of the earth, and the same is true of the length of the warmer or the colder part of the curve, which is the same as saying that the length of the warm season differs in different places. If we plot such a curve as this for a place over the ocean, we find that it is relatively flat, because the difference between the winter and summer temperatures is not very 50 PHYSICAL GEOGRAPHY. great. On the other hand, in the central parts of continents, where the winter is relatively cold, and the summer warm, the curve rises to a much greater height. At the equator, the curve is much flatter than in temperate and Arctic latitudes, where the difference between summer and winter temperatures is great. In any one of these zones there may be marked differences even in neighboring places. Upon examining one of these seasonal curves, it will be noticed that the time when the temperature is highest does not correspond with the period when the greatest amount of heat is received from the sun ; nor is the coldest time of winter coincident with the shortest days. In other words, there is a lagging, and this is due to the cumulative effect of the heat or cold. In the early summer, the ground is still cool from the effects of the last winter, and in high latitudes there is still snow upon the ground. It takes some time for the sun's rays to warm the ground and the air ; and when this is done, the effect of solar energy becomes greater than before, even though the days be shorter and the amount of energy coming from the sUn less than in mid- summer. In the opposite season, the effect of radiation during the long nights becomes most marked after the middle of winter, which is really the 22d day of December. Therefore January is almost invariably colder than Decem- ber, and February also may be colder than December. For the sake of diagrammatic illustration, the seasonal curve is represented as being a continual rise and fall of tempera- ture. It represents the average temperatures for the several parts of the different months. In reality there is no such regular and uniform rise, but it is interrupted by daily risings and fallings (the daily curve, pp. 60-62), and by irregular interruptions (Fig. 23). For days at a time the normal seasonal rise or fall may be interrupted, and even be Face page 50, Isothermal < >r the year. DISTRIBUTION OF TEMPERATURE. 51 replaced by a temporary descent (Fig. 25). This happens in our latitude when storms or cold waves pass over us, and pre- vent the effect of the sun's heat from becoming apparent. Thus in winter we may have thaws, or in midsummer the heat may be tempered by several days of cool weather ; but there are more irregularities during our winter than during the summer. The temperature curve shows only the average of these, its chief value being to illustrate the effect of the sun's rays as the season changes, and to show how differ- ently this effect is manifested in various places. DAILY MEAN TEMPERATURES OF THE STATE FOR 1891, WITH NORMAL VALUES 90 80 70 f 60 m 50 £ 40 ° 90 20 10 „L. | -UCST SE P«-«» OCT0U. .0....I. 90 SO 70 60 60 40 SO 20 10 _j_i » ,1 11 j ;> ,7 |7.| 7 .7 1,| * " " 10 *• 60 75. 90 1H HI US. lit DAY OF THE YEAR Fig. 25. Seasonal curve for New York state. Irregular variations shown by the lighter line. Isothermal Charts. — The best graphic way to show the distribution of temperature over the earth, is by means of isothermal charts. The isotherm is the line of equal tem- perature ; and the chart may show these lines for the day, or for the month, or for the year. If for the year, they represent the average of all the temperatures during that time ; or if for the month, the same average for day and night throughout the month. Every place which has the same average temperature for the period represented on the chart, has the same isothermal line. That is, if the 52 PHYSICAL GEOGRAPHY. average temperature for a given month is 50° at London, Boston, Buffalo, etc., the 50° isotherm for that month is made to pass through each of these places. On the isothermal chart which shows the average tern- Fig. 26. Isotherms for February, 1878-1887. perature for the year (Plate 2), it will be noticed that in general the temperature decreases from the equator toward each of the poles ; but in each hemisphere there are numer- ous exceptions (Fig. 26). The rate of decrease is very DISTRIBUTION OF TEMPERATURE. 53 variable in different latitudes. While there is a general tendency for the lines of equal temperature to run parallel with the lines of latitude, at times the divergence is so great that the isotherms extend in a north and south direction. There is much less irregularity in this respect in the southern than in the northern hemisphere ; and this is easily explained by the fact that the land is mostly in the northern hemisphere. One is able to see the disturbing influence of the land in many places. Another effect of the greater abundance of land in the northern hemisphere, is that the line of greatest heat, or the heat equator, is north of the true geographic equator. The land becomes much warmer than the ocean, and hence the highest temperatures are found in the interior of conti- nents. This is not because more energy is received, but because the amount that does come is much more effective in warming the land and the air. Since radiation proceeds more readily from the land than the water, the average temperatures in northern regions are lower than in the southern hemisphere. Other general influences are notice- able upon the chart of annual isotherms. For instance, in the northern Atlantic, where the warm Gulf Stream extends toward the Arctic circle, the isotherms are bent northward ; and along the eastern coast of the United States, where the cold Labrador current flows close to the continent, and isothermal lines are bent southward. On the western side of North America, the influence of the prevailing winds is well shown where they blow from the warm Pacific upon the coast. This is particularly well illustrated on the isothermal charts of the United States (Plate 3), where we see a very marked difference in the temperature of the east and west coast. Thus there is a great range in temperature between Key West, on the Face page 35. Isothera I " H'or July. DISTRIBUTION OF TEMPERATURE. 55 extreme southern end of Florida, and the northern part of the coast of Maine, while in the same distance on the west coast the temperature differences are much less. From Key West to Cape Hatteras the influence of the warm Gulf Stream is felt, while on the New England coast the temperatures are lowered by the cold Labrador current ; but on the Pacific coast the influence of the warm ocean is manifest from Southern California to Washington. A study of the charts will show many other variations in the isotherms. In the isothermal charts which represent the typical sum- mer and winter conditions, similar phenomena are noticed ; and in some cases they are more strikingly illustrated than on the annual chart. The heat equator of July (Plate 4) follows the sun well up toward the tropic of Cancer, but it does not follow the sun as far when it takes its southern journey during our winter ; and in the Atlantic, where there is much more neighboring land, the migration of the heat equator is more marked than in the broad Pacific. We notice also that the influence of the Gulf Stream in deflecting the isotherms is more important in January than in July, when the neighboring ocean waters are themselves warmed by the summer sun. In the two hemispheres there is also a difference in in the amount of migration of the isotherms for the lower temperatures. In the southern hemisphere the isotherm of 50° in July barely reaches Africa and Australia, and its position in January is not greatly different (Plate 5). This shows the influence of the prevailing condition of water in that hemisphere ; and the same fact explains the general parallelism of this isotherm with the lines of lati- tude ; but in the northern hemisphere, where there is more land, the isotherm of freezing in July is in the Arctic 56 PHYSICAL GEOGRAPHY. circle, while in January it extends below the 40th parallel in several places. The isotherm of 50° migrates from northern Scandinavia, Iceland, and Labrador, in July, to Spain and the Carolinas in January. In the higher latitudes of the northern hemisphere, the influence of the land is shown by the fact that in January excessively low temperatures occur in the interior of conti- nents. Thus so far as we know, the coldest parts of the earth are in these continental interiors, such as Asia. The winter " cold pole " of the world is not found high up in the Arctic latitudes, but in central Siberia near the Arctic circle (Plate 5). This is due to the fact that in these dry land interiors, radiation causes excessive cold during the long winter night. It is possible that when the Antarctic conti- nent or the interior of Greenland are better known, we may find upon these snow-covered lands even lower winter tem- peratures than those of northern central Asia. On the January and July charts of the United States (Plates 6 and 7), we find the greatest difference in tem- perature in the dry interior regions of Dakota and Mon- tana, and the least at Key West and on the southern coast of California, where the equable ocean waters prevent either excessively high summer temperatures, or excessive cold in the winter. Another place where the temperature of the United States is subjected to a great range in the different seasons, is in the desert region of the Great Basin. Here the sun's rays of the summer day readily pass through the dry, cool air and raise the temperature of the ground, and the lower air layers, to a very high degree. At night and in the winter, radiation proceeds with rapidity, because the air is clear and offers little ob- struction to the passage of the radiant heat ; and therefore in the winter nights the temperature becomes very low. Face page 56. ■Id 0-20 f dc'30 --^-t^irri r January. DISTRIBUTION OF TEMPERATURE. 59 The influence of topography is also well shown in several por- tions of the charts for the United States, and also on the New York chart (Plate 8), where the isotherms are seen to extend up the valleys, showing that they are warmer than the hills. 60 PHYSICAL GEOGRAPHY. Daily Temperature Curve. — The daily curve represents for the day what the seasonal curve does for the year. It shows the rise in temperature during the daytime and its fall at night (Fig. 27). Unless interfered with by some accidental cause, the temperature rises from sunrise till early afternoon, and then descends until late in the night. As in the case of the seasonal curve, | . | the time of highest temperature is not I i I : 5 I when the sun's rays are strongest, nor is the coldest part of night at midnight. The explanation is the same, the heat of the sun in the morning being partly ex- pended in warming the earth which was cooled in the preceding night ; and the temperature at night time continues to descend after midnight, because the radia- tion of the heat that came during the day proceeds uninterruptedly, and its influence is not checked until the sun again rises. There is much variation in the daily curve in different latitudes (Fig. 22), and even in different places in the same latitude. The daily change in tempera- ture is relatively slight on the seashore, and very great on the land ; and the range is much greater in temperate latitudes than in the tropics. In the Arctic regions, where the sun is above the horizon in the summer and below it in the winter, the daily curve is of very little importance, and may be entirely masked by acci- dental causes. Since in many parts of the earth there is a great variation in the length of day and night during the different seasons, the daily temperature curve varies with the season. Thus r V \ \ / / \ ^ J Fig. 27. A normal daily range for summer and for winter in New York. DISTRIBUTION OF TEMPERATURE. 61 in our latitude the temperature rises much higher in summer than in winter (Fig. 27). While normally the temperature curve is that which has just been described, in reality it is subjected to many variations and interruptions (Fig. 28). The tendency is for the tern- - -»»»D«/ -^4— ^J — I— | -a ">' _6_^U_6_ rp — i — i 1 | — 1 — — 1 — z^fc yl — i *=?- E£^ ' '■ — P^ I — ! — ' — r— r 1 -] — — 1 — i — —h~ — i — I 1 — 1 — 1 — i — — 1 — i — j — — I — I — 1 — — I — i i i i i i i i 4li"r i " 9 to 11 13 14 15 16 17 18 18 20 MAY 21, MAYS TEMPERATURE 1893 ITHACA Fig. 28. Normal daily curve followed by an interruption of several days. perature of the day to rise above the average for that season, and to fall below it at night (Fig. 23). Oftentimes the daily curve is so changed (Fig. 29) that instead of a rise during the daytime, we have a fall in the temperature (Fig. 64) ; or the temperature may continue to rise through- ,0. 1 I i : 1 I I i I 1 I 1 . I I >.-/|w 1 1 »«v I •v ^ I I DEC.27, 1892 28 29 30 3 1 JAN. 1,1893 2 3 4 6 6 JAN. 7,1893 TEMPERATURE ITHACA Fig. 29. Daily temperature record, showing interference with the normal rise and fall of temperature. out the night, the opposite of what would normally be the case. Cold waves or storms are often the causes for these changes, and many local and temporary effects may thus be produced. The presence of clouds, or of much, moisture in the air, or of winds from the ocean (Fig. 39), may very 62 PHYSICAL GEOGRAPHY. effectually modify the normal daily rise and fall of tempera- ture. Temperature Ranges. — The study of the isotherms of a region gives us an idea only of the average temperatures of different places. In a study of climate it is necessary to know something of the changes in temperature, both with reference to the amount (Fig. 30) and the rate. Fig. 30. Temperature ranges in the United States in degrees Fahrenheit, 1892. No better illustration can be found of the differences that may exist between places on the same isotherm, than that of St. Louis and San Francisco, which are on the same annual isotherm (55. 7°) and on nearly the same parallel of latitude. In San Francisco, the average for September, the warmest month, is a little less than 60°, while the January isotherm is about 50°, the actual range between the averages being about 9.5°. At St. Louis, the January isotherm is 31°, while DISTRIBUTION OF TEMPERATURE. 63 the July isotherm is 78°, a range of about 47°. Taking the highest and lowest temperatures for each place, the differ- ence is even more striking, for we find a range of 61° in San Francisco, while in St. Louis the range is 128°. There- fore, though they are on the same isotherm, the climates of the two places are quite different. The lowest temperature ever accurately observed on the earth was less than —90°, the highest over 127°, the former 30 X l/T/^klX^^^^^ /I //'-I i\H v VC/iA 1 ~\y\. \f 1/ 1 / 4^W-70 v. ^>C/* L,30 sty 40 7Q~50 32^ Fig. 31. Minimum temperatures observed in the United States, 1892. in Siberia, the latter in Algeria. This is a range of nearly 218°. Such extreme ranges are of course impossible in any single place ; but in some of the dry interiors of continents, very extreme temperature ranges are sometimes experienced. In Siberia, where the greatest ranges are found, temperatures 181° apart have been observed ; and in the northwestern states of this country, ranges of over 150° have been meas- ured. On the other extreme, ranges of only 40° or less are 64 PHYSICAL GEOGBAPHY. observed at Key West and on the coast of California. In some of the tropical islands of the Pacific, the greatest differ- ence in temperature during the year is often not over 18° or 20°. More than half of our country experiences ranges greater than 100° (Figs. 30-32). If these temperature changes came slowly, their effect would not be so very difficult to endure ; but in places of great annual change, there is almost always great change S?f> %r. 7 °f VA^H^PP^ v f/^' so i ^\v/ A Jv~^ /^^^^C^y \ — 1 [ ;_ foy\ y^M \W \ \ ^Xi^ V -95 V \\ ^ / \V Q^^ 1 ^ 1 \ x ^n\ x 7uo \J^90 Fig. 32. Maximum temperatures observed in the United States, 1892. in short periods. In Montana (in December, 1880) in less than eighteen days, the temperature fell 117°, the thermome- ter on the 12th registering 58°, and on the 29th — 59°. In the greater part of northern United States we are accustomed to similar changes in winter, though they are very rarely so extreme as this. After a few days of moderate warmth during the unseasonable winter thaws, a cold wave spreads over the eastern states, and zero weather prevails, not un- DISTRIBUTION OF TEMPERATURE. 65 commonly causing a drop in temperature of 60° or 70° in a few days. We are even liable to very excessive changes in a single day. Where the air is dry, as in parts of the arid regions, a change of 40° is not uncommon in the summer, as the result of the heat of the day, followed by the coolness of the night, which is caused by the radiation through the clear dry air. Near the ocean the difference of the day and night temperature is often very slight, particularly in the winter. At Key West the day and night temperatures differed only about 7° in December, 1877. Aside from these regular daily ranges there often occur ex- ceptional changes (Fig. 64). In winter a cold wind may fol- low a rain storm and cause the temperature to descend below zero with a change of 35° or 40° in a few hours. In this case the nocturnal radiation is an aid in the fall of tempera- ture. A daily change of 50° is not uncommon in Montana ; and in Texas the thermometer has been known to fall 63° in sixteen hours. It is said that in Thibet the temperature has fallen 90° in fifteen hours, or from 68° in midday to — 22° at night. In summer there are also great ranges ; but they are not so noticeable, nor are they so severe, as those which come at times in winter. Earth Temperatures. — At the very surface of the earth the ground is warmed when the sun's rays are present, and cooled when their effect is absent. Below a depth of a few feet the influence of the sun is not very noticeable, and from this point downward, the temperature of the earth is practi- cally permanent, and is determined by the heat of the interior. There is much difference in the effect of changes in tem- perature in different parts of the earth. At the equator the ground is very warm at the surface, and there is a slight varia- tion throughout the year. At the depth of five or six feet 66 PHYSICAL GEOGRAPHY. the intensity of the heat has decidedly decreased, and soon the zone of no variation is reached. In temperate latitudes, the difference between summer and winter temperatures is so great that the surface becomes warm in summer, and in winter cools down to temperatures lower than the freezing- point. In the winter, in such regions, frost exists in the ground often to a depth as great as six or eight feet. In the Arctic regions, where the sun's rays are of little power, and where radiation is excessive, the ground is often per- manently frozen to a depth of several hundred feet. During the summer the surface layers lose their frost and thaw, and plants grow over earth which is per- manently frozen. The ground is such a poor conductor of heat that it takes many weeks for the effect of the summer heat, or the winter cold, to reach to a depth of ten or fif- teen feet. Therefore at such depths the seasonal curve lags behind that of the air ; and at the same time the temperature range is less. At the very surface, the earth temperature changes more than that of the air (Fig. 33). This is because the earth readily absorbs heat and radiates it with almost equal rapidity. For this reason the ground at midday is warmer than the air, Fia. 33. Daily ranga at the surface of the earth (dotted line) and of the air ten feet above (heavy line). Ithaca, N. Y., July, 1893. DISTRIBUTION OF TEMPERATURE. 67 while at night time its temperature is lower than that of the air. These facts of earth temperatures are important in explaining the heating or the cooling of the lower air layers. REFERENCE BOOKS. See also Buchan's memoir referred to at the end of the next chapter ; and also the books on general meteorology, notably those by Davis, Scott, Waldo, Greely, Abercromby, Blanford, Woeikof, Hann. and Croll. The Berghaus Atlas, volume on Meteorology (Hann, Atlas der Meteo- rologie. Justus Perthes, Gotha, Germany, 1887. 15 marks 1 ), although in German, contains many charts upon temperature distribution, etc., which will prove of value in the schools. The Annual Reports of the Signal Service, and now of the Weather Bureau of Washington, contain much information relating to the tempera- ture, wind, rain, etc. , of the country. 2 Hazen. — The Climate of Chicago. Bulletin X, U. S. Weather Bureau, Washington, 1893. (Describes some interesting effects of the lake upon temperature. The other bulletins of this series are also of value.) 1 Under the present law governing' the importation of foreign books no duty is charged on those in other languages than the English. Foreign books may be ordered direct from the publishers, or through some New York, or other importers. With all charges added, the mark becomes equal to about $0.25, the franc to about $0.21, and the shilling to about $0.25; but in the last case a duty may also be charged. While this does not give the actual price, it furnishes a close approximation. 1 Where no price is given for government publications it indicates that they are distributed free of cost ; but in many cases all of the copies are exhausted, and the only way to obtain them is from the large city second-hand bookstores. Sometimes it is not possible to obtain government publications without the aid of a congressman ; but this will be easily obtained by most schools. CHAPTER IV. GENERAL CIRCULATION OF THE ATMOSPHERE. General Statement. — Since the air is very elastic, and its condition easily changed by variations in temperature, it is readily caused to move. No better illustration can be found of the mobility of the air under these circumstances, than that which is so often noticed on heated deserts. The ground becomes warmed, the air is heated by contact with it, and this causes the air to expand and become lighter so that a tendency to rise by convection is produced. Soon this ten- dency becomes so strong that the lower air must move up- ward, thus starting a dust whirl on the desert. The move- ment thus started by the effort of the denser air to take the place of the warmed layers causes very violent, though very local, winds. In a room, a warm stove, lamp, or an open- grate fire, causes the air to move, and starts a circulation which is often very noticeable. The reverse process of cooling the lower air layers causes a condensation which necessitates a settling down of other air. We may often see an illustration of this on a cold winter night when the air is quiet. If the window in a warm room is then opened, the cold, dense outside air flows in, producing a very perceptible current. If in place of these local illustrations, we substitute large areas of the earth's surface, we find an explanation of many of the greater features of the atmospheric circulation. Over equatorial regions, the air is constantly being warmed during the day, and therefore expanded. Accompanying this ex- GENERAL CIRCULATION OF THE ATMOSPHERE. 69 pansion, there is rising caused by the greater density- of the surrounding air, and so a circulation is produced which exerts its influence over a very large area. This circulation consists of four parts : (1) the inflowing surface winds, (2) the uprising currents, (3) outflowing winds at high N Ferrel's ideal diagram of the planetary circulation. Dotted arrows show upper currents of air. elevations, and (4) down-settling air at some distance from the equator. There are other features of this great circu- lation which we will soon consider. Similar winds upon a smaller scale are produced over continents, and even on the land along the seashore. When warm air is expanded and raised it pushes away the air above it, the barometric pressure is decreased, because the air column is lighter ; and when the air is cooled, it becomes 70 PHYSICAL GEOGRAPHY. denser, and hence the barometer registers a higher pressure of the air. Therefore the relation between air pressure and wind is very intimate ; and where, for any reason, low-pres- sure areas exist, winds are found blowing toward them (Plate 9). This is the case in certain areas which are permanently warmer than the surrounding regions, and also in those disturbances of the air which are classed as storms. A barometric gradient is produced, and the winds move as if they were going down grade. The air moves away from high and toward low pressure areas. Classification of the Winds. — For the sake of simplicity in the consideration of the movements of the atmosphere, it seems well to adopt some classification of air movements. The one here proposed is a logical division; but other classi- fications might be used, the only object of such a division being to group like kinds. Planetary or Perma- nent. (Due to plan- etary causes of a permanent nature.) Periodical. (Due to periodical causes.) Irregular. (Due to causes apparently of an irregular na- ture.) Trades. Anti-trades. Doldrums; or equatorial calms. Horse latitude winds and calms. Prevailing westerlies. J Migrating winds and calm belts. [ Monsoons. f Land and sea breezes. \ Mountain and valley breezes. Seasonal winds Diurnal winds Eclipse winds. Tidal breezes. Storm winds Accidental winds ' Desert whirlwinds. Cyclonic winds. Anticyclonic winds. Thunderstorm winds. Tornado winds. Landslip blasts. Avalanche blasts. | Volcanic winds. L Waterfall breezes. Face page 70. Winds and isol 'or January. GENERAL CIRCULATION OF THE ATMOSPHERE. 71 Planetary or Permanent Winds. — There are certain winds whose force and direction depend upon the fact that there is a variation in the amount of heat received in different lati- tudes of the earth, and that the earth is rotating about its axis. These may be called planetary winds, because they would be developed upon any planetary body where similar conditions prevail. Or we may call them permanent, because, compared with other winds, their direction and force are prac- tically permanent. In some places they are greatly modified by other causes; but they are so strongly developed that their influence is felt all over the earth. They are, as it were, the general atmospheric winds ; and together they form the fundamental circulation of the atmosphere. They may be described under several headings. Trade Winds. — Since the air over the equatorial regions is warmed more than that on any other part of the earth's sur- face, the denser air moving in toward the warmer region causes currents. The equator may be fairly compared with a stove, over which air rises by convection, and toward which currents flow. These inmoving winds are called the trades, because they blow with marked permanency and steadiness ; and in planning their journey, whenever possible, vessels choose a course which will allow them to take advantage of the trade winds. These winds move toward the equator (see Plates 9, 10, and 11), but instead of blowing directly toward it, they are deflected by the effect of the earth's rotation. North of the equator their direction is from the northeast, while south of it they move from the southeast. They are much less well developed over the land than over the water ; and when they blow from the water to the land, they are often deflected because of its influence. Over the land they may even be destroyed. The air in the trade winds is moving from colder to 72 PHYSICAL GEOGRAPHY. Plate 10. General circulation of the Atlantic for July. GENERAL CIRCULATION OF THE ATMOSPHERE. 73 7 7Z7(r n Z r ? Plate 11. General circulation of the Atlantic for January. 74 PHYSICAL GEOGBAPHY. warmer regions, and therefore its capacity for water vapor is constantly increasing. Therefore they are drying winds, and when they blow over the ocean, evaporation is rapid, while on the land, where water vapor is not readily obtained, they pro- duce deserts in many places. Since the temperature of this air is high, when blowing over the ocean the amount of water vapor which it is enabled to carry is very great; and much rainfall is caused if the air is made to rise, as is the case when the trades blow upon rising coasts. Doldrum Belt. — Over the heat equator, the air in this great planetary circulation rises by convection ; and in this place a condition of almost permanent calm is produced (Plates 10 and 11). This is particularly the case over the oceans, but over the land other causes may interfere. The doldrum belt is situated between the north and south trade winds, and it migrates from season to season as the heat equator changes its position. Since the air in this belt is warmed, it contains much water vapor, and it is a very rainy belt because this humid air rises by convection, and cools dynamically until the dew-point is reached. Therefore, during the day the sky almost invariably becomes cloudy, and rains fall. Anti-trade Winds. — The inflowing of surface air, and its uprising, makes necessary an outflow of air at a higher level. This outflowing air moves away from the equator, in either direction, and produces what is known as the anti-trades. These winds are not felt on the land, excepting on those rare peaks which rise to a height of 10,000 or 12,000 feet above sea-level. They move in a northeasterly direction in the northern hemisphere, and toward the southeast in the south ern, in each case being turned from a true north or south direction by the deflective influence by the earth's rotation. Their permanency is shown by the fact that the upper clouds GENERAL CIRCULATION OF THE ATMOSPHERE. 75 move in these directions. This upper air movement con- tinues in the temperate latitudes. Horse Latitude Winds. — After traveling for a certain dis- tance from the equator, the air commences to settle, and some of it reaches the earth's surface near the poleward margins of the trade wind belts. These regions of settling air are known as the horse latitudes, and because the air is descending in these belts, the prevailing condition is that of calms or light, variable winds ; but these calm belts are not so pronounced as those of the doldrums (Plates 10 and 11). Over the land, the horse latitude belt is not so distinctly developed. Prevailing Westerlies. — Apart of the upper circulation of the anti-trades continues toward the poles; and because the polar regions are places of permanent low temperature, there is a tendency for the upper air to move toward them. These air currents are deflected to the right in the northern, and to the left in the southern hemisphere, so that in the upper latitudes there is a whirl of air known as the circumsolar whirl, which, in both hemispheres, produces a condition of prevailing westerly winds, both near the surface (Plates 10 and 11) and in the upper layers of the atmosphere. These whirls produce a condition of permanent low pressure in the polar regions; for there is an eddy produced, which is somewhat analogous to that formed by the escape of water from a bath tub. In the upper air the east-moving winds are remarkably permanent, as any one may see by watching the movements of the upper clouds. At the surface, the tendency toward the development of east-moving air currents is greatly inter- fered with by other causes. This is much more strikingly shown in the northern than in the southern hemisphere, where land is less abundant. In the latter hemisphere, sail- ing vessels may go around the earth with prevailing fair winds driving them onward (Plate 9). To do this, they 76 PHYSICAL GEOGRAPHY. must go past Cape of Good Hope, and return by way of Cape Horn. In the northern hemisphere, the most striking influence of the prevailing westerlies upon the surface, comes from the fact that they determine the path of movement of the greater number of our storms. Periodical Winds. — There are certain changes of a period- ical nature, which tend to start the air in motion in a definite way ; and this tendency is repeated as these periods return. The most important of these changes are those which arise from the variation in supply of solar energy in the different seasons, and in the change from day to night. The periodi- cal winds may therefore be classed as seasonal and diurnal winds ; and in the group may also be included two minor classes of periodical winds, eclipse and tidal breezes. Seasonal Winds. — We get a large supply of heat in one season, and a very much smaller amount in the opposite season ; and the differences in seasons are very much greater far from the equator than they are in the equatorial belt. Therefore the seasonal effect upon the atmosphere, is less marked in the equatorial belt than elsewhere on the earth's surface. Still, even in equatorial regions, as the season changes, the movement of the sun in the heavens produces a very decided effect upon the atmospheric circulation. Migrating Wind and Calm Belts. — The wind charts of the Atlantic Ocean (Plates 10 and 11) show that there is a migration of the belt of calms as the season changes. The trade wind belts also move northward and southward ; and therefore in one season a region within the tropics may experience the calms of the doldrums, while in the opposite season, the dry and permanent trade winds blow steadily day after day. In the same way, a region situ- ated near the northward or southward margin of the trade wind belts, may in one season feel the trade winds, while GENERAL CIRCULATION OF THE ATMOSPHERE. 77 Fig. 35. Summer monsoons in India. in the opposite season the variable winds of the horse lati- tudes prevail. Monsoon Winds. — In latitudes outside of the tropics, where large land masses exist, very interesting winds are often caused by the difference in temperature between the land and. water. During the summer the land areas become warmed, and are covered by an area of permanent low pressure, to- ward which air currents move from all directions (Fig. 35). The air rises over the warm land, and its place is taken b}^ air from the relatively cool oceans. In the winter sea- son, the land becomes cooler than the water, and the air is caused to settle over the land and to move out from these areas of high pressure (Fig. 36), toward the then relatively warm oceans (Plate 9). This class of wind, which is very pronounced in Asia, is known as the monsoon wind. Similar winds are noticed in other continents, and we now know this class of air movements as the monsoons. In Asia, the monsoon winds blow in toward the central regions, across India, China, and other countries. During the winter they blow in the opposite direction. In Australia and other continents, the monsoon system of winds is very well developed ; and on the Spanish peninsula, the same Fig. 36. Winter monsoons in India. 78 PHYSICAL GEOGBAPHT. tendency toward inflowing summer and outflowing wintei winds is quite pronounced. Even where the distinct monsoon condition is not pro- duced, a tendency to the production of this class of wind often expresses itself in the disturbance of the wind direc- tion. This is very well illustrated along the Texas coast; where the summer trade winds are deflected until they blow upon the land. This phenomenon is shown on Plates 10 and 11 ; and on these charts it will also be noticed that in the winter, the pre- vailing winds of the coast of northern United States are from the land toward the ocean, while in summer their direction is much less definite. That is to say, the prevailing wester- ly winds are strengthened in winter and weakened in summer by the monsoon tendency, which in sum- mer is not sufficiently powerful to entirely invert the prevailing westerlies. In cold regions, such as Greenland, there is a tendency toward the production of outflowing winds in summer, because the snow-covered land is colder than the water. Another continental effect, not, however, dependent upon the temperature differences, is the retardation of winds in their passage over the land. As a result of friction, the winds of the land are less violent than those of the water; and mountain ranges may effectually check the winds and destroy them. Fig. 37. The sea breeze. GENERAL CIRCULATION OF THE ATMOSPHERE. 79 Diurnal Winds : Sea and Land Breezes. — Since the heat of the day is followed by the coolness of the night, the atmos- phere is often caused to move locally. During the summer this is particularly well shown along the seashore, when the familiar sea and land breezes are often produced on calm days and nights. During the day the land becomes warm and air tends to flow toward the warm areas from the cool sea, thus producing a very refreshing sea breeze (Fig. 37). This breeze begins to blow late in the morning, and continues until the power of the sun has decidedly diminished. It does not come every day, but only when other atmospheric disturbances are not mark- edly developed. It causes a peculiar disturbance of the normal daily tempera- ture curve. In ordinary cases the highest tempera- ture of the day comes in the mid-afternoon ; but when the sea breeze com- mences to blow, the temperature usually falls, so that the highest point reached during the day may be before noon (Fig. 39). The sea breeze does not generally extend far inland ; and ordinarily at a distance of ten miles from the coast, it is hardly perceptible. At night, when the land has become cooler than the ocean, a very gentle breeze often blows out upon the water (Fig. 38), rarely more than a few miles from the shore. This is the land breeze ; and thus it will be seen that by the daily Fig. 38. The land breeze. 80 PHYSICAL GEOGRAPHY. change in temperature there is produced a local circulation resembling in a small way the more extensive continental or monsoon circulation which depends upon seasonal changes in temperature. Where prevailing winds blow upon the coast, as is often the case in the trade wind belt, the intensity of these winds is sometimes considerably increased during the day, by the combination of the sea breeze and the normal wind. Even on the land, where no tendency to the production of the sea breeze is present, the change in tem- perature between day and night pro- duces an effect upon the winds. The heat of the daytime causes the air to rise, and freshens the winds by increasing their strength. Thus during a day which is calm in the morn- ing, strong winds may arise, and die down as the sun sets. Along the shores of large lakes, a lake breeze analogous to the sea breeze may arise during hot summer days. This is particularly noticeable along the shores of the Great Lakes of North America, and it is one of the reasons for the strong winds which prevail in such lake shore cities as Chicago. Mountain and Valley Breezes. — Where the topography of a country is very irregular, as in mountainous regions, the Fig. 39. Diagram showing normal daily curve on a hot summer day, and the effect produced hy the sea breeze. GENERAL CIRCULATION OF THE ATMOSPHERE. 81 Fig. 40. Mountain breeze. change in temperature between day and night often pro- duces a set of winds known as the mountain and valley breezes. During the nighttime, the air near the surface becomes cool by radiation, and it therefore becomes more dense and contracts. This dense air slides down the moun- tain sides into the valleys, down which it flows, often with sufficient velocity to cause gales. Just as streams gather water, and thus constantly increase their ve- locity by additions from their up- per branching tributaries, so these down-moving air currents become concentrated near the outlets of the mountain valleys. Among the valleys in the Rocky Mountains, during the calm clear days of sum- mer, when no other influences are present to disturb the tendency, these mountain breezes, or we might say mountain gales, are of nightly occurrence. The heating of the mountain sides during the day causes an updraft of air, which is the valley breeze. This is much less intense than the mountain wind, partly because the air is obliged to ascend against the action of gravity, and partly because the upflowing air is not concentrated during its ascent, but is rather disseminated over the mountain and valley sides. The day breeze becomes most intense in the mid-afternoon ; and the night breeze attains its maximum development just before sunrise. This type of wind is by no means confined to mountains, but is very well developed in plateau regions, such for instance as that of central New York, near Ithaca (Fig. 40). This plateau is dissected by numerous deep valleys which converge toward the broad depression occupied by Lake 82 PHYSICAL GEOGRAPHY. Cayuga. At nighttime the air flows down these valleys, producing perceptible breezes. Concentrated in the largei valley occupied by the lake, the wind often develops into a strong breeze during the calm summer nights ; and the wind lasts until eight or nine o'clock in the morning. No breeze of the valley type has been noticed here. It is probable that in the other similar valleys of this region the same breeze is produced ; and it may be expected in almost any place where the land is deeply cut by valleys. Eclipse and Tidal Breezes. — These are practically unim- portant. During total eclipses of the sun, breezes have been noticed whose origin seems to be due to this unusual inter- ference with the sun's rays. Where tides rise to a great height, as for instance in the Bay of Fundy, local observers report that an increase in the wind accompanies the rising tide. Little is known about this type of tidal breeze, and it is possible that the breeze is due to other causes. Irregular Winds. — These winds are irregular in direction and intensity, and they depend upon causes which do not return with regularity. In these respects they are quite dis- tinct from the permanent planetary winds, and from the regularly recurring seasonal and daily winds. They there- fore deserve to be grouped in a separate class. Storm winds, the most important of this group, are considered in Chapter V. Their important influence in disturbing the planetary circulation is well shown on Plates 9, 10, and 11. Accidental Winds. — These winds are rare, and depend upon some accidental cause which starts the air in motion. Per- haps the most common wind of this class is the landslip or avalanche blast. In mountains, and more rarely in other places, large masses of earth and rock are sometimes pre- cipitated for a considerable distance down some steep slope. These landslides, or avalanches, displace a considerable mass GENERAL CIRCULATION OF THE ATMOSPHERE. 83 of air, and form exceedingly violent local winds, which at a distance of a few hundred yards have in some cases been known to overturn trees and houses. During volcanic eruptions of a violent nature, vast quan- tities of air are started in motion ; but the effect of these volcanic winds is not usually important upon the surface, because the displaced air is high above the ground. The waterfall breeze is a gentle breath of air extending out from the base of a waterfall. The Nature of Winds. — The wind is a bodily movement of the air, but it is not necessarily a steady movement. Every one has noticed that the wind blows in gusts, and that now it is strong, and again very light. In some respects these pulsations are like waves ; and it has lately been found that even when the wind appears to be blowing steadily, it is really made up of a large number of gusts or pulsations, which can be detected only by very delicate instruments. Even during strong winds there may be momentary calms. Nor are the winds a perfectly horizontal movement of the air. As the air moves over an irregular country, it is in some cases deflected upward, and in other cases downward. Another reason for the introduction of a vertical element into wind movements, is the fact that upper air is sometimes settling, while in other cases, as a result of convection, the air near the surface is ascending. These irregularities in air movement make the wind an exceedingly complex series of motions, in which the pre- dominating direction is horizontal, but in which also there are a number of vertical movements. It seems very proba- ble that it is these vertical movements which birds make use of in soaring. Such birds as hawks and eagles are able to float about in the air, and even to rise apparently without 84 PHYSICAL GEOGRAPHY. making movements with their wings. There may be an internal work of the wind which these birds have found, and made use of in their flight, sorting out those movements which they need, and not being retarded by those which are opposed to their motion. It has been suggested by Professor Langley that it is not impossible that these air movements may be employed in aerial navigation by man himself. REFERENCE BOOKS. Ferrel. — A Popular Treatise on the Winds. Wiley & Sons, New York. Second edition, 1890. 8vo. $4.00. (In part a republication of Kecent Advances in Meteorology, Report of U. S. Signal Service for 1885, Part II., Washington.) Buchan. — Report on Atmospheric Circulation, Challenger Reports, Physics and Chemistry, Volume II. Eyre & Spottiswoode, London, England, 1889. 4to. 52s. 6d. (Contains a remarkable series of charts relating to temperature, pressure, and atmospheric circulation.) See also the general books by Davis, Waldo, Greely, and others, referred to at the end of the other chapters. CHAPTER V. STORMS. Cyclonic Storms. — As used here, a storm is any con- dition of cloudiness accompanied by rain. On coasts that Fig. 41. Ideal diagram of a storm. Large arrow shows path of storm; small arrows, inhlowing winds ; circles, lines of barometric pressure ; and shaded areas, dis- tribution and intensity of rain. rise in the paths of moist winds, clouds and rain are often caused by the condensation of water vapor, which results 85 86 PHYSICAL GEOGRAPHY. from the rising of the air, and the consequent cooling until the dew-point is reached. In the same way, air that rises as a result of convection may reach the dew-point, thus form- ing clouds and rain. These kinds of rainstorms are not of particular importance in northern United States, and there- fore need not be considered in detail. These causes aid in the formation of the very important group of storms which bring the greater part of the rain that falls in the northern half of this country. To these the name cyclonic storms may be given ; and these are not of BAROMETER ITHACA — " ^— S~" \^- \^ 1 ^ Aug.23 24 25 28 29 1893 Fig. 42. 31 Sep,] Fall of the barometer during the passage of the center of a hurricane near Ithaca, New York. importance merely in this part of the United States, but they are developed on many portions of the earth's surface. We may divide them into two groups, the tropical cyclones, or hurricanes, and the temperate latitude cyclones. Hurricanes. — Description. In the warm Atlantic tropical belt north of the equator, violent storms begin and move toward the American coast, along which they pass in their course, which is then usually northeastward across the At- lantic. These are the typical hurricanes ; and in the North Pacific similar storms occur, which are there known as typhoons. Storms of this nature are also found in the STORMS. 87 ■v, y ?h=- South Pacific and Indian oceans; but none occur in the South Atlantic, and none appear to originate on the land. They are typical productions of the tropics, and in these regions often attain extraordinary violence ; but in our latitude, although they are the most severe storms that we experience, they have lost much of their tropical violence. As one of these hurricanes or typhoons approaches a place, the sea is calm and glassy, and the air quiet and sultry. The pressure decreases (Fig. 42), and wind begins to blow with increasing violence, while clouds overspread the sky, at first as a thin hazy veil, which gradually changes to a solid mass of dark clouds from which rain falls. The wind increases to a gale and gradually shifts its direction, while at the same time the barometer falls. If the place of observation happens to be in the path of the center of the storm, as this is neared the wind decreases in violence and sud- denly changes to a calm, while the sky overhead becomes clear. This is known as the " eye of the storm." As the storm passes onward, the wind begins as suddenly as it ceased ; but this time it is from the opposite quarter, and then, in reverse order, con- ditions are experienced which resemble those noticed as the storm approached. These conditions indicate that the storm is a mass of \\\\^' t, r ' X F. 1 \S V V I s r /u £ £"**%* ^>A *V fcwV' ^«\n s -to 'h ' 1 <<■ — v~- Fig. 43. Diagram of spirally inflowing winds of the hurricane, together with the path pursued by the storm. Spiral movements greatly exaggerated. PHYSICAL GEOGEAPHY. whirling air, toward the center of which the winds are blow- ing from all directions, along spiral courses, as is shown in the accompanying diagrams (Figs. 41, 43, and 44). The hurricane is therefore not unlike the desert dust whirl which was described in the last chapter. Air is moving toward a central area, where it ascends and flows away in the air above. This outflowing of the air in the upper parts of the storm, is shown to exist by the movements of the upper clouds, which extend outward as long streamers. Effects. — The violence of the winds in a hurricane is almost incredible, and many a ship that has been drawn into the dangerous whirl has not been able to escape de- struction. The tendency is for a vessel to be whirled around the storm center; and if it happens to pass through the center or "eye of the storm," the sudden change in the direction of the wind may come so quickly that the ship is not able to adjust its course in time to prevent foundering. When these hurricanes pass over oceanic islands, they often cause much devastation ; and the destruction of sev- eral war vessels at the Samoan Islands, in 1889, was a result of one of the South Pacific hurricanes. The history of the West Indies, and of the southeast coast of Asia, is replete Fig. 44. Diagram showing conditions of wind and pressure in an actual hurricane. STORMS. 89 with instances of the destruction of stout vessels that have been overtaken by hurricanes or typhoons. Accompanying the storms, there are often great ocean waves which sweep over low-lying coasts, sometimes com- pletely destroying all life and property in the areas visited. On September 15, 1875, three-fourths of Indianola, Texas, Fig. 45. Tracks of August hurricanes, 1888-1893. was destroyed, 176 lives were lost, a million dollars' worth of property was destroyed, and much destruction was done elsewhere along the coast. The same town was again devas- tated on August 19 and 20, 1886. On the Ganges delta, many hundreds of thousands of lives have been lost as a result of these waves. In one storm alone, that of October 90 PHYSICAL GEOGRAPHY. 31, 1876, 100,000 people were killed. Even along the At- lantic coast of the United States, where the hurricanes are of much less violence than in the tropics, a vast amount of destruction is done by them. Not only are ships destroyed, but the low coasts are swept by storm waves (Fig. 82), as has frequently been the case on the New Jersey coast and on the Sea Islands of the Carolina coast. Path. — In the North Atlantic, the hurricanes usually move first toward the northwest, then they curve and pass along the Atlantic coast of the United States until the latitude of Cape Hatteras is reached, when they generally turn to the right and pass in a northeasterly direction out into the Atlantic, which they often cross (Fig. 45). However, at times they diverge from their path and enter the United States, passing northward into Canada. Thus, while we usually experience only the western part of the hurricane, at times the very center moves over the Atlantic coast states (Fig. 42). In the North Pacific, their path is about the same ; but south of the equator, instead of turning to the right, they are guided to the left by the deflective influence of the earth's rotation and the prevailing westerlies. The size of these storms varies very greatly ; and while sometimes they are very large, the area covered by the violent portion of them is usually not more than one or two hundred miles in diameter. When most violent, the area of the hurricane is small, and this is normally the case near the tropics, not far from the place of origin. By the time they have progressed well into the temperate latitudes, their area is greatly increased, and they sometimes cover several hundred thousand square miles. At the same time their energy decreases, and they may even become worn out, so that they lose their distinctive features, particularly when passing over the land. STOBMS. 91 Time of Occurrence. — Another notable feature connected with hurricanes, is the fact that they occur most commonly in certain months of the year. Between the years 1493 and 1855, 355 supposed hurricanes have been recorded at the West Indies ; and out of these, 287 occurred in four months, 42 in July, 96 in August, 80 in September, and 69 in October. In the regions south of the equator, the hurri- canes come most commonly in the months of the southern autumn and late summer, or in other words in January, February, March, and April. In the North Pacific, the time of occurrence of the typhoons is the same as that of the Atlantic hurricanes. The so-called " line storm " of the Atlantic coast, which is expected about the middle of Sep- tember, is in reality one of these hurricanes. Cause. — In the explanation of hurricanes there are several peculiar features which call for consideration. We must bear in mind that the storms are whirling areas of air, in which the winds move violently in a spiral direction toward a center, which is a place of ascending air. The whirling of the winds is in a uniform direction (Figs. 41, 43, and 44), in the northern hemisphere being toward the left hand. The storms begin over the ocean and are by far the most abun- dant in the late summer or the autumn. Their path of progression is first toward the northwest, and then toward the northeast, after having curved around with a parabolic curve (Fig. 45). They are found most commonly in the northern hemisphere and appear to be entirely absent from the South Atlantic. Any explanation which does not account for these peculiarities cannot be satisfactory. Since the storms are confined to the regions near the tropics, or occur outside of them only after having moved to the north or south, we naturally look to the heat of these regions as the cause of the storms. The warm air is ascend- 92 PHYSICAL GEOGRAPHY. ing and winds are blowing toward the place of ascent. As a result of the directly inflowing air, a whirling cannot be produced; and some cause must be found which will originate the spiral motion of the air. A possible cause for this is the deflective influence of the earth's rotation ; but ordinarily this can produce little effect near the equator, because the difference in the velocity of rotation of different latitudes in this belt is very slight (Fig. 21). Upon examining the temperature charts of the world, we find that the heat equator is farthest from the geo- graphic equator in the late summer and early autumn, and that it migrates farthest from the equator in the northern hemisphere, while in the Atlantic it is never far south of the equator. When the place of maximum heat is far from the equator, the influence of rotation will tend to turn the winds to the right as they blow in toward the place where the air is ascending. The farther these currents are from the equator, the more strongly is this tendency developed; and conse- quently those winds that blow toward the equator, are turned more than those that move in from the equator. Thus a whirl is begun, which in the northern hemisphere, always has its winds turning toward the left hand. This whirl may best be started in the summer or late autumn. The con- ditions are never favorable to the production of hurricanes in the South Atlantic, because the heat equator does not migrate far into that ocean. The almost exclusive development of hurricanes over the oceans, is probably due to the presence of moisture-laden winds in these regions, as well as to the very uniform condi- tions that exist there. Water vapor is a great storehouse of energy, and it is estimated that the heat needed to form a pound of water vapor, would melt several pounds of iron. STOBMS. 93 When the vapor condenses, this heat adds to the energy of the storm, and thus violent storms form over the ocean, where there is much vapor in the air ; but over the land the condi- tions are not so favorable. The condensation of the vapor aids the air in rising, and the very rising causes the con- densation of more vapor, so that air is drawn toward the center with great velocity ; and this is maintained for days, and possibly for over a week, by the constant supply of the necessary energy in the form of heat which was latent, and which becomes apparent when the vapor condenses. As the storm progresses into colder latitudes, its energy de- creases, and in time it dies out. We are able to find a satisfactory explanation of the path of the hurricane, in a combination of the prevailing winds and the earth's rotation. The storm starts in the trade-wind belt, but it rises above this belt into the upper air of the anti-trades. The one set of winds tends to blow the storm toward the southwest, the other toward the northeast (in the northern hemisphere), and the hurricane often remains nearly stationary for a day or two, as if in doubt which way to move. Eventually it begins to move in a northwest direc- tion toward the land, and soon it comes under the influence of the earth's rotation and the prevailing westerlies. This increases in effect as the path more nearly approaches a northerly direction, and the storms generally turn in the latitude of the region between Florida and Cape Hatteras. Temperate Latitude Cyclones : Resemblance to Hurricanes. — In many respects these storms bear a resemblance to the tropical cyclones ; and until quite recently it was common among meteorologists to consider the two classes as related phenomena dependent upon similar causes. These storms are the ones which bring the greater part of the rain to the northern United States, and upon which depend most of 94 PHYSICAL GEOGRAPHY. the weather changes of the northern temperate latitudes. The " northeast storms " of New England, so called because they bear damp northeast winds, belong to this class. Every part of the east experiences them, and their importance is very great. So close is the resemblance between hurricanes and tem- perate latitude cyclones, that when the latter are violent, it Fig. 46. Map showing path pursued by a storm and the conditions which accompany it. is quite impossible to distinguish the two kinds of storms. There is a resemblance in form, in winds, and in general behavior (compare Figs. 44 and 46). Both kinds of storms are great whirling masses of air, in which there are clouds from which rain falls ; and the storm area progresses from one place to another. The winds move along a spiral track toward a central area of low pressure, where the air is apparently ascending. In a part of their course, where they STORMS. 95 cross the North Atlantic, the paths of the two kinds of storms are practically the same (Fig. 48). Differences from Hurricanes. — Notwithstanding these resemblances, there are so many differences that we are warranted in considering hurricanes and temperate latitude cyclones as separate phenomena. One of the most striking differences is that of size ; for while the hurricanes usually Fig. 47. Paths of low-pressure areas, December, 1892. Large figures show the number of the storms, the small figures are days of the month. begin as small storms, they may cover a large area when they have passed far into the temperate latitudes ; but the temperate latitude cyclones may cover great areas even shortly after their formation. The cyclonic disturbances may extend over the entire eastern third of the country, from Canada to the Gulf, and from the Atlantic to the Mississippi. The hurricanes are most violent shortly after 96 PHYSICAL GEOGRAPHY. tliey are formed, while the temperate latitude cyclones often develop violence as they proceed on their course. While cy- clones may at times become very violent, they never attain the intensity which is noticed in some hurricanes. The whirling of the air in the temperate latitude cyclones is not so dis- tinct as in the hurricanes (Figs. 44 and 46), and, in them, there is rarely if ever a distinct "eye." Fig. 48. Average Sturm tracks. Relative abundance indicated by numbers showing the total number between the years 1878 and 1887. While hurricanes are most commonly developed in the autumn, temperate latitude cyclones occur in all seasons of the year, but are most numerous and violent in the winter. They do not develop in tropical latitudes, but are formed in various parts of the temperate zone. Some of them begin in the Pacific Ocean, others start in the southwestern part of this country, while others are first noticed in the northwest. STORMS. 97 Their path of progression does not show the peculiar curving so noticeable in the tracks of hurricanes ; but their direction is usually toward the east or northeast (Figs. 47 and 49). If they begin in the Pacific or the northwest, they move in an easterly direction across northern United States or southern Canada ; and the center very commonly passes over the Great Lakes and down the valley of the St. Law- Tracks of first decade of month Tracks of second decade of month .-.-.^n-TTv^TraclcB fxom_21st to 31st, inclusive *•:■:-. '■.'/.^■Indicates fog "belts ^^s^Indicates the position in wEiehneld'ioa or icebergs were observed B.D.Smmi.Tl.T. Fig. 49. Tracks of low-pressure areas (both hurricanes and temperate latitude cyclones) , October, 1892. Number of the storm indicated by large figures, dates by small figures. reuce. If they have their beginning in the southwest (Fig. 47), they first move northward, then curving to the right, they pass out upon the Atlantic. The paths of the hurricanes, and nearly all of the north tem- perate latitude cyclones, converge toward the Nova Scotia- Newfoundland region, and then remain nearly parallel across the Atlantic. Sometimes these storms begin in the Pacific, 98 PHYSICAL GEOGRAPHY. and pass across the United States, the Atlantic, and Europe, thus going nearly around the earth (Fig. 48). While the path of progression is usually regular, there are many minor irregularities of a peculiar and rather exceptional nature (Fig. 49). The origin of these is not well understood. Effects. — The effects of these storms in northern United States are very important ; and they are not confined to this region, but occur in Asia, Europe, and the south temperate latitudes. In the United States, the storms usually come from the west, and hence from the interior, while in Europe they come from the ocean. They bring to us the greater part of our rain and snow ; they are the main cause for thunderstorms and tornadoes ; they produce many of our most striking winds ; and they are the cause for many of the changes in temperature which we experience. The warm south winds of the winter, and the heated spells and droughts of the summer, as well as the cold northwest blasts of winter, have their origin in these cyclonic disturbances. At times the violence of the cyclones is so great that much destruction is accomplished both on the land and on the water. They are particularly destructive on the ocean, and nearly every winter the fishing fleet and coasting vessels suffer from their destructiveness. Winds. — The winds of the temperate latitude cyclones vary in force, as well as in direction. Some storms have gentle winds, while in others they are very violent ; and in dif- ferent parts of the same storm the velocity may vary greatly. On the land the)' are usually less violent than on the water, because the irregularities tend to destroy them by friction. If a storm is passing over a given place, the direction of the wind changes during its progress ; and the points of the compass through which the wind veers, depend upon the position of the storm center. If it is north of the place of STORMS. 99 observation, the kind of change will be very different from that which occurs when the storm center is toward the south. The best way to understand these changes is to study the weather maps and notice the change of wind as the storms progress on their path. Certain special kinds of winds are generated in cyclonic disturbances. On the southern side of a storm, warm winds are drawn in from southern latitudes ; and in winter these may cause a snowstorm to change to rain. In Italy, these warm southern winds come from the heated desert region of northern Africa, and hence are usually dry. In that country they are known as the sirocco ; and this same type of wind is also developed in the United States. Here, how- ever, the sirocco is not dry, but is generally warm and often damp. In southern New England it brings damp air from the Atlantic Ocean ; and this air is warm because it comes from the area influenced by the Gulf Stream. A peculiar type of wind known as the foehn is developed in Switzerland, where air is drawn over the Alps by the passage of a storm center over central Europe. This air, drawn over the Italian side of the mountains, is caused to give up much of its moisture as it rises and cools. It is drawn down the northern side of the Alps with considerable velocity, and as it descends it warms dynamically. There- fore, the foehn is a dry and very warm wind, which in win- ter will often remove a layer of snow by direct evapora- tion. Its dryness is so remarkable that it has been thought to be a hot breath from the Sahara. A similar wind is caused by the passage of storm centers east of the Rocky Mountains ; and in that region it is known as the chinook. It is developed along the eastern base of the Rockies from Colorado to Montana, and its peculiarities are the same as those of the foehn. In the winter it often 100 PHYSICAL GEOGRAPHY. causes an unseasonable rise in the temperature, and snow disappears before it with great rapidity. On the western or rear side of cyclones, instead of warm there are cold winds. Here the air comes from cold north- ern lands, and in a measure also from the upper layers of the air. When very violent, these cold north or northwest winds are known as blizzards, and they often bear with them vio- lent squalls of snow. The true home of the blizzard is the northwest ; but even in the plateau region of central New York, true blizzards of a somewhat milder form, often suc- ceed the severe winter snowstorms. In Europe, the same form of wind is developed; and in Texas the norther is a wind of similar origin. Anticy •clones. — Between well-developed cyclones, there are usually areas of high pressure, which are known as anti- cyclones. In these, the air is slowly settling l from upper parts of the atmosphere, and violent winds are not produced. The air is dry and clear, and hence radiation proceeds rap- idly, so that at night the temperatures often descend to very low degrees. While the air in these anticyclones is quiet, violent winds are often present at the margin, and particularly when the margins merge into the rear side of cyclones. Indeed, there seems to be a certain association between the cyclones and anticyclones, as if the down-settling air of the latter entered as a part of the whirl of the former. These conditions give us the cold waves of winter and the cool spells of summer (Figs. 63 and 64). Cause. — Until recently it was quite commonly believed that the origin of temperate latitude cyclones was the same 1 When air settles slowly, the dynamic heating is not marked. Hence this settling air in anticyclones usually reaches the earth with a low temperature ; but there has been some warming, and the air is not so cold as when it started. STORMS. 101 as that of hurricanes. In objection to this theory it may be said that convection does not seem capable of accounting for these great disturbances. In the first place, they cover an area often having a diameter of more than a thousand miles, but extend to a height of only two or three miles. Moreover, they are most violent and best developed in winter, when con- vection is least active. Recent studies seem to show that the cause for these storms is aloft, not at the ground. While it cannot be considered proven that convection is not the cause, there are so many reasons for doubting this explanation, that it certainly cannot be accepted ; and we are now without an explanation for these remarkable, though common, atmospheric disturbances. They pass across the country like a series of waves in the air ; and it is possible that the great circumpolar whirl is thus thrown into waves, and that these disturbances are merely a secondary part of this planetary circulation. Recent studies seem also to show that there is some relation between them and magnetism; but we cannot feel certain of these suggestions. The path followed by these cyclones and anticyclones is easily explained. They are borne along in the whirl of air which moves about the pole, and hence their direction is from west to east. As a result of the influence of the earth's rota- tion, upon the air currents the storms are carried along their regular paths. The winds in the storms cause a whirling in the same direction as that of the hurricane, and for the same reason, — those on the northern side are most deflected. Secondary Storms. — Aside from the greater general dis- turbances, there are certain minor phenomena of cyclonic storms, which attract much attention because of their vio- lence. The two most important of these are thunderstorms and tornadoes. Thunderstorms. — When moist air rises as a result of 102 PHYSICAL GEOGRAPHY. convection, if the ascent carries the air high enough for the dew-point to be reached, clouds may form and rain fall. In such cases electricity may be generated, and lightning and thunder may accompany the rain. In the belt of doldrums, the ascent of the moist air causes frequent thunderstorms during the day ; and in summer, the rising air among the mountains may cause the formation of thunderclouds and rains. In this class of storm there is no distinct whirl, but a simple ascent of moist air. In central and eastern United States, thunderstorms are common in sum- mer ; and they also are the result of uprising moist air. That this is so, is shown by the fact that they occur almost ex- clusively in sum- mer, and near the close of hot, sul- try days. On these days, one may often witness the development of such a storm, if the place of observation is sufficiently elevated to command a wide-extending view (Fig. 50). Clouds begin to develop ; and if they are seen from below, their bases are found to be flat, marking the plane at which the rising air reaches the dew-point. When seen at one side, mound-like masses of clouds, often of mountainous heights, are found to rise above the even base. If the observer is well to one side of the cloud, it will be noticed that as the storm develops, the form is quite like Fig. 50. Photograph of a distant thunderstorm. STORMS. 103 that of an anvil (Fig. 50). At high elevations, the clouds extend out in front of the storm, marking the upper outflow of the air. The great elevation of the cloud mass is due to the fact that the air continues to rise to these heights, and the vapor to condense as the temperature descends. Most of our thunderstorms are a part of moderately developed cyclonic disturbances, and they occur most com- monly in the southern part of these storms. Here warm moist air is being drawn in toward the storm center, and hence the conditions favoring the development of thun- der-storms are pro- duced. As the storm center progresses, the area in which thunder- storms may develop al- so moves eastward, and any single storm will be found to have the same path (Fig. 51). Some thunderstorms have passed entirely Fig. 51. Progression of a thunderstorm in Massachu- setts. The figures represent the hours at which the storm front reached the places indicated by the line. across New England, while others die out after traveling a few miles. Some pass over a broad path, while the width of others is only a few hundred yards. When the path is long, the storm may continue into the night ; and most night thun- derstorms have originated, during the preceding afternoon, at some point far to the west. The rate of progression is usually not greater than 40 or 50 miles an hour. In the thunderstorm, after the first violent squall, that usually blows out from the base of the storm, the winds 104 PHYSICAL GEOGRAPHY. are generally not violent; but there is a steady and often heavy downfall of rain, with accompanying thunder and lightning. In some cases the downpour of rain is exces- sive ; and among the mountains of the west, there are often such torrents of water that the name cloudburst is given to them. The name is certainly warranted, for the water falls in sheets, in a manner which can be appreciated only after having seen one. These excessive rains may be due to a supersaturation of the air. Tornadoes and Waterspouts. — These extraordinarily vio- lent storms are fortunately small, local, and not common in most of the country. Like the dust whirl of the desert, or like the hurricane, they are whirling bodies of air, in which the winds blow toward a center, where they rise (Fig. 52). The winds blow at such terrific rates that houses are torn down and the parts carried away (Fig. 53). The news- papers furnish vivid descrip- tions of them ; and while they are often exaggerated, almost no story concerning the action of tornadoes is too incredible for belief. 1 In the center, where the air is ascending, the air pressure is often so low that a partial vacuum is produced ; and the walls of houses may then be blown outward by the sudden expansion of the air within. As the tornado approaches, it appears as a great funnel- shaped column of black cloud (Fig. 52), in which there are signs of violent commotion. As it comes nearer, a roaring noise is heard ; and as the cloud overspreads the sky, rain or 1 In newspaper accounts they are usually called cyclones. Fig. 52. View of a tornado. STORMS. 105 hail falls ; but this ceases in the violent part of the tornado, where the air is rising so rapidly that these forms of water cannot descend. At first there is no wind, then suddenly a gale springs up, and almost' immediately its violence becomes so great that houses and trees are felled. On opposite sides of the storm the wind moves spirally toward the center. The tornado usually progresses at a rate of from 25 tc» 40 miles an hour. Its width is rarely as great as a mile, and more often i.,t- only a few hundred yards, or even feet, so that it cuts a swathe, on either side of which no de- struction is ac- complished. The distance traversed by one of these storms is gen- erally not more than 30 or 40 miles, and it rarely lasts more than an hour. They do not occur in large numbers outside of the central states of the Mississippi valley, although they do occasionally occur in the east. West of Dakota they are not known. They not uncommonly occur in association with thunderstorms ; and like these, they come after hot, sul- try days, in areas covered by the southern portions of cyclonic storms. Their movement is almost invariably eastward. In part at least, tornadoes are due to convection ; and the reason for their abundance in the Mississippi valley seems to be that warm, moist air is drawn up that valley toward the Fig. 53. Effect of a tornado at Lawrence, Mass., July 26, 1890. 106 PHYSICAL GEOGRAPHY. storm center, while above it there is a colder layer of eastward moving air. Therefore the conditions of the atmosphere are peculiarly unstable ; and the increased heat caused by the sun, starts an overturning which soon takes the form of a violent whirl. This is not possible in the far west, where the lower air is dry; and in the east, the atmosphere is rarely in a sufficiently unstable state for this violent overturning. When the tornado develops or passes over the sea, or over a large lake, it takes the form of a waterspout. It is doubtful if these waterspouts are col- umns of water, as is often stated ; but there is probably a conical wave in the center. Fig. 54. Distribution of tornadoes 1794-1881, the in- tensity of shading showing greatest abun- dance. Darkest more than 35, medium shade 25-35, lightest shade less than 25. REFERENCE BOOKS. Ferrel. — A Popular Treatise on the Winds. (For price, etc., see refer- ence at end of Chapter IV.) Davis. — Whirlwinds, Cyclones, and Tornadoes. Lee & Shepard, Boston, 1884. 24mo. $0.50. (Reprinted from " Science.") Finley. — Report on the Characters of Six Hundred Tornadoes. Professional Papers No. 7, U. S. Signal Service, Washington, 1884. Finley. — Tornadoes. Hine, New York, 1887. 12mo. $1.00. (Based mainly upon previous publications in the U. S. Signal Service Reports, etc.) The Monthly Weather Review and the Daily Weather Maps, published by the Weather Bureau at Washington, and the Pilot Charts, published by the Hydrographic Office of the Navy Department at Washington, will be found invaluable in laboratory instruction. Teachers who are inter- ested can probably obtain these upon application. CHAPTER VI. THE MOISTURE OF THE ATMOSPHERE. Dew. — When the temperature of the air descends far enough, a point is reached when there must be a condensa- tion of some of the contained moisture, because the ability of the air to carry water vapor, depends in large measure upon the temperature. With dry air, the temperature must be lowered much farther than with damp or humid air ; and on the sultry days of summer, a pitcher of ice water lowers the temperature of the air in contact with it sufficiently to cause the condensation of some of the vapor on the outside of the pitcher, which is said to "sweat." When the ground becomes cold at night, the lower air is also cooled, and that which is in contact with the ground may give up some of its vapor as dew. The temperature at which this will happen, naturally depends upon the amount of vapor in the air ; and in the tropics, where the hot air is very humid, the amount of dew that forms at night is often very great. Even the coolness of the late afternoon is often sufficient to cause the condensation of dew in the tropics ; and during our own summer days, one often notices that the grass is wet with dew even before dark. Dew forms most readily on those bodies that cool by radia- tion most quickly. Thus grass and leaves are dew-covered sooner than soil. During some nights, even when the air is quite humid, dew is not formed. By interfering with radia- tion, clouds tend to prevent the formation of dew ; and as a 107 108 PHYSICAL GEOGRAPHY. result of the stirring of the air, and the inflow of new sup plies of air, wind tends to check dew formation. Because the air is more humid, dew is formed more readily near streams or swamps than in dry places. Dew is heavier in valleys than on hills, partly because of the greater damp- ness of the valleys, partly because cold air slides down into them from the hillsides, and partly because the air in valleys is more quiet than that on the hilltops. While the main cause for dew seems to be condensation of vapor from the air, recent studies show that this is not the only cause. At all times plants are furnishing moisture to the air by transpiration. Ordinarily this is evaporated; but at night this evaporation is checked, when the air is cooled, and its power for evaporation reduced because it is either saturated or has a high relative humidity. Then the moisture forms drops of water on the leaves. Thus dew is a result of the combination of two processes, in both of which the cooling of the air by contact with the earth is the important cause. Frost. — When the temperature of the dew-point is below freezing, the condensation of vapor takes the form of frost. It is not frozen dew, but vapor that has become condensed as a solid, instead of a liquid. In cause, and in occurrence, frost may be described in the same terms as those used in the description of dew. However, the effect of frost is quite different, for it causes vegetation to suffer, while dew refreshes vegetation. Frost is not likely to occur on windy or on cloudy nights, and it comes earlier in damp valleys than on dry hilltops. This is why the autumn foliage first assumes its brilliant tints in the swamps. A cover- ing, such as a sheet, by interfering with radiation, will prevent a light frost ; and in this way delicate plants may be protected when there is danger of frost. THE MOISTURE OF THE ATMOSPHERE. 109 Fog. — This is merely the condensation of water vapor into the form of very tiny drops, which are so light that they do not readily fall to the ground. When we breathe the warm moist breath into the cold air of a winter day, we produce a tiny fog. Many of the great ocean fogs owe their origin to a similar cause. On the banks of Newfoundland, where the warm Gulf Stream is side by side with the cold Labrador current, fogs are produced when the winds of the one region pass over the other. Very extensive fogs are thus caused, and this has made that part of the Atlantic famous. When warm air is drawn northward toward storm centers, fogs are particularly liable to occur here. These and other ocean fogs often extend upon the land, as for in- stance on the coasts of Maine and Nova Scotia. A fog sometimes surrounds an iceberg, because the air around it is chilled. Over the surface of lakes we some- times see fogs developed by the chilling effects of air cur- rents. At times the cool water produces a fog by contact with warm air. In damp valleys, a valley fog is often formed when the air is chilled and the vapor condensed into particles. This is particularly liable to happen during nights when the conditions favoring a heavy dew are pres- ent. Every one must have noticed the cool dampness of valleys, which is so noticeable in passing along a hilly road just after nightfall. This often increases until some of the dampness forms into fog particles. One often sees valley fog among the mountains (Fig. 55~), and many clouds are nothing but fogs. As one looks down upon a valley fog, there is a white rolling surface, above which may rise the tree tops or church steeples, while everything else is hidden. The appearance is not unlike that which one sees in the mountains when above the clouds. By furnishing a nucleus about which the vapor may con- 110 PHYSICAL GEOGRAPHY. dense, " dust " particles are important in the formation of some fogs. It is believed that the fogginess of London in part depends upon the large amount of dust in the air. Haze. — At times, and particularly during dry weather, a thin veil of blue haze extends through the atmosphere and partly obscures the distant landscape. Often it is so indis- tinct that one notices it only when an unobstructed view Fig. 55. Valley fog in the Himalayas. Mount Everest in the background. of far-distant hills is obtained; but during some days it becomes so thick and dense, that points near at hand are almost completely obscured, and even the sun loses its in- tensity, while the sky becomes dull. Haze is not damp like fog, and there is reason to question whether it is often due to water particles. Probably the greater part of the haze results from dust in the air ; and during droughts the air is often very hazy, because at such times rains have not THE MOISTURE OF THE ATMOSPHERE. Ill occurred to clear the sky, and the air is often supplied with much dust from forest fires. Mist. — At times the air is filled with minute particles of water, which are larger than those in a fog, and which therefore cause greater dampness. The mist is intermediate between fog and rain, and possibly it is made of numerous fog particles which have united. Clouds. — Clouds are composed of particles of moisture due to the condensation of water vapor. Sometimes these particles are very small, like those in fog, at other times they are made of mist, or even of raindrops, and in many cases of ice particles or snow crystals. They are formed when- Fig. 56. The banner cloud, caused by a moist wind blowing against a mountain peak. ever vapor-laden air has its temperature lowered to the dew- point ; and this may be caused in several ways. When damp air encounters a cold mountain top, clouds are formed, and these may surround the mountain peak or ex- tend beyond it like a banner (Fig. 56). Where high mountains extend upward in the path of the trade winds, these banner clouds are often produced. Air that is caused to ascend, fre- quently has its temperature lowered below the dew-point; and when this point is reached, clouds are formed. This may happen when air rises by convection, or when it ascends land elevations. During the summer, and in mountains, such clouds are commonly formed. The mixture of air of 112 PHYSICAL GEOGRAPHY. various temperatures often causes cloud formation, and this appears to be the origin of many of the clouds of the upper atmosphere. In reality, fog is a form of cloud ; and Fig. 57. Photographs of five common cloud forms. Cirrus. Nimbus. Cirro-cumulus. Cumulus. Cumulo-stratus. during storms, when the clouds are low, we may find our- selves enveloped in a true cloud mist. The forms of clouds are very beautiful and varied, and the various kinds are known under different names. The THE MOISTURE OF THE ATMOSPHERE. 113 following is a classification of clouds based partly upon their form and partly upon their elevation : — Cirrus. Cumulo-stratus. Cirro-stratus. Nimbus. Cirro-cumulus. Stratus. Cumulus. The cirrus cloud (Fig. 57) is the highest form known, its elevation often being greater than five miles. It is so high that the condensation of water vapor forms ice spicules, and this is the reason why these clouds appear thin, white, hazy, and almost transparent. They drift along at very rapid rates, and in northern latitudes usually move toward the east, being carried along in the circumpolar whirl. Their form is variable and often remarkable. They are commonly pro- duced by the upper outflow of air in a cyclonic storm. At times the cirrus clouds occur in the form of distinct bands, and they are then known under the name of cirro- stratus. This form of cloud may completely overspread the sky, but its transparency is so great that the sun is visible through it, and during such conditions of cloudiness halos and coronas are commonly formed. Many varied forms of cirrus clouds are recognized, and various names are given to them. Sometimes they are frayed and torn as if by violent air currents. At other times they occur in bunches, arranged often in lines, as if produced by waves of the air, the groups of clouds resembling a choppy sea. When these bunches of upper air clouds are quite distinct, they are known as cirro- cumulus (Fig. 57). Oftentimes the sky is speckled with these clouds, and then sailors call it the mackerel sky. Among the most beautiful of clouds are those known as cumulus (Fig. 57). They are produced at a lower elevation than the cirrus, and are often composed of fog particles in- 114 PHYSICAL GEOGRAPHY. stead of ice. When best developed, as is the case in summer, they are the typical thunder heads, which rise from a flat base, at an elevation of about a mile, and extend into the air, often to a height of several thousand feet above this. They consist of a mass of rounded, dome-like clouds, which often produce a very fantastic and beautiful effect, partic- ularly when lighted by the rays of the setting sun. These clouds are common, every-day occurrences in the belt of calms, and in summer they are often produced around mountain peaks, and over the heated lowlands. In these cases their cause is the ascension of warm moist- air ; and during hot summer days they may often be seen to form. Over the land, they are much more readily formed than over the water, and the presence of land is often indi- cated by their occurrence. When sailing along the coast of Florida in summer, the position of the land is often shown by a line of these clouds. At nighttime, when convection ceases, the clouds melt away and the sky clears. Clouds that resemble the cumulus, but differ from them in being more massive and banded, are known under the name of cumulo-stratus (Fig. 57). When this form of cloud is very massive, so that large parts of the sky are covered, the name stratus is applied. These often entirely overspread the sky, forming a gray, illy-defined cloud mass. Their elevation is usually between 600 and 3000 feet, but at times they are so low that they touch the earth. This is the kind of cloud that occurs during cyclonic storms, and then they may cover the sky over an area of thousands of square miles. When rain is falling from a cloud, it is known as nimbus (Fig. 57). Rain. — There is every gradation between dew and rain, and raindrops are often made by the union of numerous fog particles. The exact means by which these particles are gathered together, cannot be stated ; but perhaps in many THE MOISTURE OF THE ATMOSPHERE. 115 cases it is the result of the contact of particles driven against one another by wind, or as a result of their descent through the air. There is a very definite relation between clouds and rain, and the causes which produce the one form the other. The most important of the causes are the mixture of currents of different temperature, the uprising of air, and the con- tact of warm moist air with cold land surfaces. The greater part of the rain of the world falls either (1) from cumulus clouds, or (2) from cy- clonic storms, or (3) where moist winds blow from the water upon the land. Away from places where these conditions occur, the rainfall is usually light. Sometimes, though rarely, rain- drops fall from a clear sky. Snow. — This is the crystallized form assumed when water vapor condenses at temperatures below the freezing-point ; and the forms thus produced are often very beautiful and fantastic (Fig. 58). There is an intimate relation between snow and rain, and the same storm may produce snow on the highlands and rain on the lowlands. Many of our winter rainstorms are due to the fact that the snow crystals have been melted in their downward passage ; and the damp snows are a partial step in this direction (Fig. 59). The difference of a few degrees thus produces a very marked change. In the one case rain falls and speedily flows away, Fig. 58. Photographs of actual snowrlakes. 116 PHYSICAL GEOGRAPHY. while in the other case a cold covering of solid snow is laid upon the land, perhaps to stay for months. The clouds of the upper air are mostly made of ice or snow, and mountain peaks that extend into these upper layers, rarely receive any other form of precipitation. Hail. — At times, particularly in summer, balls of ice known as hailstones fall from the clouds, especially from *-•# Q »* Fig. 59. Photograph taken after a fall of damp snow, showing how it clings to vegetation. those accompanying thunderstorms and tornadoes. They are usually oval or rounded in form, and are often made of successive shells of clear and clouded ice. The mode of formation is not known ; but there is some reason for believ- ing that they are formed in violently moving and rising air currents, and that this is the reason why they so commonly fall on the margins of rather violent storms. EXPLANATION I OVER 200 i ' 1 '''""■ 130-200 n 60-130 20-60 LESS THAN 20 Face page 117. Raini «l GBAMATIC MAP SHOWING L Or THE WORLD ^TIMETERS PER YEAR B.D.Seruoas.H.Y. d. THE MOISTURE OF THE ATMOSPHERE. 117 Distribution of Rainfall in the World. — As used here, the term rainfall includes both rain and snow. In general there is a difference in the amount of rainfall according to latitude and altitude. Since in high latitudes and high altitudes the temperature of the air is low, and therefore contains little vapor, the amount of rain that can be condensed in these places is less than in the warm tropics, where the air is humid. Still, there is much variation in this respect, as will readily be seen by a glance at Plate 12. Without entering into the subject in great detail, a few notable facts shown on this chart may be pointed out. It will be noticed that in the belts where the trade winds blow upon the land, the rainfall is heavy, while in those places where they blow over the land, the rainfall is slight. Thus, as a result of this, the dry desert of the Sahara exists in the same latitude with several very rainy districts. Where the winds blow against steeply rising mountains, such as the Himalayas, the precipitation is very heavy. Even outside of the trade-wind belt, when the winds blow from the warm ocean upon the land, the amount of rainfall is often very great. If mountains intercept these winds, they are drained of their moisture, and pass to the opposite side as dry winds, producing deserts. Thus there are two important causes for deserts. In the belt of calms, where the air is almost constantly rising during the day, the precipitation is quite uniformly heavy ; and as these belts migrate, the rainy conditions are carried with them. Thus we may have one very wet season, and an opposite dry season, when the calms are replaced by the trades. This is the case on both sides cf the equator in Africa and South America. Usually the rainfall is heavy near the coast ; but where this is not the case, the winds are blowing from the land to the 118 PHYSICAL GEOGRAPHY. sea. With the seasonal change in the wind direction, some coasts have a dry and a wet season. In the interior of continents, a condition of relative dryness usually prevails. This is not always a true desert condition, but often one of semi-aridity, in which the rainfall is not sufficient for suc- cessful agriculture. There may be every gradation be- tween the humid country and a desert, passing through the stages of semi-aridity and the climate in which droughts are common. The greatest irregularities of rainfall are noticed in temperate latitudes ; and these depend in part upon the winds, the topography, the neighborhood to the sea, and the occurrence of cyclonic storms. In parts of the area, nearly the entire rainfall comes in association with these storms. Bearing in mind the previous discussion of tem- perature, winds, and storms, the student will be able to understand most of the irregularities in rainfall distribution indicated on the accompanying charts. Distribution of Rainfall in the United States. — In this country (Plate 13), most of the features noticed on the rain- fall charts of the world are well illustrated ; but we have not the tropical conditions. On the Texas coast, the inblowing trades of the summer cause a heavy rainfall ; and in Florida, much of the rain depends upon the neighborhood of the warm ocean waters. The rainfall of the eastern coast is less than that of the western, because in the former the winds are mostly from the land. Still, because this region is frequently visited by cyclonic disturbances, there are no deserts pro- duced in the east. From Florida to Maine, the rainfall decreases quite uni- formly, as it should in passing from warm to cooler regions. On the western coast, the reverse is true, and the most humid part is in the north, while the southern portion is quite 120 PHYSICAL GEOGRAPHY. arid. This is due to the fact that in the northern part, the winds blow from the ocean against the mountains. Because of this, the rainfall also decreases very rapidly from the immediate coast toward the interior. Beyond the mountains of the coast, the country is either arid or in a truly desert condition ; and this extends even to the plateau states east of the Rocky Mountains. Throughout the greater Fig. 60. Rate of evaporation in the United States. Based upon observations for a year, in 1887-1888. part of the western half of the country, the rainfall is very slight, because there are no great water bodies to supply the winds with moisture. Even in the states just west of the Mississippi valley, the rainfall is light and quite irregular, because the winds are dry. Here evaporation is rapid, and in some parts, where the total annual rainfall is less than 10 inches, it amounts to 100 inches (Fig. 60). THE MOISTURE OF THE ATMOSPHERE. 121 Distribution of Snowfall. — Over a very large part of the earth' s surface snow is impossible, and a considerable part of the human race has never seen it. In the United States, snow falls nearly everywhere except in Florida and south- ern Texas and California ; but it is only in high tem- perate and Arctic latitudes that much snow can fall upon b|b 3 rllSlilSg |2 llllsillssls ?slll|i;| = ls lsllil|l|sls lsIillSf|g|l l'5Its||l'sllg SSllli5s|§ls ... • IK. ? T.TOWNSEN SPOKANE FALLS FT. SHAW FT. BENTON FT. BUFORD FT. TOTTEN ST. VINCENT ,,„ .,». WASH. WASH. MONT. MONT. N. DAK. N. DAK. M 1 NN. 2tN i IIIH...II hll!..llll ...llllll... ^JllW ...llllhl.. ..ill 11.11. ^lll ill.. PORTLAND 1 WALLA WALLA FT. E' LIS FT. CUSTER FT.KEOQH FT. A. LINCOLN MOORHEAD j 1 ORE. WASH. MONT. MONT. MONT. N. DAK.. MINN. 4 IN J , 1 11 J IN ] 1 ll..l 1 11I1. .11 ml niiii ..Ill llllll ...'Uii.,.. .1,11 II.... ..llllll.. EOLA BOISE CY. FT. HALL FT. WASHAKIE FT. FETTERMAN FT. SULLY FT. RANDALL .,] 1 ° RE J IDAHO IDAHO WYO. WYO. S. DAK. S. DAK. .1 IN ■- 1 . I .1 1 , |(j -] 1 I.mI 1 Illll. ..ll .lllllll.l 1 ..lllllll.r. ..lllllliP't ..ill .... ..ll ll.l FT. Bl DWELL WINNEMUCCA PROMONTORY FT. BRIDGER CHEYENNE NORTH PLATTE GENOA • IN CAL. NEV. UTAH WYO.. WYO. NEB. NE B. 2>N 1 .. . 1 ;:n II....I Illll.. ...II Illll.. ..Ill I.llll.ll.li ...llllll... ll.l lull 1 Is* Fig. 61. Monthly rainfall in the West, showing the heavy winter rains of Washington, in contrast with the normal condition of heaviest rainfall in summer. Also showing differences in amount in inches of rain. the lowlands. Even under the equator it may fall on high mountain peaks. There is much variation in the distribution of snow, both from season to season, and from place to place. Where the temperatures are low, the snow remains upon the ground during the winter ; but in many places it stays only for a short time. In high mountains, where the snow- fall is great, and where there is very little melting, it may 122 PHYSICAL GEOGRAPHY. niijim ii_a| 2 t 1860.9 1870.9 1880-9 FT.TOTTEN DAK. FT.SISSETON DAK. .2 en _*H 22 * ^"S I => < i3H s m o ^m I L -■-___. ■ .QL _.- — 1 — , — .a _^H "? b I" Oh- "■■ S< z u z 2 _i O * O 3 g " 2 " 5 E " i 1 = jl- JI ""^-■g-p-t 31 = z z ~^H i; z Mbri £ ™ O J • O ^" JJt* "t* 1 8 1 JI = '5 2 < C ■is: ■ BE-— ■ 1 < p~^' produce glaciers as a result of the accumulation of many winters' snow- fall. The same is true in parts of the Arctic and Antarctic lands, and in these cold places, even the summer precipitation is mostly in the form of snow. Seasonal Distribution of Rainfall. — Many parts of the earth have dry and wet seasons ; and as has already been explained, this is usually due to a change in wind. In equatorial Africa, among the headwaters of the Nile, the migration of the belt of calms causes such a condition, and the same is true of the llanos of Venezuela and the campos of Brazil. The blowing of the monsoons upon the coast of Asia, and elsewhere, causes very rainy conditions which are quite absent when the monsoon winds blow from the land. At Cher- rap unji, where the rainfall is as great as 500 inches a year, the amount fall- ing in December is only 0.2 inches, while in July it is over 130 inches. This excessive rainfall, which is the greatest on the earth, is caused by the blowing of the monsoons against a steeply rising mountain face. On the western coast of the United States, particularly in Washington and Oregon, the winter rainfall is THE MOISTURE OF THE ATMOSPHERE. 123 heavy, while in summer it is light (Fig. 61). This is due to the damp winter winds from the Pacific. In the central and eastern states, the distribution of rain- fall is very irregular, and it depends upon the nature and frequency of cyclonic storms. Some seasons are very dry, and then droughts may occur ; but there is no regularity in the recurrence of these periods. Fig. 62 illustrates this variation in the western states. Irregularities of Rainfall. — The normal rain is a steady and rather quiet downpour ; but at times, particularly in connection with thunderstorms, the rainfall may be very heavy, and then more rain may fall in a few minutes than during an ordinary cyclonic storm lasting for a day or two. For instance, at Syracuse, New York, 8 inches of rain fell in one day, June 8, 1876 ; and in June, 1886, over 21 inches fell in 24 hours at Alexandria, Louisiana. The effect of such a sudden deluge of water in swelling the streams and wearing away the land is very important. The cloud-bursts of the Rocky Mountains furnish other instances of very remarkable rainfalls occurring in a short period of time. Where the rains are excessive in violence, the aoil is some- times washed away from steep slopes, leaving the bare rock exposed to the air. This is the case in that region of remarkable rainfall in India. REFERENCE BOOKS. Tyndall. — The Forms of Water (International Scientific Series). Appleton & Co., New York, 1872. 12mo. $1.50. Schott. — Precipitation, etc., of the United States. Second edition, 1885, Smithsonian Contributions to Knowledge, Vol. XXIV., 1885. Smithsonian Institution, Washington, D.C. $6.00. Harrington. — Rainfall and Snow of the United States. Bulletin C, Weather Bureau, Washington, 1894. (Many valuable charts.) CHAPTER VII. WEATHER AND CLIMATE. Weather. — Climate is the sum and average of weather, which includes the daily change in temperature, pressure, wind, rain, etc. The climate shows the general condition, while weather deals with the special instances of changes in the atmosphere. The data obtained in a study of the weather furnish the basis for a knowledge of the climate, and thus the two subjects grade into one another. Already, in the previous pages, much has been said concerning weather and climate -, 1 but now a few statements upon the subject are made as a kind of summary. Tropical and Arctic. — There is much difference in the variety of weather in various parts of the earth. Over the ocean, the weather conditions are less variable than on the land, and the greatest variation is found in temperate latitudes. Day after day, the weather in the belt of calms is nearly the same, the clear nights being followed by cloudy days with frequent rains, and the temperature being high and not very variable. In the belt of trade winds, the air moves rather steadily toward the equator, and the tempera- ture is high. When these winds blow over the land, their dryness produces desert conditions ; and when they blow upon the land, heavy rains are caused. Thunderstorms 1 Many of the foregoing figures and plates illustrate this chapter as well. 124 WEATHER AND CLIMATE. 125 may occur, and now and then a hurricane may develop, bringing with it violent winds and heavy rains. In the polar regions the winter season is marked by uniform cold, and the storms always bring snow. During the summer there is no marked day and night alternation in temperature ; and although the air is warmer than in winter, the temperature is uniformly low and snowstorms may occur. Temperate Latitude Weather. — Taking the United States as typical of the temperate latitudes, we will examine the weather conditions of several sections. On the Pacific coast, north of central California, the days of summer are dry and warm, and the nights become quite cool. In the winter the warm winds from the Pacific blow upon the land, producing frequent rains during the day ; but the temperature of the day, and even of the night, is moderate. In the high mountains east of this, the air is cold, and even the summer storms often produce snow instead of rain. The temperature of day and night is low. In the desert regions between the mountains, storms rarely occur, and the air is quite constantly clear and dry. Occasionally, espe- cially in summer, there are heavy thunderstorms, particu- larly among the mountains ; but in some of the deserts, as for instance that of Arizona, there is almost no rainfall (Plate 13). During the summer day, the ground and air become highly heated, and at night low temperatures are produced by radiation. On the plains of Dakota, Montana, Manitoba, etc., the air is prevailingly dry ; and during the summer, the tern perature of the day becomes high, while the nights are cool. During the winter, excessively cold spells are liable to occur, and temperatures as low as —30° are not uncom- mon. This region is subjected to the influence of cyclones 126 PHYSICAL GEOGRAPHY. and anticyclones, with their accompanying conditions of rain or clear weather and variable winds. During the winter, there may be very heavy snowstorms, and at times extremely violent blizzards ; and, following these, the warm chinook wind may cause an unseasonable rise in temper- ature. Farther south similar conditions prevail, but the weather changes are less intense. On the dry plains of Texas, the temperature ranges are extreme ; but neither the chinook nor the blizzard occurs, though a cold norther some- times produces a very severe weather change. Along the coast of the southern states, high temperatures are experienced, and the ranges from season to season, and from day to night, are not great. Rainstorms are produced by the blowing of the winds from the warm ocean upon the land ; and in the autumn, violent tropical hurricanes often visit the coast. No snow falls, but during the winter, when a cold wave spreads over the country, freezing temperatures may at times extend into this belt. In the more northern states of the Mississippi valley, the weather of the winter is cold, snowstorms accompany cyclonic disturbances, and extremely low temperatures are produced during anti-cyclonic conditions. There are great daily temperature ranges, as well as some of an irregular na- ture. During the summer, cyclonic storms are less common, and they come at irregular intervals, and there may be long periods of drought. These droughts occur when the cyclonic disturbances are of moderate intensity, and the storm centers far to the north, in the Canadian territory. During these conditions, warm air is drawn in from the south toward the storm center, and this raises the temperature but does not produce rain. Under favorable conditions, thunder- storms and tornadoes may arise during the passage of low- pressure areas. WEATHER AND CLIMATE. 127 In southern Canada, New York, and New England, the weather is very variable and irregular. In the winter, snow- storms occur, and these are sometimes very heavy, particu- larly in the northern part of the area. Over a large part of the region, storms are of sufficient frequency, and the cold sufficiently intense and uniform, to allow the snows to ac- cumulate during the winter and remain upon the ground Fig. 63. Conditions of wind, pressure and temperature accompanying a cold wave, March 14, 1895. until spring. In the southern part of the district the snow- storms may change to rain, or they may be followed by warm weather, causing the winter thaws, as a result of the inblowing wind from the south, which is drawn toward the storm center. Along the coast, cold east winds are often drawn from the ocean, particularly during storms. The cold waves which often follow the storms, cover the land with a blanket of very cold air (Figs. 63 and 64), through 128 PHYSICAL GEOGRAPHY. which ladiabion proceeds with ease, giving us our coldest winter weather ; but the cold is not so intense as in the dry interior area of the northwest. In the summer, storms are less frequent and less violent ; but still they produce an effect upon the weather. When they are not intense, the warm air drawn in from the south, produces days of excessive heat and sultriness, during which thunderstorms may come ; or a continuation of this condi- tion may cause summer droughts. Along the seacoast, fogs are sometimes blown in upon the land, or the cool sea breeze may temper the heat of the summer day (Fig. 39). Well- developed cyclonic storms may arise : and in the autumn 50° 40° Tues.Mar.12 NOON March 13 March 14 NOON March 15 NOON March 16 NOON March 17 NOON 50 c 30- 20 : 10' 0° I I | i j ■N | 1 20° 10° 0° 1 L — ^_ __^-T ' I ' ^- — '1 1 I 1 1895 Fig. 64. Sudden descent in temperature during passage of a cold wave at Ithaca, N. Y. these become more frequent, and the region may then be visited by one of the West Indian hurricanes. During both summer and winter, the winds are very variable in force and direction. This, which may be considered the typical weather of the temperate latitude, has for its main feature extreme irreg- ularhVy and variability. From place to place there is much variation; and even at the same place, the weather of suc- cessive years is quite different. These are essentially the conditions that prevail in Europe ; but here the winds are damper because their prevailing direction is from the west, and hence from over the Atlantic. Since there is less land, WEATHER AND CLIMATE. 129 there is much less variability in the weather conditions of the temperate latitudes of the southern hemisphere. Climate. — The earth may be divided into climatic belts, the three primary zones being based upon difference in lati- tude, and hence in supply of solar energy. These zones are Fig. 65. Climatic zones. the tropical, temperate, and arctic (Fig. 65}. Speaking generally, the tropical climate is characterized by high tem- peratures throughout the year, the arctic by low tempera- tures in all seasons, and the temperate by variability, and a marked change in the two opposite seasons of summer and winter. There are many exceptions to this general state- 130 PHYSICAL GEOGRAPHY. ment, and each zone must be subdivided into oceanic, insu- lar, interior, and upland climates. Tropical Climate. — Between the tropics, the climate of the oceans and coasts is mostly warm and equable. The rainfall is considerable, though to this there are numerous exceptions, as for instance on those coasts from which the trade winds blow toward the sea. The doldrum belt is one of excessive rain and very uniform conditions of tempera- ture ; but that of the trade winds has a more variable climate. In the interior of the continents, there is much variation, though the uniform condition is that of high tem- perature. The temperature ranges are greater than on the ocean, and the average temperature is also higher. There is every gradation between the regions of heavy equatorial rains, and deserts. In the belt of calms the rains are heavy, while in the trade- wind belts, the dry, south- moving winds often produce a truly desert condition. Thus the desert of Sahara, with a rainfall of less than 20 inches, and in some places with almost no rainfall, is on the same latitude with the region of eastern Central America, where the rainfall is over 200 inches. In the narrow zone which is alternately occupied by the belt of calms and the trade winds, the climate of one season is dry, and that of the other is very damp. As a result of the monsoon condition, a peculiar climate is produced in India. There are three seasons : one cool and dry, when the winds blow from the interior ; the second hot and dry, when the sun's heat becomes intense ; and the third a wet season, when the monsoons blow upon the land. Temperate Climate. — For the most part, the climate of the north temperate zone is very variable, and the year is divided into two seasons of extremes, — the summer and the winter, — with intermediate seasons of spring and autumn, which are WEATHEB AND CLIMATE. 131 gradations between the summer and winter. However, this belt is divisible into several minor zones, of which we may consider five : the west coast, the east coast, the interior, the mountain, and the inter-montane zones. The climate of the west coasts is comparatively equable and damp, because of the influence of the ocean, from which the winds blow as a part of the circumpolar whirl. On the eastern coast, the climate is largely influenced by the condi- tions of the interior, because the wind comes from this direc- tion ; but the neighborhood of the ocean somewhat modifies the climate, and it is not so extreme as that of the interior. As has been said, the interior climate is very extreme ; and the cyclones and anticyclones which affect nearly the entire temperate belt, are much more marked than in other parts of the zone. The climate cf the mountains resembles that of the region in which they are situated ; but it is colder and usually more humid. Thus frost-covered mountains may rise from desert plateaus. Among the high mountains, much of the precipitation is in the form of snow. Between the moun- tains, and on the leeward side of them, arid and even desert conditions result from the fact that the winds are drained of their moisture in passing over the mountains. This great variability in condition is strikingly shown in passing around the earth on one of the parallels of latitude, such, for instance, as that of 50° N., as described by Davis in his " Elementary Meteorology." Passing from the equable climate of the Atlantic, it crosses the European continent through regions so temperate that they are densely popu- lated. In Asia, great plateau deserts are encountered, and on the Pacific coast the climate is quite severe ; but in that ocean very equable conditions are found. The parallel enters British Columbia, where the climate is moderate and moist, passes over the high snow-covered mountains, and crosses 132 PHYSICAL GEOGRAPHY. the great interior region of extreme cold, north of the Great Lakes, emerging across the Labrador peninsula to the Atlan- tic, in the middle of which the climate is modified by the warm Gulf Stream. Arctic Climate. — The arctic climate is one of extreme and prolonged cold, and the ground is covered with snow for the greater part of the year. During the winter, the sun remains below the horizon, and in summer it does not set. On high mountains which rise into the cold layers of the upper air, 1 many of the conditions of the arctic climate extend into the temperate, and even into the tropical zones (Fig. 65) . Between the tropics, a temperate climate is found at moderate elevations on the mountain sides. Minor Variations. — Aside from the larger divisions of climate, there are many smaller ones. The climate may change very perceptibly even in a short distance, „as, for instance, in going from the southern sunny aide of a mountain to the shaded northern side (Fig. 68). Even a lake of moderate size may produce a perceptible influ- ence upon climate ; and it is not uncommon to find a belt adapted to fruit-raising whose boundaries include but a small area. Changes in Climate. — There is an abundance of geological evidence to prove that there have been great changes in climate in parts of the earth's surface ; and there is some reason for believing that there have been changes in climate within historical times. Recent studies in Europe seem to show that there is a period of slight variation in climate, extending over thirty-six years. The change is from drier and warmer, to cooler and moister conditions ; and we are at present in the midst of the warm, dry part of the cycle. 1 Of course, this applies only to the cold ; for the position of the sun does not resemble that of the Arctic. WEATHER AND CLIMATE. 133 This is merely a suggestion, and cannot be accepted as a definitely established fact. Of the geological evidence we are more certain. During the earlier ages of the earth's history, the climate of the globe seems to have been more moderate and uniform. Fossils of animals and plants that are at present confined to warm latitudes, are found preserved in rocks which are buried beneath perpetual snow. When these forms of life existed, this part of the earth must have been much warmer than now. On the other hand, during one of the recent periods of geological history, arctic conditions extended down into tem- perate latitudes. The north temperate zone has just emerged from this period ; and during its existence, sheets of ice, form- ing great continental glaciers, extended down into regions now densely populated. Northern Europe and northeastern United States were covered by these glaciers (see Chap- ter XVII.). At about the same time that this ice sheet extended over northeastern United States, the climate of parts of the Great Basin region of the West was transformed from an arid condition to one of relative humidity ; and, during this time, great lakes existed where now there are only desert plains and salt lakes. Perhaps the causes for these changes in climate are to be found in conditions which we do not as yet understand ; but they may in part be due to variations in the movements of the earth about the sun. These are too difficult for simple state- ment ; but they depend upon slow changes in the distance between the sun and earth, and upon the rotation of the earth's axis, known as the precession of the equinoxes. 1 1 The teacher will find this theory fully stated in Croll's "Climate and Time," to which reference is made at the end of this chapter. A shorter, but very clear statement of the theory, will be found in Geikie's " Text-book of Geology," pp. 23-30. 134 PHYSICAL GEOGRAPHY. Since climate varies so remarkably with differences in the elevation of land, or in the relation between land and water, it is possible that changes of a purely geographic nature may account for some of the variations. If large areas of land should be raised to greater elevations, or considerable tracts be lowered, or if the ocean currents should have their courses decidedly changed, the climate of parts of the earth would be very different from the present. Such changes actually have occurred, and in this way some of the climatic variations may be explained ; but at present, only hypothesis can be offered to account for the change. REFERENCE BOOKS. Greely. — American Weather. Dodd, Mead & Co., New York, 8vo. $2.50. (Valuable information, particularly relating to United States.) Abercromby. — Weather (International Scientific Series). Appleton & Co., New York, 1887. 12mo. $1.75. (Refers more particularly to Europe.) Blanford. — Climate and Weather of India. Macmillan &Co., New York, 1889. 8vo. $3.50. Woeikof. 1 — DieKlimate der Erde. Gostenoble, Jena, 1887. 8vo. 22 m. Hann. — Handbuch der Klimatologie. Englehorn, Stuttgart, 1883. 8vo. 15 m. Croll. — Climate and Time. Stanford, London (Appleton & Co., New York agents). Fourth edition, 1890. 12mo. $2.50. A series of publications on the climate of the states and territories included within the arid belt of the West contains much valuable information. It is by Greely and Glassford, and was printed by the Signal Service at Washington. There is an admirable discussion of some of the climatic features of New York, by Turner, in the "Fifth Annual Report of the New York Weather Bureau," 1894. This is distributed free of cost, and application for it should be made to the director, Professor E. A. Euertes, Ithaca, New York. 1 In most cases reference is not made to works published in languages other than the English ; but these books are of especial importance. For a much fuller bibliography of the literature, reference may be made to the author's larger book, now in preparation. The present lists are intended to do no more than to refer to a few standard books in which reliable information may be found upon the several subjects which of necessity are very briefly treated in this book. CHAPTER VIII. GEOGRAPHIC DISTRIBUTION OF ANIMALS AND PLANTS. General Statement. — There are three great zones occupied by life, — the air, the water, and the land. None of the animals of the air exist in that medium alone, but they are in part terrestrial or aqueous, — largely the former. Aerial animals belong to several groups of the animal kingdom, but for the most part are either birds or insects. On the land, nearly all the great groups a^e represented, though some of the truly aqueous animals are absent. In the ocean, the fishes and lower forms of animals are predominant, though there are groups of birds that dwell on the ocean for a greater part of the time ; and some groups of mammals, such as the whales and seals, have adopted this zone for their home, although nearly all of their fellow-mammals live on the land. The group of reptiles is also represented in the sea, but the land is their main home. There is much difference between the life in fresh and salt water bodies, the main life characteristics being the same, but with many noticeable minor differences. For instance, insects are common inhabitants of the lakes and rivers, but are nearly absent from the sea ; and the plant life of lakes is much more varied and high in type than that of the ocean, m which only a few species allied to the land plants are known to occur. The Ocean. 1 — The zones of air and land maybe classed 1 See also Chapter IX., pp. 168-174. 135 136 PHYSICAL GEOGBAPHY. together ; but the ocean is so different that it must be con- sidered separately. The mobility of the water, and the moderate temperature ranges over the greater part of the ocean, are both favorable to the widespread distribution of marine life ; and thus in great oceans we find some species ranging almost from one end to the other. The limit of temperature is the main check to the spread of ocean animals, and this is well illustrated by the distribution of reef -build- ing corals, which are practically excluded from zones where the water temperature descends below 68°. Because the temperature of the ocean water descends as the depth in- creases, the forms of life change with the depth. In the case of ocean plants, as indeed those of fresh-water bodies, depth is a very important factor in limiting distri- bution. Below a depth of 200 feet in the sea there is a practical absence of any form of vegetable life, because below this limit the sunlight is not powerful enough to perform the work which plants demand of it. The plants of the ocean are partly floating seaweed, so well illustrated in the gulf weed or sargassum, which drifts in the Gulf Stream, and accumulates in great areas, or Sargasso Seas, in the eddies within the whirl of the oceanic currents. Many of the plants are attached along the shore line, and the most favorable place for this is the rocky shore, which furnishes a firm base for attachment. Therefore, along rock- bound coasts, the area between tides is covered with a mat of seaweed (Figs. 89 and 204). Another favorable place for oceanic vegetation, is in quiet, partly enclosed arms of the sea, away from the reach of the waves. Here many forms of plants exist, some of them belonging to the true grasses ; and in such places these help to build level, swampy plains, known as salt marshes (Fig. 206). Among the most striking features connected with the GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 137 oceanic life, are the wide distribution of its species and the great abundance of individuals. A striking difference is noticed between the animals existing in the warm waters of the tropical belt, and those occurring on the storm-beaten coasts of the cold temperate and arctic zones. The latter appear hardy, while the former are often exquisite in their beauty of color and delicacy of structure. There is much less difference in this respect among the inhabitants of the mid-ocean ; for here the changes in physical conditions are less marked. Fresh Water. — In fresh-water bodies there is much less variety of life, and usually there is not much opportunity for the study of distribution. Land animals, notably insects, often go to these water bodies for purposes of breeding ; and in many cases marine fishes enter fresh water for the same purpose. In certain cases, owing to some accident, these ocean animals find their place of outlet cut off, and they become land-locked. As a result of this, we sometimes find true ocean fishes living in lakes. Sometimes fresh-water lakes become transformed to salt lakes, and this change gradually exterminates the animals. Finally, these water bodies may become dead seas in which practically no life exists, as is the case in the Great Salt Lake and the Dead Sea The way in which lakes become inhabited is mainly by the migration of life along the streams, or else by the entrance of land animals ; and if a pond is made in the course of a stream, it will soon become stocked with both animal and plant life. The Land. Effect of Temperature and Moisture. — On the land, one of the main factors determining distribution of life is that of temperature. As a result of this, we find very great differences between the faunas and floras of the tropics, and those of arctic latitudes. This difference affects both 138 PHYSICAL GEOGRAPHY. variety and abundance ; for while there are many vicissitudes in the colder zones, everything favors the development of life near the tropics. The animals of the Arctic must prepare them- selves for the long, cold winter, and at all times the condi- tions surrounding them are severe. Only a few forms of mammals exist there, and these are very hardy and well pro- FlG. 60. Near the timber line, Colorado. tected with fur. Many of the mammals, and most of the birds, leave the coldest part of the zone during the winter; and some of the birds that spend the summer within the Arctic circle, in winter pass southward to the southern portion of the temperate zone. Reptiles are nearly absent from this cold region, and the few that do exist there are of small size. In summer, the land is clothed with vegetation up to the limits GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 139 of perpetual snow ; but there is a very marked difference between the scanty flora of the Arctic and the luxuriant, almost impassable tropical forest. Within the Arctic, the trees are prevailingly evergreens, and near the snow fields all trees disappear, while their place is taken by shrubs and the smaller forms of plant life. Intermediate conditions exist within the temperate belt. Fig. 67. Above the snow line, Mt. St. Elias, Alaska. There is a great variety of plant and animal life, in the southern part merging into the tropical conditions, in the northern portion assuming arctic characteristics. Many of the birds of the colder parts of the zone migrate to the south in the winter ; but the insects, reptiles, and many of the mammals, spend a part of the winter in a torpid or dormant condition, in this respect resembling the winter behavior of plants. They have adapted themselves to the annual 140 PHYSICAL GEOGBAPHY. climatic change from the rigorous winter to the warm summer. The influence of temperature upon the abundance and kind of life, is also illustrated in the ascent of mountains (Fig. 66). Within the tropics, one may pass upward into places where plants exist, which in many respects resemble those of the temperate zone ; and in this zone a flora of arctic habit exists upon many of the highest mountain tops. In studying the distribution of animals and plants, altitude is found to be a very important factor ; and as one ascends o a mountain range, he finds familiar plants and animals disappearing one by one, and their place only partly taken by species which rapid- ly decrease in num- ber as the ascent continues. When the snow line is ap- proached (Fig. 67), the limit of plant life is practically found, although in favored places some few species may extend even above this line. Before the snow line is reached, one passes the timber line, and goes from the forest-covered slope to one on which trees do not grow (Fig. 66). The elevation at which this is reached, varies with the latitude (see Fig. 65~), and even with the portion of the mountain. If one side is exposed to cold winds, the limit is reached at a lower altitude than on the opposite side ; and the same is true of the side of the moun- tain which receives the least sunlight, for in such places the average temperature is lower than on the sunny side (Fig. 68). Effect of sunlight in raising the zone of vegetation higher on the southwest than the northeast side of a mountain. GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 141 The moistness of the climate also affects the spread of animals and plants ; and in absolute deserts we find an almost entire absence of life, while in those arid regions which are not true deserts, the plant and animal life are peculiar, for the conditions are very unfavorable (Figs. 69 and 70). Reptiles and a few insects, mammals, and birds constitute the fauna ; and the flora is characterized by stunted, spiny bushes, and a brown grass that becomes transformed to hay as it grows in the dry atmosphere. Here the cactus thrives, and other prickly and unusual forms of vegeta- tion exist. With abun- dant moisture, vegetable and animal life flourish, and this is one of the reasons for the luxuriance of the tropical forests; and with abundant plant growth, animal life also becomes abundant. How marked this effect is, can readily be understood by consider- ing the difference between the sandy wastes of the Sahara (or the arid regions shown in Figs. 69 and 70) and the tropical forests in the same latitudes (Fig. 71). While these are extremes, even slight differences in rainfall will cause marked changes in life. Plant and Animal Habits. — Aside from those differences among animals and plants upon which the zoological and bo- tanical classifications are based, there are certain differences in habit which are of some interest from the present stand- point. Plants are for the most part fixed in a definite place, and the opportunity of distribution comes only from Fig. 69. Arid land vegetation. 142 PHYSICAL GEOGRAPHY. the seeds. Therefore in the study of geographic distribu- tion of plants, the seeds are of much interest. Some are heavy, and these drop to the ground close by the tree ; but in some cases, these heavy seeds are enveloped in a fruit, which is eaten together with the seed ; and since the germ is often protected by an indigestible shell, the vital part of these seeds may be carried for long distances, and then be left upon the ground unharmed. Some seeds cling to the Fig. 70. Arid land vegetation in the Canon of the Bio Grande, northern New Mexico. fur of animals and are thus distributed, and many are drifted to distant regions by the winds. In these, and other ways, plants are spread from one place to another. Among land animals, there are great differences in habit. Some move slowly, others rapidly, and some are able to fly in the air. Most animals of the land dwell on the surface ; but for a part of the time, many make their home either in the air or in the water, and some spend a part or all of GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 143 their time beneath, the surface of the ground. Naturally, because of these variations, there is much difference in the distribution of animals, and in the means by which they are distributed. Life Zones. — As a complex result of these animal and plant peculiarities, together with their physical surround- ings, and certain inherent conditions which cause life to grow and develop, there has resulted a peculiar distribu- Fig. 71. The tropical forest. tion of life on the land. The most perfect adjustment to conditions is noticed upon the connected continents ; and here we find three great zones, the tropical, temperate, and arctic, in each of which there are sub-zones which are due to irregularities in climate or in topography (Fig. 72). There are mountain and desert irregularities, as well as others. Not only do these zones exist upon the several continents, but quite different species, both of animals and plants, char- acterize the separate continental areas. The plants and ani- 144 PHYSICAL GEOGRAPHY. mals of Europe are quite different from those of America ; of South America from Africa. 1 Yet there is remarkable uniformity in the fact that the same large groups are pres- ent in each, as if by some means there had been an occa- sional connection or communication. Evolution teaches us that animals and plants have been developing from simpler Fig. 72. Diagrammatic representation of the life zones of the United States, showing influence of latitude and topography. to higher forms, and we now know considerable concerning the steps along which this has proceeded. The fact of the difference between the life of the several continents, shows that there has not been constant connection ; but the resem- blances prove that there has been some communication. 1 To illustrate, we have temperate and tropical zones in both South America and Africa, and in each of these also the subdivisions of coast, desert, and mountain belts. But the tropical forest of Africa bears only a general resemblance to that of South America. However, these resemble each other much more closely than do the forests of tropical and arctic zones. GEOGBAPHIC DISTRIBUTION OF ANIMALS, ETC. 145 Even more strikingly is this proven by the resemblances and differences between the life of the oceanic islands and that of the continents. These land bodies are separated, and in some cases have always been separated, by a great ocean barrier ; yet at times, as for instance in the Bermudas and the West Indies, the life of the islands resembles that of the neighboring mainland, although numerous species are absent. On the other hand, there are many cases in which the insular life is entirely unlike that of the mainland. For instance, the only native mammals of New Zealand, are two species of bat, which have been able to spread themselves by means of flight. Other vertebrate animals are scarce on this land, and in most cases the animals are of peculiar types. The animals of the East Indies are quite like those of Asia; but Australia, which lies only a short distance south of these, forms a perfectly unique life zone. Excepting a few species of bats, rats, and mice, all of the mammals belong to peculiar types, such as the kangaroo group, and the group to which the duck-bill belongs, — an animal which combines the habits and characteristics of mammals with that of laying eggs. The birds are also peculiar, including among their number many king-fishers, parrots, birds of paradise, and other peculiar forms. The Spread of Life. — The prime cause for the wide dis> tribution of land animals is found in themselves, as a result of voluntary movement from one place to another. Many birds may easily pass for long distances, and in this way they may reach far-distant lands ; but usually they are con- tent to stay in their normal home. During storms they may be blown far from their home, and when they alight, it may be upon some distant island, or in some other place from which return is not easy. Ships a hundred miles from shore, often serve as a resting place for 146 PHYSICAL GEOGRAPHY. some land bird which is lost on the open sea. In the water there are floating logs which may serve as resting places, and in this way flying animals may be distributed. A few years ago, during a violent storm, a sea gull fell exhausted not far from Ithaca, New York, at a distance of two or three hundred miles from its ocean home, which was certainly not north of Philadelphia. Naturally, because of this aid of the wind, winged animals are most widely distributed. Land animals that cannot fly, distribute themselves by moving over the land, each generation pushing its frontier line farther than the preceding, provided other conditions are favorable. As is stated in the next section, there are certain limitations to this natural spread of life. In a second way, land animals may be distributed by accidental means. Drifting in the ocean currents, there are often logs of wood which have floated down some river to the sea. Tree trunks from the American coast are in this way borne to the European coast and there stranded. Ani- mals may be carried upon these, and if they survive the journey, they may begin to increase on some land remote from their old home. This is particularly likely to happen to animals like reptiles, which can live for a long time with- out water or food, or to insects which are in the egg or in the cocoon. In rare cases, even the higher types of life may pass through such a journey ; but such animals must be small, for only these can be thus floated. It is for these reasons that the large mammals are so rarely found upon the islands of the ocean, even though the distance to the mainland is slight. By a change in climate, such as that which produced the glacial period, animals may be forced to migrate, and in so doing they may cause other animals to abandon their homes. When the ice enveloped the northeastern United States, the GEOGRAPHIC DISTRIBUTION OF ANIMALS, ETC. 147 nimals were either killed or driven away into the southern ,nd more favorable regions. The effect of this migration Qust have been felt far from the ice front ; and there are till signs of its influence in the distribution of animals ,nd plants in eastern United States. Barriers to the Spread of Life. — The great barrier is the »cean. More effectually than any other feature of the earth t serves as a check to the spread of animals and plants. In he case of Australia, it has served as an effectual check ipon the spread of the large and powerful animals of the £ast Indies, and in a less perfect way, even upon the more fasily distributed forms of life. Short arms of the sea are iot so effectual, but even these serve as a partial barrier, rhe study of the problem offered by the Australian fauna, eads to the conclusion that this continent has not been con- lected with the Asiatic lands since the higher animals began :o exist ; and in other parts of the world, a study of the dis- ;ribution of life, proves that some of the ocean barriers of the )resent have not always existed. Next to this great oceanic barrier, the most important )bstacle to the spread of life is probably to be found in high nountain chains, such as the Andes and the Rockies. Many mimals and plants are completely checked by these. Nearly ;he same is true of deserts ; for if it is not possible to pass iround these, many species find it impossible to pass from me side of them to the other. In some cases even large ?ivers serve as a boundary line, separating a zone occupied 3y a species from one in which it is absent. Effect of Man. — The above remarks hold only for the natural distribution. Now man has come upon the scene as i disturber of the natural order, and everywhere in the world we see the result of his interference. We have European and Asiatic trees in the garden, and, in some places, 148 PHYSICAL GEOGRAPHY. even in the forests. There are foreign weeds in the field, foreign birds, insects, and mammals (notably the rat), as pests, or as unnoticed additions to the flora or fauna. The ancient marsupials are no longer the most important mam- malian possessors of the Australian zone, but man has caused an invasion of their territory. Man is killing here and adding there, with the result that intentionally or unintentionally, he is changing the life zones ; but while thus interfering with the natural spread of life, and, in some cases, succeeding in domesticating plants and animals of one zone to the conditions of another, he is not able to disturb the great divisions of tropical, temperate, I and arctic, of mountain and desert. These depend upon i physical conditions of too fundamental importance. The camel may be domesticated in the desert of southern Cali- fornia, but it cannot thrive in New England ; the tiger might be introduced into South America, but not into Scan- dinavia ; the palm of the central Pacific might be made to grow on the islands of the central Atlantic, but not on the slopes of the Rocky Mountains. Thus while man will greatly aid in the distribution of animals and plants, in general he will succeed only in disseminating them over i zones in which the prevailing conditions are similar. REFERENCE BOOKS. Wallace. — Island Life. Macmillan & Co., New York, second revised edition, 1892. 8vo. $1.75. Wallace. — The Geographical Distribution of Animals. (Vols. I. and II.) Harper & Brothers, New York, 1876. 8vo. $10.00. Part II. THE OCEAN. CHAPTER IX. FORM AND GENERAL CHARACTERISTICS OF THE OCEAN. Distribution of Land and Water. — A glance at a globe shows a very marked irregularity in the distribution of land and water in the different hemispheres. It is possible to divide the earth into two hemispheres, in one of which there is little land, while in the other the water area is small (Fig. 2). Nearly three-fourths of the earth's surface is covered by water, the total area of water surface being about 145,000,000 square miles. Land rises from the water in the form of continents and islands, which differ greatly in outline and topography. Composition of Ocean Water. — The ocean is between 96 and 97 per cent pure water, the remainder being divided between several salts, of which the most abundant is com- mon salt. In addition to this common salt, there is an appreciable amount of chloride of magnesium, carbonate of lime, some sulphates, and very minute quantities of other substances. Probably some compound of every known element is dissolved in the ocean, in such minute quantities that they can be detected only by the most careful analysis. In addition to these slight impurities, the water has absorbed a considerable amount of atmospheric gases. It is upon this that the ocean life depends. In different parts of the world, there is a considerable variation in the percentage of salt impurities, the range being between 3.3 and 3.73 per cent. At the same time 151 152 PHYSICAL GEOGRAPHY. with this change in amount of salt, there is a variation in the density of the water. Representing fresh water as 1, the average density of sea water is 1.026. There are many reasons for variation in the salinity of sea water. Where rivers enter the ocean, the density is decreased by the addi- tion of fresh water ; and also where rains are abundant, as they are in the belt of doldrums, the surface water has its density decreased. On the other hand, where evaporation is great, the removal of the fresh water tends to concentrate salts and therefore to increase the density. In the Mediter- ranean and the Red Sea, the ocean water is relatively dense ; and the same is true of the belts of ocean water over which the dry trade winds constantly blow. Color and Phosphorescence. — The color of the ocean i? naturally blue. This is partly due to the fact that the blue- ness of the sky is reflected upon the water surface, and partly to the scattering of light rays which enter the water, this cause being analogous to that of the blue color of the sky itself. The color of the bottom often imparts to the water a different shade from the typical blue of the ocean ; and where the water is shallow, green shades are often produced. The Red Sea owes its color to the presence of many minute forms of vegetation, belonging to the group of Algse, while the color of the water near some coasts is due to the pres- ence of great quantities of mud brought down by the river. At times, particularly on quiet nights, the ocean waters are aglow with a silvery gleam of light, which is known as phosphorescence. It is similar in origin to the glow of the fire-fly which we see on warm summer nights. In the sur- face waters of the ocean, there are countless millions of microscopic animals, nearly all of which are able to emit a tiny spark of this strange light ; and their power to do this seems to vary from time to time. Therefore on some GENERAL CHARACTERISTICS OF THE OCEAN. 153 nights the surface is free from this light, while at other times every ripple causes a silvery gleam. In rowing upon the surface of the sea at such times, a trail of light follows behind the boat, and drops of gleaming water fall from the tips of the oars. Exploration of the Ocean Bottom. — It is only recently that the bottom of the ocean has attracted much attention. Until thirty years ago, it was supposed that after passing below a depth of a few hundred feet, the bottom of the ocean was a great, uninteresting desert. And thus, until that time, we were almost entirely ignorant of the condi- tions existing on more than one-half of the earth's surface. To the naturalists of the time it seemed absolutely impossi- ble that life could exist in the depths of the sea. When oceanic cables were laid, the beginning of the study of the deep sea was made, and proof was soon obtained that animals did live in the great ocean depths. This proof first came from the Mediterranean, where a submarine cable was drawn to the surface for repair. Attached to it were a number of animals, which therefore must have lived where the cable lay ; and the depth of water at this place was much greater than the supposed limit of life. The fact that conditions favoring the development of animals proba- bly existed over the entire ocean bottom, immediately created a desire for exploration ; and to this scientific inter- est was added the practical one, which resulted from the necessity of obtaining a knowledge of the physical features of the bottom, in order to make more easy the extension of oceanic cables ; and soon governments began the study of the ocean bottom. Methods Used in Deep-sea Explorations : Sounding. — In a study of the ocean bottom, we wish to discover something concerning the life that exists there, something about the 154 PHYSICAL GEOGRAPHY. topography, and something concerning the kind of bottom, as well as the character of the water, and the various physical conditions. For this purpose, one thing is of prime impor- tance, namely the depth ; and in every deep-sea exploration this is the first fact obtained. 1 For this sounding, many ingenious contrivances have been invented, the one best adapted to deep-sea work be- ing the Sigsbee deep-sea sound- ing machine (Fig. 73). A weight attached to the end of a fine steel wire, is carefully low- ered until the bottom is reached. The ball of the sounding ma- chine sinks by its own weight ; and when it touches bottom a jar is sent through the wire, which is felt even at the sur- face. The entire machine is very delicately constructed, and it records ocean depths with great accuracy. The wire used is so fine that it would be impossible to draw the weight back to the surface, and the instrument is so contrived that this is left behind when it touches the bottom of the ocean (see Fig. 73). The weight is nothing but a cannon ball through which a hole had been bored. Into this hole is placed a cylinder, which remains open during the passage of the weight to the bottom, and which is automatically closed when the line is drawn in. Usually the bottom of the cylinder is covered 1 Ocean depths are measured in fathoms, a fathom being six feet. 73T Fig. 73. Deep-sea sounding machine, with and without the sinker. GENERAL CHARACTERISTICS OF THE OCEAN. 155 with wax or soap, to which, clings a sample of the mud of the ocean floor ; so that as the instrument is drawn to the surface, we have both water and mud from the bottom. Near the weight a thermometer is attached to the line ; and this is so made that it is inverted when the wire is reeled in, and an automatic register of the tempera- ture at the time of inversion is thus made. Very often several ther- mometers are attached to the line at different distances, so that we obtain a knowledge of the tempera- ture of the ocean water from the surface down to the very base of the column. Dredging. — In order to obtain a knowledge of the kind of life that exists in these great ocean depths, another method, that of dredging, must be followed. The dredge, or deep-sea trawl (Fig. 74), is an iron frame several feet in length, to which is attached a bag net. This is lowered to the bottom and dragged over it, usually for several hours. The sounding apparatus is lowered perpendicularly ; but the dredge is lowered to the bottom, and then more rope is reeled out, so that it may be kept upon the bottom and dragged over it. This is done partly by attach- ing weights to the dredge, and partly by the natural sagging of the wire rope. After the dredge has been down for a Fig. 74. Deep-sea trawl. 156 PHYSICAL GEOGRAPHY. sufficient length of time, it is drawn to the surface and its contents examined. Imagine a balloon sailing through the air at a height of three miles or more, and dragging a frame a few feet in length, over a distance of a few miles. If the operators of this apparatus should imagine that, as a result of a few trials, they had obtained a fair knowledge of the life existing on the surface of the earth, it will readily be seen that they would be very much mistaken. All swiftly moving animals would escape, and only those would be taken which were small enough to enter the dredge, and so slow that they could not escape from it. In a measure this is true of our explorations of the deep sea. If large animals exist there, our methods of exploration are not calculated to discover them, nor should we expect to obtain many animals that are capable of rapid movement. Topography of the Ocean Bottom : Gen- eral. — There is a very profound dif- ference between the outline of the ocean bottom and the fea- tures of land, as we know them on the continents. In both places the crust of the earth is subjected to a tendency to wrinkle, and therefore to form mountain folds ; and in both cases also, volcanoes are produced. But on the land, there are forces at work which are absent from Fig. 75. Diagram contrasting land and ocean bottom topog- raphy, a, a, a, land surface ; B, B, height to which mountain would rise if denudation were not acting; c, c, undulating ocean bottom; d, d, ocean sediment partly obscuring mountain fold E, E ; V, volcanic cone. GENERAL CHARACTERISTICS OF THE OCEAN. 157 the ocean. The rain, rivers, changes in temperature, and wind, are engaged in the combined action of carving and sculpturing the land, the result of which is to make the surface very irregular, and at the same time to gradually lower it (a, a, Fig. 75). None of these tendencies exist in the ocean. The oceanic areas are the gathering grounds for the waste of the land. Materials worn from the continents are borne to the sea in rivers, or are wrested from the land margin by waves, and distributed over the sea bottom. Materials car- ried in solution by river waters also find their way to the ocean ; and from these the animals that dwell in the sea, are able to take the materials which they build into their skel- etons, and which upon death they leave as a contribution to the ocean floor. Therefore the tendency of deposition in the ocean is to smooth the surface. Thus in the sea, while excessive elevations are occasionally found, the general topog- raphy is remarkably uniform. There are great elevations, because nothing is present which tends to destroy the diver- sities produced ; but the absence of the agents that are carv- ing and sculpturing the land, makes the sea bottom a place of great regularity. In the ocean, there are prevailing conditions of great, wide- stretching oceanic plains or plateaus ; and where there are elevations, these are usually so gentle that they would appear to be nearly level. Occasionally, where volcanic peaks rise in the ocean, we find exceptionally steep slopes. The agents of the air are not present to carry away the materials which are building the cone, and therefore most of the material that is ejected is piled into one mass. In a distance of about 70 miles from Porto Rico, the depth of the ocean descends to 4561 fathoms ; and in this region there is a difference in elevation of fully 30,000 feet in a 158 PHYSICAL GEOGRAPHY. 1 distance of about 80 miles. Within sight of the Bermudas, at a distance of from 10 to 30 miles from _ land, the ocean reaches a depth of from ■J 2700 to 2900 fathoms. Among the oceanic t> islands of the Pacific, differences in eleva- s tion fully as great as these are frequently | discovered. On the land there are no g, such excessive differences in elevation as ® those which exist among the volcanic .§ islands of the ocean. •g The Atlantic Ocean. — Perhaps the best m way to obtain an idea of the topography J of the Atlantic Ocean, is to make a see- 's tion across it, following approximately « "S the line traversed by the oceanic steamers U f, (Fig. 76). Starting from the shore of ^ ^ to New York, an even, gently sloping plain 2 r « is found stretching eastward to a distance -g +| of from 50 to 75 miles. It is almost ®" go level, and its features are quite like some | of the very level plains on the land. This A plain extends far above the mouth of the ^ St. Lawrence, including nearly all of the 3 area between the present New England ° coast and a line about 100 miles from a the shore. South of New York this sub- ^ marine plain, or continental shelf, rapidly © narrows until off the Carolina coast it is .2 a very narrow strip. Such a continental S shelf as this, is found along the bordo^ of o nearly every continent on the earth, though in width there is much variability (Plate 14). GENERAL CHARACTERISTICS OF THE OCEAN. 159 Passing eastward, and for a while leaving the track of the ocean steamers to the northward, a region of very rapid slope is encountered. This is known as the continental slope, and in many places the rate of its descent is as great as that of a mountain. In a distance of a few miles, one passes from the edge of the shelf, whose depth is usually about 100 fathoms, to oceanic depths as great as 1000 fathoms. After the 10 00 -fathom line is reached, the exces- sive rapidity of the slope decreases ; but still the ocean depth rapidly increases to 1500 or 2000 fathoms. In a distance of from 50 to 100 miles, the depth has increased from 100 to 2000 fathoms, where the true oceanic plateau is reached. Almost the entire ocean is included in this deep plateau area. Extending northward and southward, to' the Arctic and the Antarctic circles, there is a monotonous, level plain, with ocean depths varying between 1000 and 3000 fathoms, and only rarely broken by some slight interruption. Passing eastward, this plateau extends just beyond the middle portion of the ocean, where the bottom gradually begins to rise, forming the Mid-Atlantic Midge. It extends with considerable uniformity from Iceland to the southern limit of the Atlantic Ocean ; but it reaches the surface only here and there, as in Iceland, the Azores, St. Paul, Ascen- sion, and Tristan da Cunha. It is not a continuous ridge, but an elevated portion of the ocean bottom, whose broad crest now reaches the surface, and again is fully 1000 fathoms below it. Almost everywhere along this area, the ocean depth is less than in other places far from land. After passing the crest of this rise the depth again increases, until soundings of over 2000 fathoms indicate another approach to the great submarine plateau. The plateau on the eastern side is less extensive than that on the western ; and as the European coast is approached, the deep 160 PHYSICAL GEOGRAPHY. oceanic plateau rises toward the continent. Here the con- ditions that were noticed off the American shore are prac- tically repeated. There is a slope and then a continental shelf, which merges into the continent itself. In the vicinity of the British Isles the shelf is broader than it is along the coasts of France and Spain. Other Oceans. — Much less is known concerning the con- ditions in the depths of the Pacific ; and almost nothing is known concerning the Arctic and Antarctic oceans. So far as our knowledge of the Pacific and Indian oceans warrants any definite conclusions, we may say that the conditions of the Atlantic are in a general way repeated. The great monotonous plain is more broken by volcanic peaks ; and a greater depth is found in the Pacific than in any other part of the ocean (see Plate 14). Depths greater than 4000 fath- oms have been discovered in several places ; and in one place, near the Kurile Islands, a sounding of 4655 fathoms was made. The deepest place in the Atlantic (4561 fathoms) is near Porto Rico. It is a noticeable fact that these excessive depths are found close to the land. While the greatest ele- vations occur on the land, the average oceanic depth is very much .in excess of the average land elevation 1 ; and the great land elevations are at a considerable distance from the sea, so that the elevation of the high mountain peak above its base is much less than its elevation above sea level. The greatest ocean depths descend almost directly from the land. Topography near the Coast. — While this description of the ocean bottom will serve to present the features of the deep sea, it does not convey any idea concerning the irregu- larities near the coasts. Along all continent margins, and particularly among archipelagoes, the form of the bottom is 1 The average depth of the ocean is as much as 12,000 feet, while the average elevation of the land above sea level is not much more than 2000 feet. H o hi crt Ph ■d +3 Ml fl 162 PHYSICAL GEOGBAPHT. exceedingly irregular. Without entering into the subject in very great detail, these irregularities could not be ade- quately described ; and indeed, our knowledge of the larger part of the ocean floor is so slight, that as yet we know only the general features. Temperature of the Ocean Bottom. — In the neighborhood of continents, where the depths of the sea are relatively slight, the temperature is more or less irregular, and deter- mined by local conditions. It changes with the season, and is influenced by the oceanic and tidal currents, and even by the prevailing winds. After passing this shallow and variable zone, very uniform temperature conditions are encountered. As a general state- ment, it may be said that throughout the ocean, there is a decrease in temperature with the increasing depth. Starting from the variable zone of relatively high temperatures, there is at first a rather rapid descent until the zone of about 40° is reached at a depth of a few hundred fathoms. After this, there is a very gradual descent in temperature (Figs. 76 and 83), until the cold becomes as great as that of fresh water at the point of freezing. Over a very large part of the ocean bottom the temperatures are between 32° and 35°. On some parts of the ocean bottom, particularly in the South Atlantic and the South Pacific, temperatures of 32° and even of 31° are found. 1 While this is true as a general statement, there are numer- ous exceptions to it. In the Mediterranean (Fig. 77), there is a decrease until the level of the bottom of the Straits of Gibraltar is reached, after which the temperature remains uniform, while in the Atlantic there is a normal decrease. 1 In this connection it must be remembered that the freezing-point of salt water is lower than that of fresh water ; and therefore temperatures lower than that which we call the freezing-point may be found in the ocean. GENERAL CHARACTERISTICS OF THE OCEAN. 163 This is because the Mediterranean is enclosed, and its water enters over the Straits, and hence with the temperature at that level. Also, in the Gulf of Mexico, the temperature in the deepest part is only 39^-°, which is the same as that at the Straits of Yucatan. The deep part of the Gulf of Mexico is 2119 fathoms, while that of the Straits of Yucatan is only 1127 fathoms. It will be seen, therefore, that this decrease in temperature does not depend upon increase in depth. The peculiar distribution of temperature in the deep sea, is probably due to movements of water on the bottom. In the Arctic regions the cold water sinks, 1 1 while at the tropics warm water rises : Atlantic 68° 2 =S ~ 03 <3 Mediterranean 75° 54° 55° 52° *S«?*S>y 55° 38° /■'' '■:0;:-; : l *\ 55° 37° '.\'.'-/\ 55° xyt- ; -- ;;: : :; k55° 200 and there is a con- I 500 stant passage of water from one of these zones to the other, giving a sur- Fig. 77. Diagram showing the temperature peculiarities of face movement to- the Mediterranean. ward the poles, and a bottom movement toward the equator. If a barrier exists in the line of this deep-sea circulation, the normal decrease in temperatures is interfered with. At the Straits of Gib- raltar, the water which passes into the Mediterranean does not come from the bottom of the ocean, but from a level determined by the bottom of the Straits. Light on the Ocean Bottom. — It seems certain that sun- light cannot possibly penetrate through several miles of salt water ; and if this is true, the greatest depths of the ocean are practically dark, so far as sunlight is concerned. Although it is probable that no sunlight penetrates to these 164 PHYSICAL GEOGRAPHY. zones, it still seems certain that some kind of light does exist there. This conclusion is forced: -upon us by the fact that many of the animals in the depths of the sea have well- developed eyes ; and, further, that many of them are brill- iantly colored. Animals living in dark caves become blind ; and it seems hardly probable that these inhabitants of the deep sea would continue to develop eyes for ages after their usefulness had ceased. Phosphorescence is a possible source of light on the ocean floor. After nightfall, whenever a dredge-load of materials is brought from the deep sea to the surface, it is aglow with the dull white light of phosphorescence. Each animal, each particle of mud, gleams with this light. Materials composing the Ocean Floor: Mechanical Sedi- ments. — There are two classes of substances spread over the ocean bottom : one mainly derived from the land, or j from fragments of rock emitted from volcanoes ; the other, from animals which have lived in the ocean. The latter covers by far the greater part of the ocean floor. The sandy and clayey fragments of rock which are derived from the land, are spread over the bottom of the sea only in the neighbor- hood of the coasts. Giobigerina Ooze. — One of the most striking facts con- nected with the ocean, is that the floor, covering an area greater than one-half that of the entire earth's surface, is made up of the remains of minute animals. When seen with the unaided eye, the deposit is a blue mud or ooze ; but when examined with the microscope, it is found to be com- posed of fragments or entire shells of tiny animals, generally belonging to the group of Foraminifera. The most abun- dant of these are members of the genus Giobigerina ; and these are so characteristic of the deposit, that it is known as the Giobigerina ooze (Fig. 78). GENERAL CHARACTERISTICS OF THE OCEAN. 165 It covers the greater portion of the Atlantic, and large parts of the Pacific and Indian oceans. Its rate of accumu- lation must be extremely slow ; for although the animals which compose it are very abundant in the surface waters of the ocean, they are so small that it must require long periods of time to form any considera- ble depth of ooze. Each par- ticle must depend upon the life and death of a tiny animal. The chalk of England, and other regions, is a rock whose origin was similar to that of the Globigerina ooze. Med Clay. — At a depth great- er than 2000 or 2500 fathoms, the bottom ooze changes its character and becomes known as red clay. This form of ocean deposit is particularly abun- dant in the Pacific, although it is not entirely absent from the Atlantic. It is one of the most remarkable deposits being made in the ocean. In these great ocean depths, the power of the salt water to dissolve the lime of shells has increased until this substance is taken in solution as the shells drop from the surface. Therefore the insoluble por- tions, of which there are tiny amounts in every shell, are the only parts of the Globigerina that reach the bottom. There- fore the ooze is in part a residue of the shell after the soluble portions have been removed. And if the shells were small at the beginning, how much smaller must these tiny remnants be! Fig. 78. Globigerina ooze from the ocean bottom. 166 PHYSICAL GEOGRAPHY. It is not exclusively made of the residue of the shells of surface animals, but contains contributions from other sources. The most common addition comes from pumice rocks, which were ejected from volcanoes, and after floating for some time settled to the ocean bottom at some distant point. Therefore, remnants of volcanic ash or pumice are common in the red ooze. Aside from this, there are frag- ments of meteorites which have dropped to the bottom, indicating exceedingly slow accumulation. This deposit covers an area of over 51,000,000 square miles, which is a little more than that covered by the Globigerina ooze. Each kind of deposit covers an area equal to about one-fourth of the earth's surface. Life in the Ocean : Pelagic or Surface Faunas. — The ocean is the great meeting ground of the life of three provinces, — the air, the land, and the water. Forms belong- ing to all the great groups of the animal kingdom find it possible to live in the conditions which exist in the ocean. There the conditions of life are remarkably uniform ; for there are few changes in temperature, and few variations such as animals on the land experience. Day after day, and year after year, the surrounding conditions are nearly the same. No such difference exists between the surface faunas of the ocean in different latitudes, as between the land animals of the tropics and the temperate latitudes. This is partly because the temperature of the water changes very slowly and very slightly, and it is also in part due to the fact that the waters of the ocean surface are in movement, so that the temperatures of one latitude are distributed to another. From the tropics, the currents bear bodies of warm water, and in them animals of tropical origin ; and these may be distributed far over the surface of the ocean. So uniform are the conditions of temperature, that even GENEBAL CHARACTEBISTICS OF THE OCEAN. 167 very slight differences will cause marked changes in the faunas. In the Gulf Stream, which flows at a distance of 100 miles or more from the land, there are found many creatures of tropical origin, which cannot exist in the colder waters near the coast. At times, during strong prevailing winds from the south, these creatures are driven into the colder waters ; and, as a rule, they are unable to survive the change. The ocean surface is particularly favor- able to the wide distribution of animals. It is constantly in motion, and as a result of this, hardy animals may be distrib- uted from one end of an ocean to the other. Many of the oceanic animals are free-swimming creatures, others are drifting animals, and still others are attached to floating objects. This last group is particularly liable to be found attached to the floating seaweed or Sargassum, which at times, particularly in the eddies between the ocean currents, exists in such abundance that these areas are known as sargasso seas. All except the largest of the surface animals are in a measure at the mercy of the winds or currents. At the surface, and on the ocean bottom, there is abun- dant life. Between the surface and the bottom, over the greater part of the ocean, there is a zone of water, at least two miles in depth, whose conditions as regards habitation are not known. It is the greatest unexplored area on the earth, and we are unable to say whether it is a great desert region, or whether it is actually inhabited. It is exceed- ingly difficult of exploration ; but since animals have been found in every explored nook of the ocean, and have become adapted to each place, it seems probable that some have found this zone and have adapted themselves to it. Littoral or Shore Faunas. — Along the shore line, the con- ditions more closely resemble those of the land than in any 168 PHYSICAL GEOGRAPHY. other part of the ocean. There is no such monotony of conditions as we find, at the surface of the ocean away from the land. But from day to day, from season to season, and from place to place, there are very marked differences in the conditions upon which the animals depend for their existence and variety. Here, as in every part of the ocean, tempera- ture is a very important cause for differences in faunas and for variation in animal forms. Even a few degrees of temperature will cause a very marked difference in the abundance and variety of ani- mal life. This is well illus- trated on the coast of Massa- chusetts, where the end of Cape Cod serves as a dividing line between two quite distinct faunas, because on the northern side of the cape the water is cool, while on the southern side it is comparatively warm. The influence of the Gulf Stream is felt south of Cape Cod, while north of it, in Massachusetts Bay, the cold Labrador current reduces the temperature. Another limitation upon the spread of animals along the shore, is that of food supply. Perhaps the best illustration of this is found in coral regions. At the points reached by food-bringing currents, the abundance and variety of life is Fig. 79. Coral reef on the Australian coast. GENERAL CHARACTERISTICS OF THE OCEAN. 169 very great (Figs. 79 and 207), and the coral polyps select from the water the food that they need. Soon the waters are robbed of their food supply, and in passing on are unable to support abundant coral growth. It has been noticed among the coral reefs, that on one side of a coral bar, the polyps grow readily and in great numbers, while on the opposite side, they are very scarce and not well devel- oped. In the one case there is an abundance of food, in the other, the food supply has been taken from the water by those corals which have the more favorable situation. It follows from this that circulation of water must take place in order to bring fresh food supply to the animals which are fixed in one place, and which are not able to move about for the purpose of obtaining the food which they need for existence. Therefore we rarely find coral reefs in other places than those bathed by currents. The animals that dwell upon the shore line are of several kinds : those that are free swimming and able to move about ; those that are drifted against the shore by accident ; those that crawl about ; those that are attached firmly to the rocky coasts ; and those that burrow in the clay and sand which are found in certain places. Since animals that are in the habit of attaching themselves permanently to one place, can find no opportunity for this attachment in places where sand and clay form the coast line, it follows that as a result of the differences in kind of rock, there may be very marked changes in the faunas from one place to another. On the rocky coast, the animals are almost entirely of types which are attached or which crawl about, while on shores that are sandy or clayey, the animals are almost all of the burrowing and crawling types. Faunas of the Ocean Bottom. — Every dredge load that is brought to the surface during deep-sea exploration, proves 170 PHYSICAL GEOGRAPHY. the presence of a great pressure of water in the depths of the sea. The more highly organized animals, such as the true fishes, are unable to accommodate themselves to this change in condition ; and when they are drawn to the surface, they are commonly broken by the expansion of the gases within the body. Their eyes protrude from the head, the air bladder extends from the mouth, and the skin is cracked and fissured. Thus while they may live with immense pressures upon every particle of the body, they are unable to exist when the pressure is removed from the outside, while it still partly remains on the inside. As a result of deep-sea exploration, it has been found that all the ordinary types of marine animals exist on the ocean bottom, and that in certain favorable places they exist in great variety and abundance. Fishes of types not unlike those found at the surface, swim about in the depths of the sea ; starfishes, crabs, and shrimp, crawl over the bottom ooze ; shells not unlike those which we find along the sea- shore, live on the bottom or burrow into it ; and some forms exist attached to solid parts of the bottom, while others are permanently attached by means of root-like extensions of the body, which ramify through the mud. Among the animals of the ocean bottom, are found certain types that in an earlier stage in the history of the earth were quite abundant, but which do not now exist elsewhere, — as if they had retreated to this place as an asylum where changes and struggles are practically absent. As in other portions of the ocean, temperature is the main cause for variations in the kind of animals dwelling on the sea floor. A change of one or two degrees causes an almost absolute change in the faunas. This is in large part because of the unvarying conditions of the ocean bottom. There is no effect of day and night, nor of season ; but year after year, GENERAL CHARACTERISTICS OF THE OCEAN. 171 and age after age, the conditions of temperature remain the same. Therefore animals which have become accustomed to a practically permanent condition of 35°, will find a de- crease in temperature to 33° so great that they cannot survive the change. Since these deep-sea animals live amid conditions of un- varying temperature, there is naturally a very great decrease in vitality as the temperature decreases. And with perma- nent temperature conditions of 32° (or as in some cases even of 31°), the possibility for the existence of life becomes very much decreased. Therefore in the coldest zones of the ocean, the abundance of animals is not great. Another feature upon which the life of the ocean bottom depends, is that of food supply. So far as we are able to judge, the animals of the ocean bottom exist partly upon one another, but mainly and ultimately upon a supply of food that rains down upon them from above. The death of the animals of the surface constantly supplies the bot- tom creatures with the necessary food. As it sinks, each tiny Globigerina serves as a morsel for some animal of the ocean bottom ; and the lack of abundance of this kind of food supply, seems to place a limitation upon the excessive development of animals on the ocean floor. This is probably one of the reasons why the variety and abundance of the bottom animals is not greater. There is not food enough for many more to exist. The animals of the ocean depend upon a supply of oxygen for breathing ; and this is as true of the animals of the ocean bottom as it is of those at the surface. It is not difficult to understand how the creatures that dwell in the surface waters are able to obtain their supply of oxygen, for the surface of the ocean is in constant contact with the great body of air. In the case of the animals of the ocean bottom, 172 PHYSICAL GEOGRAPHY. this is far from being true ; and yet they are constantly supplying to the water a certain amount of carbonic acid gas which in the course of time would tend to so vitiate the water that life could not exist. This is one of the strongest arguments in favor of a cir- culation of the waters along the bottom of the ocean, from polar to tropical regions. There must be some supply of oxygen furnished to these deep-sea animals, otherwise they could not exist ; and there is no other supply known than that which may be brought by this great oceanic circulation. Since everything points to the conclusion that this series of ocean movements along the bottom is very slow, it is not unlikely that another limitation to the spread of deep-sea animals, is the lack of abundant oxygen. For if there is not much supplied to the water, there cannot be much taken out. Therefore the existence of life on the ocean bottom, appears to depend upon several conditions which are more or less important ; one of these is temperature, another is food supply, and a third is a supply of oxygen. REFERENCE BOOKS. Williams. — The Geography of the Oceans. Philip & Son, London, 1881. 16mo. New edition in the press. (An accumulation of fact and purely descriptive matter.) Shaler. — Sea and Land. Scribner, New York, 1894. 8vo. $2.50. (Much information and discussion, particularly with relation to the coast line.) Thomson. — The Depths of the Sea. Macmillan & Co., New York, 1873. 8vo. $7.50. (A general discussion of the life and conditions of the ocean depths.) Thomson. — The Atlantic. McDonough, Albany, N.Y. 8vo. Vol. T. and II., $3.00. [Published originally by Harper Bros.] (Very full account of the conditions existing on the ocean bottom, as revealed by the explora- tions of the British ship Challenger.) GENERAL CHARACTERISTICS OF THE OCEAN. 173 Keports on the voyage of the Challenger. — Narrative. Vol. I., Parts I. and II. Eyre and Spottiswoode, London, 1885. 4to. £5 16s. 6d. Pub- lished for the British government. See also Summary, Vol. I. Price 80s. (The best and latest account of the history of the deep-sea exploration. Contains several excellent charts of the ocean bottom.) Agassiz. — Three Cruises op the Blake. Houghton, Mifflin & Co., Boston, 1888. 8vo. Vol. I. and II., $8.00. (The most recent and accurate description of the depths of the Atlantic, particularly of the Gulf and Caribbean region.) Wild. — Thalassa. Marcus Ward & Co., London, 1877. 8vo. 12s. (Much on depth, temperature, and currents. ) Thoulet. — Oceanographie (Statique). Baudoin, Paris, 1890. 8vo. 10 fr. (Much of importance on the physical questions relating to the ocean.) Sigsbee. — Deep-sea Sounding and Dredging. United States Coast Survey, Washington, 1880. (A splendidly illustrated description of the methods employed in deep-sea exploration.) Holder. — Living Lights. Scribner, New York, 1887. 8vo. $1.75. (A popular description of phosphorescent animals on the land and in the sea. ) Murray and Renard. — Volume on Deep-sea Deposits, in the Challenger Reports. Eyre & Spottiswoode, London, 1891. 4to. 42s. (A very complete discussion of deep-sea deposits. Beautifully illustrated. ) Moseley. — Notes by a Naturalist. Murray, London, 1892. 8vo. 9s. (Narrative based upon the voyage of the Challenger, and containing much on animal distribution and peculiarities.) The immense mass of information on this subject accumulated by the Challenger is published in an extensive series of over thirty quarto volumes. The set is very expensive ; but many of the points of most general interest are found in the two volumes of Narrative and the Summary referred to above. The Annual Reports of the U. S. Fish Commission also contain much on deep-sea exploration ; but it is scattered, and mainly found in the earlier volumes, which are now difficult to obtain free of cost. CHAPTER X. OCEAN WAVES AND CURRENTS. Wind Waves. 1 — As a result of friction between wind and water, the ocean surface is readily started in motion in a Fig. 80. Ocean waves. Copyrighted, 1871, by Proctor Bros., Gloucester, Mass. series of wave-like risings and fallings. Normally these wind waves are swells, with alternate ridge-like troughs and crests ; but where broken by violent winds, they may 1 For discussion of the effect of waves on the coast, see Chapter XVIII- 174 OCEAN WAVES AND CUBBENTS. 175 be cut into a series of chops or angular crests (Fig. 80). The water movement consists of oscillatory risings and fall- ings of water particles, while the waveform passes across the water in the direction toward which the wind is blowing. As the wave passes on, a floating object rises and falls as the troughs and crests of the waves pass over the surface, show- ing that the water itself is not in horizontal movement. In reality, the friction of the air does drive some of the sur- Fig. 81. Breakers on the coast. face water along, and therefore if a body could float entirely submerged in water, so as to be out of the direct influence of the wind, as each wave passed on, it would continue to rise and fall, but it would also move a short distance in the direction toward which the wave was moving. When a wave approaches the shore, its form and behavior are greatly changed. The rising and falling particles of water encounter the bottom, the top of the wave combs over, 176 PHYSICAL GEOGRAPHY. and it dashes upon the coast in the form of a breaker (Fig. 81). The wave is such a shallow movement in the water that it is readily destroyed upon reaching ah irregular coast. Thus in harbors or bays, the violent ocean waves lose their force, largely because of friction upon the shores and bottom. A very slight breeze will cause a series of wave-like move- ments or ripples ; but as the wind continues, and its force increases, the water surface may be thrown into a series of great undulations. The water is so mobile that these wave movements are transmitted for great distances, and they often extend far beyond the place of origin. One may see this illustrated upon the surface of almost any lake over which a steamer is passing. The series of waves started by the movement of the steamer through the water, extend out- ward for miles before losing their form. Upon the ocean it is not uncommon to find great swells or rollers, although the sky is clear, the air calm, and the water glassy, — their origin generally being some distant storm. During almost all times of day, even when the air is quiet, the waves beat upon exposed coasts. When the winds are severe, waves often rise to unusual heights and beat against the coast with terrific violence. They dash against the exposed highlands, sending spray into the air, often to the height of two or three hundred feet ; and at these unusual times, great boulders may be wrested from the rocky shore and hurled above the line of the ordinary ocean surface. In some cases, in times of unusual storms, lighthouses h°^ r e been washed away. Usually the effect of the waves is con- fined to that part of the coast which is within a few feet of high-tide mark. But during these unusual storms, the action of the wind waves may be extended a number of feet above this point, reaching places which for many years had been OCEAN WAVES AND CUEBENTS. 177 considered safe from wave attack. During a storm, a few- years ago, many summer cottages on the sandy coast of New Jersey were attacked and destroyed by the waves, and a railroad that crosses the beach was torn down (Fig. 82). These effects of waves attract our attention because they are unusual ; but the every-day action of the wind waves is also of great importance. They are constantly battering Fro. 82. Effect of storm waves on the New Jersey coast. against the coast and tending to wear it away, while the wind-formed currents and the undertow are important aids in the removal of the loose materials thus wrested from the shore. In many places, as for instance in Boston Harbor, it has been found necessary to build sea walls in order to save from destruction some of the exposed islands which are com- posed of unconsolidated gravel. 178 PHYSICAL GEOGRAPHY. If we watch the rushing of the waves against exposed coasts, or the breaking of the rollers upon the sloping beaches, we are able to form some conception of the vast amount of destructive work that these oceanic agents may do in the course of long periods of time. With every rush of the water upon the beach, pebbles and sand are dragged backward and forward ; and this constant friction of one particle upon another, in the course of time will cause even the hardest rocks to wear away. In the course of a few years fragments of brick or glass become rounded, so that they resemble the form of the true beach pebbles ; and in a year or two a brick may be reduced to a pebble only a small fraction of the size of the original. Earthquake Waves. — When an earthquake shock disturbs the waters of the ocean, a great wave is formed, which extends from the bottom of the sea to the surface, and which is therefore much more profound in its effect than the shallow wind waves. In the mid-ocean these earthquake waves may not be perceptible ; but as they reach shallow coasts, they may become noticeable as their elevation is increased in the shallowing water. Upon reaching coasts not far from the point of origin, they may have a height of from 50 to 100 feet, which gives them the power of rushing upon the shore to a much greater distance than ordinary waves are capable of reaching. During some earthquake shocks, the water wave has extended over low coasts and destroyed scores of thousands of lives. Fortunately this form of ocean disturbance is rare, and it is a type of wave which is not common in the Atlantic Ocean. Along the west coast of South America, and on the Asiatic coast, where earthquakes and volcanic eruptions are frequent, the earthquake wave assumes very great importance. It travels at a rate of from three to four OCEAN WAVES AND CURRENTS. 179 hundred miles an hour, and may extend for a distance of six or seven thousand miles from the place of origin; but at such great distances it has so lost its force that it produces no destructive effect. Among the important effects of these rare waves is the destruction of life in the ocean. An explosion of dynamite in water will kill the fishes that are exposed to the shock ; and near its source, the earthquake wave tends to cause the same kind of destruction. Storm Waves. — When the great whirling storms of cyclonic origin (the hurricanes and temperate latitude cyclones de- scribed in Chapter V.) pass over the ocean, the spirally inblowing winds tend to heap up the water near the center of the storm. In the center the air pressure is less than on the margins, and this also causes the water near the center of the storm to rise. Therefore during these storms there are two tendencies to the production of unusually high water. When the storm centers pass along the coast, the ocean sur- face is raised to a height often as great as six or eight feet above the average ; and if violent wind waves accompany this high state of water, their destructiveness along the shore becomes greatly increased. Any strong prevailing wind blowing upon the coast, tends to raise the water to an unusual height. During the passage of waterspouts over a portion of the ocean, there is raised a cone-shaped wave, a few yards across the base, which, on a small scale, resembles that caused by the passage of hurricanes. Ocean Surface Temperatures. — Latitude is the most impor- tant cause for differences in atmospheric temperature, and the same is true for the ocean. Near the equator the oceanic waters are warmed, while near the poles their temperature remains approximately at the freezing-point throughout the year. There are all gradations between these two extremes 180 PHYSICAL GEOGRAPHY. Fathoms i SO _Fahr, 200 300 (Plates 15 and 27). As in the case of the atmosphere, this regularity of distribution is interfered with by outside causes, mainly the influence of land, and air and water movements (Plates 15 and 27). The influence of the ocean currents is shown in both of these maps ; and they also show the greater regularity of the ocean surface isotherms in the southern hemisphere, where there is little land. Near the coast the temperatures of the ocean surface are subjected to very marked variations. This is particularly true in the temperate zones, where the difference be- tween summer and winter temperatures is Yerj great. Thus, on the New England coast, the water in summer is warm enough for purposes of bathing, while in winter it is not uncommonly frozen in the shallow har- bors. Even at a distance of a number of miles from the shore, this variation from summer to winter is quite marked ; but in the mid-ocean, and in the tropical and arctic zones, the summer and winter temperatures are very nearly the same. In the ocean there is a vertical change in temperature. Since water warms very slowly, the effect of the sun extends only to a dis- tance of a few score of feet, even in the trop- ics ; and below this, the temperature throughout the year is practically uniform, while it rapidly descends until the cold waters of the great ocean depths are encountered (Fig. 83). Because radiation from a water surface is a slow process, the temperature of the water does not become rapidly lowered during the night. Therefore there is very Via. So. Diagram to show the normal de- scent of temper- ature in a col- umn of water in the ocean at the equator. Plates 15. 182 PHYSICAL GEOGRAPHY. little reason for decided changes in temperature, either between the day and night, or between the seasons. Ocean Currents : Planetary Circulation. — As a result of differences in temperature between polar and tropical re- gions, the air is engaged in a series of great movements. There are many reasons for believing that a similar circu- lation exists in the ocean. The fact of the difference in temperature suggests the probability of such a circulation, which would consist of a rising of the water under the equator, a surface outflow from equatorial to polar regions, and then a downsinking to the bottom, from which there would be a return to the equatorial regions along the ocean bottom. That this theoretical circulation actually exists, is sug- gested by the fact that the bottom of the sea is inhabited by large numbers of animals. If some such circulation as this did not exist, it would be difficult to account for the supply of oxygen which these creatures need for their existence (pages 171 and 172). Such planetary circulation seems also demanded by the temperature conditions of the ocean bottom. Unless there has been a downsinking and passage of Arctic waters over the bottom of the ocean, we cannot explain the fact that temper- atures as low as the freezing-point of fresh water exist over great areas in the depths of the sea. A circulation is also suggested by the peculiar distribution of temperature on the ocean bottom. It was stated on page 162 that in some deep parts of the sea, the tempera- tures are higher than in other portions whose depth is not so great, the apparent explanation being a barrier which inter- feres with the passage of the slowly moving water over the ocean bottom. Then also, under the equator, a temperature of 41° is encountered at a depth of 250 fathoms, while in the Face page 183. <£/"*' - 1 Jt.D.Stno,,.ll.Y. Warm, Currents ^Hp Cold Currents ffel OCEAN WAVES AND CURRENT 8. 183 northern hemisphere this temperature is reached at a depth of about 600 fathoms, and in the southern hemi- sphere at a depth of about 400 fathoms. This indicates that under the equator the cold water of the ocean bottom is rising. The System of Ocean Currents. — (Plate 16.) We know little concerning the circulation of the water on the sea floor; but at the surface there are certain very distinct movements, to which the name ocean currents is given. In the Atlantic there is a drift of surface water toward the equatorial portion of South America. This slowly moving surface water divides against the triangular coast of South America, one portion passing southward, the other and larger part moving northward, still as a slowly moving drift. A considerable part of the north-moving drift passes into the North Atlantic outside of the West Indies, and this may be called the North Atlantic Drift. The portion which enters the Caribbean, passes through the Straits of Yucatan into the Gulf of Mexico, where a part of it circles around, and finally emerges past Key West at the southern end of Florida. Some of the water passes northward between the West Indies. Therefore, by the time it has reached the latitude of the Carolinas the warm current of the North Atlantic is composed of several parts. The portion which emerges from the Gulf of Mexico between Cuba and Florida, is known as the Gulf Stream ; and this passes northward along the coast as a very percep- tible current, on the seaward side of which is a portion of the Atlantic drift, which did not enter the Gulf of Mexico. At about the latitude of Cape Hatteras, the Gulf Stream is turned to the right as a result of the influence of the earth's rotation, and it then passes out into the Atlantic until the 184 PHYSICAL GEOGRAPHY. European shore is neared. 1 A branch extends northward into the Arctic, while a part returns as a surface current along the coast of Spain and Africa, there joining the equa- torial drift. The water thus eddies around in the North Atlantic, moving northward, then eastward, southward, and southwestward, thus establishing a complete whirl. Another important current in the North Atlantic is the cold Labra- dor current (see p. 189). In the South Atlantic a similar whirl of water is caused ; but this is distinctly less pronounced than that of the North Atlantic, and there is no current so marked as the Gulf Stream. Cold water from the Antarctic extends northward into the South Atlantic. In the North Pacific, a circulation is established which very closely resembles that of the North Atlantic. A broad equatorial drift passes westward toward the Asiatic coast, then becoming in part a north-moving current, it proceeds as a very distinct stream, in many respects resembling the- Gulf Stream. This is known under the name of the Kurc Siwo, or better as the Japanese current. It passes north- ward, is turned to the right, then moves southeastward, bathing the western coast of the United States, then curv- ing to the southwest, it joins the equatorial drift. Owing to the fact that land practically excludes the Arctic waters from the North Pacific, there is no distinct Arctic current in this ocean, nor is the Japanese current able to extend a large branch into the Arctic. Still a small current of cold water does pass through Bering's Straits into the North Pacific. In the South Pacific and Indian oceans, distinct whirls of 1 Numerous observations on the movements of wrecks and floating bottles have given us much information concerning this current. One set of obser- vations upon a floating wreck is shown on Plate 16. OCEAN WAVES AND CURRENTS. 185 water are produced, which more nearly resemble the whirl of the South Atlantic than those of the northern oceans, but which nevertheless are better developed than the South Atlantic system of currents. Cold currents from the Antarctic extend into both of these oceans. Thus we find a great series of whirls in the oceans, one on each side of the equator in each ocean, the water passing poleward as surface currents and in part returning to com- plete the whirl. The north-moving system is better devel- oped than the return south-moving currents, and the system of ocean currents is much better developed in the northern than in the southern oceans. Particularly is this true of the Atlantic. In any explanation of oceanic circulation these facts must be accounted for. Cause of Ocean Currents. — Although there is evidence that a planetary circulation exists in the ocean, there are many reasons for doubting whether this cause is sufficient to explain the system of surface currents which is so well devel- oped. The difference in temperature does not seem suffi- cient to account for the great oceanic whirls. While the al- most constant cold of the Arctic and Antarctic oceans causes a continual descent of water, it cannot be said that the heat at the equator is sufficient to so expand the water as to cause surface currents to start here. The comparison has been made between the ocean circu- lation and that of the air ; but this is only partly warranted, for the air is warmed from below by contact with the earth, and when warmed this lower air must rise. In the ocean, the heat of the sun is practically confined to the immediate surface ; and this relatively thin layer is not warmed to a very high degree, so that its expansion would not seem to be sufficient to cause a flowing away. If the sun's heat penetrated to the ocean bottom, the warming of the lower 186 PHYSICAL GEOGRAPHY. layers of water would cause sufficient expansion to necessi- tate their rise to the surface ; but this is not the case. If temperature differences account for ocean currents, the fact of the greater development of the system in the north- ern oceans would be difficult to explain. The Antarctic is practically open to both Atlantic and Pacific, and in that hemisphere there is an excellent opportunity for an exchange of polar and tropical waters. But the Arctic is almost com- pletely shut off from the Pacific, and is only open to the Atlantic through narrow and rather shallow channels. Therefore, in the hemisphere where the least favorable con- ditions for an exchange of water exist, we have the best developed currents. In the North Pacific there seems abso- lutely no chance for the general passage of cold northern waters along the bottom to the equator. This and other reasons, such for instance as the presence of cold surface currents returning from the Arctic, cause great doubt as to the validity of the temperature theory which has been held by many physicists. It seems that we are forced by these arguments to return to the theory which was proposed by Benjamin Franklin, who pointed out the fact that nearly permanent winds are blowing toward the equator throughout the year. These trade winds necessarily drive large quantities of surface water before them, just as the winds along the coast will cause the surface water to drift before them. Thus the water is being heaped up in equatorial regions, and this seems sufficient to account for the great whirls ; and there are many facts tending toward the conclusion that winds are the prime cause for these currents. Among other things, the whirls are best developed in the northern oceans. The belt of calms, which separates the two systems of trade winds, is mostly north of the equator, and the northern belt OCEAN WAVES AND CUBBENTS. 187 of trade winds does not extend south of the equator during the northern winter, while the southern belt does extend north of the equator during the northern summer (Plates 10 and 11). Therefore there is a greater drift of water in the northern hemisphere than in the southern. Probably the differences in temperature aid in this circulation ; but to this cause we must assign a secondary importance. The course of the various currents is in part determined by the outlines of the continents. If there were no conti- nents, the effect of the trade winds would be to produce a surface drift, which would tend to pass around the earth, approximately in the belt of calms. The north and south extension of land in the form of continents, interferes with this circulation, and causes the moving water to pass north- ward or southward. After this deflection, there is a con- tinued tendency for the currents to turn, under the influence of the earth's rotation, to the right in the northern, and to the left in the southern hemisphere. These two facts of continental interference, and deflective effect of the earth's rotation, are mainly responsible for the paths pursued by the currents, and for the great system of whirls. The cold sur- face currents which come from the Arctic and Antarctic, are probably a partial return of the warm water that drifts into these zones. The Gulf Stream. — This, which is the best-known of ocean currents, is of so much interest to us, and so well illustrates some minor phenomena of ocean currents, that it is well to examine it in a little more detail than has been done (Plate 17). The Gulf Stream proper is that portion of the equatorial drift which has passed through the Caribbean and the Gulf of Mexico. During its passage through these warm gulfs, its temperature has been increased so that it emerges into the Atlantic as a very warm current. It is 188 PHYSICAL GEOGRAPHY. one of the most rapidly moving of ocean currents, and its rapidity depends upon the peculiar effect of irregularities in CHART OF THE aTJXF STREAM SHOWING ITS AXIS AND LIMITS Boston c^ cifpe'.Cod--'"" NANTUCKET I. Washington P.LAXE H. the continental outline. Passing into the Gulf of Mexico without difficulty, it finds itself partially enclosed. The one OCEAN WAVES AND CUBBENTS. 189 place of easy escape is in the narrow passage between Key West and Cuba. In a measure, it is concentrated here, in a manner somewhat analogous to the concentration of water in the nozzle of a hose. When it passes through the channel at the end of the Yucatan peninsula, its velocity is only about ^ of a mile an hour, and its width is about 90 miles, while its depth is approximately 1000 fathoms. When it emerges past Key West, its velocity is from four to five miles an hour, its width only 50 miles, and its depth about 350 fathoms. If it were not for this concentration, the Gulf Stream would not be such an important factor in the North Atlantic. Soon after passing through this narrow channel its velocity decreases ; and by the time it has reached the Banks of Newfoundland, its rate of movement is less than half that which it had on the Florida coast. It has been estimated that every day, the Gulf Stream carries past Florida the enormous amount of 136,000,000,000,000 tons of water. The Labrador Current. — The Labrador current comes from the Arctic between Greenland and Labrador, passing down the coast of Nova Scotia and New England, and keeping close to the coast, because of the influence of the earth's rotation, which tends to make it curve to the right. It remains as a surface current until Massachusetts Bay is reached, where it sinks to the bottom, owing to the fact that it has a lower temperature, and therefore greater density, than the surrounding water. But its influence is felt upon the continental shelf nearly down as far as Cape Hatteras. Effects of Ocean Currents. — The most striking effect of currents is upon the temperature. If it were not for the existence of this oceanic circulation, it is probable that a large part of the now habitable earth would be ren- dered unfit for habitation. Much of the heat received in 190 PHYSICAL GEOGRAPHY. equatorial regions would remain there, while the cold of high latitudes would increase, and the temperature in these regions would be reduced to very low degrees. The equatorial regions would be much hotter than at present, while the high temperate and arctic belts would be colder. The immense influence of these currents is shown by the fact that in the latitude of Labrador, — a bleak, inhospitable land, — there are powerful and well-populated countries on the other side of the ocean. In the one case, cold Arctic currents flow along the coast; in the other, the climate is tempered by the warm ocean current. Ocean streams carry vastly more heat than air currents are capable of doing. Croll has estimated that the Gulf Stream alone carries as much heat as falls upon a surface of 1,560,935 square miles at the equator. He says that this stream carries from tropical regions, nearly one-half as much heat as is received directly from the sun in the entire Arctic. The temperature of the Pacific coast of the United States is greatly moderated by the warm Japanese current, which carries into the North Pacific a large store of heat ; and, both on this coast and on the western coast of Europe, the warm bodies of water which are off-shore, not only supply quantities of heat, but they furnish to the land much moist- ure which is condensed in the form of rain. Franklin's attention was called to the Gulf Stream by reason of the fact that sailing vessels made their voyage from the colonies to the mother country in a shorter time than on the return. Therefore, in some cases, currents in the ocean are an aid to navigation (see Plate 16, showing the drifting of a wreck in this current). Where a cold and warm current are side by side, as is the case near Newfoundland, fogs are abundant, and this interferes with navigation. OCEAN WAVES AND CURRENTS. 191 Since the currents temper the ocean waters (see Plate 15), they tend to modify the conditions upon which the spread of marine animals depends. This influence is particularly noticeable among the coral reefs. The warm tropical cur- rents carry large quantities of food and of clear water to the banks upon which corals are developing ; and it may be said that in this indirect way, ocean currents are an impor- tant cause for coral reefs. Thus, where the Gulf Stream bathes the coast of Florida, reefs and coral keys are pro- duced ; and even as far north as the Bermudas, coral life is possible because of the presence of the warm tropical current. REFERENCE BOOKS. In many of the books referred to at the end of the last chapter, there is something on oceanic movements. See particularly Agassiz, Wild, and Thoulet. For a very complete discussion of ocean currents, see Croll's " Climate and Time," referred to at the end of Chapter VII. Maury. — The Physical Geography of the Sea. (There are many edi- tions of this book, most of them, and the best, being out of print and obtainable only in the second-hand condition.) Pillsbury. — The Gulf Stream, " U. S. Coast Survey Annual Keport for 1890," Appendix 10. Washington, 1891. Issued by the survey in sepa- rate form. (The most complete discussion of the Gulf Stream which has been printed.) Berghaus Atlas, volume on Hydrography. Justus Perthes, Gotha, Germany, 1891. 15m. (Contains numerous excellent charts of currents, ocean temperatures, etc.) CHAPTER XI. TIDES. Nature of the Tidal Wave. — Each day the ocean surface is disturbed by two waves which pass about the earth with great rapidity (fully 500 miles an hour in the Atlantic), and affect the entire ocean, from surface to bottom. The actual height of the tidal wave is very slight, and sailing vessels in the mid-ocean are never aware of its existence. When it approaches the shore, the wave is subjected to a variety of complex changes, which make it an important feature of the ocean. On the coast, the water gradually rises or flows, and as gradually falls or ebbs; and this is repeated, with marked regularity, approximately twice each day. Cause of Tides. — In origin, the tide is directly associated with the effect of the moon and sun upon the earth. All bodies in space are engaged in a mutual attraction which we know as gravitation ; and the effect of this gravitative at- traction is proportional to the product of the masses, and inversely proportional to the square of the distance. Every member of the solar system is exerting an attraction upon the earth. Since the attraction varies with the mass, such a large body as the sun would produce a great effect if its distance were not so great. The moon, although relatively small, is so near that its influence upon the earth is much greater than that of the larger and more distant sun. Leaving the sun out of the question for a time, let us see what effect the attraction of the moon will have upon the 192 TIDES. 193 earth. If the earth were all liquid, the attractive action of the moon would tend to destroy the sphere and change it to an ellipse. The ellipse would project toward the moon, for that portion of the earth's surface which was nearest the moon would be most attracted. Since the rotation of the earth causes the moon to appear to pass through the heav- ens, this ellipse would constantly change in position, with its axis always pointing toward the moon. Therefore the liquid sphere would be thrown into a series of waves, one crest being beneath the moon, while the other crest was on the opposite side of the earth farthest from the moon. This is approximately what happens in the liquid ocean. The surface of this partial liquid covering is disturbed by the gravitative attraction of the moon, and a wave is thus produced beneath it, while one is also formed on the opposite side of the earth. As the moon appears to pass around the earth, these two waves also move. They are much disturbed by the irregularity of the continents, and their movements are rendered very complex as a result of this influence. They are not able to remain directly beneath the moon, but lag behind and follow it, instead of passing around the earth with it. In a similar manner the sun causes two waves ; and these combine with those produced by the moon to produce the tidal wave. Since the movements of the sun, the moon, and the earth are very irregular, there are many complexities introduced into the tidal movement. In the latter part of the chapter some of these irregularities are considered, and their cause pointed out. Effect of the Land. — The tide waves tend to pass about the earth from east to west, following the direction of the path of the moon through the heavens. This west-moving tide wave is much better developed in the great expanse of 194 PHYSICAL GEOGRAPHY. water in the southern hemisphere, than in the northern. The continents interfere with the movement of the wave, and in some cases successfully check it. When the wave enters the Atlantic, its direction is changed from west to north, and soon the wave is so changed that it advances more rapidly in the middle part of the ocean than on the margins (Plate 18). This is due to the effect of the shallow waters near the continents. The wave is retarded near the shore and advances more rapidly in the central portion of the ocean. As a result of this, the crest of a wave may have reached the latitude of Newfoundland at the same time that the margins are affecting the coast of northern Africa and the West Indies. In a similar way this effect of friction is also shown in the bays and larger estuaries. Thus as the wave passes up the Bay of Fundy, friction with the shore causes it to be re- tarded, while in the central part of the bay the wave advance? more rapidly. Nowhere can this effect of the land be better illustrated than in the vicinity of the British Isles (Plate 19). The wave passes up the Atlantic, and without serious inter- ference moves to the northern extremity of Ireland and Scot- land, while the same wave has advanced to the southern coast of England, and begun to pass into the English Chan- nel. The shallowness of the water in this region prevents the rapid movement of the wave ; and by the time it has passed through the English Channel, the part that went out- side of the British Isles has gone entirely around the islands, and entered the North Sea, where the two parts of the same wave meet (Plate 19). On the American coast a similar influence of the land is noticed in the approaches to New York Harbor. The tida] wave passes readily up the bay toward New York, while the Face page 194.. Diagrammatic representation of the advanc ives. Figures refer to noon and midnight. Plate 19. Magrammatic representation of the tidal wave near the British Isles. Figures refer to hours of the day. 196 PHYSICAL GEOGRAPHY. same wave goes around the eastern end of Long Island into Long Island Sound (Fig. 84). Here its rate of motion is retarded, the dis- tance traveled is greater, and at Hell Gate Chan- nel the two parts of the same wave arrive at entirely different times (Fig. 85). This is one of the rea- sons for the vio- lent currents at Hell Gate, coastal irregularities upon Fig. 84. Diagram to show path pursued hy the tides on the two sides of Hell Gate. Figures represent height of tides at different places. Aside from this influence the time of approach of the tidal wave, these pe- culiarities also influence the height to which the tide rises. The normal tidal rise, as observed in mid-ocean and on ex- posed coasts, is only one or two feet. Along the eastern coast of America, we find the tide rising in one place only two or three feet, in other places 10 or 20 feet, and in the Bay of Fundy often 50 or 60 feet. These irres'u- of HOURS AFTER TRANSIT III IV V VI VII VIII IX X XI 1 1 1 /* /P '£ * h 4° *v Fig. 85. Diagram to show time of arrival and height reached by the tides on the two sides of Hell Gate. larities are due to the influence of the coastal outline. TIDES. 197 There are two ways in which, the tide may be almost entirely destroyed. It is a familiar fact, that if two waves meet trough to crest, they extinguish one another. It is believed that the two tidal waves which meet in the North Sea (Plate 19), actually come together in this way. When the tide enters a large body of water through a narrow inlet, the tidal rise is almost entirely destroyed, as is very well illustrated in the Mediterranean. Outside of the Straits of Gibraltar, on the coast of Spain, the height of the tide is from five to six feet. The wave enters the Mediter- ranean through this narrow inlet, then expands, and con- sequently loses in height, until almost no tide is left. In portions of the Mediterranean there are slight tides, but these appear to depend in part upon another cause. The opposite effect of increase in height of the tide is by far the most common influence of coast irregularities. When, instead of entering a large body of water through a narrow inlet, the tidal wave passes into a narrowing bay through a broad mouth, the effect of the converging shores is to pile up the wave, and therefore to increase the height of the tide. This is the cause for the very high tides of the Bay of Fundy, and many other V-shaped bays and estuaries. It is well illustrated in Massachusetts Bay, where the rise of the tide is between 8 and 12 feet. As a result of the influence of coast irregularities, some peculiar tidal effects are produced. In two neighboring, and possibly connected bays, the height of the tide may be quite different. This is the case in Vineyard Sound and Buzzard's Bay, on the south coast of Massachusetts, where, in the lat- ter, the tide rises one or two feet higher than in Vineyard Sound, which is open on both ends. In the channels which connect these two bays, violent currents are produced; and this whole region, between the Elizabeth Islands and 198 PHYSICAL OEOGBAPHT. Nantucket, is one of relatively rapid tidal currents. The rapid currents in the straits between two such bodies of water, may be called tidal races. A tidal race is produced at Hell Gate, near New York City, mainly because the tide rises higher in Long Island Sound than it does in the bay of New York Harbor (Figs. 84 and 85). The very rapid currents in this shallow strait, are in part due to this cause, and in part to the fact that the time of high tide is different on the two sides of Hell Gate. Similar tidal races occur on many parts of the irregular northern shore, and at times currents are produced which are as violent as rapidly moving streams. In some cases it is impossible to row a boat against the current. In a rapidly narrowing bay, particularly at the mouth of a river, the rising tide is sometimes transformed to a wave, which in form resembles the wind wave ; and there is an advancing wall of water, instead of the gradual, almost imperceptible rising of the ocean surface, which is the normal form of the incoming tide. To this peculiar phenomenon the name tidal bore is given. This wave is produced in the Amazon, the Severn, the Seine, and many other rivers. Other Causes for Variation in Tidal Height At any given point on the coast, the height of the tide is liable to vary from time to time. This variation may be of an irregular nature, due to the effect of winds upon the surface of the water. Sometimes, when strong winds blow upon the coast, the height of the tide may be increased several feet. A mere change in the pressure of the air also appears to cause fluc- tuations in the surface of the sea ; and upon lakes, these causes produce fluctuations in level which are often of quite noticeable size. In the Swiss lakes these irregular variations in the level of the water are known as seiches, and they are also found upon the Great Lakes. TIDES. 199 The main variations in the height of tide depend upon astronomical causes. Since the tide is the com- bination of two waves, one produced by the sun, and the other by the moon, the height of the tide naturally varies as the position of these bodies in the heavens changes. During new moon, the sun and moon are nearly in the same line, and they therefore pull approximately along the same line, so that the unusually high tide then produced (known as spring tide), is the result of a combination of the two waves. During full moon, the sun and moon are again in line, one on either side of the earth, and then the two waves again tend to combine. Therefore every month there are two sets of rather strong or high tides (Fig. 86). Between new and full moon, — that is, during the first and third quar- ters, — the sun and moon 3 So g SO •2. w 3 P t? orq" 2 a B » & -■ CD a: go £ co o w CD H*» 0» PS 55 ^ 9 : = = = = = !»£ r _: f == T"i :::::::«=|E: ::g:J Ltvj, „,,==:::; -r::zZ'l. . .._.{! c« ----- ■-== ==« =====-=?- ""*> ' """■" - «===:=: =i:^=t— -t» s ....:i«i:::: — -^^^-— --- ----==.. ! :"::::]:;--; -- + ""---•-4-50 - + :-:: = = = - = = = i±- r S j. . __* -- =-- ^ r ...,-ZZT.Z-.- zl llffinl iN i==|=— - & ----- = %22: :=g- ^ -.:::•::::: -$ i 1 - ==^r:r: ::=;:::..l. j.$ -—«===:: ::g ^ ,.$:j; i===-r— -- — J| I .„s;; iJ=S:r — " — U HIE::- 9 y tff?T --; = = - ** T-— '!« = = «; ^= T ^~. ... .NO -----». INS Ijl... -i::;:::| ==? = r-L.I JN3 — . ia ;~"":::::: r * ..._:::=::===-: "te ±-= = — -- £o --.- T Si 1"::::^:- ffiftlrr ;xcci- r§ L -= t =*r r 1 05 :::::: i « ...-■r = - :::::: -— i| 4 E plains as the most prominent g features. Usually there are 5' two plateau areas, one upon g either margin of the continent ; £ and above these rise more or rt (3* less continuous ridges, which £ we know as mountain chains. £ This feature of continents is 8 well illustrated in North Amer- sr ica (Fig. 129), which may be considered a typical continent ; and it will be described in more detail in later parts of the chap- ter. Plains usually occupy the S interior portion of the continent, ^ and these are sometimes in the form of low plateaus ; while Mid Atlantic Bidge Spain 252 PHYSICAL GEOGRAPHY. in some cases they are even interior basins. The land surface is very irregular, the irregularities being partly due to origi- nal features of the earth's crust, and partly to the sculpturing of these by the agents of denudation (see Chapter XIII.). Geological conditions conclusively prove that the con- Fig. 129. Relief map of North America (Lambert's projection). tinents are subject to changes, and that the present form is merely the result of an evolution which has long been in progress. Even at present, in some cases, there are changes of considerable moment still in progress. The mountains which form the border of the continents have been elevated Face page 253 TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 253 by successive foldings of the rocks ; the plateaus have been produced by great elevations of the land ; and many of the plains have been caused by the filling of seas from the waste of the mountains. Many forces have cooperated to build the continents, but this subject is properly one of pure geology. From the present standpoint, it is enough to know that these changes are going on, and to recognize the fact that the continents are not of the same form at all times. Indeed, there seems good reason for believing that the true Ameri- can continent does not end at the present shore line, but that its proper boundary is along the margin of the conti- nental shelf, which on the northeastern side, extends to a distance of from 50 to 100 miles from the present shore. At this point the great ocean abysses commence, and from the land to this point there is very little depth to the ocean. (See Fig. 128.) Physical Geography of the United States. — The best way to illustrate the typical features of the earth's surface, is not to make a hasty survey of the entire surface, but rather to consider a single area in some detail. Thus we may select the United States (Plate 22) as a typical part of the con- tinent, and by examining the physical geography of this area, form an idea concerning the main features of the earth's sur- face ; for we have very nearly every important topographic form represented within the boundaries of this country. For the sake of completeness, it will be well to extend the boun- daries of the area described, for a short distance beyond the Canadian boundary, in order to include a portion of the con- tinent which is essential to its proper consideration. This area, including the United States and southern Canada, forms a true section of a typical continent. We may divide this area into five great divisions, each having characteristic geographic features. These are the Atlantic Coast Province, 254 PHYSICAL GEOGRAPHY. the Eastern Mountain Ranges, the Canadian Highlands, the Mississippi Valley Plains, and the Cordilleras of the West. Atlantic Coast Area. — This properly includes the conti- nental shelf, which is now submerged beneath the sea, but which is a submarine plain bordering the continent and appearing to form a true part of it. Above the sea level, the continuation of this area is represented by a narrow strip of level country with an elevation of but a few feet above sea level, and extending from New Jersey to the Rio Grande. It forms a low plain which is nearly featureless, and which in some parts is in the condition of a swamp. It is but a few miles in width in the northern portion, and varies in width as we proceed southward, but gradually increases until the Gulf States are reached. A large part of Florida is included within the area, and along the Mississippi valley the coastal plains expand and extend inland to a considerable distance. The present delta and floodplains of Mississippi, Louisiana, and a part of Arkansas, belong to this coastal area ; and in Texas there is a strip whose width is often as great as 50 miles. On the landward side of this low-lying plain, is a more elevated area of level country, which is also a true plain, but which is more ancient in origin. The low swampy plains are scarcely drained ; but these higher, inland plains, are cut by river valleys, and in some cases carved into a series of rounded hills. For the most part, the low swampy plains, near the coast-line are of little use to man, their swampiness prohibiting their occupation, though this does not apply to some parts of the plains, such as the delta and floodplain region of the Mississippi. The higher plains on the land- ward side of these, are much better adapted to occupation, and it is upon these that the greater part of the agriculture of the Southern and Gulf States is carried on. The Eastern Mountains. — In nearly all cases mountain TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 255 chains are found rising above basal plateaus. This is true for the great system of eastern mountains, the Appalachians. Both on the eastern and western sides of these chains, there is a highland country which is a true plateau, though in most cases deeply carved by stream valleys. There are two parts to this system of eastern mountains, one much older than the other, and both considerably destroyed by the sculpturing action of the agents of denudation. The oldest series of mountains date back to the first beginning of the known history of the North American continent, when they Were formed as very high mountain chains. A considerable part of New England is included within this area of ancient mountains, and the chains extend southward through the hills of New Jersey, and thence along the eastern base of the modern Appalachians into the Carolinas. In many places these would not be recognized as mountains, but are now in the form of low hills. They have been worn down to their very roots, and nothing but hills are left where once existed very lofty chains. The highest remnants of these mountains are found in New England and North Carolina. The true Appalachians were much more recently formed; but yet they are among the ancient mountains of the conti- nent. For a long period of time they also have been exposed to the destructive action of denudation, so that their original form is very much altered. They are no longer high chains, and in point of size and grandeur bear no comparison with such recent mountains as the Rockies, the Andes, or the Alps. Formerly they were much higher than now, and probably their features were much more like those of the grander mountains of the globe. At present they consist of a series of ridges and ranges, extending in a northeasterly direction, usually with nearly level tops, and in no case rising to great heights. 256 PHYSICAL GEOGRAPHY. The highest part of the eastern mountains is Mitchell's Peak in North Carolina, whose elevation is 6688 feet. In these mountains there are vast stores of coal, building stone, iron, and other products which are of use to man. The Canadian Highlands. — These are another ancient series of mountains, once much more extensive than now, and they enter this country in only one or two places. The Adirondacks may be considered a part of this highland area, and the same holds true for the hilly region near Lake Superior. At present this region is occupied by a series of low, rather rounded hills, never rising to great mountain heights, and rarely being over a mile above the level of the sea. Among the Adirondacks the highest point is Mt. Marcy, which is 5379 feet above sea level. For the most part this hilly region is of little value, partly because it is situated far in the north, and partly because it is composed of rocks that do not favor the formation of even slopes and deep soil. There are considerable areas of valuable mineral materials, mainly iron and copper. The St. Lawrence valley forms quite another province. The Central Plains. — Extending from the western base of- the Appalachians to the Mississippi, there is a great area of plains, which gradually decrease in elevation toward this river. From the Mississippi westward, the plains con- tinue until the base of the Rocky Mountains is reached ; and here also, as the mountains are neared, the elevation gradually becomes higher. At the base of each of these mountain systems, the plains have become transformed to true plateaus, in the case of the Appalachian plateau with an elevation of 1000 or 2000 feet, and of the Rocky Moun- tains with an elevation of over 5000 feet. This great area of plains is not everywhere level or roll- ing, but in some of its parts is broken by truly moun- TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 257 tainous irregularities. This is true, for instance, in Indian Territory, in Arkansas, in part of Missouri, and elsewhere. Aside from these limited areas of mountainous character, there are other regions which have been very much cut and dissected by stream action. However, the general condition of these plains is that of gently undulating country. They form the great farming belt of the continent, and also contain deposits of valuable minerals, such as coal, iron, petroleum, building stones, etc. This area of plains is equal to fully one-fourth of the total area of the country, and the elevation is generally less than 2000 feet, while nearly one-half of the area has an elevation of less than 1000 feet. Since occupied by man, the greater part of this area has been free from timber. In the plains of the far west this is due to the fact that the climate is dry; but among the prairies of the east the cause is less easily ascertained. Some think that, because of its compactness, the soil was unfavor- able, others that the timber has been burned off by fires ; but neither theory can be considered proven. The Cordilleran Area. — This is the most complex of our geographical areas, and perhaps should be subdivided, though for our general purpose it may be considered as one great area. In the main it consists of a great plateau, with an average elevation of over a mile above the sea level, above which rise several mountain chains. Commencing on the eastern base of this Cordilleran region, we will examine it in cross-section until the Pacific is reached. A high plateau reaches to the very base of the Rocky Mountains, which then rise to great elevations, not only above the sea level, but also above the plateau itself. The highest part of the Rockies is in Colorado, in which state there is a total area of nearly 13,000 square miles with an 258 PHYSICAL GEOGRAPHY. elevation greater than 10,000 feet, while several peaks rise above 14,000 feet. The chains, which extend northward and southward, are of varying heights and differ also in extension. There is not one mountain chain, but a series which together make the Rocky Mountains. They pass entirely across the United States, entering Canada on the north and Mexico on the south. West of these mountains is a region of interior drainage, known as the Great Basin. In reality there are numerous interior basins, some of which combine to form a Great Basin (Plate 23 and Fig. 151), while others exist as separate smaller basins of interior drainage. The basin region is a great plateau area, generally above sea level, and usually more than a mile above the level of the sea. It is commonly surrounded by high mountains, and the interior plateau itself is broken by ridges, known as the Basin Ranges, which extend in a north and south direction. Bordering the Great Basin on the west, is the Sierra Nevada range, which passes in a nearly north and south direction, from the northern part of California to the southern border of the country. 1 It is a high mountain region, but its average elevation is less than that of the Rockies. West of the Sierras is a great valley, which, with minor interruptions, extends from Canada to Southern California, where it is interrupted by a mountain mass, to the southeast of which the broad valley continues into the Great Basin, to the Gulf of California, which is really a part of this valley. Death Valley of Southern California, which is a part of the Great Basin, is an illustration of the rather rare feature of an interior basin below sea level. It is 175 miles long, and in one place is at least 225 feet below the level of the sea. 1 There is no uniformity in the usage of the term Sierra Nevada, and the boundaries of the range are vaguely and variously drawn. TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 259 West of this valley, and rising almost out of the Pacific, is a fourth series of mountains, the Coast Ranges, which extend from Lower California to the northern boundary of the United States, and apparently as far as Alaska. They are rugged mountains, and among them are found some of the highest peaks on the continent. These mountains of the Cordilleras are much more recent than those of the eastern part of the continent. Many of them were formed in the Tertiary period, and there is evi- dence that some of them are still growing. It is as a result of this that they are so rugged and so high ; for they have not been long enough exposed to the action of denudation to be reduced to low, rounded forms. No mention has been made of volcanoes, for the reason that within the borders of the United States, outside of Alaska, there are none known to be active. That this has not always been the case, is shown by the vast number of volcanic cones, in all stages of destruction, which dot the Cor- dilleran region. There are thousands of these (see Chapter XX.); and on every hand, the evidence is conclusive that in very recent times large areas have been deluged in lava and ash deposits. Along the eastern margin of the country there is no sign of recent volcanic activity, although during the time of formation of the higher mountains, volcanoes did exist. Aside from being the largest geographic zone of the country, the Cordilleran region contains minerals in extraor- dinary variety and abundance. It is the great precious metal zone of the earth, and from it is produced more gold and more silver than is supplied by any other region. The Drainage of the Country. — Three oceans receive the waters that fall in the United States. The accompanying map (Plate 23) shows this so graphically that description may be omitted. TOPOGRAPHIC FEATURES OF EARTH'S SURFACE. 261 The Shore Line. — The coast of North America, like that of several other continents, encloses a land area which is triangular in form, the apex of the triangle being toward the south (Fig. 129). There is much variety in the form of the coast ; but the coast line is by no means so broken as that of Europe (Fig. 127). There are several great promontories, notably on the east coast, and many minor irregularities along this coast line ; and these are much more prominent in the northern than in the southern part (Plate 22). In the east, islands abound along the coast of Maine, and near Florida. On the Pacific coast there are very few islands, excepting those which begin to be numer- ous at the northern boundary of the United States, and in- crease in abundance toward Alaska. REFERENCE BOOKS. Whitney. — The United States. Little, Brown, & Co., Boston, 1889. 8vo. $3.00. (In the main reproduced from an article by the author in the Encyclopedia Britannica.) Shaler. — The United States of America. Appleton & Co., New York, 1894. Two volumes. 8vo. $10.00. (Particularly Vol. I, in which the relation between man and nature is pointed out.) Shaler. — The Story of Our Continent. Ginn & Co., Boston, 1891. 12mo. $0.75. (Of value to the student for elementary reading.) As a type of the kind of book needed to adequately describe the features of this country, attention may be called to Geikie, — Scenery of Scotland. Macmillan & Co., New York, 1887. Second edition. 8vo. $3.50. For known elevations of points in the United States, see Gannett. — Dictionary of Altitudes, etc. Bulletin 5, U. S. Geological Survey, Washington, 1884. 8vo. $0.20. Second edition of same, revised, Bulletin 76, 1891. 8vo. 393 pages. $0.25. For average elevation of the states and country, see Gannett, Thirteenth Annual Beport, U. S. Geological Survey, Washington, 1893. CHAPTER XV. RIVER VALLEYS. General Description. — A river is a natural drainage lme on the land, and it usu- ally occupies a valley which has certain quite definite characteristics. On either side of the river are the valley sides or walls, sometimes ris- ing gently, sometimes steeply : at times to great heights (Fig. 130), and again with only low elevations. In the lowest part of the valley, and generally near its middle portion, the river flows, usually with a meandering course. In most cases the river is immediately bounded by rather steeply rising banks, only a few feet in elevation, beyond which the slope is more gentle, and sometimes even a plain, which in times of flood is covered with water (Fig. 157). 262 Fig. 13U. A deep mountain valley. RIVER VALLEYS. 263 Fig. 131. Stream issuing from a limestone cave. Here and there, at irregular intervals, tributaries enter the main stream, and these themselves branch into other tribu- taries, until very often there is finally produced a branching network of minor streams, all directly or indirectly contribut- ing to the supply of water in the main stream. This princi- pal supply often comes from springs, and some- times the source of the river is a spring (Fig. 131). Together these form a river system; and such a system may have an area of only a few miles, or it may drain an area of many thousand square miles. The line or plane which separates one stream or system of streams from another, is known as a divide or water parting. All streams have these general characteristics ; but when their valleys are examined in detail, there are found to be many differences, not merely between different rivers, but even in the several parts of the same river. The valley of one stream, or a part of a stream, may be a precipitous gorge (Fig. 142), or it may have very gently sloping sides (Fig. 132). In some rivers there are floodplains and deltas, in others these are absent ; and in some cases the rock walls of the valley may rise directly out of the river. In some there is a permanent flow of water, in others the supply is inter- mittent, and in some extreme cases water flows only once or twice a year. In most river systems the tributaries are numerous, but in some cases they are few ; and while some of these join the main streams at a high angle, in many cases they enter it at an acute angle. There are many reasons for 264 PHYSICAL GEOGRAPHY. these differences in streams, the two most important being the position and kind of rock in which they occur, and the stage in development which they have reached. Only a very few years ago, river valleys were believed to have been formed by some earth convulsion, or some un- usual force, and it was thought that rivers occupied them Fig. 132. Brink of Niagara Falls. A valley having very gently sloping sides above the falls and precipitous sides below. merely because they were valleys ready made. It was believed that the crust of the earth had been contorted and fissured, that ocean floods had swept over the land, and that the rivers had practically no share in the formation of the valleys which they occupied. We now know that the major- ity of rivers have formed their own valleys, that they have formed them in a very slow way, and that most of them are RIVER VALLEYS. 265 still engaged in the work of val- ley carving. If this be so, then river valleys have had a de- velopment and a history ; and in this history we may hope to find an explana- tion of many of the differences. Development of River Val- leys. — We must bear in mind Fig. 134. Royal Gorge, Col. A mountain gorge where erosion is in rapid progress. Fig. 133. A gorge near Ithaca, N.Y., illustrating the down-cutting of a stream valley and its broadening by weathering. that the river valley is the result of the combined action of stream erosion, which tends to deepen (Fig. 133), weath- ering which tends to broaden the valleys (Fig. 123), and the transportation of sediment fur- nished by these means. Ero- sion proceeds more rapidly than weathering, but there comes a time when its action is checked. In no part of the valley can the stream cut be- low the sea level, or below base level, as it is called ; and since it must carry water 266 PHYSICAL GEOGBAPHY. down a slope, in its erosion the river reaches lower levels near its mouth than higher up in its course. Until a line of easy slope is reached, erosion exceeds weathering (Fig. 134) ; but then, since erosion is checked while weath- ering continues, the latter produces its most marked effect, and the valley gradually broadens, while the hills slowly melt Fig. 135. Oxbow cut-off in Connecticut Valley, Northampton, Mass. A broad matuiv. river valley with rounded valley walls. down. Unless interfered with, this would continue until the surface was base-leveled, or reduced to a nearly level con- dition. Different parts of the river will work at different rates ; and under variable conditions, this development from the canon or gorge-like valley of youth, to the broad valley with rounded sides (Fig. 135), which characterizes the more RIVER VALLEYS. 267 mature stages, may proceed at different rates and with, dif- ferent results. Naturally the part of the river which is nearest the mouth is first and most easily developed, for it is nearest the sea level. There- fore a stream val- ley may have the narrow gorge-like condition among its head waters, while the lower portion is broadened into mature form. A part of the stream may flow through soft rocks, and another por- tion through hard lay- ers ; and then, even in short distances, the form of the valley may change ; for the process of broadening by weathering is much more rapid in the soft than in the hard strata Fig. 136. Development of the canon. A and C, layers of hard rock, B and D soft. Fig. 137. Development of the canon profile in an arid region. 1 and 3 hard, 2 and 4 soft strata. , \ \ K //, ■AU—4-i -.&.&■'■ A stream may be able to cut in one part of its course, and be obliged to deposit sediment in another por- tion ; and in the latter case, erosion is checked, while weathering continues to produce a perceptible effect. In an arid climate, where weathering is relatively ujiimportant, the valleys are almost all in the condition of Fig. 138. Diagrammatic representation of develop- ment of a young valley (aa) to old age (hh). 268 PHYSICAL GEOGRAPHY. the gorge or canon (Figs. 136, 137) ; and among moun- tains, where the elevation and slope are great, erosion so exceeds weathering that the gorge is the characteristic valley (Fig. 134). During the time when erosion exceeds weathering, — that is during youth, — the resulting valley is deep and relatively narrow ; and wherever we see this kind of valley, we may Fig. 139. The Yellowstone, a young valley broadening by weathering and being deepened along a narrow line by the river erosion. be certain that, for one reason or another, erosion is now, or has recently been in progress. That weathering is also pro- ducing an effect, is evident from the fact that the valley is wider at the top than at the bottom, because the former has for a longer time been exposed to its action (Fig. 139). In such cases the river is often a series of cascades or falls, because (see Chapter XVI.) in its rapid down-cutting, the RIVER VALLEYS. 269 Fig. 140. A bit of drainage in Illinois, showing slight development of tributaries. stream finds rocks of different powers of resistance, and therefore cuts its bed irregularly. Therefore, in addition to gorges, waterfalls characterize youthfulness in river valleys. In many cases lakes are also present ; and since the process of lake destruction or re- moval is a simple and brief task, they do not long re- main in the river valley. The development of the stream proceeds most rap- idly near its mouth, and later in the headwaters ; and consequently, tribu- taries are not numerous at first (Fig. 140) ; but one by one they begin to develop, until all of the area is brought under the influence of some stream or rill (Fig. 141). At first the divides are not very definite, and they may be flat-topped and swampy ; but in maturity these become quite sharply defined, and usually every part of the area is drained. When vertical erosion has ceased, the work of the river be- comes merely that of a transporter of sediment, except that in swing- ing about, the river does some lateral erosion on its banks. The characteristics of youth disappear, waterfalls are worn down, lakes are filled and destroyed, the gorge is broadened to the gently sloping valley side (Fig. 138), and the number of tributaries Fig. 141. A bit of West Virginia drainage, illustrating well - developed tributaries of maturity. 270 PHYSICAL GEOGRAPHY. increases. With the broadening valley, and the decrease in river slope, the conditions favoring noodplains are brought about ; and since the first and most rapid development is in the lower part of the river, in this stage the valley may con- sist of three quite different parts, — a lower flood-plained course, a middle portion, and an upper torrential part, with gorges and waterfalls. The l|| majority of streams have reached this stage, and this is why, in describing a river, it is commonly said that it consists of these three parts ; but really this is to be considered as merely a stage in develop- ment, to reach which other stages are passed through, and which is normally suc- ceeded by others. Since all rivers are not in the same stage of development, a careful examination of the valleys of a country shows many exceptions to this condition of early ma- turity. Naturally there is much difference in the rate of develop- ment, and in the result produced under different circum- stances. Whether the river develops in a mountain or on a plain, or in an arid or humid climate, the main fact is the same, — that there is this development from immature gorge to broad valley. On a low plain near the sea level, the rate of development in the soft clay is much more rapid than in Fig. 142. Canon of the Colorado. RIVER VALLEYS. 271 a high plateau ; but while in the former there are produced only shallow trenches a few feet in depth, in the latter a canon may be cut with a depth of thousands of feet. The former we see in the plains bordering the coast of Texas, the latter in the Colorado canon (Fig. 142). In the hard rocks of the Colorado the form of the canon is preserved, and this is also favored by the dry climate ; but the soft, clay banks of the _ _... Texas streams readily crumble under the action of weathering in a moist climate. The development of the latter to the state of maturity, will therefore be much more rapid than that of the former, — just as some animals or plants pass through life in a few weeks, while others live for a century. In a mountainous country the elevation is so great, and the rock structure so complex, that gorges will remain for long periods of time ; and ages must elapse before the erosive action of the river becomes less rapid than weather- ng. Now and then a deep mountain lake may check the vork of the river, and serve as a temporary base level, below fhich, for the time being, the stream cannot cut ; and so lere, for a short distance, the valley may become broadened. Fig. 143. A broad Alpine valley. 272 PHYSICAL GEOGRAPHY. While the prevailing type of mountain stream valley is that of the gorge (Fig. 134), there are mountain valleys of great breadth and depth. These are not true stream valleys, but great synclinal valleys of rock folding (Fig. 143) which the rivers have occupied because of their convenient location. After passing through a deep defile (Fig. 144), a tiny stream may emerge into one of these great, park-like valleys (Figs. 143 and 221); and then we see, side by side, the valley of stream forma- tion and that of rock folding. With the aid of weathering, even the mountain gorge will in time broad- en out into a wide valley. Adjustment of Streams. — When a river begins to cut its valley upon a new land, there is no necessary relation between stream course and rock structure. The stream may flow across hard and soft layers alike, the course being consequent on the topography, because the river was guided down the original slopes. However, as the river develops, it often gradually changes its course in order to follow soft la3 r ers of rock; and therefore, in regions where the rock layers are inclined, many river courses are adjusted to the rock Fig. 144. Mountain gorge in the Alps. RIVER VALLEYS. 273 structure, soft layers being the site of valleys, while the hard strata stand out as ridges. This is characteristic of mature streams which have had a long period of development and change. At first the topography guides the stream course, but finally the river course determines the topography. In such regions as New England, we find the large river valleys cut in the softer beds of rock, while the harder strata stand up as ridges. Still, here and elsewhere, there are numerous exceptions to this statement, which is only generally true. This mature adjustment is well shown in many of the New England and Appalachian streams. Some of the ways in which these changes take place are described in the next section. The River Divide. — Between any two streams there is a line, or an area, which divides the waters, sending a part one way and the rest in an opposite direction. These divides or water partings are by no means permanent, but are con- stantly and usually very slowly changing. The stream that has the most power pushes the divide into the territory of the other, and there are various ways in which one stream may have more power than another. One may have a shorter course to the same level, and hence have a greater slope (Fig. 145); or one may be cutting through soft rock, while the opponent is working in hard layers (Fig. 146); or (as in many islands in the trade-wind belt) the rainfall on one side of the divide may exceed that on the other. Gradually the divide moves into the area of the stream laving the least rainfall, or the least slope, or the hardest rock. ■"H v ^ Fig. 145. 274 PHYSICAL GEOGRAPHY. Fig. 146. A still more important cause for the change of divides is found among tilted rocks. If the layers of a series of strata stand in the monoclinal attitude, and if these alternate in hardness, the soft layers weather more rapidly than those which are hard, and which, because of this fact, tend to re- main above the general level (Fig. 261). In such a case, the highest points do not sink verti- cally as the ridges wear down ; but they move downward and back- ward in the direction of the dip of the strata (Fig. 147). This is so permanent a condition that it may be stated as a law, that in rocks of monoclinal attitude the divide migrates in the direction of the dip. This law of monoclinal shifting applies also to changes in river courses. In their down-cutting, the valleys also tend to migrate in that direction, and this is one of the reasons why streams adjust themselves c to soft layers ; for once finding them, they tend to re- main in them. Usually the mi- gration of a divide is an extremely slow process, and in the course of a lifetime one would not notice any change ; but under exceptional circumstances it may become rapid, and in a brief time the divide may change for many miles. Fig. 147. Illustrating monoclinal shifting of divides. BIVER VALLEYS. 275 This will happen when a river with a more favorable situa- tion, for some reason gradually pushes its divide back until it taps its opponent. Then the stream receives a large acces- sion of drainage area and carries a part of another system across the old divide (Fig. 148). Before the diversion, the divide was low and nearly on the same level as the stream about to have its course changed ; and then, perhaps during some time of flood, the new course was chosen and after- wards maintained. While these cases undoubtedly occur, it is doubtful if they are at all common ; and the ordinary change in the divide is a very slow one. By these changes in divides, the adjust- ment of streams is also favored. Accidents to Streams. — River valleys tend to pass through a regular cycle of development, from the young to the old stages ; and if nothing intervened to prevent, we should find them all in some stage in this regular cycle. Some would be young, others mature, and others old ; some would be upon plains,^others on plateaus, or among mountains. There svould be great variety in river valleys, but it would be of . regular kind. In reality, the development of rivers is ubject to many interruptions of various kinds, and the cycle s never entirely passed through by any single river. The ccidents to which rivers are subjected, sometimes increase, ometimes decrease, the power of the stream. In the course Illustrating sudden shifting of a divide (aa) to (bb) by carrying the headwater (e) across the old divide at (c). J 276 PHYSICAL GEOGRAPHY. of its development, the different parts of a river may experi- ence entirely different accidents, and the resulting valley will be complex or composite. Any single part of a stream may also suffer a variety of accidents. Land Movements. — Land movements are among the most common accidents which interfere with normal development ; and these are of three kinds : (1) broad uplifts, (2) down- ward movements, (3) folding which accompanies mountain formation. With the general uplift of a country, streams are given new life, or rejuvenated, and we may then have a narrow gorge cut in the center of a broad valley. After a long period of denudation the uplift gives new powers to the stream, and it then cuts a nar- row valley (Fig. 149). Such an uplift may affect great areas; and in the rivers thus revived, waterfalls again begin to develop, and nearly all of the appearances of youth may return. Nearly every stream system shows some sign of this kind of rejuvenation, which has affected its recent development. If this elevation happens near the sea- coast, a part of the ocean bottom is raised to the condition of dry land, and the streams of the old mainland extend across it ; and perhaps by this means separate streams may be united to form one system. Depression of the land would rob streams of some of their force by decreasing their elevation, and hence their slope. Along the coast, the lowering of the land causes the ocean to extend up the valleys, drowning parts of the streams, and transforming their mouths to estuaries or straits, while Fig. 149. Diagram showing the result of an elevation, which caused the inner canon of the Colorado to he cut be- tween the older walls of the outer and broader valley. RIVER VALLEYS. 277 Plate 24. River valleys drowned by submergence beneath the sea. 278 PHYSICAL GEOGRAPHY. numerous islands are formed where the hilltops rise above the sea (Figs. 193, 211 and Plate 24). This entrance of the sea produces a reverse effect from that of elevation ; for the lower parts of streams may be dissected, and parts of one system may enter the ocean through separate mouths. This is very well illustrated in many cases on the coast of Maine, and particularly well in the Chesapeake (Plate 24), which, with its tributary streams, represents a part of a river system drowned by the sea. When the strata are folded in the form of mountains, stream erosion is interfered with and often entirely checked. As the mountains rise, a dam is built in the path of the rivers ; and unless their rate of down-cutting is as rapid as the rate of elevation, which in most cases would not be true, the streams will suffer interruptions. If they persist in their course, and cut their channels as rapidly as the moun- tains rise, they are known as antecedent streams. It is doubtful if there are many cases of rivers now crossing a large mountain in exactly the same course which they occu- pied at a time antecedent to the mountain formation ; but many geologists believe that the Green River, where it crosses the Uinta Mountains of Utah, is an illustration of this type of stream. Ordinarily the folding would locally transform the river to a lake, and as the dam continued to grow, the lake would gradually become deeper and more extensive. With the formation of the lake the erosive power of the stream decreases ; for when it flows from the body of quiet water, it has been robbed of its sediment supply, and is therefore unable to do much erosive work. If the mountain growth is rapid, it may even cause a stream to flow in a direction opposite to the course which it originally had — it maybe diverted or even inverted. Where the rocks in the middle BIVER VALLEYS. 279 course of a river are rapidly folded during mountain growth, a stream may even be separated into two parts. With the growth of the mountain, since the river slope is increased, new tasks are set before the streams. Gorges and waterfalls are caused, and because of the great elevation of the mountains, these continue for a long time ; and thus long- continued youth is impressed upon the mountain valleys. Every mountain furnishes illustrations of these latter feat- ures ; and in many, such as the Alps, there are also lakes, which are the result of mountain folding, and which repre- sent the interference with stream erosion which is brought about by the growth of mountain dams. Climatic Accidents. — A change of climate to a condition of dryness, robs streams of their erosive power ; but even more markedly does it decrease the power of weathering. Hence such a change favors the angular type of valley. It reduces the number of streams (Fig 150), and causes those which remain, to be dry for a large part of the time ; and hence in a dry country, there are large areas unoccupied by drainage lines. A rare, heavy rain, falling upon such an un- drained surface, carves a temporary valley, or arroya, which may never again be occupied by water. Stream valleys may be permanently abandoned, while others may be only withered or shrunken. By the increasing dryness of the climate, lakes may be evaporated and great basins of interior drainage be formed. Therefore, stream systems may be dissected by this cause also, and channels of outflow of lakes may be aban- doned (Fig. 151), while the direction of the drainage changes from the sea to the lowest point of the old lake bottom ; and this causes many other peculiar changes of a minor nature. By this action, a part of the Great Basin which was once tributary to the Pacific, through the Columbia, is now trans- formed to the Great Salt Lake interior drainage area. 280 PHYSICAL GEOGRAPHY. The change in climate which produces glaciation, first covers all the country with ice and buries the valleys. Near the margin of the snow-covered area, streams may be sepa- rated, and an entire change in the drainage be caused. Fig. 150. The drainage of an arid region Among the effects of the ice front, is the interference with streams that flow toward the ice, which acts as a dam, trans- forming them to lakes, and causing them to overflow across some divide. When the ice of the North American conti- RIVEB VALLEYS. 281 \*ru *r.!<; o^maqx s/s3 /' W i '? h nental glacier (see Chapter XVII.) was melting from the surface of the country, many such lakes were produced, and some of them were of great size. The St. Lawrence system was dammed, and lakes were produced in different positions from those occupied by the present Great Lakes. During the same period, the valley of the Red River of the North was transformed to a great lake which overflowed to the Gulf of Mexico, instead of to the Arctic, as the present drainage directs. Some streams had their courses permanently changed and even reversed. When the ice melted, it left much drift material upon the surface ; and this soil sometimes completely buried the old valleys, so that entirely new channels had to be cut. More often this filling was only partial, and streams were turned from their course for short distances, and often dammed into lakes, which in many cases are now repre- sented by swamps (Plate 25). Hence in a glaciated region we may have very complex streams ; for in broad, mature valleys, local post-glacial gorges may be cut, while here and there falls and lakes exist. The streams are often given new life, or rejuvenated, either through their entire course, or for a short distance. Often the course forced upon them is very much more roundabout than that pursued before the Fig. 151. The Great Basin. The lighter shad- ing shows the former extension of lakes when the Great Salt Lake overflowed into the Columbia. 282 PHYSICAL GEOGRAPHY. glacial period (Fig. 152). Illustrations of the various effects of glaciation abound by the thousand in the glacier belt of New England, New York, and other of the Northern States. Nearly all of the gorges and lakes in this belt are the result of the condition of glaci- ation. While the most notable in- stances are those of the Great Lakes and Ni- Fig. 152. Diagram of a river caused to flow irregularly because of glacial deposits in its course, which prevented it from entering the main stream by its preglacial course, now partly occupied by a tiny stream. agara, these are merely large examjjles of a great group. Other Accidents. — Interference with river valley develop- ment is commonly noticed in regions of volcanic eruptions. Sometimes the valleys are filled with lava ; at times the streams are forced to cut new valleys in a part of their course ; again they are transformed to lakes ; and they may even be forced to flow in a reversed direction. Here again, the valleys are rejuvenated, and gorges and falls are pro- duced. Illustrations of these features may be seen on almost any map of a region of volcanic activity. An avalanche in a mountain may produce one or all of these effects, and there are other minor accidents to which streams are subjected. Sometimes river valleys are again and again subjected to one or several of these accidents, and their cycle of development much interfered with. This is why youth and early maturity are the characteristic features of most valleys ; for the stage of old age cannot be reached, BIVEB. VALLEYS. 283 o K 1 Plate 25. Drainage in the glaciated region of Wisconsin showing the abundant swamps (indicated hy dashes) between the drift hills, and the interfer- ence of these hills with the stream course. I 284 PHYSICAL GEOGRAPHY. because in the conflict between denudation and the internal forces of elevation, the latter are more powerful and keep the streams either constantly or intermittently at work in valley formation. REFERENCE BOOKS. From the text books of geology, previously referred to, one may obtain additional information upon some parts of the subject treated in this chapter. Important articles on the Development of Rivers will be found in the National Geographic Magazine, Washington, D.C., Volumes I. and II. These are from the pen of Professor W. M. Davis. This magazine is a very valu- able one for teachers of geography. Six volumes have been published, the price to the public being $2.00 each for the first two, and $3.00 for the others. To members, they are sold at a lower rate ; and each member received the Magazine. The cost of membership is $2.00 a year, and any one inter- ested in geography is eligible. The remarkable Colorado Canon is fully described by Dutton in Mono- graph II (with Atlas), IT. S. Geological Survey, Washington, 1885. $10.00. The Atlas is splendidly illustrated. For a shorter account, see Second Annual Report IT. S. Geological Survey, 1 Washington, 1882. Powell's "Exploration of the Colorado River of the West," Washington, 1872, now unfortunately out of print, but still on sale at the second-hand stores, is a most fascinating description of travel, as well as a scientific description of this wonderful region. The same author has published upon the same subject "Canyons of the Colorado." Flood & Vincent, Meadville, Penn., 1895. 4to. $10.00. Huxley. - Physiography. Macmillan & Co., New York, 1891. 12mo. $1.80. (A study in physical geography, in which the Thames Basin is taken as the central topic.) 1 Many of the annual reports of this survey may be obtained by the aid of congressmen, though the earlier ones are now exhausted. They contain much valuable material, written in a sufficiently popular manner for the non-geological reader. Reference is made to many of these articles in the later chapters. CHAPTER XVI. DELTAS, FLOODPLAINS, WATERFALLS, AND LAKES. Deltas. — Nearly all streams carry sediment ; and if for any reason the velocity is suddenly checked, some of this material must be deposited. The most favorable situation for the deposit of river sediment, is where the stream enters another body of water. In such places the material is deposited near the stream mouth, and a delta often results. Where streams come from steep mountain valleys upon relatively level plateaus, the sudden change in slope causes the deposit of some of the sediment at the mountain base. This material is dropped most abundantly near the moun- tain, and the rapidity of deposit decreases away from it. As a result of this, a fan-shaped deposit is produced, to which the names alluvial fan, fan delta, or cone delta are commonly given (Fig. 155). These deposits are very com- mon in arid regions ; and although relatively rare elsewhere, when they occur in moist countries, they are usually natter and less distinct. The apex of the fan extends up the stream toward the mountain base. The formation of true lake or ocean deltas, depends upon a variety of circumstances. There are many large streams which are not forming deltas in the sea. In some cases this is due to the fact that the streams carry very little sediment; in other cases, the sediment brought to the sea is mostly car- ried away by currents. In general, delta formation is not favored in open seas, where tidal currents and waves are 286 286 PHYSICAL GEOGRAPHY. present to distribute the sediment over the ocean bottom. Nearly tideless seas, such as the Gulf of Mexico (Fig. 153), or the Mediterranean, are particularly liable to have deltas opposite the stream mouths. APPROACHES'TO THE MISSISSIPPI RIVER Sj>.Armi,JEE Fig. 153. Delta of the Mississippi. If the ocean bottom is sinking, the rate of deposit of mate- rial opposite the river mouth is often not sufficiently rapid to build a delta above the level of the sea ; and therefore for the rapid development of deltas, the ocean bottom must either remain in its position, or else be slowly rising. On many seacoasts, one or the other of the conditions which favor DELTAS, FLOODPLAINS, WATERFALLS, ETC. 287 delta formation is absent, and this is the reason why deltas in the sea are so uncommon. In many cases the submer- gence of the coast has transformed the river mouths to estuaries, instead of admitting of the formation of deltas. By far the most favorable conditions for the formation of deltas are found in lakes. Here there are no tides, waves are only moderate in effectiveness, and the depth is compara- tively shallow and usually not increased by subsidence of the bottom. The lake water acts as a filter, removing all the sediment which streams bring, and the greater part of this is deposited, almost immediately opposite the mouth of the tributary. With these very favorable conditions, in nearly every lake deltas occur opposite the mouths of most of the streams ; and in some cases, by the growth of two deltas from opposite sides, lakes are divided into two parts, as at Interlaken in Switzerland. Over large deltas, the streams flow in uncertain course, sometimes changing their channel from one side of the delta to the other, as is so frequently done on the delta of the Yellow River of China. In this way much destruction of life is accomplished. Over the nearly level delta, the main stream divides and often subdivides, entering the sea through a number of branches, which may be called distributaries, in distinction from the tributaries, which bring water to the stream, while these distribute the river water to the sea (Fig. 153). As a result of this branching of the streams, and the changes in river channel, in course of time all parts of the delta are traversed by sediment-bringing water ; and in this way the delta front is made to advance into the sea, while the delta itself is built up above the sea level (Fig. 154) . In the course of the growth of the delta, the advance is often irregular, and arms of the sea may be enclosed in the form of lakes (Fig. 153). The form of the delta is 288 PHYSICAL GEOGRAPHY. roughly triangular, or like the Greek letter Delta (A), whence its name. This is really a partial though somewhat distorted cone, not unlike the fan delta itself (Fig. 155). Floodplains. — Rivers are very often given more load than they are able to carry, and of necessity they are obliged to deposit some. The material is sometimes deposited in the form of bars in the stream channel, or at other times it is spread over the valley at one side of the channel, particularly when the stream has quantities of sedi- ment during flood times. In this way, by laying aside parts of the sediment load, the stream is forming floodplains. ZZZ - KEVB /"" ^ BOCA GRANDER WOMAN KEY»- ,■■' MAN KEY 1 ' S.D.Strm... 2T.T. TTest Ch*« ntX Fig. 208. Atoll-like keys on the Florida coast. 342 PHYSICAL GEOGRAPHY. at the southern end of Florida (Fig. 208). In the mid- ocean, particularly in the South Pacific, the coral growth forms ring -like islands, which are known as atolls (Fig. 209). Sometimes these are nearly perfect rings, enclosing an area of water which is connected with the sea by a small opening. The atoll rises above the level of the sea to a height suffi- cient for the growth of trees, and many of these islands are inhabited by man. The reason for their elevation above the sea is the washing action of the waves, combined with the Fig. 209. An atoll in the South Pacific. blowing of the wind, which drifts the coral sand into mounds. On all of these reefs, corals are still living and growing where exposed to the action of the waves and currents which are bringing food to them. It is found that the coral reefs are better developed on coasts which are exposed to the oceanic currents of tropical origin. As is so well illustrated in the Bermuda Islands, which are in latitude 32° N., corals may be developed where these currents extend their warmth into latitudes well beyond the tropics. ■ THE COAST LINE. 343 The cause for atolls is at present in dispute, and it does not seem desirable to consider the question as to which ex- planation is correct. The one which has been before us for the longest time (having been proposed by Darwin), and is accepted by many geologists, is that the atolls are nothing more than reefs which once surrounded volcanoes that have since disappeared by submergence (Fig. 210). As the cone sank beneath the water, the corals built the reef higher and higher, so that even after the cone had entirely disap- peared, its position was indicated by the ring-like reef. Cer- tainly this seems to be a true explanation for some atoll reefs ; but for others, another explanation is very likely necessary. The great barrier and fringing reefs are merely formed by the growth of the coral along or near the coast line. Changes in Coast Form. — With change in the conditions, a coast may assume entirely different characteristics. If, for instance, the clear waters of some coral coasts are for any reason changed to muddy water, coral life is driven out, and the muddy or sandy shore takes its place. If land is ele- vated, or if it is depressed, the form of the coast is very greatly changed. The agents of denudation are always at work tending to alter the coast form. Therefore the shore line which we know at present, is merely a temporary fea- ture, merely the stage which has been reached at the present time; and it is far from the condition which has existed in Fig. 210. Diagram to illustrate one explanation of the origin of atolls. V, volcanic cone; aa, bb, cc, successive levels of the sea ; del, ee, and ii showing corresponding condition of the reef, finally producing the atoll ii when the volcano was entirely submerged. 344 PHYSICAL GEOGRAPHY. the past, and probably from the condition which will exist in the future. In imagination we are able to look back to the time when the eastern coast of the United States had not its present irregularity; and by geological evidence we are also certain that but a short time ago Florida was not present as a peninsula. The delta of the Mississippi is a growth of very recent date ; and preceding its formation an estuary extended up the valley of the Mississippi, at least as far as Arkansas. Our knowledge of the geology of the coast line is not suf- ficiently detailed to allow us to study all the changes that are going on ; but any one who dwells by the coast, will be able to see that there are some changes now in progress. A visit to the seacoast in time of storm, or indeed to the lake shore, will convince any one that there are changes in progress, which, as a result of the repetition of this action through scores of years, must produce perceptible changes. Islands. — There is a very great variation in the size of oceanic islands, in the distance from the shore, in the form, and in origin. It is quite customary to speak of two classes of islands : oceanic and continental, the oceanic being those which occur far from the land. These oceanic islands are generally of three classes : (1) those that are formed by volcanoes, (2) those that are produced by the folding of mountains, and (3) the mid-ocean coral reefs. Generally they are small, and they often occur in chains, as if they represented tops of mountain peaks along some ridge that is partly beneath the ocean. In some instances, soundings have shown that this is actually the case. Near the coasts of continents we have the same kinds of islands. The Japanese archipelago is apparently a moun- tain chain which is now in process of being formed. In the Mediterranean, among the West Indies, and elsewhere, there THE COAST LINE. 345 are many instances of volcanic peaks which form islands not far from the coast ; but probably nine out of ten of the islands of the world have resulted from causes other than these. Most of the islands are derived either from the submergence of the land (Fig. 211), or from the building up m. /^fj Same's Pt Moore's neJk' Pt. ^\V^ ^'LANE'S I Parker's r/Wm , fly"--*. - .>-' v ~._W ! . '^v since the strata vary in character, the rate of mo- tion differs, and conse- quently the spherical form is distorted (Fig. 244). The point on the earth's surface directly above the focus is known as the epicentrum, and this is the place where the shock first reaches the surface. The waves come from the "earth at equal distances from this point, and on all sides of it. If the rock texture were uniform, the shock would be felt at the same time at all points whose distance from the M, 7 Fig. 244. Earthquake waves of Charleston earthquake, showing effect of folded rocks of Appala- chians. VOLCANOES, EARTHQUAKES, AND GEYSERS. 385 epicentrum is the same. The most violent part of the earthquake is in the immediate vicinity of the center, while it decreases quite uniformly away from this (Fig. 245). Even during violent earthquakes, the amount of movement of the rocks is not very great; but the effects of the jar are often very disastrous. Parts of cliffs are thrown down, landslides produced, houses destroyed (Fig. 246), trees overturned, and general destruction caused. The destruc- tion of human life is greatly increased by the fact that houses are readily thrown down by earthquake waves. When earthquake shocks occur in the ocean, great sea waves are often produced, and these, sweeping upon the coasts, devastate the lowlands. Any jar in the earth will produce an earth- quake. During the ex- plosion of dynamite at Hell Gate, near New York, a few years ago, a shock was started which was measured as far away as Washington on the one side, and Boston on the other. The great earthquake shocks are evidently connected either with volcanic eruptions or with faulting in the rocks. The violent eruption of a volcano, like that of Krakatoa, sends a series of earthquake waves through the rocks ; and in 2c Fig. 245. Earthquake shock in Japan. 386 PHYSICAL GEOGRAPHY. the time immediately preceding volcanic eruptions, earth- quake shocks are very common, being apparently the result of unsuccessful efforts of the lava to force its way to the surface. As the rocks are broken apart, each step in its prog- ress toward the surface produces a jar. When mountains Fig. 246. Effect of earthquake in Japan, 1891. are being formed, rocks are often broken and faulted; and as they break and slip, waves are started which produce earth- quake shocks (Fig. 247). Many of the most violent earth- quakes of the world appear to be attributable to this cause. Geysers and Hot Springs. — Underground water, after a passage through the earth, often finds its way back to the surface in a heated condition. In such cases hot springs are produced, and these are generally mineral springs ; for VOLCANOES, EARTHQUAKES, AND GEYSERS. 387 hot water, in passing through the crust, finds many mineral substances which it can dissolve (Fig. 105). Hot springs are quite commonly found in association with volcanoes; and it is very probable that the heat of the water is in most cases furnished by some supply connected with volcanic ac- Breaking of the earth along the fault line which caused the Japanese earthquake shock of 1891. fcion. Even after volcanoes have ceased activity, hot springs may remain in the neighborhood. Sometimes hot springs have the peculiar habit of bursting forth into eruptions of steam and hot water, and then a geyser s produced (Fig. 249). A geyser may be defined as a hot pring which has a habit of intermittent eruption. One of ;he geysers of the Yellowstone Park region, the Artemesia, rvas for a long time known as a hot spring, and then suddenly 388 PHYSICAL GEOGRAPHY began eruptions like the other geysers of the Park. While hot springs are very widely distributed, geysers are quite uncommon. There are only three places in the world where they are features of importance, one being the Yellowstone Park, the second in Iceland, and the third in New Zealand. In all of these cases, the geysers are bringing to the surface Fig. 248. Crater of Oblong Geyser, Yellowstone Park. large quantities of chemically dissolved mineral matter ; and in the Yellowstone region, craters are built around the geyser (Fig. 248). There is much difference in the time between the eruptions of geysers, some being in eruption every few hours, others hav- ing very irregular periods of action. Hot water slowly boils in the tube, then it overflows gently, and suddenly, with very VOLCANOES, EARTHQUAKES, AND GEYSEBS. 389 little warning, bursts forth into eruption, when the air is filled with a great column of hot water and steam, which in the case of the larger gey- ser usually rises to a height of 100 or 200 feet (Fig. 249). REFERENCE BOOKS. VOLCANOES. Dana. — Characteristics of Volcanoes. Dodd, Mead & Co., New York, 1891. 8vo. $ 5.00. (A very complete and valuable discussion of the sub- ject.) Hull. — Volcanoes: Past and Present. Scribner, New York (Contemporary Science Se- ries), 1892. 12mo. $1.25. Judd. — Volcanoes. Appleton & Co., New York (Inter- national Scientific Series), 1881. 12mo. $2.00. For Eruption of Krakatoa, see The Eruption of Krakatoa (edited by Symons). Trubner & Co., London, 1888. 4to. 30s. Eor Hawaiian Volcanoes, see Button, Fourth. Annual Report, U. S. Geological Survey, Washington, 1884. earthquakes. Milne. — Earthquakes. Appleton, New York, 1891 (International Scien- tific Series). 12mo. $1.75. For a description of the Charleston Earthquake of 1886, see Dutton, Ninth Annual Report, U. S. Geological Survey, Washington, 1889. * 1 Nearly all of these articles in the U. S. Geological Survey Reports are well illustrated ; and since many of them are readily obtained free of cost, they should be widely used. Fig. 249. Old Faithful Geyser, Yellowstone Park. CHAPTER XXL THE TOPOGRAPHY OF THE LAND. General Statement. — Land forms are of two kinds: (1) those that have been built by some agency and (2) those that have resulted from the combined action of building and carving. By far the greater number of land forms are of the last origin, and there are few that have resulted exclu- sively from constructive action. There are two sets of forces working upon the earth in an effort to modify its surface : the one internal, which tends to make the surface diverse, the other mainly external and tending to level. As a result of the action of the former, the earth's surface is thrown into a series of waves, great and small, and some of these are even now in process of formation. If nothing had interfered, these earth waves would have made the surface very irregular, and the mountain chains would have risen to vastly greater heights, and often with much steeper slopes than we really find. In opposition to this force there are the agents of denudation, which derive their power chiefly from causes outside of the earth itself, and are mainly manifestations of solar energy, combined with complex causes, some of which are described in the first chapters of the book. By removing materials from the higher parts and spreading them over the lower areas, the agents of denudation are engaged in the work of leveling. In the course of this, it is often necessary, or most easy, to temporarily increase the irregularities, as is done by the Col- orado in its work of valley formation in the great Arizona- 390 THE TOPOGRAPHY OF THE LAND. 391 Utah plateau (Plate 28). The present land form is the result of the complex interaction of these forces, and it is still in process of change. Plate 28. Brink of Marble Canon, Colorado River. 392 PHYSICAL GEOGRAPHY. Some parts of the earth are now being built up, others are being worn down by one cause or another. As a result of this, the surface of the earth presents most complex features ; but if we look at the causes and influences that are at work, it becomes a much more simple task to account for them. These may be briefly summed as follows : The crust of the earth is in movement, in some places upward, in others down- ward, here by broad uplift or downsinking, there by the more local and intense upfolding or downfolding which accompa- nies mountain growth. Some regions are therefore nat- urally high, others low ; some are mountains, others plains, and still others plateaus. Denudation is everywhere at work ; and since the conditions are variable, the results are quite different. Its action upon plains differs from that upon plateaus ; and in regions of horizontal strata, its effect is quite different from that produced when the rock position or attitude is complex. Not merely does the difference in rock position produce a perceptible effect, but the variations in resistance to weathering and erosion are of most funda- mental importance. These agents of denudation are also engaged in the work of construction ; for the materials taken from one place find rest in another, and often the two processes of tearing down and building up overlap. Constructive Land Forms : By Internal Forces. — It is to be borne in mind that in nearly every part of the land, no matter what the origin of the surface features, there is evi- dence of the action of the destructive denudation ; and there- fore in this section we deal merely with the skeleton, not with the perfected form. The larger diversities of the earth's surface, although greatly sculptured, owe their main features to the action of contraction of the earth's interior. 1 1 Accepting the contractional hypothesis, as we may fairly do, for a work- ing hypothesis. THE TOPOGTtAPHY OF THE LAND. 393 Thus the continents and mountains, considered without ref- erence to details, are true constructional forms, being built by the folding of the roots. In the same way, many of the plateaus, such as those which lie at the base of the Rocky Mountains, are due to the elevation of a part of the earth's crust; and many plains have also been given their present condition by land movements. This is the case with the coastal plain which forms the eastern margin of the country south of New York. This represents an old, nearly level sea bottom, very recently raised into the condition of land ; and another elevation of this part of the continent to a height of 600 feet, would add a plain which in some places would be more than 100 miles in width. The volcano is also a constructional form dependent upon the heated condition of the rocks beneath the crust (Figs. 234-241). It is built up and is formed into a typical topo- graphic feature ; but under different circumstances this form varies somewhat. The cone results from the piling up of materials derived from beneath the crust, and accumulated into a conical heap around the place of ejection. Partly because of denudation, and partly because of the explosive action of some eruptions, the cones are much less perfect than they normally tend to be. By Agents of Denudation. — Some topographic features are produced directly by the building action of the agents of denudation ; but these are usually of minor importance. As a cliff crumbles away, talus deposits accumulate at its base, and these often produce great sweeping slopes at the foot of steeply rising mountains (Figs. 118 and 219). Some- times this curve unites with that caused by denudation, and a double curve is then produced. The wind often blows sand into mounds, and these may cover great areas, com- pletely burying the underlying topography. These are par- 394 PHYSICAL GEOGRAPHY. ticularly liable to be formed near seacoasts (Fig. 120); but sand-dune areas are also common in arid regions. When filled with sediment, and transformed to swamps or plains (Fig. 172), constructional forms of monotonous regu- larity are often built in the site of lakes. The same condition results when lakes are displaced by other causes, as is the case when they evaporate ; and many of the great alkaline plains or fiats of the Great Basin are old lake bottoms (Fig. 150). The disappearance of a glacial lake often leaves an exten- sive plain, as is so well illustrated in the great wheat plains of the valley of the Red River of the North (Fig. 215). In these cases the shore lines are also left, and these topo- graphic forms, though of minor importance, are often strik- ing features in the landscape (Fig. 170). Deltas (Fig. 154), bars (Fig. 213), and spits (Fig. 196) are built up in the lake waters ; and upon the disappearance of the lake these are left upon the valley sides (Fig. 170). Rivers also build deposits, the most notable being deltas and floodplains ; but in somes cases, terraced valley sides result from the constructive action of the river floods. One of the most important causes for the details of the topog- raphy in northern United States, is found in the recent glaciation ; and much of this topographic variety is due to the building action of the ice. With the debris that it car- ried, the glacier formed great plains, either by direct depo- sition from the ice, or in a secondary way through the intervention of water produced by ice melting. Much of the prairie country of the Central States owes its present levelness to these causes. In other places, hills of peculiar and irregu- lar form were built by the ice. To the majority of people who live in the glaciated belt, these hills of gravel and unstrati- fied till must be familiar features; and in the morainal regions they are strikingly developed. (See Chapter XVII.) THE TOPOGRAPHY OF THE LAND. 395 The ocean is the great receiving ground for the waste of the land ; and for the most part the debris is spread quite evenly over the bottom, producing a plain, which in some cases is partly raised above the sea. But along the shore line, the constructive action of the ocean is producing many irregularities, though here, as elsewhere, the actions of tear- ing down and building up ar so intimately associated that it is often difficult to draw th line between them. Still, the beaches (Figs. 200 and 201), the bars, the long sandy islands (Fig. 194), and other similar coastal features, are often mainly the result of the action of the waves and cur- rents in building up materials furnished by various means. When, for any reason, the level of the sea is changed in its relation to the land, these shore-line formations are either submerged, or, if the land rises, are left as ancient shore lines, which then resemble those remaining when lakes dis- appear. By Animal and Plant Life. — In various ways, both animals and plants are engaged in constructing land forms. The salt marsh of the seashore (Fig. 206), and the swamps of the land, in part represent this action ; but the most notable action of life in this respect is that of the corals, which are building reefs (Fig. 207). It is true that the corals do not build the reefs above the sea level ; but a slight elevation of the bottom has oftei raised them to the air. Also, the action of the waves may pil the coral fragments above the reach of ordinary waters ■ and wind action, by blowing the coral fragments into dune-like hills, then causes them to rise still higher. By these constructive processes combined, many islands are built in the sea (Figs. 208 and 209). Effect of Rock Structure upon Topography. — The land forms constructed in the ways above described, are subjected to attacks from all the agents of denudation ; and as a result 396 PHYSICAL GEOGRAPHY. of this, the land surface presents many diversities. Under uniform conditions, denudation affects rocks differently according to (1) their elevation, (2) their position, and (3) their structural features. Moreover, the intensity of denudation varies ; and as a result of these facts, land forms differ from place to place. It ; s impossible here to enter into this subject in any considerable detail ; but some of the main principles may be briefly stated. Much depends upon the ease with which materials may be Fig. 250. View in Brazil, showing hard layer etched into relief by the removal of the less resistant enclosing rocks. removed (Fig. 250). In high mountains, where the grade is steep and d udation intense, the etching of the rocks is very sharply done (Fig". 224, 225, 230, and 261); and hence, in such places, we have J '\e characteristic ruggedness of high mountains ; but when the mountains are low, even though the difference in rock hardness may be great, the outlines are less angular and more rounded and flowing. In this connection one may contrast the Alps (Figs. 143, 144, and 220) with the Highlands of Scotland, or the Rockies of THE 10P0GRAFHY OF THE LAND. 397 Plate 29. Navajo Church, Arizona, showing sharpness of denudation in an arid region. Soft clay, capped hy harder rock, in foreground. 398 PHYSICAL GEOGRAPHY. Colorado (Figs. 221 and 223) with the Appalachians or the Adirondacks (Figs. 263 and 264). With conditions of aridity, the soil covering is readily removed from the rocks, so that they are exposed to the air ; and hence, here also, angularity and ruggedness of topography prevail (Figs. 122 and 142 and Plates 28 and 29). Often- times the streams cannot carry the material furnished to them, and instead of trenching the highlands, they flow on the sur- face of a plateau. This is the case with the river Platte. The in- tensity of denuda- tion is therefore of great impor- tance, and this varies with the stage of develop- ment, so that there is an intimate relation between topography and the age of topo- graphic forms. The young valley is a sharply denned feature (Fig. 133), while the mature valley, in which the intensity of erosion has ceased, is rounded under the more widespread, but more moderate action of weathering (Fig. 135). Altitude is an important element in this connection, but it is by no means the only one. Fig. 251. A cliff in the Yosemite. THE TOPOGRAPHY OF THE LAND. 399 Much depends upon the rock structure. Even though it be soft, a rock of uniform texture produces massive effects. The granite of the Yosemite (Figs. 164 and 251) is com- posed of materials uniformly arranged, — hence the bold, regular outlines. Massive beds of limestone produce the same effect ; and among some of the ranges of the Rocky Mountains, where the rock is a thick bed of limestone of quite uniform texture, there are places of great precipitous- Fig. 252. Cliffs in the loess clay of China. ness. Even when the surface is covered with consolidated clay, this uniformity impresses itself upon the topography, as is so well illustrated in the Chinese region (Fig. 252). Upon the seashore these massive rocks are often cut into cliffs, which frequently rise to great heights, as in the case of the chalk cliffs of England. On the other hand, if the rock is in layers, or if for any other reason it is rendered mechanically weak in places, the boldness disappears. On the seacoast, the weakness of the 400 PHYSICAL GEOGRAPHY. rocks is taken advantage of by the waves, and the weak places indicated by an indentation in the coast. Where the rocks are jointed or broken, or where one layer is softer than another, sea caves (Fig. 198), chasms (Figs. 199 and 253), and even small bays, may be I produced. The cliffs are not so high nor so angular as in the massive rocks (Figs. 254 ) and 255) ; for, both by the waves and by weathering, they are caused to crumble and to as- sume a more gentle slope. Since hard strata (meaning those resistant to denudation) are worn down with much less rapidity than soft ones, where these alternations exist in such a position as to be exposed to denudation, there is much ir- regularity introduced. Accord- ing to the attitude of the rocks, there is much variety in the topography. If the strata are horizontal, the hard layers tend to remain ; and between the rivers, there are relatively flat-topped hills, capped by these hard rocks. Their margins are steeply sloping, but the slope decreases where the layers are soft (Figs. 256 and 257). These features of the land are particularly well developed in arid regions, where differences in rock hardness are always etched with greater intensity than in moist coun- tries; and in such places terraces are often produced. These, which have been called terraces of differential degradation, are flat-topped where hard layers exist, while between two Fig. 253. Rafe's Chasm, Cape Ann, Mass wave-worn chasm in granite THE TOPOGBAPHY OF THE LAND. 401 Fig. 254. A rugged coast in massive granite, Cape Ann, Mass. such areas there is a steep ascent. In such a place, in traveling across country, one passes over a series of steps on the land (Fig. 217 and Plate 28). Such topography Fig. 255. A granite coast where the rock is much jointed, Gape Ann, Mass. 2d 402 PHYSICAL GEOGRAPHY. is typical of plateaus, and particularly of those in arid lands. On the seashore, the tendency to produce a step-like coast exists where the horizontal rocks outcrop in cliffs composed of layers of different hard- ness. With gently dipping rocks, very nearly the same kind of topography is produced ; but the flat- topped areas are less dis- tinct. In passing across a country in which the differ- ences in hardness of slightly inclined rocks are well brought out, as in the central part of Texas, the aspect of the country changes entirely, according to the direction pursued. If one Fig. 256. Effect of hard layers (unshaded) in the denudation of nearly horizontal strata. Signal Butte, Texas. An outlying hill protected by a hard cap of horizontal rock. travels at right angles to the dip, he may pass for long dis- tances upon a flat-topped terrace, bounded on one side by a steeply rising face, and on the other by a steeply descending THE TOPOGRAPHY OF THE LAND. 403 slope (Fig. 258). If going in the direction of the dip, one ascends a steeply sloping hill, then passes over a bench to H Fig. 259. H, hard stratum. Fig. 258. Step topography in region of inclined strata. H, H, H, hard layers; S, S, S, soft. another sloping hill, and this may be repeated many times. If the journey is in the oppo- site direction, there are a se- ries of descents with interme- diate terraces. Looking in the direction of the dip, one sees a series of hills, while the view in the opposite direction is over the surface of the plain. The flat areas are determined by hard layers, and the steep slopes are also due to their presence ; for they serve to protect the softer underlying layers from destruction. Where such a series of rocks occurs on the seacoast, the Fig. 260. H, hard stratum ; S, soft. 404 PHYSICAL GEOGRAPHY. form of the coast differs entirely according to the direction of the dip. If the waves beat against a series of rocks dip- ping toward the sea, they produce a gently-sloping shore, whose form and position are determined by a hard layer (Fig. 259). On the other hand, if the dip is away from the sea, the waves beat against a bluff (Fig. 260.) When the strata are inclined at a high angle, the hard Fig. 261. A ridge of hard rock etched into relief by more rapid removal of softer strata. layers tend to stand up above the surrounding country in the form of ridges, while the position of the softer strata is indi- cated by valleys (Figs. 230 and 261). These peculiarities are particularly well illustrated among mountains, where the ridges and peaks are quite commonly the result of the resist- ance of some hard layer which is tilted into the mountain form (Figs. 219, 225, and 230). Many complexities of moun- THE TOPOGRAPHY OF THE LAND. 405 tain topography are the result of this etching of folded rocks which present differences in hardness. This is seen among the Appalachians (Fig. 262), where nearly all of the ridges are made of hard strata, and where they form ridges because they are more resistant than the surrounding rocks. Not merely are there ridges where hard layers exist, but peaks (Fig. 220) are often produced where unusually hard rocks are found ; and very often, where the general rock structure is harder than that of the surrounding regions, these places stand up as more elevated areas. Thus the Adirondacks, the New England area, etc., are high mainly because their rocks are prevailingly hard. When, by land movements, these carved areas are brought beneath the sea, their irregularities impress themselves upon the coast line, as for instance on the coast of Maine, which is a land area partly drowned by the sea (Fig. 211). The hard rocks which formed hills, now exist as promontories, capes, or islands, while the sites of the softer layers are occupied by bays or straits. From this brief statement, it is seen that the causes for topographic irregularities are most complex. They are to be found in a combination of internal and external forces. The land is in movement, and the forces of denudation are at work carving and removing, and often locally build- ing. With variations in altitude, position, and kind of rock, many complex results may be produced. Above all, it must be borne in mind that these changes are now in Fig. 262. Effect of hard layers (unshaded) in the de- nudation of mountains. 406 PHYSICAL GEOGRAPHY. progress ; that the land forms are still changing ; that they have been different in the past ; and that the future will find them different still. Some forms have reached one stage, and some another ; but all are developing along cer- tain lines of a more or less definite nature, notwithstanding the fact that the conditions are complex, and are even undergoing change themselves. Any intelligent study of the earth's surface must be made with these facts clearly in mind. REFERENCE BOOKS. There is no easily accessible book in which the relation between scenery and geology is more clearly shown, than in Geikie's "Scenery op Scot- land." Macmillan & Co., New York. Second edition, 1887. 12mo. $3.50. Powell. — Physiographic Features (Natural Geographic Monographs, Vol. L, No. 2). American Book Co., New York, 1895. 4to. $0.20. (Some suggestive descriptions of the origin of land forms.) CHAPTER XXII. MAN AND NATURE. General Statement. — The relation between man and the physical conditions of the earth's surface is most intimate, although in his present civilized state they are very much less important than in the past. Formerly, even slight bar- riers were almost impassable, while now we cross them with ease. Less than a half -century ago, the journey from the Mississippi to the west coast was of the most dangerous kind, while now, in a few days, we pass over the mountains and plateaus with ease and comfort. While, with the advance of civilization, man is becoming less dependent upon nature, at the same time he is increasing his power to control and modify the surrounding conditions. So the subject of the relation between man and nature naturally divides itself into two parts, (1) the influence of nature upon man, which is of decreasing importance, and (2) the influence of man upon nature, which is all the time increasing. These subjects can be treated only very briefly. Modifying Influence of Man In many small ways man is engaged in the work of modifying the natural conditions of his surroundings. He protects himself from the rigorous climates, and thus makes his existence possible in zones where otherwise he could not dwell. He modifies the forces of nature so that they become his servants. The winds, the rivers, and even the tides are converted into forces which serve him. He confines the river within its banks and pre- 407 408 PHYSICAL GEOGRAPHY. vents the flood ; and he turns the river waters from their course to lead them where he wills. Deserts are transformed to fertile gardens ; swamps are made dry ; the sea is excluded from the marshy lands of the coast lines; 1 and almost every- where we find evidence that man is at work in modifying the surface. The earth is pierced with mining shafts and tun- nels ; new water connections are made by canals across the narrow isthmuses; inland towns are connected with the sea; and seashore towns are made into seaports by the construc- tion of artificial harbors. Notwithstanding the importance of these effects, there is no influence of man more potent than that which he exerts upon the life of animals and plants. Many species are being perpetuated under domestication, and much is being done to- ward their modification. New fruits are constantly being pro- duced, and in this respect the influence of man is very im- portant. Man is doing a great work in distributing animal? and plants over regions which are not properly their homes. Sometimes the effect is beneficial, but very often it is most disastrous. For instance, the rabbit introduced into Australia has become a national pest ; and the English sparrow is com- pletely overrunning this country. Insect pests and diseases are also spread, and these attack not merely man, but also the plants and animals. However, it is in the destruction of life that the most baneful influence of man is noticed. Animals of nearly all kinds, particularly some of the largest, are disappearing before his advance. Several species have been entirely ex- terminated, and some, such as the bison, which was formerly so abundant, have been so reduced in numbers that they are almost exterminated. By the destruction of birds, the num- ber of insects has been increased ; and so both directly and 1 It is estimated that one-tenth of Holland is land reclaimed from the sea. MAN AND NATURE. 409 indirectly the influence of man in this direction has been harmful. Man and the Forest. — Probably the most important single influence of man comes from his habit of destroying the forest (Fig. 263). In many ways the forest covering is important. It protects the soil from being washed away, Fig. 263. A part of the Adirondack forest. (Copyrighted, 1888, hy S. R. Stoddard, Glens Falls, N.Y.) and when it is removed, and the soil turned by the plow, both weathering and the removal of the loose materials are increased. In some places, notably in France, the mountain sides, from which the forests have been stripped, have been transformed to barren wastes of rock because of the re- moval of the soil by the rain. In other places, the soil has 410 PHYSICAL GEOGRAPHY. been so gullied that it is unfit for cultivation. A part of Mississippi lias been transformed to a barren waste of clay, the features of which resemble those of the Bad Lands of South Dakota (Plate 21). The effect of the absence of forests is well illustrated in the arid lands, where the forest covering is absent because of natural climatic conditions. Here every rain gullies the land ; and on the steeply sloping hillsides, the removal of the soil by rain and wind action has exposed the bare rock (Figs. 90 and 121). Fig. 264. Deforesting in the Adirondacks. The forest serves to prevent excessive river floods ; for it protects the snows from rapid melting, and prevents the rain from readily passing away in the streams. The mat of leaves and moss, the forest Utter, serves as a great sponge which holds the water. This is important in many ways : for it makes the stream less liable to violent floods ; it furnishes a constant and rather steady supply, both to springs and streams; and it furnishes moisture to the air. With the removal of the forest covering, the rain and the melting snow MAN AND NATURE. 411 pass rapidly into the rivers, and thence to the sea (Fig. 264). At times, exceptional floods are produced ; and then, when these have passed away, the river rapidly loses in size, until it may perhaps become nearly, if not quite dry (Fig. 124). The greater part of the water passes through the river in a few days. Every person of maturity who has dwelt by the side of a stream heading in a region once forested, but now bared of its tree covering, will bear testimony to the fact Fig. 265. Bare rock exposed to weathering by removal of the forest, Mt. Desert, Me. that streams which were formerly moderate, clear, and per- manent, are now transformed to trickling streams, which at times become raging torrents, clouded with sediment. This influence of man is very disastrous. It not merely causes the removal of soil from the mountains (Fig. 265), but distributes this over the lowlands ; and in some places, farms have been rendered uninhabitable by the deposit of sediment during times of flood. Besides this, the floods themselves are very destructive both to life and property ; 412 PHYSICAL GEOGRAPHY. and, with the removal of the forest covering, they are becom- ing ever more destructive. Mills cannot count upon the same steady water supply that they formerly had ; springs quickly become dry; and there is some reason for believing that the removal of the forest also affects the climate. This latter point has been suspected ; but it has never been proven that the forest makes the rainfall more uniform or greater in quantity. The reasons for suspecting this forest influence are (1), that the damp winds, when coming in con- tact with the cool forests, are made to give up their moisture more readily than elsewhere ; and (2) that by holding the water in the litter beneath the trees, a greater opportunity for evaporation is furnished than when the forest is removed. It is held that, as a result of this, the air is rendered moist and is more liable to give up its moisture. These influences are so important, that one of the needs of the present, is greater care, intelligence, and patriotism in the relation of man to the forest. The conditions need to be carefully studied, destruction ought to be checked so far as possible, and the damage of past destruction should be repaired in every possible case. The state and national governments are in some cases engaged in this work ; but it is possible for nearly every one to do something toward it. Unless something is done, the heritage of the land which we have received, will not be transmitted to our descendants in so good a condition as it is our duty to leave it. Influence of Nature upon Man It is quite impossible at present to estimate the effect of nature upon man ; for in most respects we have risen above its immediate and most important modifying influences. Without serious difficulty, we cross mountains and continents, rivers, lakes, and even oceans ; and in a few weeks we may pass around the entire world. Every generation sees an increase in the independ- MAN AND NATURE. 413 ence between man and nature, and the completeness of the conquest of the latter. This has not always been so, and many of man's most marked characteristics have had their origin in, or have been impressed upon him by his environment. Even now we find a marked difference between the miner, the ranchman, and the farmer ; and, except in the most general way, the effect of climate upon man's condition cannot even be estimated. Both extremes of heat and cold introduce habits of mind and body quite the reverse from the lively mental and physical activity of the inhabitants of the temperate zone. The in- habitant of the Arctic loses vitality because of the unequal struggle ; and where no severe struggle for existence is necessary, the enervating influence of the tropical sun also decreases vitality. Under the bracing air of the temperate latitudes, and with the necessity for preparation for the win- ter, man's physical and mental powers have been improved; and this is probably the most potent reason for the very striking fact, that the most important development of the race has taken place in these regions ; and why to-day, nearly every nation of marked inrportance is situated within the temperate belt, and mostly near the arctic limit of it. If we glance back to the time when man was less inde- pendent of nature, when his railway trains were not present to transport him across river and mountain, nor his steam- ships ready to bear him across oceans, we find a very close relation between the life of men and nations and the sur- rounding physical conditions. When a people migrated to a new land, they often found conditions favorable to a rapid development ; and if they were sufficiently enclosed and isolated for protection from invasion, they often developed to a high state. However, in time the very isolation caused degeneration ; and we see this illustrated in the history of 414 PHYSICAL GEOGRAPHY. such people as the Chinese and the Egyptians. Because of the climatic conditions under which they lived, some' of the early people became nomadic, others developed into agri- cultural nations, and still others into seafaring races. With the growth of commerce, the most rapid progress occurred in the nations most favorably situated for its devel- opment. Thus Italy, nearly isolated from other countries, became an important center of commerce. Fresh blood and new ideas were constantly introduced, and gradually the power of the nation increased and extended, until decay came mainly as a result of the rapid development, and the nation crumbled because of its very success. In ancient Greece we find another instance of the influence of surrounding condi- tions. The very rugged topography favored independence : for even in small areas, different states could exist independ- ently ; but because of their very smallness, they were com- pelled to unite against common foes. The Mediterranean was the seat of early development; and this was made possible by reason of the short distances between countries, the possibility of navigation in the enclosed sea, and the interchange of materials and ideas. Here the people learned lessons in navigation which made the explora- tion of the Atlantic less hazardous ; and then, by land and by sea, the peoples from the shores of the Mediterranean taught lessons to the more northern races, which, when well learned, made their ultimate success possible ; and then the students themselves became leaders. The shores of the Mediterranean were the great training schools, in which were learned most of the fundamental ideas upon which the progress of the human race has depended ; and even now its influence is felt most markedly in all the nations of the world. Perhaps there are no better illustrations of the influence of surroundings upon the development of nations, than those MAN AND NATURE. 415 furnished by Scandinavia and England. The roving North- men, born on a rocky coast, which was deeply indented with fjords, made the sea a second home. Instead of farming, they fished ; instead of remaining to develop their inhospitable coast, they roved the seas and invaded their neighbors. With that hardiness born of the sea, they roamed not only along the European shore, but sought and discovered new lands in the west. They not only learned much themselves, but taught much to others ; and the lessons that they learned and imparted were mainly due to their neighbor- hood to the sea. In England, we find a most remarkable illustration of the influence of environment. The climate gave vigor to the people ; and the mixture of races, that had come in earlier days, made a nation of men with great mental and physical power. The mineral wealth did much to make the subse- quent development possible, for it became sought after by many nations. Because of the insular condition, this store of wealth was protected without great difficulty ; and yet the islands were readily visited for purposes of friendly com- merce, and the stores of wealth were distributed over the world to the profit of the people of the islands. A com- merce was readily developed ; and largely upon the basis of this, England became what she is to-day, — ■ the great naval power of the world, and the possessor of colonies in every part of the earth. It never can be told how important an event it was in the development of nations, when, in some prehistoric time, the sea first passed through the English Channel, and separated the British Isles from the mainland. With land connection, the history of Europe and the world might have been quite different. When we look at the maps of Europe and America, two differences of a most striking nature attract our attention. 416 PHYSICAL GEOGRAPHY. The one is the extreme irregularity of the European coast line, the other the great number of nations in that land. The latter fact depends upon several causes. The very irregularity of the coast, and the great diversity of the topography, have made possible the development of distinct nations. As the race was progressing, mountain barriers, and even rivers, served as boundary lines between separate tribes ; and some of these are preserved to this day. We find Switzerland completely enclosed between other nations, because no ancient tribes could drive these people from their mountain fortress. To fully appreciate the importance of these influences, one needs but examine a physical map of Europe, and notice how the mountains and the seas almost universally serve as boundaries, and how upon every penin- sula, there is one, or more, independent nation. This is not so in America, partly because the conditions are not so diverse, but chiefly because the settlement of America was made by races which had already developed. With the development of knowledge and power, there came an era of exploration, in the course of which America was discovered. Even this discovery depended upon peculiar physical conditions ; for had Columbus undertaken to make his voyage either to the north or south of the trade-wind belt, the chances are that he would never have succeeded. With the favorable trades furnishing fair winds, the journey was a relatively easy one. The explorers and settlers found the American land occu- pied by nomadic races, whose power of resistance to invasion was not equal to the skill of the invaders ; and with the discovery of America there began a new era. Navigation increased ; and Spain, who had learned her lesson from Italy, and who was important in maritime affairs because of her extensive coast line, became a powerful nation. As in Italy, MAN AND NATURE. 417 success caused almost utter collapse, and Spain lost more than she gained. In America, the invigorating climate, the necessity of work, and the great possibilities, developed a race which has become renowned for its vigor and energy. At first the Appalachian barrier, with its almost impassable forests, prevented entrance to the central regions. Therefore, of necessity, settlements were made close by the coast ; and it is said that in 1700, one could go by stage from Portland, Maine, to Virginia, spend- ing every night in a good-sized village, while to the west there was an impassable wilderness traversable only on foot, along the Indian trails. The large waterways leading into the interior were guarded, the Mississippi and the St. Lawrence by other nations, and the Mohawk by a powerful Indian population. This forest barrier caused a concentration of population, upon which much of the success of our later development has dependedo It determined the location of most of the great centers of population ; and it protected the English until their strength had sufficiently increased to admit of pushing into the western region, and displacing the unfor- tunate savage occupants. The success of the Revolution also in great measure depended upon the concentration of pop- ulation thus induced. Had the people been less connected, they could not have cooperated so well as they did. When, finally, a definite roadway was established across the Appalachians, which was first done over Cumberland Gap in Tennessee, the most difficult step in the western progress was taken. The great treeless prairies were then reached, and upon these agriculture was easily pursued, while further progress was not difficult ; and hence the Mississippi valley became speedily developed. When once the way was found, other openings were soon made across the forest barrier. 2 E 418 PHYSICAL GEOGRAPHY. Then came the discovery of the wonderful mineral wealth of the west ; and the eagerness to obtain some of these stores from the bosom of the earth, caused an almost magical de- velopment of this great realm. Cities sprang up among the mountains, farms were developed on the desert, railroads crossed the mountain chains, states grew out of hitherto un- settled territories ; and, in a quarter of a century, a great region was transformed from an unknown waste, inhabited only by savages, to the most remarkable mineral-producing region of the world. Such progress as this could be made only after man had so far developed as to be able to defy and overcome the most formidable of obstacles. In this country, the influence of topography upon man is seen in many small ways. In New England, particularly in central Massachusetts, the old interior towns were on the hills, which were fortresses where the people were, in a measure, safe from Indian attack ; and even now we find many of these hilltop villages, which at present are scarcely more than relics of a past stage in development. With the devel- opment of the industries, manufacturing determined the posi- tion of the more important interior towns ; and these were naturally placed in the valleys which afforded a good supply of water power. Hilly New England became a manufacturing region ; the states of the level and fertile prairie formed an agricultural district ; the drier plains and plateaus of the west became the seat of the cattle industry ; and the mountainous region of the far west developed into a mining territory. Many of the larger cities were situated on the seacoast, because here communication and commerce with other countries were pos- sible. Even the sites of these large cities were determined by the form of the coast line ; and everywhere that we may go in the world, we find an almost universal relation between MAN AND NATURE. 419 man's condition and his surroundings. The delta lands are farming districts, the semi-arid plains and plateaus are devoted to cattle raising, etc.; but while man is largely a creature of his environment, he is much less so now than ever before ; and, little by little, he is rising above the necessity of direct dependence upon the surrounding physical conditions. Formerly he was guided by nature, but now, in many respects, he governs and guides nature to suit his needs. REFERENCE BOOKS. Shaler. — Nature and Man in America. Scribner, New York, 1891. 12mo. $1.50. Guyot. — The Earth and Man. [Translated by Felton.J Scribner, New York. Kevised edition, 1893. 12mo. $1.75. Marsh. — The Earth as Modified by Human Action. Scribner, New York, 1885. 8vo. $3.50. CHAPTER XXIII. ECONOMIC PRODUCTS OF THE EARTH. Soil. — The crust of the earth furnishes to man most of the material which he needs for life and comfort. The rocks crumble to form soil, and upon this exist the plants which furnish us directly or indirectly with most of our food supply. In this the trees grow, and all of the animals of the land depend upon the plant life which exists by virtue of this soil covering. This is by far the most important mineral product of the earth, for upon it depends our exist- ence as inhabitants of the land. Building Stones. — Within the earth, as a part of the crust, there are many substances which man finds it possible and profitable to remove for his own use. For instance, there are the building stones, of which we have many kinds. The great masses of molten rock, which have been intruded into the earth's crust from below, and then cooled, and finally reached by denudation, furnish us with great quantities of granite, which is such excellent building stone, both with regard to durability and appearance. Sometimes other forms of igneous rocks are employed for building pur- poses ; and among these we find great variety both in color and texture. Granite is imitated among the metamorphic rocks, where as a result of the process of alteration, a structure closely resembling that of granite is sometimes introduced into the gneissic layers. Indeed, many gneisses are sold as granites, 420 ECONOMIC PRODUCTS OF THE EARTH. 421 and their resemblance is often so close that one can tell the difference only by a slight banding which characterizes gneisses, but is not usually present in granites. There are other metamorphic building stones, chiefly slate and marble. Slate represents a clay rock formed as a deposit in water, and then subjected to heat and pressure, so that its peculiar cleavage is introduced. Marble is the metamor- phosed product of limestone, in which the carbonate of lime has in some cases been transformed to crystals of calcite, causing the white sugary marble, such as that found in Ver- mont. In other cases no crystals are produced, but a re- markable and often very beautiful banding is introduced. The causes for this metamorphism are usually a combination of heat, pressure, and motion during the folding of the rocks. The sedimentary rocks themselves also furnish us with much building stone, chiefly in the form of sandstone and limestone. Among these there is great variety, both as re- gards texture and color ; and this class of building stone is extremely common. Indeed, these rocks are so abundant that only the best can be extensively used; and in many places a stone is quarried for home use, but is never trans- ported far beyond the quarry. Few stones, and these mainly ornamental, will pay for transportation to great distances, for there is an abundance of stone for ordinary purposes, and nearly every place has its quarry. From the unconsolidated clays and sands, we obtain much material for building purposes. The sand for plaster, the clay materials for some cements, and the clay for bricks, are among the most important of building materials, and their sources are varied. Some are decayed rocks, others ocean deposits, others have been formed by rivers or lakes, and many, particularly in northern United States, have been brought to their present position by glacial action. 422 PHYSICAL GEOGRAPHY. Economic Deposits of Sedimentary Origin. — Aside from the sedimentary building stones, and some of the ores, the crust of the earth contains numerous valuable deposits formed in water. Some of the sandy rocks are sufficiently rough to be used for grinding purposes ; and the tiny shells of silica, which are left in fresh-water swamps and ponds by certain low forms of animals and plants (Infusoria and Diatoms), furnish a white polishing powder. When lakes have their outlets cut off, and evaporation exceeds the supply of water, they gradually become salt; and finally they may become so concentrated that some is deposited in the bottom of the lake, and then there is formed a layer of rock salt. These layers may be buried beneath other strata, and at some later time be discovered as a salt mine. In the Great Basin there are many beds of this kind, now exposed at the surface, where in some recent times a salt lake has completely dried up ; and the ranchmen visit these beds with wagons, and shovel up from the surface all the salt that they need. At times there are other materials deposited with this precipitated salt. In this way such substances as bromides, borax, natural soda, and even gypsum, which is used as a basis for plaster of paris, are deposited in layers. Left to itself, the soil furnishes the plants as much food as they require ; but when man interferes and tries to draw more than this from the soil, in the course of time he ex- hausts much of the supply of plant food, and it is then necessary either to abandon the land until it can recover, or else to artificially supply the needed substances. For this reason fertilizers of one kind or another are added. Sometimes the fertilizer is only a limestone, or it may be a marly clay in which there are many fossil shells, or it may be one of the natural phosphates. Phosphatic materials are ECONOMIC PRODUCTS OF THE EARTH. 423 among the substances needed by plants, and phosphate is present in the bones of many mammals. In some places, as for instance in South Carolina, near Charleston, and in many parts of Florida, there are beds of a phosphatic rock which owes its peculiar character to the presence of large numbers of bone fragments. These are great mammalian burial grounds, and man is now drawing upon them with profit. Miscellaneous Substances. — There are many other products of the earth, which though valuable, are of minor importance. Springs containing mineral matter in solution often have medicinal properties, and this ensures a wide sale for these mineral waters. Artesian wells (page 229) are of no little importance. Sulphur is often found near volcanoes ; graph- ite, which occurs in metamorphic rocks, furnishes the black lead for our pencils ; mica, asbestos, etc., are also found in the metamorphic rocks ; valuable mineral paints are usually colored earths due to rock decay ; and to these many other minor products might be added. Coal. — Seams or beds of coal are often found between layers of sandstone and limestone. These enclosing rocks bear evidence of having been formed in water, and the fos- sils which they contain often prove that they were deposited in salt water. Yet the coal is composed of the remains of land plants, and even tree trunks are sometimes found pre- served in the beds. In some cases these fossil tree trunks stand upright, with their roots in the clay beneath, showing that the coal bed is near the place where the plants grew. It is further evident that they were then covered by the sea, and that in this the marine sediment was deposited. Often there are several beds one above another, each prov- ing some such change as this. Much about the origin of coal cannot be considered to be finally settled ; and there are many theories for its origin. 424 PHYSICAL GEOGRAPHY. Since we cannot enter into a discussion of these, it will be necessary to confine ourselves to a statement of what seems to the author to be the most probable explanation. Without doubt, different coal beds have had a very different history, Some represent the drifted fragments of wood that have been deposited in an ancient bay or estuary, and then buried beneath marine deposits. Thus if the Mississippi delta should be consolidated into rock and be elevated, there would be coal seams formed where rafts of logs have been stranded. There also seems to be no doubt that some coal beds are nothing more than swamps which were formed either on shores of lakes, or as the last stage in their disappearance, — in a measure being like peat bogs consolidated to mineral fuel. In the southern part of Florida there are a great num- ber of swamps, and swampy lakes, in which there is a vegeta- ble accumulation several feet in depth. This muck is made almost entirely of plant remains with practically no clay impurities. If this low, swampy land were to be lowered beneath the sea, these beds of vegetable matter would be covered with sediment, and a coal bed would be begun. Later the same conditions might be repeated and another bed be formed, etc. Even at present, some trees (the mangrove, Fig. 205) grow in salt water; and in the early geological ages many others probably had this habit, for the land vegetation of these early times was evolved from marine plants. At this time there were probably great salt-water swamps, in which many of the coal beds were formed. Very likely each of these theories accounts for some of the beds. The coal is a mineralized form of vegetation, produced by a slow change, in the course of which many of the volatile gases have been driven off. There is every gradation from ECONOMIC PRODUCTS OF THE EARTH. 425 wood to peat, from this to lignite or brown coal, then to bituminous, next to anthracite, and finally even to graphite. This does not require great heat, but slow, steady change. The ash of the coal is an impurity, often bits of clay and sand that were deposited with the coal. It was once supposed that coal was formed only at one period in the history of the earth, and this was given the name Carboniferous; but with the exploration of the Cor- dilleras, this has been shown to be a wrong idea. Workable beds of coal were not formed before the Carboniferous time, because in those early ages there was not enough land vege- tation; but ever since this time, coal has been formed where- ever the conditions have been favorable. In the west there are vast quantities of Cretaceous and Tertiary coal. Indeed, in such places as the swamps of Florida, the Dismal Swamp, and the peat bogs of the north, it is quite probable that we are even now witnessing the first stages in coal accumu- lation. Natural Gas and Petroleum. — In some places, wells drilled into the sedimentary rocks reach layers containing either a natural illuminating gas, or petroleum. These products are very useful, the gas for fuel and light near the wells, the oil for the basis of kerosene, and numerous other products. These substances occur rather irregularly ; and wells upon neighboring farms may in the one case find oil, while this is not discovered in the neighboring well. However, certain layers are liable to be oil bearing, while others are never known to contain oil or gas. After awhile both the oil and gas wells gradually decrease in volume, and must finally be abandoned. Therefore the supply is not constantly fur- nished at as rapid a rate as the drain. These substances are the product of a slow natural distil- lation of the organic remains of the rocks; and they quite 426 PHYSICAL GEOGRAPHY. closely resemble substances which we produce artificially. The oil is not markedly different from that produced from fish refuse ; and the gas resembles the illuminating gas caused by distilling coal. The change is a slow one, and in the course of time, enough accumulates in a certain layer to make a gas or oil deposit. In some cases this accumulation is in the same layer in which the distillation took place ,• in others, the substances have migrated into a neighboring layer. Like water, they are able to slowly seep through the rocks ; and in their passage, they may come into a coarse sandy rock, and be imprisoned there by an overlying clay layer which is too impervious for easy passage. In these cases there is a resemblance to the con- ditions favoring artesian wells. This is the common case in the Pennsylvania wells ; but in Indiana these substances occur in a limestone. Ore Deposits. — Some of the metals which occur in the earth possess qualities which make them useful to man; and, as we know, great effort is made to obtain them. Iron, gold, silver, copper, etc., serve us in many ways. In the earth they generally occur in association with other elements, in the form of minerals ; and when mined, these have first to be separated from their companion minerals, with which they are mechanically mixed ; and then it is usually necessary to separate the metal from the elements with which it is chemi- cally combined. Therefore in obtaining these substances from the earth, many complex and often very costly methods are employed. In order that this may be profitable, the ores of the metals must occur in a somewhat concentrated condi- tion, and they must be in a place from which they may be obtained without too great expense. Thus a copper mine that would pay in New England or New York, might not be profitable if situated among some of the nearly inaccessible ECONOMIC PBODUCTS OF THE EARTH 427 mountains of the west. Where the deposit is very rich, it is often profitable to tunnel into the earth to a depth of several thousand feet. Ores occur in the rocks of the crust under many different conditions. Sometimes the ore is a native metal, as is the case with most of the gold which is mined ; but more commonly it is a simple compound of a metal. It would be quite impossible to state in a few words the various ways in which the ores occur, and only one or two of the most common kinds can be described, and these only in a general way. Some of the ores have been deposited in beds, by a process of replacement. That is, some mineral or rock, such as quartz or limestone, has had its place taken by the ore, — this being deposited bit by bit, while the water which car- ried the solution took away an equal amount of the original mineral. This resembles the replacement of wood tissue by silica — a process known as petrefaction. Some ore deposits represent the mere gathering together of substances into bunches, known as concretions, the cause for the accumu- lation being still unsolved. Much more commonly, ores are deposited in some cavity in the earth ; and the most common of these is the fissure which accompanies faulting. In this break in the strata, which often extends to great depths, ore is deposited from solution in water. This underground water is often highly heated, and contains in solution alkaline or acidic substances which give to it great power of dissolving and altering min- erals. By complex chemical reactions, which are not well understood, these ores are deposited in veins, usually in bands, and commonly associated with other minerals which are not of value. Even ores of gold or silver are frequently deposited in this way. 428 PHYSICAL GEOGRAPHY. Another important way in which ores occur, is in surface deposits of sedimentary origin. For instance, when a gold- bearing rock decays, the nearly indestructible gold resists weathering ; and being a heavy substance, as it is being washed down toward the sea, it tends to accumulate on the stream bottom, forming what is known as stream or placer gold deposits. This is the condition in which a great deal of the gold of the world has been found ; and this precious metal occurs in such deposits in the west, in Siberia, Aus- tralia, and many other places. Both tin and platinum are also found in a similar condition. Distribution of Ore Deposits. — The valuable ore deposits which are found in fissures, are not present in all parts of the crust ; but for the most part they are confined to mountainous regions. The Cordilleran region of the west is a most strik- ing illustration of this ; for these mountains form the most remarkable mineral district of the world. While this dis- trict produces only a few of the metals, it is not because the others (such as iron) are absent, but because in that region the conditions are too unfavorable for the extraction and marketing of those which are not very valuable. The reasons for the great importance of the Cordilleras in the production of metals, are mainly two. In the first place, among these mountains there are many faults, and other cavities, in which ore may be deposited. There are also numerous volcanic rocks of recent date, a point of consider- able importance. The heat from these lava intrusions fur- nishes to the underground water a temperature sufficiently high for important action. Probably even at present some of the hot springs of that region receive their heat from buried lava intrusions ; and probably also, mineral deposits are being made in their tubes at a considerable distance from the surface. A second reason why the presence of igneous ECONOMIC PRODUCTS OF THE EARTH. 429 rocks aids in ore formation, is that there is a larger percent- age of metals in these than in others. Therefore water which is percolating through and altering them, finds a greater supply of metals for solution than would be the case if passing through most sedimentary rocks. Mineral Wealth of the United States. — Mainly because of the Cordilleras, the United States is the great mineral country of the world. Of the following metals it produces more than any other country : gold, silver, iron, and copper, which are the most important of metals ; and in the pro- duction of lead, zinc, and mercury, it holds second rank. Its output of coal is greater than that of any other nation excepting Great Britain, while no other country supplies so much petroleum and natural gas. In some of the minor substances it also holds a high rank. Indeed, we produce nearly every important mineral substance found in the earth's crust ; and usually our production is very great. The importance of the mineral industry of this country, is shown by the fact that in 1892 the mineral production was valued at nearly $700,000,000, of which about $300,000,000 came from the metals, — mainly iron, silver, gold, copper, lead, zinc, and mercury. For the most part this represents the crude product ; and in the utilization of this in manu- facturing, there are industries also worth many hundred millions of dollars : so that, directly and indirectly, the min- eral industry of the country is one of the most important. The few facts that follow, will serve to furnish an idea of the distribution of this product. According to the census, the leading mineral state is Pennsylvania, which produces more coal, petroleum, gas, and stone, than any other state. In 1889 the value of its product was $150,000,000. Second in rank is Michigan, which produces most iron and salt, and is the second in the production of copper. Then comes 430 PHYSICAL GEOGRAPHY. Colorado, which leads in the production of silver and lead, and is second in the production of gold ; and Montana fol- lows, leading in the production of copper, and second in the output of silver. The east excels in the production of non- metallic substances, and the west in metals. This astonishing mineral wealth has, in no small degree, been responsible for our development as a nation ; and there are still great undiscovered stores. There seems to be almost no limit to the possibilities in this direction, and our Alaskan territory promises to add to this wealth. Nature has been most prodigal in lavishing her favors upon this country, for she has given us nearly all that man could request : great variety of climatic conditions, an almost infinite variety of topography, a soil wonderfully rich over a great area, a forest covering from which we have been able to draw heavily for over a century, water power for the mills, harbors for the commerce, mineral deposits of marvelous wealth, — these are things which mark our country as one of great possi- bilities, and which have made possible our present prosperity, and upon which we may predict so much for the future. REFERENCE BOOKS. Kemp. — The Ore Deposits of the United States. Scientific Publishing Co., New York, 1893. 8vo. $4.00. Phillips. — Ore Deposits. Macmillan & Co., New York, 1884. 8vo. $7.50. Tarr. — Economic Geology of the United States. Macmillan & Co., New York. Second edition (revised), 1895. 8vo. $3.50. APPENDIX I. METEOROLOGICAL INSTRUMENTS, APPARATUS, AND METHODS. By instruments we are able to measure the temperature, pressure, wind force and direction, rate of evaporation, percentage of moisture in the air, amount or percentage of sunshine, rainfall, and other weather phe- nomena. In order to understand these instruments, it is necessary to handle them just as the meteorological observer does. Mere descrip- tion can serve only to explain the principle upon which they depend. Thermometric Records. — In measuring the temperature, use is made of the principle that certain substances expand when heated and contract when cooled. Ordinarily it is more convenient to employ a liquid, and that best adapted to this purpose is mercury. However, where tem- peratures below the freezing-point of mercury are liable to be experi- enced, alcohol is used. The thermometer is graduated into degrees according to some scale, and different scales are employed, the most common in use being the Fahr- enheit, which is adopted in nearly all English-speaking countries, and is used in this book. The two points of importance in the Fahrenheit scale are the freezing-point, which is placed at 32°, and the boiling- point, which is placed at 212°. In the Centigrade scale, the principle is the same ; but in this case the degrees are larger, the freezing-point being placed at 0°, and the boiling-point at 100°. Therefore, in converting the Fahrenheit to the Centigrade scale, 1° of Centigrade is equal to 1.8° of Fahrenheit, and to this must be added 32°. All are familiar with ther- mometers, and the principle upon which they depend is easily under- stood. Much care is needed in the construction of a good and accurate ther- mometer, and there are some cheap and very inaccurate instruments. This is one reason why the observations of temperature made by different 431 432 PHYSICAL GEOGRAPHY. people may vary so widely, even though made in almost the same location. Another very important reason for this difference is the fact that the thermometer is not always wisely placed. In order to obtain a true measure of the temperature of the air, it is necessary that neither the sun, nor any warm body on the earth, shall influence the air whose tem- perature is to be measured. At meteorological stations, the thermometers are placed in a thermometer shelter, which consists of a frame, open so that the air may pass through it, and yet sufficiently closed to prevent the sun's rays from striking upon the thermometer. This is raised about 10 feet from the surface, and is placed away from buildings. Of late, metallic thermometers have come into use ; and these depend upon the effect of heat and cold on metal strips or springs enclosed within a clock-like case. They are not so accurate as the well-made mercurial thermometers, and their chief value is in obtaining a continu- ous record. The self-recording thermometers, or thermographs, are mostly of this class. As the metal expands or contracts, it causes an index hand to move back and forth over a dial, and upon this index, a pen or pencil may be fixed in such a manner as to press against a sheet of paper. As the temperature rises and falls, the needle is made to move backward and forward, and therefore the pen is also moved over the paper. For the purpose of obtaining a record of the time at which these changes occur, the paper itself is also made to move by means of a clock-work attachment ; and therefore a record of all the temperature changes throughout the day, may be automatically registered. It is often found desirable to have a record of the highest and lowest temperatures of the day made by a mercurial thermometer. For this purpose the maximum and minimum thermometers are used, which, by a special contrivance, record the very highest and lowest temperatures of the day, but do not give any record of the time at which these occurred. The thermometer itself gives us a record of the air temperature, which is very different from the energy which comes from the sun. If the bulb of a thermometer be blackened by black paint or lampblack, and the instrument be placed in the direct rays of the sun, it is found that the temperature rises very much higher than in the case of a thermom- eter in the shade, or even of a natural thermometer exposed to the sun's rays. Such an instrument is known as the black-bulb thermometer. Barometric Records. — The air has weight, and at the sea level this weight, or air pressure, averages approximately 15 pounds on every square inch. The air pressure at any given place is liable to many varia- APPENDIX I. 438 tions, and it is the purpose of the barometer to detect these changes. The principle of the barometer is that a column of air will exactly counter- balance a column of equal weight of any liquid. Thus water in a vacuum will be made to rise to a height of about 32 feet. In other words, it counterbalances the pressure of the air, and the pump is based upon this principle, water being forced into the partial vacuum caused by pumping. We could use a column of water for a barometer just as well as mercury, which is ordinarily used ; but such a barometer, since it would need to be at least 35 feet in height, would be most unwieldy. The mercurial barometer consists of a tube of glass, sealed at one end and partly filled with mercury. Above the column of mer- cury is a pi'actical vacuum, and the. lower part of the tube is immersed in a cistern of mercury. As the air pressure varies, the mercury is caused to rise in the tube, or to descend from it into the cistern ; and when the air is heavy, we speak of a high barometer; when it is relatively light, of a low barometer. In meteorology, these terms have come to be synony- mous with high pressure and low pressure. The tube of the instrument is graduated in inches, and at the sea level the average height of the mercury in the barometer is about 30 inches. The method of reading the barometer, and the use of the vernier scale, can be understood only by handling an instrument. Several forms of barograph are employed to convert the record of the change in pressure into graphic, continuous records. The rising and falling column of mercury may be automatically photographed, or the rise and fall of the column may be recorded by electricity; but most commonly some form of aneroid barometer is employed. The aneroid depends upon the effect of air pressure upon a metallic diaphragm ; and as the index hand moves one way or the other, it carries a pen, which marks the changes upon a sheet of paper revolving on a cylinder, just as in the case of the self-recording thermometer. Measurement of Wind Direction and Force. — To-day the direction of the wind is measured in very nearly the same manner that it has been for centuries. The wind vane is a familiar feature. The force of the wind, or its velocity, may be roughly estimated by any observer. A state- ment that the wind velocity is 40 miles an hour, means that in one hour the wind travels that distance. For accurately measuring this velocity, an instrument known as the anemometer is used. It consists of four cups, fixed Upon a cylinder, which are revolved by the wind at rates depend- ing upon its velocity. The air enters these cups and whirls them about, 2f 434 PHYSICAL GEOGBAPHY. very much as water enters a turbine wheel and causes it to revolve. By means of a series of wheels, each revolution of the anemometer is re- corded, and this may be transmitted by electricity to some place where an automatic record is kept in miles per hour. Measurement of Evaporation. — The measurement of evaporation is made in inches of water evaporated from a surface exposed to the air. Almost any dish can be used, and the scale of inches be marked upon it ; or the measurement may be made with a graduated rule. Since the rate of evaporation varies with the temperature, it is best to attempt to imitate natural conditions as nearly as possible, though this is not ordi- narily done. The best way is to place the evaporating pan in a quiet body of water, allowing it to float on the surface. There are various contrivances for obtaining a continuous record. Measurement of Moisture in the Air. — The measure of the relative humidity is often obtained by the hair hygrometer, which is a bundle of human hair from which the oil has been extracted. As the amount of moisture in the air increases, the hair absorbs more and more, and as it does so, expands; and, since one end is fixed while the other moves freely, this expansion may be made to record itself against a graduated glass scale. The best method is that of the use of the sling psychrometer. This instru- ment consists of two thermometers fixed side by side upon a board. One is an ordinary thermometer, the other has a piece of wet muslin placed around its bulb. The instrument is whirled in the air, and the water evaporates from the wet muslin, the rate of evaporation varying with the humidity of the air. If the air is very dry, evaporation takes place rapidly ; if damp, it proceeds with slowness. Since evaporation produces cold, the temperature of the wet bulb thermometer descends lower than that of the ordinary thermometer. By reading these two records of tem- perature, the relative humidity of the air is readily determined by means of a series of tables which are constructed for and furnished by the Weather Bureau at Washington. The relative humidity is expressed in per cents between 0, which is perfectly dry air (a condition which never occurs), and 100, which is saturated air. From this measure the dew- point may also be determined. Study of Clouds and Sunshine. — Various instruments are used to obtain a record of the amount of sunshine, and these may be found described in the books referred to at the end of this Appendix. Much work of a scientific nature is also being done in the study of clouds, in- APPENDIX I. 435 eluding the measurement of height, the photographing of cloud forms, etc. We cannot devote space to a description of these. Measurement of Rainfall. — By the rain gauge, rainfall is measured in inches, an inch of rainfall being an actual inch of water which has fallen upon the surface. This is a cylinder having a broad, funnel-shaped top, with the outlet to the funnel extending into an inner cylinder. The water falls upon the surface of this funnel, and runs into the inner cylinder; and the proportion of this to the surface of the funnel is as 1 to 10. By this means the actual rainfall is magnified 10 times in the inner cylinder, so that light rainfalls may be readily measured. The snowfall is often measured in the same instrument ; and in order to express the snowfall in inches of rain, as is usually done, the snow that is collected in the cylinder is melted. About one inch of rain is equal to 10 inches of snow; but in this there is much variation, for some snows are composed of very compact crystals, while others are light. In some cases the depth of the snow is measured and divided by 10, in order to be reduced to inches of rain. This is roughly correct. Self-registering rain gauges are made, the record of rainfall being kept either by means of a float that rises as the rainfall increases, or else by means of a pair of scales upon which the rain gauge is placed. Meteorological Methods and Results. — At present, nearly every civil- ized nation has a weather bureau from which are issued weather maps and predictions. In the United States the central bureau is at Wash- ington, and many of the states have similar bureaus. The national bureau issues daily maps and other publications describing or predicting the weather. The information obtained in this way is of much value. The pre- dictions of the Weather Bureau are very closely followed by the masters of sailing vessels, and much loss of life and property has been prevented by this means. Predictions of excessively cold weather, and of storms, give much information concerning the weather changes that are liable to occur; and by means of the warnings farmers are sometimes able to prepare against unusually early or late frosts. For the purpose of obtaining information which shall serve as a basis for predictions, the Weather Bureau has stations distributed over various parts of the country, at which observers read the records of the several kinds of instruments. These observations are made at regular times during the day, and the results are telegraphed to central stations, where they are all worked over and plotted upon a map. Then, with the 436 PHYSICAL GEOGRAPHY. knowledge of the changes that have occurred in the preceding days, and knowing what changes are liable to follow, predictions of greater or less accuracy are made, in some cases for several days in advance. In many respects these predictions are of great importance ; but in addition to this result of the work, we are rapidly obtaining much scientific infor- mation concerning the air. We are also obtaining many facts relating to the general climatic features of the country, and of the world. But in these directions much less is being done than should be ; for until we know more about the air and its behavior, we may not expect to obtain more accurate predictions. Upon a weather map (Fig. 46) the wind direction is plotted in the form of a series of arrows pointing in the direction toward which the wind is blowing. The temperature is also placed upon them, and lines of equal temperature, or isotherms, are drawn across the country. The pressure of the air is also graphically shown on the maps by a series of lines which are known as isobars, or lines of equal barometic pressure, each tenth of an inch being represented by an isobar. The amount of rainfall at the different stations is printed on the maps. Thus at a glance one may see the weather conditions of a whole country ; and by studying a series of these maps made for several successive days, one is able to trace the variations in weather conditions for different places. REFERENCE BOOKS. Waldo. — Modern Meteorology. (Contemporary Science Series.) Scrib- ner, New York, 1893. 12mo. $1.25. Russell. — Meteorology. Macniillan & Co., New York, 1895. 8vo. $4.00. Abbe. — Treatise on Meteorological Apparatus and Methods, Annual Report U. S. Signal Service for 1887. Part II. Washington, 1888. There is also a description of instruments in the first part of the Annual Report of the Weather Bureau, 1891-1892. For obtaining the Dew-point and Relative Humidity, see The Tempera- ture op the Dew-point, etc., U. S. Signal Service, 1889. For all kinds of Meteorological Tables, see Guyot, Tables : Meteoro- logical and Physical. Fourth edition, 1884. 8vo. Smithsonian Miscel- laneous Collections, Washington. $3.50. APPENDIX II. TOPOGRAPHIC MAPS. The study of the land is greatly facilitated by the use of maps, and for this reason some space may be devoted to the description of the more common kinds of topographic maps. By far the best means of representing land irregularities is the model (Fig. 266), upon which ele- vations are shown as elevations, so that one sees the actual land forms in relief, although one gains an exaggerated idea of the relation between the vertical and the horizontal. Unfortunately, the expense of prepara- tion of a model is too great for its common employment. In some instances, elevations are shown by means of shading, this being known as the hachure method. By a series of lines, the actual Fig. 266. Model of Cumberland Valley, Pennsylvania. elevations are made to appear to rise above the rest of the country, while the depressions are shown in their natural relation to the high land. This method is used by the United States Coast Survey in charting the coast line of the United States (Fig. 267), and it is employed in some of the European countries. Its effect is very vivid ; but one disadvantage is, that while the differences are shown, one does not find information concerning the actual elevations expressed in feet. The contour method is extensively used, and is employed in the large scale map which is now being prepared of this country. While from the artistic standpoint it is not so effective as the hachure method, it is 437 438 PHYSICAL GEOGRAPHY. superior to this in many respects. A contour is a line of equal elevation. It is the line to which the sea would rise if the land were depressed to the depth represented by the height of the line. If we imagine our- selves near the seashore, the coast line is then the contour line of 0, and the 100-foot contour line is that to which the sea would reach if it were raised just 100 feet. The contour map (Figs. 150, 190, 228, and Plate 25) is made upon a horizontal scale which varies in different cases. In this country the usual scale is one inch to the mile : that is, every mile of country is allowed one inch. No allowance is made for the vertical element of the country. Thus if a region of considerable irregularity is being mapped, an inch on the sheet is made to represent one mile in a hori- zontal direction. As one stands upon the side of a hill, and looks across a valley to another hillside at the same elevation, and a mile distant, Fig. 267. the horizontal line is just one mile in length ; but if the observer should start to walk from the place where he stood, to the point to which he looked, he would need to travel considerably more than a mile. On ordinary maps this greater distance is not shown ; but on the contour maps it is brought out by means of the contour lines. The inch repre- sents the horizontal mile. Each descent or ascent finds a representation in the contour lines ; and if they are close together, one sees that the vertical distance to be traveled is very great. There is much difference in the scale of elevation represented by con- tour lines. On most of the maps in the eastern part of the United States, every 20 feet of ascent or descent is represented by a contour line, and we speak of this as the contour interval. Let us suppose ourselves pass- ing over an irregular country. Imagine that we are to travel a dis- APPENDIX II. 439 tance of one mile, in the course of which we go down into one valley, up the hillside and down into another valley. The entire area on the map would be represented in the space of one inch. If the first valley had a depth of 200 feet, and the contour interval were 20 feet, on the map representing this area there would be 10 contour lines, which would need be very close together, because the descent of 200 feet in the small fraction of a mile would necessarily be rather rapid. If the hill over which we pass rises 40 feet above the valley bottom, we would ascend over a distance represented on the map by two contour lines, — a rather moderate ascent. If the valley on the opposite side of the hill should happen to be 400 or 500 feet in depth, the descent would be ex- tremely precipitous; and it would be necessary to represent this steep declivity by so many contour lines that one would merge into the other, and there would be a mass of crowded lines. From the several sections of contour maps (Figs. 150, 190, 228, and Plate 25, reproduced diagrammatically), one is able to understand the meaning of the contour lines, and to discover the irregularities which they represent. 1 REFERENCES. Nearly every European government is publishing a topographic map, and among these are to be found many excellent illustrations of land forms. In this country, the entire area of Massachusetts, Ehode Island, New Jersey, and Connecticut is now mapped, and teachers can obtain these from the Commissioners of the Topographic Map at the state capital. In all of the other states there are maps of some districts ; and copies of these may be obtained from the U. S. Geological Survey. During the year 1895-96 the Survey will issue, at a small price, a few of their most instructive maps with descriptive text. The seacoast maps of the IT. S. Coast Survey are excellent and cheap. The same is true of the maps of the Great Lakes, the Mississippi, and the Missouri. A very important pamphlet (" The Use of Governmental Maps in Schools," Davis, King and Collie, Holt & Co., New York, 1894, $0.30) has been prepared for the purpose of indicating useful topographic maps. The methods used in making the maps of the Geological Survey are described in Gannett's Manual of Topographic Methods, Monograph XXII., U. S. Geo- logical Survey, Washington, 1893. 4to. $1.00. 1 Specimen maps may be obtained from the U. S. Geological Survey. SUGGESTIONS TO TEACHERS. In the preparation of this book, the endeavor has been to state the subject in a purely descriptive manner. Nevertheless, the best way to learn physical geography is not to read about it, but, so far as is pos- sible, to work out the points for one's self. Not merely does the labo- ratory method teach the subject better, but it trains the mind of the student in a far more valuable way than is done " merely by acquiring information from a book. The following notes are appended merely as suggestions concerning the way in which simple laboratory methods may be introduced. There is very little necessary expense attached to the introduction of these methods ; but of course by the acquirement of other and more expeiisive materials one can improve the teaching almost with- out limit. Each teacher will need to work out the details of the problem for him- self; for the environment, the available materials, the time that can be devoted to the subject, etc., are so variable that at present it would be difficult to outline a course of even general value. I would urge upon every teacher the importance of introducing some laboratory work ; for it will stimulate the interest of the student, particularly if he is brought in contact with the real phenomena of nature. The land and the air are always available and full of lessons : to some, the ocean or the lake shore may also be within reach. I am so much interested in having these methods introduced that I invite teachers to correspond with me, if I can aid them in obtaining materials for teaching purposes. Chapter I. — Laboratory work in illustration of this chapter is not easy. Still, the best way which I know to give the student a clear idea of the relation of the several members of the solar system, is to have each student construct a rough model of it. This can readily be done by means of fine wire and pasteboard. By merely coiling the wire on the desk, each of the orbits can be made in its proper relation to the others. Then each planet can be made from pasteboard, the size representing a slice cut along the equatorial diameter. In order to have this produce the most 440 SUGGESTIONS TO TEACHERS, 441 good, the scale, or relative sizes and distances, should be true to nature. Upon these orbits, the bodies can be made to revolve and to rotate, so that some idea may be obtained concerning the relative movements of the bodies. The relation of the moon and earth may be studied in the same way. In order to show the movements of the earth and the cause of seasons, an excellent method is to construct an orbit of wire and cause a sphere to move around it, the sphere rotating as it revolves. There are various ways in which this may be done; a permanent orbit may be constructed in the schoolroom, and a large ball, or better a globe, may be carried around it, each student being allowed to stand near the center, as if he were in the position of the sun. Each student might be allowed to con- struct a smaller orbit and study the earth movement himself. Celluloid spheres are very inexpensive, and upon them the continents may be roughly outlined, while an axis is passed through them to represent the position of the poles. An exercise or two conducted along lines some- thing like the above will do more to teach the students the relations of the bodies of the solar system than a score of lessons from the book ; and many students go through a course in astronomy without a proper conception of the solar system. The teacher will see many means of adding to this if more time can be spared. Thus it is possible to show the relations of the comets to the solar system ; the immensity of the distance to the stars ; the size of the sun ; aphelion and perihelion ; apogee and perigee, etc. Chapter IT. — The teacher of physics will find many opportunities for illustrating this chapter by laboratory methods. Thus the various effects of heat and light are capable of very graphic illustration. Convection may be illustrated by heating dust or smoke-filled air in a cylinder. Refraction is readily shown by the prism, and nearly all of the prin- ciples of light and heat may be illustrated. Compression of air can be very readily shown. Saturation of air may be shown by placing water in the bottom of a cylinder ; and then if the air temperature is lowered, some of the water vapor may be condensed on the sides of the vessel. The various ways in which humidity is increased or decreased can be studied in detail by each student ; and they can be given hypothetical cases from which to draw conclusions concerning the condition of the air which necessarily follows. The difference in the length of the summer and winter days is readily illustrated by the use of the globe and a candle. By placing the candle 442 PHYSICAL GEOGRAPHY. in different positions, so as to throw the rays at the angles at which the solar rays reach the earth, and by causing the globe to revolve, this is easily seen by the students. Chapter III. — In illustration of this chapter, laboratory work maybe introduced by stating the latitude of a place and having the students tell the probable temperature conditions. Then add the altitude and have them state what modifying effect this would have. After this the position with reference to the sea may be given, and each student ought to be able to state the approximate conditions of temperature. They could be given prominent cities in the world, and have for their problem the deter- mination of the temperature, for which purpose it would be necessary for the student to first ascertain their position, altitude, etc., and this would also serve to teach geography. On the other hand, given a set of temperature peculiarities, the students can determine what parts of the world experience them, and why this is so. The teacher can tell the student of differences between places on the same latitude, or of resem- blances between points on different latitudes, and call for an explanation of these. Much similar work may be introduced if the time allows ; and it is safe to say that not only will the interest be aroused, but the habit of logical thought will be improved. Each student can construct a daily curve from personal observation, particularly if a maximum and minimum thermometer are available. With either fictitious or actual data, they may construct a seasonal curve. Placing a maximum and minimum thermometer in the ground at a depth of one or two feet, the difference between the range of air and earth temperatures is very vividly impressed upon the mind. In order to make this even more striking, temperature observations should be kept at the surface of the ground, and at an elevation of about 10 feet. These differences are best shown in warm weather. A study of the isothermal charts furnishes opportunity for observation and deduction, particularly if Buchan's charts (see p. 84) can be ob- tained. The student can construct an isothermal chart from data given and averaged for several places, either for the state, or the country, or for the locality near the school. For these and other purposes in which maps are needed, the set of cheap outline maps published by Heath & Co. of Boston, or Rand, McNally & Co. of New York, are valuable. Maps of all the states and territories can be obtained. The data of temperature, etc., for these purposes may be made arbitrarily; but it would be better to use the tables which can be found in the Annual SUGGESTIONS TO TEACHERS. 443 Reports of the Weather Bureau. In some states, as for instance in New York, climatic data will be found in the State Weather Reports. These and the national reports may probably be obtained free of cost, provided a statement is furnished of the object for which they are needed. One report will last for many years. With these data, temperature ranges and other illustrations may be graphically plotted by the students. The amount of laboratory work possible in this and other subjects far exceeds the time that will be available in most schools. Chapter IV. — After studying the general features of the atmospheric circulation, the students should be able to construct a summer and winter wind chart for the Pacific, — of course attempting only the general feat- ures. Upon the charts of the Atlantic, there are many problems which have not been mentioned in the text ; and a thorough examination of the wind charts will be valuable. The Challenger charts by Buchan (see p. 84) contain much of value on the winds of the globe. As an instance of how observation and deduction may be brought into the study, the following might be suggested as a fair question : What condi- tions result in the two opposite seasons in the belt where the doldrums and the trade winds overlap? The student should note the relation between wind and barometric conditions. The daily weather charts 1 are valuable for this study ; and the student can also make his own observations with barometer, thermometer, wind vane, etc. A particularly valuable study can be made with the weather maps. i$y examining a series of such maps, one may observe the force and direction of the winds, and the progression of the conditions favoring certain winds during the successive days. 2 When studied with reference to the conditions prevailing in its home region, this method becomes of much value. In this way the student can come into the possession of a knowledge of the causes for the winds that are common in his section, as well as the relation of these to the winds of the surrounding country. Observations on approximate wind force and direction can easily be made by each student ; and this will serve as a basis for a comparative study of the daily weather maps. Before the map of the day is shown them, they should be able to approximately foretell the probable conditions, on the basis of a series of simple observa- 1 The teacher can probably have these sent by mail to the school. 2 The semi-daily maps are of especial value for this purpose, and some of them may undoubtedly be obtained by applying to the Weather Bureau. 444 PHYSICAL GEOGRAPHY. tions on the wind, temperature, and pressure. Such a study will create a real live interest, and make the students observers of the things of every-day occurrence, as well as train their minds to the habit of drawing logical conclusions from a series of observed facts. Chapter V. — The study of cyclones and anticyclones receives much aid from the daily weather maps. On these the student will see the form and size of the areas, their rate and direction of progression, the amount and distribution of rainfall, the direction of the winds, their spiral tendency, the left-hand whirling, etc. He will observe how the winds change from day to day, and what relation they bear to the areas of high and low pressure. He can predict the changes and study them in connection with the weather of his own immediate neighborhood. The storm paths and their irregularities can be studied with the aid of the Monthly Weather Reviews. 1 From the weather predictions, and the printed notes on the map, the relation between the cyclonic areas and thunderstorms is readily seen. The Coast Pilot 2 for the fall months, often contains valuable material for study in connection with West Indian hurricanes. Chapter VI. — The student can be directed in the study of the forma- tion and movement of clouds, and their relation to rainfall and tempera- ture. Reports upon these observations, from time to time, will stimulate them to a deeper interest in cloud formation. Attention can be directed to the possibility of predicting weather changes by an examination of the clouds. This furnishes an excellent opportunity for bringing the student into contact with nature. A study of the rainfall charts, 3 in connection with those of temperature and wind, will give opportunity for the ex- planation of many peculiarities of rainfall distribution. Careful obser- vation concerning the rainfall of the place where the student lives will be of value in showing the irregularities in amount, as well as in occur- rence. Let him compare this with that of the doldrum belt. A sling psychrometer (see Appendix I.) may be readily constructed from two thermometers, and the relative humidity of the air be determined. From this the student can be taught to predict the occurrence of dew or frost for the succeeding nights. The value of these lessons will be greatly increased if the students are called upon for reports. If the pre- 1 These also may probably be obtained at Washington upon application. 2 Distributed free by the Hydrographic Bureau of the Navy Department. * Those recently published by the Weather Bureau are very valuable. SUGGESTIONS TO TEACHERS. 445 dictions that are made are not fulfilled, perhaps the reasons will be apparent; and there may have been dew at one place and frost at another, or dew at one home and none at another. Then the explana- tions for these differences can be obtained from the students. There are few better ways to train the habit of observation than to tell the students to look for certain things, giving enough directions so that they may be led to observe. If too little guidance is given, all but the brightest will be appalled by the difficulties; for one of the least developed parts of the student mind is generally that which directs the eye to look for details, and then to put these details together into a con- nected whole. I have often noticed how pleased secondary school stu- dents have been when their teacher has told them to look up something, and with what earnestness they have worked to have correct answers. They like to be made to feel that they are using their own minds; and it is a distinct relief from the monotony of learning what the book says. Since they are in constant contact with the problems of physical geography, each day can be made to yield opportunity for observation ; and nothing could be more profitable than to give the class a daily task in observation, devoting a part of the recitation hour to a discussion of the results. Chapter VII. — Many of the suggestions 'made for the previous chap- ters will apply to this ; but there are many ways in which these may be put together for a whole. The probable conditions of weather and climate in various parts of the earth may be inferred by a study of the charts of temperature, wind, and rain. The country near the school may furnish illustration of local differences in climate. Chapter YIII. — There is of course much opportunity for an enlarge- ment of this subject; but it would come better under a study of zoology and botany. The object sought in this chapter is to point out the rela- tion between climate and life, and to show also that the land itself presents certain obstacles to the spread of life. Chapters IX., X., and XI. — Unless the student dwells by the sea- shore, there is little of value to be obtained from an attempt at observa- tion study in the topics covered by these chapters. The charts may be studied, and reasons found for the peculiarities exhibited. If the charts of the Challenger Reports are available, particularly those accompany- ing the two final volumes of Summary, there will be found much opportunity for laboratory study. By a careful examination of the charts of the ocean bottom, much can be learned concerning the topog- 446 PHYSICAL GEOGRAPHY. raphy of that large part of the earth's surface which is submerged beneath the ocean. 1 One or two visits to the seashore for the purpose of studying the rise and fall of the tide, the action of waves, the distribution of life along the coast, etc., will be of great value. Even upon the shore of a lake some of these features may also be illustrated. The distribution of cold and warm water by the ocean currents, furnishes much opportunity for study in connection with climate. For tides, the "Tide Tables" (see reference at end of Chapter XL) give data for a very interesting study. The rise and fall of the tide is stated for various places ; and if the student is told to construct a dia- gram similar to Fig. 86, which is based upon these tables, he will learn much about the rise and fall of the tide. At the end of that book the phases of the moon and the times of perigee and apogee are stated, so that the reasons for many of the more important tidal variations will become apparent after a little study and thought. Taking the various stations for which the tidal predictions are tabulated, and locating them upon a map, one sees the geographic reasons for the difference in tidal height from place to place ; and given a place with a certain geographic location, the student can apply these principles to the approximate deter- mination of the tidal conditions. There is no better way to impress upon the student the peculiarity of tidal movement, than to have him laboriously construct a chart of these movements. For students of the interior, this is less important than for those who dwell near the sea. Chapter XII. — There is almost no limit to the opportunity for field and laboratory study upon the topics briefly outlined in this chapter, though it more properly falls to the province of geology. 2 Photographs of various phenomena, as well as lantern slides made from them, are now easily obtained. 3 An excellent method is to project a view upon the screen, and call upon students for a description of the phenomena illus- trated. This has the great advantage of placing an enlarged picture before the class, so that each student may see every feature ; and this does away with the necessity of many duplicate pictures with one in the hand of each student. Where the latter method is employed, cheap 1 The Jones relief globe, sold by A. H. Andrews & Co., 215 Wabash Ave., Chicago, 111., for $100, is of great value in this connection. 2 The study of geology could very properly be introduced here as a part of physical geography. 3 The author will be glad to advise teachers who wish to obtain these. SUGGESTIONS TO TEACHERS. 447 blueprints may serve admirably as substitutes. With the widespread introduction of electricity, it is now possible, in many schools, to make use of the electric lantern, which may be used in a room only partially darkened. Much can be done by asking the students to describe the features illustrated in the pictures in the book. Several phenomena are often illustrated in the same view. By far the best way to study the phenomena of the earth's surface is to see the actual thing; and there are usually opportunities for some such study near the school. In most cases the teacher can find some phenomena of geology, such as igneous or sedimentary rocks, fossils, folds, etc. The students will enjoy and profit by field excursions. Collections of the common minerals and rocks can be bought for a few dollars ; x and more will be learned by an hour's study of such a col- lection, than by weeks of study from the books. Some of the common rocks and minerals may usually be collected near the school. The teachers in those schools which are located within the glacial belt will find a storehouse of rock specimens in the clay and gravel banks. All of the common rocks, and many of the minerals, will often be found there. If the students can be sent or taken out for the purpose of making such collections, they will soon learn a great deal about rocks ; and this plan will be found admirable, even if the school has complete collections. Chapter XIII. — In most places the phenomena of erosion and weathering can be studied in the field. Rock specimens exposed at the surface will show the destruction in progress ; and upon exposed bluffs many instructive lessons may be studied. A journey in such a place will be found to be most profitable ; and the students will see important things that the majority of the world pass by without ever noticing. A visit to a spring may prove of value ; and if it chances to contain iron, or other substances, in solution, chemical action of water becomes something more than the mere book statement. In most parts of the country, wind and glacial action cannot be illus- trated by actual examples. Upon the lake shore, or better upon the sea- shore, wave action may be studied ; and in practically every part of the country, some form of river and rain erosion may be seen. Let the teacher have the students watch the rills and brooks and report upon the change in amount of water and sediment. This will train their 1 Ward's Natural Science Establishment at Rochester, NY., and E. E. Howell, 612 17th St., N.W., Washington, D.C., have such collections. 448 PHYSICAL GEOGRAPHY. powers of observation and arouse their interest; and the skillful teacher may make this the basis upon which to build a real understanding of the action of rivers. The key to success in this direction is to tell the student only so much as is absolutely necessary, but to make him tell the story, not from memory of what the book says, but upon the basis of a series of observations which necessarily lead to these conclusions. It is not necessary to find illustrations of all phenomena in the field, though the more, the better ; but the object is to teach the student how to see for himself, so that he may see other illustrations whenever he hap- pens to come upon them. Where it is not feasible to study the phe- nomena in the field, photographs or lantern slides make a fair substitute. Chapter XIV. — With a set of Physical Maps 1 of the continents, there is opportunity for study of the grander features of the land. These are much more naturally shown upon a relief globe. 2 The distribution of mountains, continents, seas, etc., are there shown very vividly. Par- ticularly is this the case in the second model, for here the ocean waters are not present to obscure the topography of the bottom of the sea. The larger features of the United States may be studied on the nine-sheet contour map published by the U. S. Geological Survey; and also upon the smaller shaded relief map published by the same bureau. 3 Better still, if the school can afford it, a model of the United States should be obtained. 4 With these aids, a good knowledge of the geography of the world can be obtained, and at the same time much training be gained, for the teacher will find ample opportunity to suggest problems for the pupil to study and answer. Chapter XV. — In many parts of the country, particularly within the glacial belt, two types of river valley may be seen within a short distance of the school ; and everywhere in the field it will be possible to see illustration of some stage of river-valley development. The teacher can make such an excursion, or series of excursions, the basis for an ex- pansion of the subject of river-valley development. Where these features 1 The Kiepert maps are sold by most large dealers in school supplies. Eand, McNally & Co. of Chicago and New York advertise a similar set. 2 Such as that sold by Eand, McNally & Co. of New York, or the Jones globe (see suggestions for Chapter XL). 3 The latter accompanies the Thirteenth Annual Report of the Survey. 4 E. E. Howell, 612 17th St., N.W., Washington, has a model of the country for $125, and a smaller one for $25. SUGGESTIONS TO TEACHERS. 449 are not well illustrated, recourse may be had to photographs or lantern slides. A study of topographic maps will be found of great value in this connection, as well as in ' illustration of the features described in the following chapters. For suggestions concerning the special maps needed, and their use, see the pamphlet by Davis, King, and Collie, referred to at the end of Appendix II. Chapter XVI. — Here again there is the possibility of finding illus- trations in the field, and a certainty of finding them in photographs and slides. The U. S. Geological Survey topographic map of Niagara (free), and the Lake Survey map of the same (United States Engineer Office, 34 W. Congress St., Detroit, Michigan; $0.20), are very useful. The latter bureau publishes a number of charts of the Great Lakes ; and on the Geological Survey maps, notably those of New England, many illustrations of glacial lakes and swamps will be found. The Mississippi delta is well illustrated on the U. S. Coast Survey chart 194. For flood- plain peculiarities, see particularly the maps published by the Mississippi and Missouri River Commission, whose headquarters are at St. Louis, Missouri. For facts concerning these maps see the pamphlet by Davis, King, and Collie, referred to at the end of Appendix II. Chapter XVII. — The effects of glaciers upon the surface of the land may be partly inferred from the study of a series of topographic maps of places within the glacial belt, and a comparison of these with some from outside of this belt. This may be very well supplemented by views from the two regions ; and then, if the school is situated within the glacial belt, by excursions 1 to glacial deposits. These will be of great value for the illustrations of many points. In all of these cases, the teacher should have the students observe as much as possible, and should avoid telling them things which they ought to be able to see for themselves. Chapter XVIII. — A teacher who has given no attention to the sub- ject, will be astonished to find how many lessons can be learned by an hour's tramp on the shore of a lake or the ocean. The beaches and cliffs are full of interest ; and on some ocean coasts, as well as on most lake shores, there will be found numerous instances of the minor coastal feat- 1 The number of excursions suggested may seem excessive ; but it is assumed that no one school will be so favorably located as to make it possible to study all of these phenomena in the field. 2g 450 PHYSICAL GEOGRAPHY. ures, such as bars, spits, and possibly small deltas. This kind of work may very advantageously be supplemented, or if necessary be replaced, by a study of the admirable charts of the American coast, which are sold at a very slight cost by the U. S. Coast Survey at Washington. Some of these charts should be in every school where physical geography is taught. For those who dwell near the Great Lakes, the charts of the Lake Survey (sold at $0.20 a sheet) will be found very valuable aids to the study of shore lines. Chapters XIX. and XX. — To most students the subjects treated in these chapters are inaccessible, and they must be studied upon maps, models, and photographs. Unfortunately the demand for materials for laboratory instruction in geology and physical geography, has not yet been sufficient to warrant the preparation of cheap illustrative models of such phenomena as these. In schools where modeling is done, many valuable lessons could be taught by having each student illustrate these changes by the actual construction of models ; and a well-constructed series would undoubtedly find ready sale. As soon as the rational method of instruction is introduced, and there is a strong demand for new and additional material, it will undoubtedly be supplied. In the meantime it will be necessary for the teacher to make use of the only material that is at hand; namely, maps and photographs. Much can be learned from a carefully selected series of these ; and some of the schools will be situated near or among the mountains, so that, in these cases, excursions may be made for the purpose of studying some of the mountain peculiarities. In many parts of New England, in the Catskills, and in the entire Appalachian belt, the opportunity for this kind of illustration is excellent ; and if the teacher will take the trouble to look about him, he will find numerous interesting lessons. For instance, along the eastern base of the Appalachians, there exist two sets of moun- tain ranges, the very ancient series now reduced to low, rounded hills, and the younger, but still old, and relatively high Appalachians. Among the Cordilleras, there is an abundant opportunity for the study of mountains, and in many places of volcanoes also. Chapter XXI. — The teacher will be able to illustrate these features also; for by looking about him, he will find a variety of land forms, and among these will be found illustrations of importance. They may be merely plains or swamps, or they may be mountains. For the teacher who looks with an open eye, there is abundant chance for the discovery of illustrations of the relation between structure and topography. It SUGGESTIONS TO TEACHEBS. 451 would not be necessary to take excursions to every place ; but an admi- rable method is to request the students to visit some of the places and report upon them. This method has been tried with good success, the students being sent out in squads to examine and report upon land or rock peculiarities, at times outside of the regular school hours. There are many photographs and maps which may be used in illustration of this chapter. Chapter XXII. — The teacher will find it possible to expand this subject in connection with the study of history and geography. Indeed, »hroughout physical geography there are numerous points which could properly be made to serve in the teaching of these subjects. Much good Can be done in geographic teaching by showing that, in many cases, features of geographic importance are not arbitrary, but have their origin in phys- ical causes. Most of us have learned that England is a great country, that it manufactures this and that, etc. ; but the fundamental reasons for her greatness are not ordinarily presented. We learned to bound Switzer- land or France, but did not learn what these boundaries meant. We learned the size, position, and industry of Philadelphia, but did not find out the reasons. If we had been told the causes, the isolated fact would have been more easily retained; for the average mind learns unconnected facts with much less ease than those which are philosophically related. Chapter XXIII. — This chapter is mainly intended to be one of information; and while abundant opportunity exists for laboratory work, it does not seem so essential, or so easily obtained, as in the pre- ceding chapters. Indeed, the teacher who follows the foregoing sug- gestions will probably find that the main difficulty lies in the fact that too much is suggested. Appendix I. — No part of meteorology is better capable of furnishing illustration by laboratory methods. The various instruments can be placed by the teacher, and the class be taught to make regular observations, just as is done at any meteorological station. These can be plotted upon cross-section paper, to illustrate ranges in temperature, weather changes, etc. By this means each of the more important instruments may be understood, and a knowledge obtained concerning the results of their use. A series of weather maps for successive days can be furnished each student for study, and for statement concerning the conditions and changes illustrated. Much interest in the subject will be aroused by having the weather map posted in some conspicuous place; and each student can be taught to see upon what basis the weather predictions 452 PHYSICAL GEOGRAPHY. are made. Indeed, the students may make their own weather maps and weather predictions. Furnished with outline maps of the United States, 1 each student can plot temperature, pressure, wind direction, etc., for various places, from observations which the teacher furnishes from a map. After making one or two of these, the student will be in a position to thoroughly understand weather maps. Let the teacher take a series of weather maps for successive days, and have the class plot upon their maps the conditions there recorded. After two or three have been finished, each member of the class ought to be able to make a fairly close prediction of the general weather conditions of the country for the next day. They might even embody these predictions upon another map. Not only will these methods teach students how to use weather maps, but the mind is put to work imagining and drawing con- clusions from a series of facts. Weather maps are readily obtained free of cost from the United States Weather Bureau, at Washington ; and, in some states, a teacher who is willing to maintain voluntary observations may obtain the more common instruments from the State Weather Bureau. A set of the really neces- sary instruments is not so very expensive, and some, such as the ther- mometer and barometer, may also be used in the physical laboratory. Appendix II. — There is no better way to teach the student the mean- ing of the topographic map, than to have him make one of a small area. Moreover, it impresses the meaning of elevations in a way that no other in-door method can do. In the making of models and maps, there is a training in the appreciation of proportion, in constructive imagination, and in the grouping of facts, that is most valuable, and is usually not obtained by the student. No one should be allowed to go through the secondary school without having some development of the " topographic sense." I have known educated people who have lived in a place for several years without having the points of the compass in mind, who have had no idea of the direction to a neighboring place to which they have gone by train or wagon, and whose estimate of distance is simply ridiculous. Particularly is this true of women : for most men, by contact with the outer world, learn by experience what they might easily have been taught in school, while the majority of women get little of this training, even by experience. 1 Such as those sold by Eand, McNally & Co. , of Chicago, and Heath & Co., of Boston, at the rate of a few dollars a thousand. QUESTIONS UPON THE TEXT. In the following questions, no attempt is made to include all that could possibly be asked, but rather to ask the most important, and indi- cate what class of questions seems best calculated to produce the most desirable effect, both in interesting the student, and in drawing from him what he knows. The questions frequently ask for a general view of the subject ; and it may often be necessary for the teacher to ask the pupil other questions which shall aid in obtaining a thorough answer. An excellent kind of question, is one calling for more than a mere answer from the text, but rather one in which the student groups things, partly from his own mind, and partly from the book ; such, for instance, as asking the application or bearing of a point treated in the book. The questions are arranged under sections corresponding to those of the book, and usually follow the order of presentation of the subject. CHAPTER I. The Earth as a Planet. Pages 3-22. Form of the Earth. — Of what is the earth composed ? What is its form? What irregularities are there on the surface? What are the differences between the elevation of the land and the depth of the ocean? What is the area of land and water ? What is the depth of the atmos- phere ? The Solar System. — What are the five classes of membei's? The Sun. — How does the sun differ from the other members of the solar system ? What does the spectroscope reveal ? What are the three parts of the sun? The characteristics of each? What are sun spots? What are the movements of the sun ? The Planets. — What are the important features of Mercury? Of Venus ? Of Mars ? Of Jupiter ? Of Saturn ? Of Uranus ? Of Neptune ? Asteroids. — What are these ? 453 454 PHYSICAL GEOGRAPHY. The Earth. — What reasons have we for believing that the interior is highly heated? What is the probable condition of the interior? What are the movements of the earth? What are the peculiarities of its revolution ? What is the cause of the seasons ? The Moon. — What are its movements ? What is perigee? Apogee? Why is one side of the moon never seen from the earth? What are the probable conditions on the moon ? Comets, Shooting Stars, and Meteors. — What are comets? How do they move? What is the origin of meteors? Why do they glow? The Stellar System. — What is the probable number and distance of the stars? How are they arranged? What and where are nebulae? Symmetry of Solar System. — What points of symmetry are noticed? What are the distances between the members ? Illustrate. The Nebular Hypothesis. — State it. Verification of the Nebular Hypothesis. — What points are there tend- ing to verify this hypothesis? What is the probability of its truth? CHAPTER II. The Atmosphere. Pages 23-42. General Statement. — What variation is there in the density of the air? What gases compose the atmosphere? What is dust in the atmos- phere ? Water vapor ? What is the importance of the atmosphere ? Light. — What is the source of our light ? Of what is white light composed? What is diffusion of light? Selective scattering? What effect upon light is produced by dust? What is the cause for the sunset color? What is reflection ? Give illustration. What is mirage? Loom- ing? Refraction? What is the cause of the rainbow? What is the halo? The corona? What is absorption of light? Why are bodies transparent, translucent, and opaque? Why are some objects colored? Electricity and Magnetism. — What are the indications of terrestrial magnetism? How is atmospheric electricity made apparent ? What is lightning ? Thunder ? Heat lightning ? Heat. — What is the source of heat ? How do different bodies behave toward it? What interferes with its passage through the atmos- phere? Why does the ocean surface remain relatively cool? What is latent heat ? Why does the land become warmer than the ocean ? How is the atmosphere warmed? What is radiation? Conduction? QUESTIONS UPON THE TEXT. 455 Convection ? What is the importance of convection ? What are the differences in heat effect and their results ? What is the effect of rota- tion on the temperature of the air? Of revolution? How does this differ in various parts of the earth ? What are the reasons for the short, cold days of the temperate latitude winter ? What is the normal varia- tion or range in temperature during the year ? How does this differ iD the several zones, — tropical, temperate, and arctic ? Moisture. — What is evaporation? What is saturated air? In what places is the air naturally driest ? Why do winds favor evaporation ? How does temperature effect evaporation? What is absolute humidity? Relative humidity ? Dew-point ? What is the effect upon humidity caused by oceans ? By tropical heat ? By elevation ? By descent of air from higher altitudes? By the passage of air currents from warm to cold regions ? From cold to warm ? By the rising of air ? What are the effects of variations in humidity? Pressure. — In what two ways does the air pressure vary? Effect of Gravity. — What is its effect upon the atmosphere ? Effect of Rotation. — What important effect upon moving bodies of air and water is produced by the earth's rotation ? State the reason. CHAPTER III. Distribution of Temperature. Pages 43-67. General Statement. — What is the normal distribution of temperature from equator to pole ? What are the normal seasonal and daily ranges or curves ? How are they interfered with ? Effect of Atmospheric Movements. — In what ways do the atmospheric movements modify the temperature ? Influence of Oceans. — Why are the ocean temperatures more equable than those of the land? What is the effect of the oceanic circulation in this respect? How does the temperature change from seashore to in- terior ? From tropical to arctic regions ? Effect of Topography. — How does the temperature on the hills differ from that of the valleys ? How does it differ on the north and south sides of hills ? Why are mountain tops eolder than lowlands ? What does this show as to the behavior of heat ? Seasonal Temperature Range. — What is an isotherm ? Why are iso- thermal lines not parallel to the latitude ? What is the normal temper- 456 PHYSICAL GEOGRAPHY. ature range? How is this shown on the isothermal charts? What do the curves show ? How does the range differ in various places, — ocean, land, and different latitudes ? Why do not the highest parts of the curve coincide with midsummer? The lowest with midwinter? In what ways is the normal curve interfered with? Isothermal Charts. — Why are the isotherms of the southern hemi- sphere more regular than those of the northern? Why is the heat equator north of the geographic equator? What is the effect of the Gulf Stream ? The Labrador current ? How does the temperature distribution of the west coast differ from that of the east? Why? Why is the heat equator so far north in July ? Why is it farther north in the Atlantic than in the Pacific ? Why is the deflecting influence of the Gulf Stream greater in January than in July ? Why do the isother- mal lines change in position more in the northern than in the southern hemisphere? Where are the coldest places on the earth ? Where is the cold pole? Where are the greatest seasonal ranges in the United States? The least? Why ? Why are deserts places of great temperature range? What influence of topography is shown on the chart of New Tork ? Daily Temperature Curve. — What is the normal daily range ? When do the coldest and warmest times come? Why? How does the curve differ in different places ? According to season ? By accidental inter- ruptions ? Temperature Ranges. — How closely do the isotherms give the real temperature conditions? Illustrate by San Francisco. Where are the lowest and highest temperatures found ? The greatest ranges ? Where are the greatest and least ranges in the United States? Give an example of rapid change. Contrast the range of Key West and Montana. Give an example of great daily temperature range. Earth Temperatures. — What is the normal change in earth tempera- ture ? In the tropical regions ? The temperate ? The arctic ? How does the temperature of the surface compare with that of the air ? CHAPTER IV. General Circulation of the Atmosphere. Pages 68-84. General Statement. — Illustrate mobility of the air by its action on deserts. Compare with the effect of a stove. How may this compari- son be extended to the atmospheric circulation? What are the four QUESTIONS UPON THE TEXT. 457 principal parts to this circulation? In what ways are these changes registered by the barometer? What is a barometric gradient? Classification of the Winds. — Give the classification of the winds. What are the planetary or permanent winds ? Planetary or Permanent Winds : Trade Winds. — What are the trade winds? How and why do they move? Where are they best developed? Why do they produce deserts? Why do they often cause very rainy belts? How can the same wind produce these two opposite effects? Doldrum Belt. — What are the doldrums ? Their characteristics ? Anti-trade Winds. — In what direction do they move? How do we know of their existence? Horse Latitude Winds. — Where does the air come from? What are the characteristics of the belt? Prevailing Westerlies. — What is the circumpolar whirl? How do we know the permanency of these winds in the upper air? Of what value are they in the southern hemisphere? Why not also in the northern? Periodical Winds. — What are these? Seasonal Winds. — Where is the change of the season most noticeable? What effects ai'e produced in the atmospheric circulation near the tropics? What is the seasonal effect on the land? What is the mon- soon? Where are monsoons found? How is their influence noticed in the United States? How do the winds of Greenland show the influence of the season ? What is the effect of friction between wind and land ? Diurnal Winds: Sea and Land Breezes. — What is the cause of the sea breeze? When does it come? What are its effects? What is the land breeze? What do these winds resemble? What is the effect of the sea breeze in the trade-wind belt? What is the general effect of the day- time heat on the winds of the land? What are lake breezes? Mountain and Valley Breezes. — Describe the valley breeze as to cause and effect. The mountain breeze. Why are the former more violent than the latter? Where are these breezes noticed outside of mountains? Eclipse and Tidal Breezes. — What are these ? Irregular Winds. — How do they differ from the preceding? Accidental Winds. — What is the landslip or avalanche blast? What are the volcanic winds? The waterfall breeze? The Nature of Winds. — What is the real nature of the wind? What causes introduce a vertical movement? What are the possible uses of the internal work of the wind? 458 PHYSICAL GEOGRAPHY. CHAPTER V. Storms. Pages 85-106. Cyclonic Storms. — What is a storm? What are some of the causes of storms? What are the two kinds of cyclonic storms? Hurricanes: Description. — Where do the hurricanes begin? The ty- phoons ? What changes are noticed as the storm nears and passes over a place ? What is the eye of a storm ? How is the air moving in the storm ? Effects. — What is their effect upon vessels? Upon the coast? State some instances. Path. — What is the natural path in the North Atlantic ? How do they sometimes diverge from this? What is their path in the Pacific? South of the equator? What is their size ? Where are they most violent ? Time of Occurrence. — When are they most common in the northern hemisphere? In the southern? What is the line storm? Cause. — What are the facts to be accounted for? Why may we expect that the heat of the tropics is the cause for their beginning? What would account for the whirling? What reason is there for the greater influence of right-hand deflection in certain seasons? Why should they be confined to the ocean? What is the effect of condensation of water vapor? Why do the storms lose energy when they have passed beyond the tropics? What is the explanation of the path? Describe the hurricane. State its cause briefly and clearly. Temperate Latitude Cyclones : Resemblance to Hurricanes. — How do they resemble hurricanes? Differences from Hurricanes. — How do they differ in general behavior? In time and place of development ? In path ? What is the usual path ? Effects. — Where do they occur? What are their effects in the United States? Winds. — How do these vary? What changes occur as the storm passes? What is the sirocco? The foehn? The chinook? The bliz- zard? The norther? Anticyclones. — What is their cause? What are cold waves? What are the accompanying conditions of winds? Cause. — What was the former theory? What objections can be urged to it? State a possible explanation. What is the reason for their paths? Secondary Storms: Thunderstorms. — Where do they occur? Under what conditions? What is the cause for the thunder cloud? Its form QUESTIONS UPON THE TEXT. 459 and features? What is their relation to cyclonic storms? Their path? What is a cloud burst? Describe and discuss the thunderstorm. Tornadoes and Waterspouts. — What are the form and characteristic features of the tornado? Their effects? The area covered and time occupied? In what respect do they resemble thunderstorms? What is the cause? What is a waterspout? CHAPTER VI. The Moisture of the Atmosphere. Pages 107-123. Dew. — What is the cause of "sweat" on a pitcher of ice water? How does this resemble dew formation? At what temperature and time will this occur? What conditions especially favor the formation of dew? Why does dew occur more readily in valleys than on hilltops ? What is the main cause for dew? What other causes also aid? Frost. — What is frost ? What prevents it ? Fog. — What is fog ? What is the cause for ocean fog ? What is valley fog? In what other ways may fog be caused? What is the relation of dust to fog? Haze. — What is haze? Its cause? Mist. — What is mist? Clouds. — Of what are clouds composed? Under what condition are they formed? Give the classification of clouds. Describe the cirrus ; the cirro-stratus ; cirro-cumulus ; cumulus ; cumulo-stratus ; stratus ; nimbus. Rain. — What is the cause of the drop ? Under what conditions is rain caused? What relation does it bear to clouds? Snow. — What is snow ? The difference between snow and rain? Hail. — What is hail? Distribution of Rainfall in the World. — What do we mean by rain- fall? Why are there differences according to altitude and latitude? What is the cause for variation in tropical regions ? What is the effect of steeply rising mountains? What are the two main causes for deserts ? What are the rainfall peculiarities within the belt of calms ? How does the rainfall vary from coast to interior ? Distribution of Rainfall in the United States. — What are the causes for the heavy rains of the Texas and Florida coasts? For the differences between the east and west coasts? What is the effect of the high west- ern mountains upon the rainfall of the western half of the country ? 460 PHYSICAL GEOGBAPHY. Distribution of Snowfall. — Where does snow fall ? Where are glaciers produced ? Seasonal Distribution of Rainfall. — What is the effect of the migra- tion of the belt of calms? How do the monsoons affect the seasonal rainfall? What is the reason for the winter rains of Washington and Oregon ? For the irregularities of rainfall in the east ? Irregularities of Rainfall. — What is the normal rainfall ? How does it sometimes vary from this ? What are the effects of heavy downpours ? CHAPTER VII. Weather and Climate. Pages 124-134. Weather. — What is weather ? Climate ? Tropical and Arctic. — What are the weather conditions of the belt of calms ? Of the trade-wind belts ? Of the polar regions ? Temperate Latitude Weather. — What are the weather conditions on the northern Pacific coast? In the mountains east of this? In the deserts between the mountains? On the plains of Dakota, etc.? On the more southern plains? In the southern coastal states? In the northern central states ? What is the cause for the droughts ? What are the weather conditions of the northeastern states? What are the winter conditions in this belt? The summer climate? What are the typical weather conditions in temperate latitudes? How do those de- scribed differ in Europe ? In the southern hemisphere ? Climate. — What are the climatic belts ? Their subdivisions ? Tropical Climate. — What is the general climatic condition ? The difference between the ocean and the land? The doldrum and trade- wind belts? What are the differences in rainfall? What climatic peculiarities are caused by the monsoon condition of India? Temperate Climate. — What are the characteristics of the climate of this belt? What are its subdivisions? What is the climate of the western coasts? Of the eastern coasts? The interior climate? Of mountains? Of the inter-montane district. State the climatic differ- ences noticed on the parallel of 50° N. Arctic Climate. — What are its characteristics? Minor Variations. — What are some of these? Changes in Climate. — What two classes of evidence point to climatic change? What is the supposed thirty-six-year cycle? What is the QUESTIONS UPON THE TEXT. 461 geological evidence of former differences in climate? What recent geological changes are recorded in the United States? What are the possible explanations of these changes ? CHAPTER VIII. Geographic Distribution of Animals and Plants. Pages 135-148. General Statement. — What are the life zones? What kinds of life occur in the several zones ? What are the differences between the life in fresh and salt water? The Ocean. — ■ What causes the wide distribution of ocean life ? What is the effect of temperature on distribution? Where in the ocean are plants unable to live ? Under what conditions do they especially thrive ? What is the difference between the tropical and northern animals? Fresh Water. — What are land-locked animals? What forms of life are found in fresh water ? What is the effect of change to salt lake ? The Land : — Effect of Temperature and Moisture. — What is the effect of temperature? What is the effect of arctic cold on the animals? On the plants ? Of the cold of high temperate latitudes ? What is the influence of altitude? What changes in vegetation are noticed in ascending high mountains ? How may this vary on the opposite sides of a mountain ? What are the effects of aridity ? Of great moisture ? Plant and A nimal Habits. — How do the seeds effect the distribution of plants ? What animal habits influence distribution ? Life Zones. — What are the great life zones and their subdivisions ? How do the continental zones resemble one another? How do they differ ? What do these differences and resemblances show ? How is this illustrated by oceanic islands ? In the Bermudas ? In New Zealand ? The East Indies ? Australia ? The Spread of Life. — What is the main reason for the distribution of land animals? What is the effect of the winds and storms? What animal groups are distributed by this means? What is the effect of ocean currents ? What animals are thus liable to be carried ? Why are large animals so rare on oceanic islands ? What was the effect of the change of climate causing the glacial period ? Barriers to the Spread of Life. — What is the great barrier? What does Australia teach us in this respect ? What other barriers are there ? Effect of Man. — What is the effect ? Is there any limit to it ? 462 PHYSICAL GEOGRAPHY. CHAPTER IX. Form and General Characteristics of the Ocean. Pages 151-173. Distribution of Land and Water. — What are the main features of distribution of land and water ? Composition of Ocean Water. — What are the principal ingredients of salt water ? How much variation is there in salt impurities ? What are the reasons for this ? Color and Phosphorescence. — What is the natural color of the ocean ? Why ? Are there other colors ? What is phosphorescence ? Exploration of the Ocean Bottom. — What reasons led to the belief that animals could not live here? How can the animals exist under the great pressure ? What has led to the study of the deep sea ? Methods Used in Deep-sea Explorations : Sounding. — What are the objects sought ? What is a fathom ? Describe the sounding machine. What other facts are learned during the sounding ? Dredging. — Describe the deep-sea trawl. How correct a knowledge may we expect to obtain by dredging ? Topography of the Ocean Bottom : General. — What is the fundamental difference between the land and ocean bottom topography ? Why are there greater occasional elevations in the ocean ? Why greater general levelness ? What are the general features of the ocean bottom ? State some of the excessive differences in elevation in the ocean. The Atlantic Ocean. — What is the continental shelf? The continental slope? The oceanic plateau? The mid-Atlantic ridge? What are the features east of this ? What features are shown in a cross-section of the Atlantic ? Other Oceans. — How do the features of the Pacific correspond with those of the Atlantic ? What is the deepest known point in the Pacific ? In the Atlantic ? Compare ocean depths with land elevations. Topography near the Coast. — Compare this with the ocean depths. Temperature of the Ocean Bottom. — What are the temperature features of the ocean bottom near the land? How does this change with increas- ing depth ? What is the general temperature condition of the waters of the ocean bottom? How does this vary in such places as the Medi- terranean? The Gulf of Mexico? What is the explanation? Light on the Ocean Bottom. — What is the probable source of this ? QUESTIONS UPON THE TEXT. 463 Materials Composing the Ocean Floor : Mechanical Sediments. — What are the two sources of ocean deposits ? Globigerina Ooze. — What is this? Where does it occur? How is it accumulated ? What rock resembles it ? Red Clay. — What is this ? Where does it occur ? What materials compose it ? How large an area does it cover ? Life in the Ocean : Pelagic or Surface Faunas. — What ocean conditions especially favor abundant life ? Why is the temperature uniform ? What conditions favor the widespread distribution of the surface animals? Under what conditions do they live? Do animals live in the waters be- tween the ocean surface and bottom ? Littoral or Shore Faunas. — How do the conditions in this zone resem- ble those of the land ? What is the effect of temperature here ? Illus- trate. How does the food supply influence the development of these animals? Illustrate by coral growth. What are the habits among shore- line animals ? How do these vary ? Faunas of the Ocean Bottom. — How do the deep-sea animals show the effect of pressure when brought to the surface ? What forms live on the ocean bottom? What is the main cause for limiting their spread? Under what conditions do they exist? How does the low temperature tend to diminish the abundance of animals? What is their food sup- ply ? How does this also limit their abundance ? How do they obtain their oxygen ? What do they prove with reference to oceanic circulation ? How does the oxygen supply tend to limit the abundance of life ? CHAPTER X. Ocean Waves and Currents. Pages 174-191. Wind Waves. — What is their cause? Their form? How do they move ? What change is caused at the shore ? How far do they extend ? When are they formed? How do they act on the shore? What are their effects ? Their every-day action ? How may their effects be seen ? Earthquake Waves. — What are these ? How do they behave ? What are their important effects ? How far may they travel ? Storm Waves. — What causes tend to produce these ? Their effect ? Ocean Surface Temperatures. — What is the natural change from place to place? How may this be made to vary? What influence is noticed near the coast? What are the conditions in mid-ocean? Why is the 464 PHYSICAL GEOGRAPHY. warm surface water so shallow ? Why are the surface temperatures so constant ? Ocean Currents : Planetary Circulation. — What resemblance is there between ocean and air circulation ? What reasons are there for believing in a planetary, oceanic circulation ? The System of Ocean Currents. — What is the circulation in equatorial regions? What is the North Atlantic drift? What becomes of the water entering the Caribbean ? What is the origin of the Gulf Stream ? Its course ? What is the Labrador current ? Briefly describe the general circulation of the North Atlantic. What are the conditions in the South Atlantic ? What is the circulation of the North Pacific ? What is the Kuro Siwo ? What is the circulation of the South Atlantic ? What are the main features of the oceanic circulation ? Cause of Ocean Currents. — What reasons are there for doubting the temperature theory ? What is the apparent explanation ? What facts support this? What influence has the temperature difference? What causes determine the course of currents ? What would be the circula- tion if there were no land ? The Gulf Stream. — What is the reason for its warmth? Its velocity? How does it vary in velocity? The Labrador Current. — What is its course? Effects of Ocean Currents. — What is the most important effect? What would result if there were no circulation? What indication is there of an important influence upon temperature? How much heat is carried? What is the influence upon rainfall? Upon sailing vessels? In producing fogs? Upon animal life in the ocean ? CHAPTER XI. Tides. Pages 192-203. Nature of the Tidal Wave. — What is the nature of the wave? Cause of Tides. — What is the origin of the wave? Why is the in- fluence of the moon greater than that of the sun ? Effect of the Land. — What is the natural course of the wave? What is the cause for its peculiar movement in the Atlantic? What is the change introduced in bays ? What are the peculiarities near the British Isles ? In the approaches to New York ? How does the height vary ? How may it be lessened? How may it be increased ? What is the effect QUESTIONS UPON THE TEXT. 465 of the difference in the height of the tide in connected bays? What are tidal races? Illustrate. What is the tidal bore? Other Causes for Variation in Tidal Height. — What is the effect of the wind? Of air pressure? What are seiches? How does the relative position of sun and moon influence tidal height? What are spring tides? Neap tides? What is the influence of perigee and apogee? What other astronomic causes for variation are there? Effects of Tides. — What is their influence upon navigation? In changing the coast? What is their effect in estuaries? How are the tides utilized? CHAPTER XH. The Crust of the Earth. Pages 205-223. Interior Conditions. — What reasons are there for believing that the interior of the earth is highly heated? What was the former belief? The present hypothesis? What is the apparent effect of loss of heat? Movements of the Crust. — What classes of proofs are there showing the crust to be in movement ? State some of the historic proof. The geologic evidence. Is this a movement of the water or the land ? Disturbance of the Rocks. — What is the position of the rocks of the crust? By what means are they changed from the horizontal? What is a monocline? Anticline? Syncline? What are the characteristics of the folds in mountains? What is dip? Strike? A fault? A fault- plane? How does the movement take place? Volcanic Action. — What is a volcano? A lava flow? Volcanic ash? Pumice? How do volcanoes vary in their ejections? How large an area is covered? What are dykes? Bosses? Rocks of the Earth's Crust. — What are the three groups of rocks? What is their origin? Igneous Rocks. — What are minerals? What rocks are crystalline? How do these rocks vary chemically? What minerals occur in them? Why are some igneous rocks coarse grained, while others are fine. Metamorphic Rocks. — How do they resemble the igneous? What are their characteristics? Their origin? What are the common rocks of this group? Sedimentary Rocks. — What are the three subdivisions ? Which is 2h 466 PHYSICAL GEOGRAPHY. most important? How are the mechanical sediments derived? How are they accumulated ? What are the kinds ? How do they differ ? Deposition of Sedimentary Rocks. — In what position are they depos- ited in the ocean? What is the origin of stratification? What are the characteristic deposits in the sea? What are the characteristic sedi- mentary rocks on the land? What does this prove? How thick are the sediments. What does this prove? What is an unconformity? Consolidation of Sedimentary Rocks. — How are rocks cemented? Illus- trate. What are the common rock cements? Geological Chronology. — What is the condition of the rock record? What are fossils? How has a record of early life been obtained? What does this show? Can the age be told by fossils? What is the difference between age and stage? What do the names of the geological periods really indicate? What does the name Carboniferous mean? Learn the table of geological ages. The groups of animals that lived then. Age of the Earth. — What do the estimates show? What does geology show as to the age of the earth? Illustrate by Niagara and the Colorado. By volcanoes. By the thickness of sedimentary rocks. What are the two fundamental conceptions in geology? CHAPTER XIII. Denudation of the Land. Pages 224-248. Underground Water. — How does water find its way into the rocks? How does it move through them ? What is the evidence of its existence? How is it able to dissolve? What evidence of this is there? Why should some of the dissolved mineral substances be deposited? What effects are produced by the deposits of this in the earth? What effect is produced by underground water in changing minerals? The Formation of Caverns. — What is their origin? What are stalac- tites? Stalagmites? What is the origin of the natural bridge? Springs and Artesian Wells. — In what ways are springs produced? What are the conditions favoring the accumulation of artesian water? What rock is particularly favorable? What must be the position of the rocks? Why does the water rise to the surface? Why does it not rise above the permeable layer? What is the use of this water? Durability of Rocks. — How do rocks vary as regards durability? What is the influence of texture? What is meant by a hard rock? QUESTIONS UPON THE TEXT. 467 Weathering. — What agents are engaged? What are the chemical changes? How do these affect the rocks? In what rocks are they most liable to act? What sedimentary rocks does this decay form? What is the most important mechanical agent? What conditions favor the action of this ? Where is it checked ? How do plants aid in weather- ing? Animals? How widespread is the action of weathering? Where is its action rapid? Where slow? What are the results? With what is weathering in combat? Which has excelled? What would have been the result had there been no re-elevations ? If there had been no other agent of destruction? What agents have aided the effectiveness of weathering ? What is residual soil ? Where is it important ? Agents of Erosion. — What are the most important of these? Wind Erosion. — Where is this important? What is its effect on the seashore? What are sand dunes? Why is wind erosion important in arid regions? What is its effect ? Rain Erosion. — When does this action commence? What is its effect? Where is it least important ? What is the origin of gravel slopes ? What is the importance of gravity? Percolating Water. — How does this act? How does it act mechani- cally? How are avalanches or landslides produced? River Erosion. — What tasks are rivers engaged in? What materials are furnished to them? How do these materials vary in amount and kind? In what way does the river erode? Why are most arid land rivers V-shaped? Why are newly begun valleys V-shaped? What causes them to broaden? By what means is the rate of erosion caused to vary? How do rivers vary? What is their most important office? Ocean Erosion. — How do waves act ? How are materials removed ? How does this affect the coast line. Glacial Erosion. — How does ice erosion differ from that of water? Denudation. — What is denudation ? Whence come the forces ? How do the agents interact? What has been the importance of their action? CHAPTER XIV. Topographic Features of the Earth's Surface. Pages 249-261. Continents and Ocean Basins. — What are the greater irregularities of the earth? What is the arrangement of land and water? What is the relative size of the continent and ocean areas? What are the more important features of the ocean bottom ? What is the elevation of the 468 PHYSICAL GEOGRAPHY. land compared with the ocean depth? What are the most characteristic features of continents? Are the continent forms permanent? What changes are in progress ? Where is the real continent border ? Physical Geography of the United States. — What are the five geo- graphic provinces? Atlantic Coast Area. — What is the extent of the coast plains? What are the characteristics? What are the characteristics of the plain on the landward side of this? Of what value are these areas? The Eastern Mountains. — What are their features ? What are the two parts? The extent of the older mountains? Their features'. What is the relative age of the Appalachian and the more eastern moun- tains? What are their features? Why are they less high than the Andes and Rockies? What are their most important mineral products? The Canadian Highlands. — Where do these extend into this country? The Central Plains. — What are the main features of these ? Their extent ? How are they interrupted in places ? For what are they valu- able ? Why are they not forested ? The Cordilleran Area. — What are its main features? What are the features on the eastern base? In the Rocky Mountains? West of these ? In the Sierras ? At the western base of these ? On the Pacific coast ? Why are these mountains so high ? What are the indications of intense denudation? What is the condition of volcanic activity in this region? Elsewhere on the continent? What is the importance of this area in mineral production? The Drainage of the Country. — (See map.) Into what oceans does the water drain? What part drains to the Arctic? Through what river? What to the Pacific ? Through what large rivers ? What two important rivers enter the Gulf ? What is the condition of the Appalachian drain- age? What are the features of the St. Lawrence drainage? The Shore Line. — What is the general form of continents ? What are the main features of the Atlantic coast line ? Of the Pacific ? CHAPTER XV. River Valleys. Pages 262-284. General Description. — What is a river? What are the general char- acteristics of river valleys ? What is a river system ? A divide ? How do rivers differ ? What was the former belief concerning river valleys ? What do we now know to be their origin ? QUESTIONS UPON THE TEXT. 469 Development of River Valleys. — What actions combine to produce the valley ? What is base level ? When in river development does erosion exceed weathering ? When does this cease ? What would be the ulti- mate result ? What is the valley-form in youth ? In maturity ? Where is the development earliest and most rapid ? How may the valley-form vary in different parts of the course? What is the influence of rock structure? Of sediment load? Of arid conditions? What would the canon valley show as to age ? What evidence is there that weathering is in progress? What other features of youth are there? How do the number of tributaries show age ? What is the condition of the divide ? What happens when vertical erosion ceases ? What is the condition of the river in this stage of maturity? What stage have most valleys reached? What characteristic features have led. to the division of the river course into three parts ? Why cannot this be considered universal ? How may the rate of development vary ? What would be the difference between a valley on a plain and on a plateau ? How may the climate influence this? Why do gorges remain so long in mountains? What would be the effect of a mountain lake ? What is the origin of the broad valley in high mountains ? Adjustment of Streams. — What is a consequent stream course? How may this change as the river develops? What is mature adjustment? The River Divide. — Are these permanent ? How may they change ? What is the law of monoclinal shifting ? How may divides be suddenly changed ? Accidents to Streams. — What would be the condition if no accident interfered with river development? In what different ways do these accidents affect stream valleys ? What are composite streams ? Land Movements. — What are the three kinds ? What would be the effect of a general uplift? Along the seashore? Is this rejuvenation common ? What would be the effect of depression ? How is this illus- trated on the eastern coast? How will folding influence the streams? What are antecedent rivers ? How may the river course be changed by mountain growth? What features are introduced? Climatic Accidents. — What are the effects of a change to a condition of dryness? What is an arroya? What are withered or shrunken streams ? What are the first effects of glaciation ? How are the lakes formed along the margin ? Give instances. How may stream courses be changed ? What are the results ? What effects are produced by vol- canic action ? By avalanches ? Why is the old-age stage not reached ? 470 PHYSICAL GEOGRAPHY. CHAPTER XVI. Deltas, Floodplains, Waterfalls, and Lakes. Pages 285-305. Deltas. — Where are delta deposits made ? What is the alluvial fan? What conditions favor delta formation in the ocean? Why are lakes favorable places for these ? How does the river flow over the delta? What are distributaries? How does the delta grow ? Floodplains. — Where are these found? What causes floodplains among mountains? What is the most common cause for floodplains? How may they merge into deltas? What effect would be produced by tilting the land? From changes of climate? What are the character- istics of floodplains? What is the course of the stream? What are oxbow cut-offs? How are the floodplains raised? How does the flood- plain material move down stream? What is the effect of the floodplain upon tributaries ? Waterfalls. — What is their origin ? What cause has produced most of these? What was the origin of Niagara? Its history? The falls of St. Anthony ? What other causes produce falls ? What is the fall line ? Its importance? How may waterfalls be naturally developed? What is the most common position of the rocks in which these are developed? What is the origin of such rapids as those of the Colorado? Lakes. — How do they differ? What relation do they bear to rivers? How may they be produced ? What is the most common cause ? What other accidents produce lakes? What are original lakes? How may lakes be naturally developed? How permanent are lakes? How are they destroyed ? Which of the processes is the more important ? Why ? Under what conditions may cutting at the outlet become of importance? Illustrate one of these by Niagara. What is the effect of evaporation ? What have been the changes in the Great Basin ? Swamps. — What relation do these bear to lakes? How' does the change take place? In what other ways may swamps originate? CHAPTER XVII. Glaciers. Pages 306-327. Cause of Glaciers. — What is a glacier? How does it form? What determines the terminus? Where are conditions found which favor their formation? What are the kinds of glaciers ? Alpine or Valley Glaciers. — Where are these found? What is the snow field? How does the glacier receive its supply? How does it QUESTIONS UPON THE TEXT. 471 move ? What are crevasses ? What is an ice fall ? What are the causes of irregularities on the surface ? How is the glacier supplied with rock material? What is the lateral moraine? The medial moraine? The ground moraine ? The terminal moraine ? What is the origin of the ice cave ? What are the characteristic features of the valley glacier ? What are the characteristics of the glacier at the foot of Mt. St. Elias. Continental Glaciers. — Where are these now found? How extensive are they ? How thick are these ice sheets ? What are the features of the Greenland glacier ? What are nunataks ? Icebergs. — What is floe ice ? How are icebergs formed ? How far do they journey ? How much is below water ? How high are some bergs ? Glacial Period : Area covered by Ice. — What recent changes of climate have taken place ? What was the effect ? How extensive was the glacia- tion? What were the conditions in northeastern America? In Europe? Were these two areas connected? What were the conditions in Asia? In western America ? What do we know about the cause for this change in climate? How long ago did the ice sheet disappear? Terminal Moraine. — How did the glacier resemble the Greenland ice sheet? What was accumulated at its margin? Where is the terminal moraine? What are its features? Formation of Soil. — What is till or boulder clay ? What are its characteristics ? What are the signs of a scouring action ? How deep is the soil? What other kinds of soil were left? Formation of Lakes. — How were temporary lakes formed? AVhat effect was produced in the Red River valley ? What was the size and extent of this lake? What is the proof of this? How were lakes formed by the deposit of glacial drift ? How were rock basins formed ? What large lakes were produced by the action of the glacier? Formation of Waterfalls. — How were the stream courses interfered with ? Why are the new valleys gorges ? Why were waterfalls caused ? What was the general effect of the ice upon the topography ? CHAPTER XVIII. The Coast Line. Pages 328-349. General Statement. — What changes are taking place? What agents are at work ? How do lake and sea shores resemble one another ? Effect of Elevation. — What are the effects of this ? Effect of Depression. — What are the effects of this? What would 472 PHYSICAL GEOGRAPHY. result from the depression of the land bringing sea level to the plaoe occupied by the student? What is shown on the coast of Maine? Where else is this also shown? What are the two general types of coast? Why? Give illustrations. Effect of Sediment. — What becomes of most of the sediment? When the sediment supply is too great, what becomes of it ? Why are sand bars produced in the sea? Effect of Waves and Currents. — What are these doing on exposed coasts ? Give some illustration from the English coast. From the American. What are bars? Spits? Hooks? How does the effect vary with the hardness of the rock? What is the tendency of the wave work? How are lagoons formed by beach barriers? What is the natural form of the beach ? Effect of Plants. — What is the effect of seaweeds ? Of the mangrove? Of the marsh grasses ? Effect of Animals. — Under what conditions may corals live? Why are they absent from some tropical coasts ? What do they build? What are barrier reefs? Keys? Atolls? Why are these above sea level? What is the Darwin theory for atolls. Changes in Coast Form. — What are some of the causes for change ? What are some of the recent changes on the eastern coast ? Islands. — How do these vary ? What are the classes ? What are the classes of oceanic islands? Where are these represented on the coast? What are the causes for most of the islands? What becomes of islands if left to the waves? Illustrate. Promontories. — What is the difference between capes and promonto- ries ? What are the causes for some of the larger promontories ? What is the origin of the Nova Scotia peninsula ? Florida? Sandy Hook? Lake Shores. — What are the features of these? How are capes and islands formed in them? What is the origin of the Thousand Islands? What part of the seashore do most lake shores resemble ? Fossil Shore Lines. — How are these formed ? What are their features ? How durable are they ? Give some instances of these. CHAPTER XIX. Plateaus and Mountains. Pages 350-369. Plateaus. — What is a plateau? How does it differ from a plain? With what are they associated? Where are they found? What large QUESTIONS UPON THE TEXT. 473 plateaus are covered by lava? What is the climate of the plateaus of the west ? How do the western plains differ from the prairies ? What is the condition of the river valleys? Why? What is the characteristic topography of the high plateaus? What is a mesa? A butte? Mountains : Characteristics of Mountains. — What is a mountain ? What is the origin of the features? What is a mountain system? A Cordillera? A range? A ridge? How do they resemble one another? What is a peak? What is the origin of the peak? Of what are they made? How do they differ from the ridge? What other kinds of peaks are there? What are hills of circumdenudation ? What are interior basins ? Where are they found ? What is their comparative importance in different continents ? What is the origin of the longitudinal valleys ? What are parks? What is the origin of mountain gorges? What are passes? What is the characteristic topography in mountains? What are the reasons for this ? What are the features of the flora ? Why are mountain peaks rugged ? Upon what does the form of the peak, ridge, etc., depend? When are mountains most rugged? The Origin of Mountains. — State the contraction theory. What com- parison may be made concerning the wrinkling of the crust? What is the value of this theory? What is the history of mountain folds? How do mountains grow? What happens as they grow? What would be the result if denudation had been absent ? Sculpturing of Mountains. — What determines the result of this? The Drainage of Mountains. — What determines the drainage? What are the characteristics of the mountain drainage? What are longitudi- nal streams ? Transverse valleys ? What may be said about the origin of antecedent valleys ? What is the origin of mountain lakes? Their characteristics ? Destruction of Mountains. — What are the features of young moun- tains? Why? What happens as the age increases? What is the stage reached by the Appalachians? By the eastern highlands? What changes occur in the position of the hard and soft layers? What are synclinal mountains ? CHAPTER XX. Volcanoes, Earthquakes, and Geysers. Pages 370-389. Volcanoes : Distribution. — Where do they occur with reference to the sea? To mountains? Where found in North America? What about 474 PHYSICAL GEOGRAPHY. their former abundance? Have they occurred in all parts of the world? Materials Erupted. — What substances are erupted ? What is the cause of pumice ? What are the effects of the steam ? What is a mud flow? How does the lava flow move? What is the extent of the lava? How does this differ from ash? What was the effect of Krakatoa? Eruptions of Volcanoes. — How do these vary as to violence ? Contrast the eruption of Krakatoa with those of the Lipari Islands. What is the case in Vesuvius ? In the Hawaiian Islands ? What kinds of volcanoes are the most violent ? What are the three groups ? Form of Cone. — How does a volcano grow? What tends to destroy the cone? Where are they steepest? What is their angle of slope? How do lava and ash cones differ ? Effects of Volcanic Eruptions. — What are the more important effects ? Extinct Volcanoes. — What happens after volcanoes become extinct ? What are volcanic necks ? What are dykes ? What are buttes ? Mesas ? Cause of Volcanoes. — What is the immediate cause ? What is the origin of the heat? What is the association with mountains? Why? Earthquakes. — Where do these occur ? What is the nature of the shock? What is the focus ? The epicentrum ? How does the shock travel out from the center ? What are the effects ? What may cause earthquakes ? Geysers and Hot Springs. — What is the origin of hot springs? With what are they commonly associated? What is the association with ore deposits ? What is the relation between geysers and hot springs ? Where are geysers found? What are their characteristics? CHAPTER XXI. The Topography of the Land. Pages 390-406. General Statement. — How are land forms derived ? What are the forces? What would be the result if denudation had been absent? What are the opposing forces succeeding in accomplishing ? What feat- ures and forces determine the complexity of the land form ? Constructive Land Form : By Internal Forces. — How are these compli- cated? What are the larger constructive forms? What is the origin of the coast plains ? Volcanic cones? By Agents of Denudation. — What constructive forms are produced by gravity? By wind? In lakes? By rivers? By glaciers? In the ocean? How are these forms modified ? QUESTIONS UPON THE TEXT. 475 By A nimal and Plant Life. — State some of these. Effect of Rock Structure upon Topography. — How may rock character- istics influence the action of denudation? What are the features in high mountains ? In arid climates ? What influence does the stage of devel- opment have upon topographic form? What is the effect of uniformity of texture ? Of variation ? What is the effect of position ? When the rocks are horizontal? What are terraces of differential degradation? What forms result when the rocks dip gently ? What are the features found in traveling over such a region? What results on the seacoast? What happens in mountains with steeply inclined strata? When rocks are harder than others, what happens ? What results when submergence occurs ? What is the interaction of the various forces ? CHAPTER XXH. Man and Nature. Pages 407-419. General Statement. — How does man's present condition differ from that of the past? How may the subject be divided? Modifying Influence of Man. — State some of the ways in which he modifies nature. How is he modifying animals and plants? What is his influence in spreading animals and plants ? In destroying them ? Man and the Forest. — What is the effect of the forest covering in pro- tecting the soil? How does it influence the distribution of rainfall? How does it affect the streams? State briefly the importance of the forest. What reasons are there for thinking that it affects the climate » Influence of Nature upon Man. — What change is taking place ? What differences do we find between people of different occupations ? How do the inhabitants of the several zones differ ? What was the former condi- tion of man? How did the surroundings influence the Chinese? The Egyptians? The inhabitants of the Italian peninsula? Of Greece? Why was the Mediterranean the natural seat of early navigation ? How were the Northmen influenced by surroundings ? The English? What are the reasons for the large number of European nations? What is illustrated by Switzerland? Why is America so different from Europe in respect to political divisions? What physical features aided in the discovery of America? What influence did this discovery exert? Why were the American settlements made near the coast? What was the influence of the forest barrier? Why was the settlement of the interior 476 PHYSICAL GEOGRAPHY. delayed? When reached, why was its settlement relatively easy? What caused the development of the far west? What has determined the position of the towns of New England? What relation is there between the industries of the country and the surroundings? CHAPTER XXIII. Economic Products of the Earth. Pages 420-430. Soil. — What is its origin? Its value? Building Stones. — What is the origin of granite? What other stones are sold as granite? What are the metamorphic building stones? What is the origin of slate? Of marble? What are the causes of metamorph- ism? What are the sedimentary building stones? How abundant are they? What other mineral substances are used for building? What is the origin of the clay deposits ? Economic Deposits of Sedimentary Origin. — What is the origin of the substances used for grinding and polishing? Of rock salt? What other substances occur with it ? What is the origin of the fertilizers? Miscellaneous Substances. — What are some of these ? Coal. — What evidence is there pointing to the origin of coal? How may coal have been formed by drifting wood? By accumulation in bogs? On seashore marshes ? What is the probable origin of coal ? How is the coal changed? What are these changes? In what periods in the earth's history has coal been formed ? Natural Gas and Petroleum. — What is their value ? How do they occur? How constant is their supply? What is their origin? What artificial products do they resemble ? Ore Deposits. — In what associations do metals occur ? How must they occur to be profitable? Describe replacement deposits. Fissure deposits. Sedimentary deposits. What are placer deposits ? Where do they occur ? What other substances, besides gold, occur in this way? Distribution of Ore Deposits. — Where do they most commonly occur? Why are so few of the metals produced from the Cordilleras ? What are the reasons for the importance of the Cordilleras ? Mineral Wealth of the United States. — In the production of what metals does this country take first rank? In what does it take second rank ? How valuable is the industry, and how is it distributed ? Which is the leading state ? Its products? The second? Its products? The third ? The fourth ? What has been the value of this great wealth ? INDEX. Absolute humidity, 37, 434. Absorption of heat, 30 ; of light. 28 ; of vapor, 36. Accidental winds, 70, 82. Accidents to river valleys, 275. Active volcanoes, 377. Adirondack^, 256, 304, 409, 410 ; lakes in, 299, 300 ; peaks of, 356. Adjustment of streams, 272. Aerial life, 135. Age of earth, 218, 221. Ages, geological, 220. Air, effect of heat upon, 68. Air currents, deflection of, by rotation, 39. Alaska, glaciers in, 308, 311, 312, 313. Algeria, high temperature of, 63. Alkaline plains, 394. Alluvial fan, 285, 288. Alpine glacier, 307. Alpine snow field, 306. Alps, 368; glaciers in, 308; valleys in, 271, 272. Altitude, effect upon temperature, 47. American Falls, Niagara, 295. Andromeda nebula, 17. Anemometer, 433. Aneroid barometer, 433. Animals, aid in disintegrating rocks, 235; effect on coast, 340; habits of, 141, 142 ; importance in ocean, 395 ; of ocean bottom, 156, 169, 171. Antarctic, icebergs in, 316 ; ice sheet of, 313. Antecedent valleys, 278, 365. Anticline, 208, 209. Anticyclones, 100. Anti-trade winds, 70, 74. Apogee, 13; effect of, upon tide, 200. Appalachian Mountains, 255, 368. Arctic climate, 132 Arctic, life in, 138. Arctic weather, 125. Argon in atmosphere, 24. Arid land drainage, 280 ; vegetation, 141, 142. Arroya, 279. Artemesia geyser, 387. Artesian wells, 229. Ash, volcanic, 371, 373. Asia, monsoons of, 77. Asteroids, 6, 11. Atlantic, 249; circulation of, 72, 73; coast plains, 254 ; cross-section of, 158, 251 ; temperature of, 181 ; tides^ of, 194 ; topography of bottom, 158 ; volcanoes in, 370 ; windj of, 72, 73. Atmosphere, 5 ; absorption of vapor by, 36 ; circulation of, 68 ; composition of, 23 ; cooling of, on ascension, 33 ; den- sity of, 23, 24 ; effect of earth's rota- tion on, 39; effect of gravity on, 39; effect of heat upon, 68; extent of, 23; moisture in, 35; pressure of, 39; saturation of, 36 ; warming of, 32, 33. Atmospheric circulation, parts of, 69. Atmospheric electricity, 29. Atmospheric movements, effect of, upon temperature, 44. Atolls, 342. Aurora, 29. Australia, animals of, 145 ; monsoons of, 77. Avalanche blast, 70, 82. Avalanches, effect upon rivers, 282 ; for- mation of, 241. Avalanche lake, N.Y., 299. 477 478 PHYSICAL GEOGRAPHY. B. Bad Lands, S.D., 247. Baker's Park, 357. Bank of river, 262. Banner cloud, 111. Barograph, 433. Barometer, 433 ; change during passage of hurricane, 86. Barometric gradient, Barrier reefs, 341. Bars, 331, 334, 335, 394, 395; in rivers, 288. Base level, 265. Basin of Minas, tidal flat in, 202. Basin Ranges, 258. Bay of Fundy, tides of, 196, 197. Bays, origin of, 276, 277, 329. Beaches, 335, 336, 395 ; abandoned, 349. Bermudas, depth of ocean near, 158. Black-bulb thermometer, 432. Blizzards, 100. Bonneville, Lake, 302. Borax, 422. Bosses, 212, 383. Boulder clay, 321. Boulders in moraine, 320, 321 ; on sea- coast, 336. Breakers on the coast, 174, 175. Brines, 422. British Isles, tides near, 194, 195. Bromine, 422. Building stone, 420. Butte, 353, 356, 383, 402. Buzzard's Bay, tides of, 197. Calm belts, migration of, 70, 76. Campos, 122. Canadian Highlands, 256. Canons, 240, 242, 267, 270. Canon of Colorado, 270, 352, 391. Cape Ann, Mass., coast of, 203, 334-338, 400, 401 ; moraine of, 320 ; salt marsh on, 339 ; sand dunes on, 238. Cape Cod, Mass., sea cliff, 328. Capes, origin of, 329, 345, 346. Carbonic acid gas in atmosphere, 24. Casco Bay, Me., islands of, 345. Cave, 227 ; river source in, 263. Caverns, formation of, 226. Cementing of rocks, 217. Centigrade scale, 431. Central plains, 256. Ceres, 11. Charleston earthquake, 384. Chasms, origin of, 400. Chemical deposits from underground water, 225. Cherrapunji, rainfall of, 122. Chesapeake Bay, origin of, 278. Chicago, lake breeze of, 80. Chili, changes of level in, 206. China, loess in, 399. Chinook wind, 99. Chromosphere, 7. Chronology, geological, 218. Circulation of atmosphere, 68 ; of ocean, 182; of water on ocean bottom, 163, 172. Circumpolar whirl, 75, 101. Cirro-cumulus cloud, 113. Cirro-stratus cloud, 113. Cirrus cloud, 113. Clays, 421. Climate, 129; of arctic zone, 132; changes in, 132, 317 ; effect upon lakes, 302 ; effect on man, 413 ; effect of upon streams, 267, 279; influence upon topography, 398 ; minor variations of, 132; of plateaus, 351 ; of St. Louis, 62; of San Francisco, 62; study of, 435; of temperate latitude, 130; of trop- ical regions, 130. Climatic zones, 129. Cloudbursts, 104, 123. Clouds, 111; kinds of, 112; study of, 434; of cyclonic storms, 94. Coal, 423. Coast, cause of irregularities of, 330; changes in, 343 ; destruction of, 332 ; effect of tides on, 201; effect of waves upon, 176. Coast line, 328, 395 ; of United States, 261. Coast Ranges, 259. Coastal plain, 254, 393. Cold pole, 56. Cold waves, 51, 100, 127, 128. INDEX. 479 Color, cause of, 29 ; of ocean water, 152. Colorado canon, 270, 352, 391 ; rapids in, 298. Colorado, mineral wealth of, 430. Comets, 6, 15. Complex valleys, 276. Composite valleys, 276. Concretions, 427. Conduction of heat, 32. Cone delta, 285. Cone of volcano, 378. Consequent river courses, 272. Constructional land forms, 392. Continental glacier, 307, 313, 318. Continental islands, 344. Continental shelf, 158. Continental slope, 159. Continents, 249 ; cause of, 390 ; change in form of, 252 ; features of, 251. Contorted limestone, 214. Contour interval, 439. Contour maps, 438. Contraction theory, 363. Convection, 32. Coral deposits, 395. Coral islands, 395. Coral keys, 341. Coral reefs, 168, 341. Corals, conditions favoring develop- ment of, 169 ; effect of Gulf Stream upon, 191; effect of ocean currents on, 342 ; effect of temperature on, 136 ; importance on coast, 340. Cordillera, 354. Cordilleras, age of, 259 ; minerals of, 259 ; ores of, 428, 429 ; volcanoes of, 371 ; of the west, 257. Corona, 7, 28. Crater of geysers, 388 ; of volcanoes, 379. Crevasse, 309. Crust of earth, 205 ; movements of, 206, 390 ; rocks of, 212. Crystalline rocks, 213. Cumberland valley, model of, 437. Cumulo-stratus clouds, 113, 114. Cumulus cloud, 113. Currents of ocean, 163, 172, 182, 328; deflection of, by earth's rotation, 39 ; effect of, on coast, 330, 332. Cyclonic storms, 85. D. Daily temperature ranges, 43, 59, 60 61, 65. Day, cause of, 13. Dead Sea, life in, 137. Death Valley, 258. Deccan, plateau of, 351. Deep-sea animals, oxygen supply of, 171. Deep sea, circulation of water in, 163, 172 ; dredging of, 155 ; exploration of, 153; life in, 169; light in, 163; sedi- ments of, 164; sounding of, 154; sounding machine, 154; temperature of, 162 ; trawl, 154, 155. Deforesting Adirondacks, 410. Delaware Bay, origin of, 277. Delta lakes, 300. Delta of Mississippi, 344. Deltas, 285, 331, 394 ; conditions favoring formation of, 286 ; in lakes, 287 ; rela- tion to floodplain, 288 ; rivers on, 287. Denudation, 246, 390, 393, 395-397 ; ab- sence of effects of, on ocean floor, 156 ; of land, 224 ; of mountains, 360, 361, 364, 367 ; of volcanoes, 379. Depression, effect on coast, 329. Depth of ocean, 157, 158. Desert dust whirl, 68. Deserts, cause of, 74, 117 ; life in, 141. Dew, 107. Dew point, 37. Diathermanous bodies, 30. Diffusion of light, 26. Dip of rocks, 209; relation of topog- raphy to, 402. ^ Dismal Swamp, 304, 425. u^"^ Dissection of valleys, 278. Distributaries on deltas, 287. Diurnal winds, 70, 76, 79. Diversion of streams by mountain growth, 278. Divide, 263 ; changes in, 273. Doldrums, climate of, 130; density of ocean in, 152 ; migration of, 70, 76, 122 ; rains of, 117 ; thunderstorms of, 102 ; weather of, 124. Donati's comet, 15. 480 PHYSICAL GEOGRAPHY. Dormant volcanoes, 377. Drainage of mountains, 365. Dredging, 155. Droughts, cause of, 126. Drowned rivers, 276, 277. Dust, effect of, on light, 26, 27 ; in at- mosphere, 24; importance in forma- tion of fog, 110. Dust whirl of the desert, 68. Dykes, 212, 383. E. Earth, 11; age of, 218, 221; condition of, 11; elevations on the surface of, 4; form of, 3; interior condition of, 11, 205 ; irregularities on surface, 3 ; movements of, 12, 33; movements of surface, 206; revolution of, 12; rotation of, 13 ; water on the surface of, 4. Earth columns, 232. Earth temperature, 65. Earthquake waves, 178. Earthquakes, 383 ; association with vol- canoes, 381 ; cause of, 385. Eastern mountains, 254. Eastport, Me., tides at, 199, 200. Ebb of tide, 192. Eclipse breezes, 70, 76, 82. Electricity, 29. Elevation, effect on coast, 329. Elk Mountains, Col., 354. English Channel, tides of, 194, 195. Epicentrum of earthquakes, 384. Erosion, agents of, 238 ; by glaciers, 245 ; by oceanic forces, 244, 328; by rain, 239 ; by rivers, 241, 265, 268 ; by under- ground water, 240 ; of volcanoes, 379 ; by wind, 238. Eruption of geysers, 389. Estuary, filling with salt marsh, 339. Estuaries, origin of, 276, 277, 329. Eurasia, map of, 250. Europe, glaciation of, 318. Evaporation of water, 31, 35, 39, 120; measurement of, 434. Extinct lakes, 302; volcanoes, 378, 381. Eye of storm, 87, 96. P. Fahrenheit scale, 431. Fall line, 296. Fan delta, 285. Fathom, 154. Fault, 210../-" Fault plane, 210. Faults, association of earthquakes with, 386 ; relation of ores to, 428. /■ Faunas of ocean bottom, 169 ; of ocean surface, 166. Fertilizers, 422. Finger Lakes, origin of, 325. Floe ice, 315. Floodplains, 288, 394; characteristics of, 291 ; building of, 293. Floods, influence of forest upon, 410. Florida, growth of, 347; keys of, 341; lakes of, 300; swamps of, 303, 424. Flow of tide, 192. Focus of earthquakes, 384. Foehn wind, 99. Fog, 109. Food of ocean animals, 168. Food supply, effect on ocean life, 171. Forest in Adirondacks, 409. Forest, importance of, 409 ; influence on development of United States, 417; influence of man upon, 409 ; on moun- tains, 360. Forest litter, 410. Fossils, value of, 219. Fresh water life, 137. Frost, 108; action on mountain peaks, 361 ; aid in disintegrating rocks, 234. Fusiyama, Japan, 380. G. Ganges delta, effect of hurricane on, 89. Garden of Gods, Col., 231. Gas, 425. Gassendi, lunar crater of, 14. Gay Head, retreat of, 333. Geographic distribution of animals and plants, 135. Geological ages, 220. Geological chronology, 218. Geysers, 386, 387. INDEX. 481 Glacial deposits, 394 ; formation of lakes by, 299 ; production of waterfalls by, 294 - ^ Glacial erosion, 245. Glacial lakes, 299, 317, 323. Glacial period, 133, 316 ; effect upon life, 146 ; effect upon streams, 280 ; time of, 319. Glacial scratches, 322. Glacial soil, 321. Glaciers, Alpine, 307; in Antarctic, 313, 316; cause of, 122, 306; continental, 307, 313, 318 ; effect upon valleys, 280 ; in Greenland, 313, 314 ; Piedmont, 313 ; relation to swamps, 304. Globigerina ooze, 164; area of deposit, 166. Gneisses, 420. Gold deposits, 428. Gorges, caused by glacial action, 281 ; formation of, 242, 325 ; in mountains, 271, 272, 358, 365 ; near Ithaca, N.Y., 215, 265. Graham's Island, 346. Granite, 420 ; disintegration of, 233. Gravity, aid in erosion, 240; effect on atmosphere, 39. Great Barrier Reef, Australia, 341. Great Basin, 258, 281, 357 ; drainage of, 279; mountains of, 258; temperature of, 56. Great Lakes, effect of ice on, 281 ; origin of, 325 ; winds of, 80. Great Salt Lake, 303 ; former extension of, 281. Greenland, glaciers of, 313, 314; winds of, 78. Green River, Utah, 278. Ground moraine, 312. Gulf of Mexico, temperature of bottom, 163. Gulf Stream, 183, 187; effect on corals, 191, 347 ; effect on life, 167, 168 ; effect on temperature, 51, 55, 190; map of, 188 ; velocity of, 187. Gulf weed, 136. Gypsum, 422. Hachure maps, 437, 438. Hail, 116. Halo, 28. Harbors, sea action in, 334. Hawaiian Islands, volcanoes of, 377, 380. Haze, 110. Heat, 30 ; absorption of, 30 ; distribution of, 43; effect of movements of the earth upon, 33 ; effect upon air, 68. Heat equator, 53, 55. Heat lightning, 30. Heligoland, destruction of, 332. Hell Gate, tide at, 196, 198. Hercuianeum, destruction of, 376. High pressure, 433. Hills of circumdenudation, 356. Himalayas, 110, 368. Hooks, 333, 334, 347. Horse latitude winds, 70, 75. Hot springs, 386. Humidity, absolute, 37; relative, 37; measurement of, 434; variation in, 38. Hurricane, 86 ; cause of, 91 ; cause of path of, 93; destruction caused by, 89 ; difference from temperate latitude cyclones, 95; effects of, 88; features of, 87 ; importance of vapor in, 92 ; paths of, 89, 90, 97 ; pressure in, 86, 88 ; reason for absence from South Atlan- tic, 92 ; reason for development over ocean, 92; resemblance to temperate latitude cyclones, 93 ; size of, 90 ; time of occurrence of, 91; violence of, 90; winds of, 87, 88. Hygrometer, 434. I. Icebergs, 314-316. Ice cave, 312. Ice fall, 309. Igneous rocks, 213, 420 ; relation of ores to, 428. India, monsoon of, 77. Indianola, Tex., destruction by hurri- cane, 89. Interior basins, 356. Intruded rocks, 212, 213. Irregular winds, 70, 82. Island life, 145. 2i 482 PHYSICAL GEOGRAPHY. Islands, destruction of, 346; origin of, 329, 344 ; volcanic, 244. Isothermal charts, 51. Isotherms, 51 ; of New York, 56, 59 ; of United States, 54, 56-58; relation to climate, 62. Ithaca, N.Y., change in harometer at, 86 ; cold wave at, 127, 128 ; gorges near, 265, 297; humidity changes in, 37; temperature changes in, 61, 66; valley breeze at, 81 ; waterfall near, 297. Japan, earthquake in, 385-387. Japanese current, 184 ; effect on temper- ature, 190. Jupiter, 9. K. Key West, temperature of, 53, 56, 63, 65. Keys, 341. Krakatoa, eruption of, 374, 375, 380, 381, 385. Kurile Islands, depth of ocean near, 160. Kuro Siwo, 184. L. Labrador current, 189 ; effect upon tem- perature, 53, 55, 168. Lagoon, 348. Lake Agassiz, 324. Lake Bonneville, 302. Lake breeze, 80. Lake Champlain, origin of, 325. Lake Drummond, origin of, 300. Lake Erie, destruction of, by Niagara, 301. Lake spit, 333. Lakes, 298, 394; caused by beach bar- riers, 335, 348; caused by lava, 374, 381 ; deltas in, 287 ; destruction of, 300 ; extinct, 302 ; on floodplains, 292 ; glacial, 281, 317, 323 ; in Adirondacks, 409 ; in mountains, 366, 367 ; in young valleys, 269 ; relation to swamps, 303 ; shores of, 328, 348, 394. Land breeze, 70, 79. Land, denudation of, 224 ; effect on tem- perature, 55 ; effect on tide, 193 ; ele- vation of, 206; life, 135, 137; move- ment, effect on coast, 329; topography of, 390. Land-locked animals, 137. Landslide, formation of, 241. Landslip blast, 70, 82. Latent heat, 31, 35, 39. Lateral moraine, 310, 312. Lava, 371. Lava flow, 211, 372. Lava plateaus, 351. Lawrence, Mass., tornado, 105. Levees, 293. Life, barriers to the spread of, 146 ; de- struction by volcanic eruption, 381; effect of man upon, 146, 147 ; effect of ocean currents on, 191 ; of the air, 135 ; of the arctic zones, 138 ; of the dead seas, 137 ; of the deserts, 141 ; of the fresh water, 137; of the land, 135, 137; of the mountains, 140; of the ocean, 135, 166; of the ocean bottom, 153, 156, 169; of the ocean bottom, oxygen supply of, 171 ; of the ocean shore, 167 ; of the temperate zones, 139; spread of, 145. Life zones, 135, 143; of United States, 144. Light, 25 ; absorption of, 28 ; diffusion of, 26; effect of dust on, 26, 27; on ocean bottom, 163; reflection of, 27; refraction of, 27 ; selective scattering of, 26; source of, 25. Lightning in thunderstorms, 29, 102, 104. Limestone, 421. Line storm, 91. Lipari Islands, volcanoes of, 375. Littoral faunas, 167. Llanos, 122. Loess in China, 399. Longitudinal valleys in mountains, 358, 365. Long's Peak, Col., 360. Looming, 27. Low pressure, 433. Low-pressure areas, tracks of, 95-97. Lunar craters, 15. INDEX. 483 M. Magnetic pole, 29. Magnetism, 29. Malaspina glacier, 313. Mammoth hot springs, 225. Man and nature, 407. Man and the forest, 409. Man, effect in distributing life, 146, 147 ; modifying influence of, 407. Mangrove, 338. Mangrove swamps, 424. Marble, 421. Mars, 9. Marsh grass, 339. Massachusetts, lakes in, 324. Massachusetts Bay, tides of, 197. Mato Tepee, Wyo., 383. ^^Matterhorn, 355. Mature adjustment of streams, 273. Mature river valleys, 266, 267. Maximum temperature in United States, 64. Maximum thermometer, 432. Mechanical sediments in ocean, 164. Medial moraine, 310, 312. Mediterranean, temperature of water in, 162, 163; tides of, 197. Mercury, 8. Mesas, 353, 383. Metals, 426 ; of Cordilleras, 259. Metamorphic rocks, 213, 214, 420. Meteorites, 16. Meteors, 6, 15, 16. Michigan, mineral wealth of, 429. Mid- Atlantic ridge, 159. Mineral waters, 423. Minerals, 213, 426; of the Cordilleras, 259; disintegration of, 233; effect of water upon, 226 ; of United States, 429. Minimum temperatures in United States, 63. Minimum thermometer, 432. Minnesota, lakes in, 324. Mirage, 27. Mississippi, delta of, 286, 344 ; floodplain of, 290, 291, 292. Mississippi valley plains, 256. Mist, 111. Mitchell's Peak, height of, 256. Models, 437. Moisture, effect upon life, 141; in the atmosphere, 35 ; measurement of, 434. Monocline, 208. Monoclinal shifting, 274. Monsoon winds, 70, 77 ; effect upon cli- mate, 130 ; effect upon rainfall, 122. Montana, mineral wealth of, 429 ; tem- perature changes in, 56, 64, 65. Monte Somma, 376, 380. Moon, 13 ; effect in producing tide, 192, 199-201. Moqui Pueblo, N.M., 239. Moraines, 310, 312, 317, 319, 394. Mount Dana, glaciers on, 310. Mount Desert, Me., 411; coast of, 330. Mount Everest, 110. Mount Hood, 378. Mount of Holy Cross, Col., 359. Mount Marcy, height of, 256. Mount St. Elias, 139 ; glaciers of, 312. Mount Shasta, 382 ; glaciers on, 307. Mountain breeze, 70, 80. Mountain gorges, 358. Mountain thunderstorms, 102. Mountain valleys, 271, 356. Mountain vegetation, 140. Mountains, association of volcanoes with, 370; association with plateaus, 350 ; cause of, 390 ; characteristics of, 353 ; in continents, 251 ; denudation of, 396, 404 ; destruction of, 367 ; drain- age of, 365 ; of eastern United States, 254 ; effect of growth of, upon streams, 278; effect upon temperature, 48; floodplains among, 289; glaciers in, 307; of Great Basin, 258; growth of, 363, 367; life in, 140; origin of, 362, 393; ruggedness of, 361, 396; sculp- turing of, 364 ; valleys in, 262-265 ; of the west, 257. Mud flow, 372. Muir's Butte, Cal., 379. N. Natural bridge, origin of, 228. Natural gas, 425. Natural soda, 422. 484 PHYSICAL GEOGRAPHY. Nature and man, 407- Nature, influence upon man, 412. Navajo Church, Arizona, 397. Neap tides, 200. Nebulse, 17, 21. Nebular hypothesis,19; verification of ,20. Neptune, 10. New York, isotherms of, 56, 59 ; tempera- ture of, 51. New York harbor, tides of, 194. New Zealand, animals of, 145. Niagara, effect in draining Lake Erie, 301. Niagara Falls, 264, 295, 301 ; age of, 222 ; history of, 294 ; origin of, 298. Night, cause of, 13. Nimbus clouds, 113, 114. Nitrogen in atmosphere, 23. North America, cross-section of, 251; glacier of, 318; shore line of, 261. North Atlantic drift, 183. Northeast storms, 94. Norther, 100. Nunatak, 314. O. Oblong geyser, 388. Occupation, relation to topography, 418. Ocean, area of, 4, 151; deposits in, 395; depth of, 160, 161 ; effect in checking spread of life, 146 ; effect of, on tem- perature, 45; erosion in, 244, 328; phosphorescence in, 152, 164; shores of, 328; surface temperature of, 179; volume of, 4 ; volcanoes in, 370. Ocean basins, 249. Ocean bottom, circulation of water on, 163, 172 ; dredging of, 155 ; exploration of, 153, 156; life on, 153, 166, 169; light on, 163 ; sediments of, 164 ; tem- perature of, 155, 162 ; topography of, 156, 160, 250. Ocean currents, 182; cause of, 185; cause of course, 187; effects of, 189; effect on life, 146, 166 ; effect on tem- perature, 46, 180; on ocean bottom, 163, 172 ; system of, 183. Ocean water, color of, 152; composi- tion of, 151 ; density of, 152. ' Oceanic islands, 244, 344. Oceanic life, 135, 166; habits of, 169; influence of temperature upon, 136. Oceanic plateau, 157, 159. Oil, 425. Old Faithful Geyser, 389. Opaque bodies, 29. Ore deposits, 426. Oxbow cut-off lakes, 266, 292, 300. Oxygen, in atmosphere, 23 ; supply of, to deep-sea animals, 171. Pacific Ocean, 249; topography of bot- tom, 160; volcanoes in, 370. Parks in mountains, 357, 358. Passes in mountains, 359. Path of storms, 89, 94-97. Peaks in mountains, 355. Peaks, origin of, 404. Peat bogs, 304, 425. Pecos River valley, N.M., plain of, 350. Pelagic faunas, 166. Pennsylvania, mineral wealth of, 429. Percolating water, importance of, 240. Perigee, 14 ; effect upon tide, 200. Periodical winds, 70, 76. Permanent winds, 70, 71. Petroleum, 425. Phosphates, 422. Phosphorescence in ocean, 152, 164. Photosphere, 7. Piedmont glacier, 313. Pike's Peak, 355. Placer deposits, 428. Plains, 350; of Atlantic coast, 254; in continents, 251 ; of Far West, 351 ; of Mississippi valley, 256 ; origin of, 393 ; of Red River valley, 394. Planetary circulation in ocean, 182. Planetary winds, 70, 71. Planets, 6, 8 ; relative distance of, 5, 8 ; relative size of, 9. Plants, aid in disintegrating rocks, 234; effect of, on coast, 337 ; habits of, 141 ; in the ocean, 136, 395. Plateau, 350; association with moun- tains, 350; of continents, 251; of ice, 314; of Mississippi valley, 256; of ocean bottom, 157, 159, 250. INDEX. 485 Platinum, 428. Pompeii, destruction of, 372, 376. Porto Rico, depth of ocean near, 157, 160. Prairie soil, 323. Prairies, 257, 351, 394. Pressure of atmosphere, 39. Pressure in hurricane, 88. Pressure, measurement of, 432; relation to winds, 70. Prevailing westerlies, 70, 75, Promontories, origin of, 329, 345, 346. Psychrometer, 434. Pulpit terrace, 225. Pumice, 211, 371. R. Radiant energy, 30 ; effect upon water, 31 ; effect upon the land, 31 ; passage through the atmosphere, 31 ; reflection of, 30. Radiation from the earth, 32. Rafe's Chasm, 400. Rain, cause of, 114. Rain erosion, 239. Rain gauge, 435. Rain in thunderstorm, 104. Rainbow, cause of, 28. Rainfall, distribution of, 117 ; in dol- drum belt, 74 ; effect of forest on, 412 ; irregularities of, 123; measurement of, 435 ; seasonal distribution of, 122 ; in trade-wind belt, 74 ; of the United States, 118. Ranges of mountains, 354. Rapids, relation to waterfalls, 294. Ray Brook, Adirondack^, 304. Red clay, 165. Red River valley, effect of ice on, 281 ; lake in, 324 ; plains of, 350, 394. Red Sea, cause of color of, 152. Reefs, coral, 341. Reflection of radiant energy, 30. Refraction of light, 27. Rejuvenation of river valleys, 276. Relative humidity, 37, 434. Replacement deposit, 427. Residual soil, 238. Revived rivers, 276. Revolution, effect of, upon temperature, 33. Rhone glacier, 308. Ridges, mountain, 354, 361, 368, 405. Right-hand deflection, 40. Rio Grande valley canon, 142; talus in, 236. River bank, 262. Rivers, boulders in bed of, 243; acci- dents to, 275; characteristics of, 263; deposits by, 394; divide of, 273; effect of forest on, 410; erosion of, 241, 243; on floodplains, 291 ; at margin of ice, 312, 322 ; in mountains, 365 ; relation of lakes to, 299 ; sediment in, 241 ; of United States, 259, 260. River system, 263. River valleys, 262 ; adjustment of, 272 ; drowned by sea, 330 ; development of, 265; difference in rate of develop- ment of, 270; effect of climate on, 279 ; origin of, 264 ; variation among, 244. Rock basins, 325. Rock pillars, 231. Rock salt, 422. Rocks, consolidation of sedimentary, 217; deposition of sedimentary, 215; disintegration of, 233; disturbance of, 207; durability of, 231; of earth's crust, 212 ; elevation of, 216 ; horizon- tal, 208 ; igneous, 213 ; influence of, on form of crust, 334; influence upon stream course, 272; influence upon topography, 208, 395, 402-405; in- truded, 212, 213; metamorphic, 213, 214; of mountains, 355, 362; sedimen- tary, 213, 214. Rocky Mountains, 257, 368. Rotation, deflective effect of, 39 ; effect of, on temperature, 33. Royal Gorge, Col., 265. S. St. Anthony, Falls of, 296. St. Louis, temperature of, 62. Salt lakes, 302. Salt marsh, 332, 339. Salts in the ocean, 151. 488 PHYSICAL GEOGRAPHY. Samoan Islands, hurricane of, 88. San Francisco, temperature of, 62. Sand bars, 331. Sand dunes, 239, 394. Sands, 421. Sandstone, 421. Sargasso Sea, 136, 167. Satellites, 6. Saturation of atmosphere, 36. Saturn, 10. Sea breeze, 45, 70, 79. Sea caves, 334, 335, 400. Sea cliffs, 347, 400, 401, 403; Cape Cod, Mass., 328; retreat of, 332. Seasonal temperature range, 43, 48, 49, 51. Seasonal winds, 70, 76. Seasons, 12, 13, 33. Seaweeds, importance on coast, 337, 338. Secondary storms, 101. Sediment, effect of, on coast, 330 ; on ocean bottom, 164 ; in rivers, 241. Sedimentary rocks, 213, 214, 330, 421 ; consolidation of, 217 ; deposition of, 215. Seeds, aid in distribution of plants, 141. Seiches, 198. Selective scattering, 26. Shastina, 382. Shooting stars, 15, 16. Shore faunas, 167. Shore lines, 328, 395 ; above sea level, 207 ; change in, 343, 400 ; effect of tide on, 201 ; fossil, 349 ; of lakes, 348 ; of United States, 261. Shrunken streams, 279. Siberia, low temperature of, 56, 63. Sierra Nevada Mountains, 258. Signal Butte, 402. Sigsbee deep-sea sounding machine, 154. Silver deposits, 428. Sink-holes, 226. Sirocco wind, 99. Slate, 421. Small planets, 11. Snake River valley, lava plateau of, 351, 373. Snow, 115. Snowfall, distribution of, 121 ; measure- ment of, 435. Snow field, 306, 308. Snowflakes, 115. Snow line, 139, 140. Soil, 420 ; effect of forest on, 441 ; for- mation of, 237 ; glacial, 321. Solar light, 25. Solar system, 5; symmetry of, 18. Sounding, 153. Sphagnum moss, 304. Spits, 333, 334, 347, 394. Spring tide, 199. Springs, effect of forest on, 410 ; origin of, 228. Stalactites, 227. Stalagmites, 227. Stars, 17, 18. Steam in volcanoes, 372, 383. Stellar system, 17. Storms, 85 ; conditions in, 88, 94 ; of secondary origin, 101 ; tracks of, 89, 94-97 ; waves accompanying, 177, 179 ; winds of, 70, 82, 85, 94, 98. Straits, origin of, 276, 277. Strata, 216 ; influence on topography, 401-405 ; in mountains, 364. Stratification, 216. Stratified rocks, 215. Stratus clouds, 113, 114. Stream gold, 428. Strike, 209. Summer, temperature of, 50. Sun, 6; effect in producing tide, 193, 199, 201 ; movements of, 8. Sun spots, 8. Sunset colors, 26. Surface faunas in ocean, 166. Swamps, 303, 394 ; of Florida, 424, 425 ; of glacial origin, 281, 283 ; mangrove, 339. Sweden, changes of level in, 206. Syncline, 208. Synclinal mountains, 369. System of mountains, 354. T. Talus, 236, 240, 354. Taughannock Falls, 294. Temperate climate, 130. Temperate latitude cyclones, 86 ; cause of, 100 ; cause of path of, 101 ; differ- INDEX. 487 ence from hurricanes, 95 ; effects of, 98 ; features of, 94 ; path of, 97 ; rela- tion of, to thunderstorms, IG3 ; resem- blance to hurricanes, 93 ; size of, 96 ; time of occurrence of, 96; winds of, 94,98. Temperate latitude, weather of, 125. Temperate zone, life in, 139. Temperature, of Atlantic, 181 ; daily ranges in, 59, 65; in cold wave, 127, 128; of earth, 65, 205; effect of alti- tude upon, 47 ; effect of atmospheric movements upon, 44; effect of land upon, 55, 56, 57; effect upon land life, 137 ; effect upon mountain life, 140; effect of mountains upon, 48; effect of ocean upon, 45; effect of ocean currents on, 46, 189 ; effect upon ocean life, 136; effect of sea breeze on, 79, 80; effect of topography upon, 47, 56; of Great Basin, 56; of Key West, 53, 55, 56; maximum, in United States, 64; measurement of, 431; of midsummer, 50; of midwinter, 50; minimum, in United States, 63 ; ranges in, 61, 62, 64 ; seasonal range of, 35, 43, 48 ; of St. Louis, 62 ; of San Francisco, 62; of United States, 53; variation of, 35, 43, 51, 60, 61. Temperature of ocean, 180; effect on circulation, 182, 185; effect on life, 166, 168, 170. Temperature of ocean bottom, 155, 162, 170. Temperature of ocean surface, 179, 181. Terminal moraine, 310, 312, 319. Terraces, 323, 400. Texas, bars on coast of, 331 ; monsoons of, 78 ; temperature changes in, 65. Thermograph, 432. Thermometer, 431. Thermometer shelter, 432. Thibet, temperature ranges in, 65. Thousand Islands, origin of, 348. Thunder, 30, 102, 104. Thunderstorms, 101-103. Tidal action in ocean, 328. Tidal bore, 198. Tidal breezes, 70, 76, 82. Tidal currents, importance of, 201, 333. Tidal height, causes for variation in, 193- 203. Tidal flat, Basin of Minas, 202. Tidal races, 198. Tidal wave, 192. Tide-power, uses of, 202. Tides, cause of, 192; effects of, 201; effect of coast upon, 193-198; in Eng- lish Channel, 194, 195 ; in New York harbor, 194. Till, 321, 394. Timber line in mountains, 138, 140, 359, 360. Tin, 428. Topographic maps, 437. Topography, influence upon climate, 47, 56 ; influence on man, 413^419 ; of bottom of Atlantic Ocean, 158; of glaciated regions, 320, 326 ; of the land, 390; of ocean bottom, 156, 160, 161, 250 ; relation to rock structure, 395. Tornadoes, 104. Trade-wind belt, 70, 71 ; climate of, 130; effect on oceanic circulation, 186; rain caused by, 117 ; weather in, 124. Translucent bodies, 29. Transparent bodies, 28. Transverse mountain valleys, 365. Trawl, deep-sea, 155. Tributaries of river, 263, 269 ; on flood- pi ains, 293. Tropical climate, 130. Tropical cyclones, 86. Tropical forest, 143. Tropical weather, 124. Typhoons, 86. U. Unconformity, 217. Underground water, 224, 233, 240, 386. Undulatory theory, 25. United States, drainage of, 259, 260; evaporation in, 120; ice sheet of, 318; isotherms of, 53, 54, 56-58; life zones of, 144; maximum temperature of, 64 ; mineral wealth of, 429 ; mini- mum temperature in, 63: monsoon tendency in, 78; ores of, 428; physical geography of, 253; rainfall of, 118, 119, 122; shore line of, 261; temper- 488 PHYSICAL GEOGRAPHY. ature ranges in, 62, 64 ; terminal mo- raine of, 320 ; volcanoes in, 259, 371. Uranus, 10. Valley breeze, 70, 80. Valley fog, 109. Valley glaciers, 307; former extension of, 317. Valley sides, 262. Valleys, development of, 242, 262, 267, 270; effect of climate on, 279; effect of land movements on, 276 ; in moun- tains, 356. Vapor, absorption of, 36 ; importance of, in hurricanes, 92 ; variation in amount, 36. Vegetation, in arid land, 141, 142; in mountains, 140 ; in swamps, 303. Veins, 427. Venus, 9. Vesuvius, 372, 376, 380. Vineyard Sound, tides of, 197. Volcanic action, 211. Volcanic ash, 211, 371, 373. Volcanic cone, form of, 378. Volcanic island, 244. Volcanic necks, 382, 383. Volcanic winds, 70, 83. Volcanoes, association with atolls, 343 association of earthquakes with, 385 association of hot springs with, 387 association with ores, 428; cause of, 383; destruction of, in sea, 346; dis- tribution of, 370 ; effect of eruptions, 381 ; effect upon rivers, 282 ; eruptions of, 374 ; extinct, 378, 381 ; materials erupted by, 211, 371; in ocean, 156; origin of, 393 ; of United States, 259. Vulcano, 375. W. Water, area of, on earth, 151; effect upon rocks, 231, 233; importance in volcanoes, 383 ; underground, 224, 240. Water vapor in atmosphere, 24. Waterfall breeze, 70, 83. Waterfalls, 268, 281, 294, 297, 325. Water parting, 263. Waterspout, 106. Waterspout waves, 179. Watkins Glen, N.Y., 326. Waves, 174 ; action of, on coast, 176, 244, 328, 330, 332; cause of, 176; earth- quake, 178, 385 ; form of, 175 ; storm, 179. Weather, 124 ; arctic, 125 ; temperate latitude, 125 ; tropical, 124 ; study of, 435. Weather maps, 435. Weather predictions, 435. Weathering, 233 ; effects of, 236 ; impor- tance of, 235, 265; of volcanoes, 379. Westfield River, Mass., 243. White glacier, Alaska, 311. White Mountains, N.H., 356. Whitney glacier, 307. Wind vane, 433. Wind waves, 174. Winds, accidental, 70, 82 ; action of, 393 ; aid in causing rain, 117; aid in distri- bution of animals, 145 ; of Atlantic, 72, 73 ; classification of, 70 ; in cold wave, 127, 128; diurnal, 70, 76, 79; effect upon height of tide, 198 ; effect upon temperature, 44; erosion by, 238; in the general circulation, 69; of horse latitude belt, 75 ; of hurricane, 87, 88 ; internal work of, 83 ; irregular, 70, 82 ; irregularities of, 83; measurement of r 433; migration of, 76; monsoon, 70, 77; nature of, 83; periodical, 70, 76: permanent, 70,71; planetary, 70, 71; seasonal, 70, 76 ; of storm, 85, 94 ; of temperate latitude cyclones, 94, 98; of temperate latitudes, 75 ; in thunder- storms, 103; of the tornado, 105; ver- tical movement in, 83. Winter, temperature of, 50. Winter thaws, cause of, 127. Withered streams, 279. Yellow Sea, cause of color of, 152. Yellowstone Falls, 293. Yellowstone Park, geysers of, 387-389. Yellowstone Valley, 242, 268. Yosemite, 296, 398. Youth in river valleys, 266. «7 <4 UNITED STATES, WITH BRIEFER MENTION OF FOREIGN MINERAL PRODUCTS, By RALPH S. TARR, B.S., P.G.S.A., Assistant Professor of Geology at Cornell University. Second Edition, Revised. S3.50 COMMENTS, " I am more than pleased with your new ' Economic Geology of the United States.' An introduction to this subject, fully abreast of its recent progress, and especially adapted to American students and readers, has been a desideratum. The book is admirably suited for class use, and I shall adopt it as the text-book for instruc- tion in Economic Geology in Colorado College. It is essentially accurate, while written in a pleasant and popular style, and is one of the few books on practical geology that the general public is sure to pronounce readable. The large share of attention given to non-metallic resources is an especially valuable feature." — Francis W. Cragin, Professor of Geology, Mineralogy, and Paleontology at Colorado College. 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PRESS NOTICES. **It is a book that old and young will learn much from, and that by stimu- lating an interest in natural phenomena will lead to a more systematic study of the great laws that govern the processes of growth and transformation in the material world about us." — Boston Beacon. " A delightful book. A careful perusal of the volume excites surprise that so large an amount of scientific knowledge, covering a great array of points per- tinent to the subject, could be presented in a form that can easily be grasped by readers of ordinary capacity." — New York Observer. " The book is filled full of novel discussions, of interesting facts, and of dis- coveries which could be made only by the closest study by the enthusiast in this especial department of research. With it all there is not a dull chapter or one which the reader will willingly leave unread." — Boston Daily Advertiser. THE MACMILLAN COMPANY, 66 FIFTH AVENUE, NEW YORK. HOURS IN MY GARDEN, AND OTHER NATURE SKETCHES. By A. H. J APR With 138 Illustrations by W. H. J. Boot, A. W. Cooper, and other artists. Cloth, $1.75. " A glance through the pages of Dr. A. H. Japp's ' Hours in my Garden ' leaves one with an agreeable impression of having enjoyed a summer ramble in the country. The little volume is made up of nineteen ' nature sketches,' large- ly the fruit of personal observation, and it is well freighted with the lighter lore of the woods and fields, ponds and streams, hedgerows and coppices of Old England. The style of the book recalls Richard Jefferies, but there is more lite- rary allusion, and the author has evidently looked at nature through spectacles more scientific than poetical. The little essays are pleasantly written, and are well adapted to stimulate young readers to a systematic study of nature. The one hundred and thirty-eight woodcuts are nicely done, and add to the educative value of the text." — The Dial. " Lovers of nature have for years past been accustomed to derive enjoyment and instruction from the essays and field notes of Thoreau, and, more recently, John Burroughs, Frank Bolles, and two or three others on this side the water. And now comes an echo from the other side of the Atlantic, in this handsome and admirably illustrated volume, telling us what the author has seen and deems worthy of note in the mother country. My Garden Summer-seat, My Pond, My Wood, Up in the Morning Early, The Village Well, A Scottish Trout Stream, Wild Ducks, Water Birds, and Sea Fowl are the titles of some of the chapters. He is an eager and careful observer, he finds much to tell us, and his descriptions are charming both in subject and style. The author has given reason to hope ' that his essays may not be found other than pleasant reading, and that young folks here and there may derive some stimulus to more systematic study of nature than he was fortunate enough to have the chance of making while still young.' It is a book to be read and enjoyed by old and young." — Public Opinion. " A book that should be read by every one who delights in the study of nature. . . . Bird-lovers will be especially charmed with the results of his close intimacy with English feathered folk. He has found wonderful sagacity in the dainty den- izens of the woods and fields, and he describes many of their habits that are almost human in their reasonableness and wisdom." — The Delineator. " Mr Alexander H. Japp has written a series of nature sketches which will multiply the hours devoted to them to seasons of delight and enlarge the place in which they may be read into a garden beside which that of Boccaccio was a com- mon everyday field. He discourses about the characteristic features of English scenery, cultivated and wild, if there can be said to be any wild scenery in Eng- land. . . . Mr. Japp is evidently a naturalist, but he is more than that, for his pages are instinct with feeling as well as observation, and are, if one may say so, alert and alive. It is not a book to be described, but to be read in the spirit in which it is written, carefully and lovingly." — Mail and Express. THE MACMILLAN COMPANY, 66 FIFTH AVENUE, NEW YORK. MACMILLAN'S SCIENCE CLASS-BOOKS. F'cap 8vo. ELEMENTARY PHYSICAL GEOGRAPHY. By Ralph S. Tarr, S.G.F.A., Assistant Professor of Geology and Physical Geography at Cornell University. $1.40. LABORATORY MANUAL AND PRINCIPLES OF CHEMISTRY FOR BEGINNERS. By George M. Richardson, Associate Professor of Chemistry in the Leland Stanford Jr. University. $1.10. PHYSIOLOGY FOR BEGINNERS. By Michael Foster, M.D., F.R.S., and L. E. Shore, M.D. Fully 111 ..rated. 75 cents. LESSONS IN APPLIED MECHANICS. 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Questions on the Above for Schools. By J. Forbes-Robertson. 40 cents. LESSONS IN ELEMENTARY CHEMISTRY. By Sir H. E. Roscoe, F.R.S. $1.25. " Much new matter has been added to keep the book up to date. We have always considered it the best work for those who wish to get a clear and connected knowledge of the outlines of Inorganic and Organic Chemistry..* — yournal of Education. SCIENCE CLASS-BOOKS. " It still holds its position among the very best text-books of elementary ehemistry." — School Board Chronicle. Problems adapted to the Above. By Professor Thorpe and W. Tate- With Key. 65 cents. INORGANIC CHEMISTRY FOR BEGINNERS. By Sir Henry Roscob, F.R.S., assisted by Joseph Lunt, F.C.S. 75 cents. OWENS COLLEGE JUNIOR COURSE OF PRACTICAL CHEMISTRY, By F. Jones. With Preface by Sir H. Roscoe, F.R.S. 70 cents. " It is eminently practical. The text is concise, and at the same time accu- rate. The instructions concerning experiments are clear, and calculated to develop observant habits. 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This book is intended to come to the help of the stu- dent in his transition stage, and contains an exceptionally clear concise explanation of chemical theory." — Journal of Education. LESSONS IN ELEMENTARY PHYSIOLOGY. By Rt. Hon. T. H. Huxley, F.R.S. $1.10. " It is an admirable illustration of how the greatest master of a science may teach its elements in the most simple manner." — Medical Press. " A very useful little manual, which should be received with acclama- tion." — - Spectator. Questions on the Above for Schools. By Alcock. 40 cents. LESSONS IN ELEMENTARY ANATOMY. By St. G. Mivart, F.R.S. $i.7S. " It may be questioned whether any other work on anatomy contains in like compass so proportionately great a mass of information." — Lancet. " Its utility to the general reader who desires, in a small space, to be acquainted with the leading facts and generalizatio.is of modern comparative anatomy is manifest." — Pall Mall Gazette. ELEMENTARY LESSONS IN PHYSICAL GEOGRAPHY. By Sir Archibald Geikie, F.R.S. Illustrated. $1.10. " The language is always simple and clear, and the descriptions of the various phenomena are no less vivid than interesting: the lessons are never dull, never wearisome, and they can scarcely fail to make the study of Phys- ical Geography popular wherever they are used." — Academy. Questions on the Same for Schools. i8mo. 40 cents. THE MACMILLAN COMPANY, 66 FIFTH AVENUE, NEW YORK. MAR 1959 *}SS