'••V'-V-. I m m ■ V l s vi«*/;SV'II I H HI as ■ I ■ HI ^■■IB HHH ■H ■ Jp ■ST ■^■^■H Era ■ Hi Bgsi ma mmmWmlBB^Bmm^Um^^Mm^BmlmmmW^EBSBaSm Class. . •Hi COPYRIGHT DEPOSIT THE ELEMENTS OF PHYSICAL GEOGRAPHY, FOR THE USE OF JHOOLS, ACADEMIES, AND COLLEGES. BY EDWIN J. HOUSTON, A.M., Ph.D., Emeritus Professor of Physical Geography and Natural Phi- losophy in the Central High School of Philadelphia; Professor of Physics in the Franklin Institute of the State of Pennsylvania. REVISED EDITION OF 1901. PHILADELPHIA: Published by ELDREDGE & BROTHER, No. 17 North Seventh Street. 1901. vi PREFACE. ticularly aimed at ; for which reason the names of authorities for statements which are now generally credited have been purposely omitted. The Author has not hesitated to draw information from all the standard works on Geography, Physics, Geology, Astronomy, and other allied sciences ; and in the compilation of the Pronouncing Vocabulary he acknowledges his indebtedness to Lippincott's Gaz- etteer of the World. Acknowledgments are due to Mr. W. M. Spackman, of Phila- delphia, and Prof. Elihu Thomson, of the Central High School, for critical review of the manuscript. Also to Mr. M. Benjamin Snyder, of the Central High School, for revision of the proof-sheets of the chapter on Mathematical Geography. E. J. H. Central High School, Philadelphia, Pa. PREFACE TO THE REVISED EDITION. »OXKOO Houston's Elements of Physical Geography has now been before the teaching public for more than a quarter of a century. During that time, in order to keep abreast with advances in the many separate physical sciences that are necessarily embraced under the wide scope of Physical Geography, many new editions of the book have b^en issued. But the advances, during comparatively recent years, have been so marked that at the earnest request of teachers from all parts of the country the author has prepared a new book, based on the general lines of the old work, but practically rewritten. For convenience, the shape of the new book has been changed to a duodecimo, the size of the maps, however, being retained, by making them occupy the space of two pages, across which inserts are made. PREFACE. vii In the preparation of the new book the author has endeavored so to proportion the subject matter as to give due prominence to such topics as, in his judgment, would prove of greatest value to the student. In this direction physiography has been treated more fully than in the older book. At the same time, however, he has studiously avoided giving such an undue prominence to this part of the science as would necessitate the suppression of equally important topics. The influence of geological agencies in giving the earth its present surface features has been fully treated, and yet not so fully as practically to exclude the influences of these features on the climate, and this, in its turn, on vegetable and animal life, especially on the development and civilization of earth's highest type of life, man. As in the old book, the logical order of sequence of topics has been carefully considered, since in this manner only can the best results be achieved during actual work in the school-room. The author has not hesitated to draw information from all standard works on Geography, Physics, Astronomy, Geology, Botany, Zoology, and other allied sciences. He acknowledges his indebtedness to many of his teaching friends throughout the country for valuable suggestions as to the subject matter, etc., of the new book. Especially is he indebted to his friend, Prof. B. W. Mitchell, for careful reading of the proof-sheets, of the entire book, and to Prof. Angelo Heilprin for a careful reading of the proof-sheets of the chapter on Zoological Geography. The author offers the new book to the teaching fraternity in the hope that it will prove of service to them in teaching this exceed- ingly attractive study. The Author CONTENTS. oXKc Introductory 11 PART I. THE EARTH AS A PLANET. CHAPTER PAGE I. Mathematical Geography ... 13 PART II. THE LAND. SECTION I.— The Inside of the Earth. I. The Heated Interior 40 II. Volcanoes 44 III. Earthquakes 55 SECTION II.— The Outside of the Earth. I. The Crust of the Earth ... 63 II. Distribution of the Land Areas 77 III. Islands 83 IV. Belief Forms of the Land . . 91 V. Eelief Forms of the Continents 100 PART III. THE WATER. SECTION I.— Continental Waters. I. Physical Properties of Water . 125 II. Drainage 128 III. Eivers 136 IV. Work of Eivers 142 V. Drainage Systems 149 VI. Lakes 152 viii SECTION II.— Oceanic Waters. CHAPTER PAGE I. The Ocean 162 II. Oceanic Movements 168 III. Ocean Currents 177 PART IV. THE ATMOSPHERE. SECTION I.— The Atmosphere. I. General Properties of the At- mosphere 187 II. Climate . 193 III. The Winds 201 IV. Storms 209 SECTION II.— Moisture of the Atmosphere. I. Precipitation of Moisture . 221 II. Hail, Snow, and Glaciers . . , . 236 III. Electrical and Optical Phenom- ena 250 PART V. PLANT LIFE, ANIMAL LIFE, AND MINERALS. SECTION I.— Plant Life. I., Plant Geography 267 II. Cultivated Plants 281 SECTION II.— Animal Life. I. Zoological Geography .... 290 II. The Distribution of the Human Eace 306 CONTENTS. IX SECTION III.— Minerals. CHAPTER PAGE I. Minerals 319 PART VI. THE PHYSICAL FEATURES OF THE UNITED STATES. I. Surface Structure 334 II. Meteorology 339 III. Vegetable and Animal Life . 346 CHAPTER PAGE IV. Agricultural and Mineral Pro- ductions 349 V. Alaska 355 VI. The Insular Possessions of the United States 358 General Syllabus 372 General Review Questions . . 378 General Map Questions .... 381 Index 386 oX*;c INDEX TO THE MAPS. between pages Map of Volcanoes and Regions of Earthquakes 58- 59 Map Showing Geological Formation of the Earth 66- 67 Map of Oceanic Basins, Areas, and Eiver-systems 142-143 Map of the Ocean Currents 168-169 Map of the Isothermal Lines 196-197 Map of the Winds and Ocean Eoutes 202-203 Map Showing the Distribution of Vegetation 278-279 Map Showing the Distribution of Animals 292-293 Map Showing the Distribution of the Races of Men 308-309 Physical Map of the United States 332-333 Map Showing Tracks of Storm-centres, Areas of Low Barom- eter, Weather Signals, and Storm Signals 344-345 mm® e HEAVEN THE EARTH. rvtwvwwrn THE ELEMENTS PHYSICAL GEOGRAPHY. INTRODUCTORY. 1. Geography is a description of the earth. The earth may be described in three different ways : (1) In its relations to the solar system ; (2) In its relations to government and sdciety ; (3) In its relations to nature. Hence arise three branches of geography — Mathematical, Polit- ical, and Physical. 2. Mathematical Geography treats of the earth in its relations to the solar system. Mathematical Geography forms the true basis for accurate geographical study. Here we learn the location of the earth in space, its relations to the other members of the solar system, its size, form, and movements, its division by imaginary lines, and the methods of representing all or portions of its surface on maps. 3. Political Geography treats of the earth in its relations to the governments and societies of men, of the manner of life of a people, and of their civilization and government. 4. Physical Geography treats of the earth in its relations to nature and to the natural laws which determine its phenomena. It treats especially of the systematic distribution of all animate and inanimate objects found on the earth's surface. It not only 11 12 PHYSICAL GEOGRAPHY. tells of their presence in a given locality, but it also endeavors to discover the causes and effects of their existence. Physical Geography therefore treats of the distribution of Land, Water, Air, Plants, Animals, and Minerals. Geography deals with, the inside as well as with the outside of the earth. It encroaches here on the province of geology. Both treat of the earth : geography mainly with the earth's present condition ; geology with its condition both in the past and present. Some authors make physical geography a branch of geology, and call it phys- iographic geology or physiography, but the term "physical," or, as etymology would make it, "natural" geography is preferable. A separate branch of geography, called Commercial Geography, is based on a combination of portions of physical and political geography. As the name indi- cates, commercial geography is such a description of the surface of the earth as includes the production, transportation, and interchange of commodities, either as raw or as manufactured materials. PART I. THE EARTH AS A PLANET. CHAPTER I. Mathematical Geography. 5. The Earth moves through empty space around the sun. A book or other inanimate object placed on a support will remain at rest until something moves it. If the book were thrown up into the air it would keep on moving upward unless some cause occurred to stop it. That property of matter by which it retains its state of rest or its state of motion, unless some cause occurs to change that state, is called inertia. The book when thrown up into the air soon stops moving and falls to the earth ; because — (1) The earth draws or attracts it ; (2) The moving body gives some of its motion to the air through which it moves. Were the book thrown in any direction through the empty space in which the stars move, it would continue moving in that direction for ever, unless it came near to some other body which would attract it and cause it to change its motion. Our earth moves through empty space on account of its inertia, and must continue so moving for ever. All heavenly bodies con- tinue their motion solely on account of their inertia. Space is not absolutely empty, but is everywhere filled with a very tenuous substance called ether, which transmits to us the light and heat of the heavenly bodies. Wherever the telescope reveals the presence of stars we must believe the ether also extends. 13 14 PHYSICAL GEOGRAPHY. 6. The Stars. — The numerous points of light that appear in the heavens, on clear, moonless nights, are called stars. Stars consist of immense balls of matter, that, like our earth, are moving through empty space. A few of these brilliant bodies, apparently stars, are planets. Planets differ from other stars in appearance, since they do not perceptibly twinkle; moreover, the planets shine not because of their own light, but by light thrown on them from the sun. The planets move around the sun and so change their positions as regards the other stars, which always retain, approximately, the same rela- tive positions, and are, therefore, frequently called fixed stars, although, as already stated, they are moving through empty space. Our sun is believed to be one of the fixed stars, and, immense though it be, is, nevertheless, far smaller than many of them. The fixed stars, like our sun, are highly heated and shine by means of the light they throw off in all directions. Many of them are more highly heated than our sun. Others again are less highly heated, and still others are dark, or do not shine at all, being too cold to emit light. The actual number of stars that can be seen by the unassisted eye, under favorable conditions, is probably about 7000. An ordinary opera-glass, however, increases this number to about 100,000. A telescope, whose larger glass is about 2£ inches across, increases this number to about 300,000, while the great Lick telescope brings it to about 100,000,000. It is possible that in the remote realms of space are countless stars whose distances are too great to permit them to be seen by even the most powerful telescopes. There are, moreover, numerous dark bodies, or suns, that have completely cooled, thus again greatly increasing the number of the heavenly bodies. 7. The Solar System comprises the sun, eight large bodies called planets, and upward of four hundred and fifty smaller bodies called planetoids or asteroids, besides numerous comets and meteors. Some of the planets have bodies called moons or satellites moving around them. These also belong to the solar system. Fig. 1, represents the solar system. In the centre is the sun. The circles drawn around the sun show the paths or orbits of the planets. These orbits are represented as circular, but in reality they are slightly flattened or elliptical. The elongated orbits mark the paths of the comets. The drawing shows the relative sizes of MATHEMATICAL GEOGRAPHY. 15 the planets and their order from the sun, their common centre, the diameter of the sun being approximately the diameter of the orbit of Saturn. 8. Names of the Planets. — The planets, named in their regular order from the sun, are : Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The first four — Mercury, Venus, The Solar System. Earth, and Mars — are comparatively small ; the second four— Jupiter, Saturn, Uranus, and Neptune — are very large. The initial letters of the last three planets, Saturn, Uranus, and Neptune, taken in their order from the sun : s, u, and n — spell the name of their common centre. 16 PHYSICAL GEOGRAPHY. Data Concerning the Solar System (Peck, 1900). Mean diameter. Volume. Density. Mean dis- tance from Names. Actual, in miles. Relative, Earth = 1. Earth = 1. Earth = 1. Water = 1. sun, round numbers- miles. Sun Moon Mercury . . . Venus .... Earth Mars Asteroids . . j Jupiter .... Saturn .... Uranus .... Neptune . . . 866,500 2,163 3,000 7,700 7,918 4,230 From a few miles to 300 86,500 71,000 31,900 34,800 110. 0.83 0.97 1.00 0.53 } ... 10.92 8.97 4.03 4.39 1330.000 0.0203 0.056 0.92 1.00 1.15 13.09 7.21 .65 .85 0.255 0.604 2.232 0.86 1.00 0.72 0.42 0.13 0.22 0.20 1.41 3.44 12.4 4.9 5.6 4.0 ... { 1.3 0.7 1.2 1.1 92,900,000* 238.818* 36,000,000 67,200,000 92,900,000 141,500,000 204,400,000- 325,200,000 483,300,000 886,000,000 1,781,900,000 2,791,600,000 The asteroids are mostly found in the space between Mars and Jupiter. It is probable that all these bodies were once collected in a ring around the central nebulous sun, but that this ring failed to collect in a single planet, owing to the presence of the large planet Jupiter, whose powerful attraction tore the ring into the separate fragments that now form the asteroids. It is difficult to obtain clear conceptions of distances that are represented by millions of miles. Should a man travel forty times around the earth at the equator he would have gone over only about 1,000,000 miles. Now, Mercury, the nearest of the planets, is thirty-six times farther from the sun than the entire distance the man would have travelled, while Neptune is nearly three thousand times the distance he would have travelled. In estimating the enormous distances of the fixed stars it is convenient to use for our unit of measurement what is called the light-year, or the distance light would travel in one year. Since light moves through space with a velocity of about 186,000 miles a second, the enormous value of this unit will be readily understood. It is equal to about 63,000 times the distance between the earth and the sun. Most of the stars that can be seen by the naked eye are probably situ- ated at distances of from 200 to 300 light-years. The distance of the remotest stars in the stellar system is unknown. It is believed, however, to be at least from 10,000 to 20,000 light-years. 9. The Satellites. — A satellite is a body that revolves around another body : the planets are satellites of the sun ; the moon is a satellite of the earth. Mars has two moons or satellites. So far as now known, neither Mercury nor Venus has a satellite. All the planets whose orbits are beyond the orbit of the earth have moons : * Mean distance from Earth. MATHEMATICAL GEOGRAPHY. 17 Jupiter has five, Uranus four, Saturn nine, and Neptune one. Besides its moons, Saturn has a number of curious ring-like accu- mulations of separate solid or liquid particles revolving around it. 10. The Earth's Moon.— The earth's moon is about 240,000 miles from the earth. Fig. 2.— Full Moon. Taken from a photograph. Our moon always turns the same face toward the earth. Its opposite face is never visible to us. This is because the moon turns or rotates on its axis in the same time that it revolves in its orbit around the earth. The irregularities of light and shade seen in the face of the moon, as shown in Fig. 2, are due to irregularities in its surface. The moon has its highlands and 2 18 PHYSICAL GEOGRAPHY. lowlands, its mountains and its valleys. These mountains far surpass in height those of the earth. With telescopes of comparatively high magnifying power, these mountains are seen to he volcanoes, their craters in many instances being clearly discernible. An exceedingly careful study of the moon has failed to show any signs of the presence of either air or water on the surface that is turned toward us. 11. The Sun is the great central body of the solar system. Around it move the planets with their satellites, receiving their light and heat from it. The sun is a huge heated mass about 1,330,000 times the size of the earth. Its diameter is about 866,500 miles. It appears the largest self-luminous body in the heavens because it is comparatively near the earth. Many stars, which appear as mere dots of light, are much larger than the sun. Were the sun hollow and the earth placed at its centre, there would not only be sufficient room to enable the moon to revolve at its present actual distance around the earth, but it would still, in all parts of its orbit, be nearly 200,000 miles below the surface of the sun. Only a comparatively small portion of the sun is visible to us. The surface we ordinarily see is bounded by what is called the photosphere, a mass of lumi- nous clouds formed by the condensation of gaseous substances that exist lower down in the hotter body of the sun. Beyond the photosphere, resting on it, and partly penetrating it, is an enve- lope of permanent gases, mainly hydrogen and helium. This envelope is called the chromosphere. As seen during a total eclipse of the sun, the chromosphere resembles a scarlet sheet of flame, from which prominences or protuberances arise, often to the height of hundreds of thousands of miles. Above the chromosphere and resting on it, is another layer called the corona. This layer extends far beyond the distances to which the higher prominences of the chromosphere rise. The corona appears to consist of highly rarefied gaseous substances, together with some variety of meteoric dust or fog. The corona is visible only during eclipses of the sun at the time of totality. The sun is a body heated to luminosity, and gives out or emits light and heat like any other highly heated body. 12. Position of the Solar System in Space. — The sun, with all the bodies which move around it, is in that portion of the heav- ens called the Milky Way. The sun is an insignificant star among the millions of other stars the telescope has revealed to us. It was formerly believed that the sun was stationary, for it was not then known that the positions of the fixed stars were undergoing slight variations MATHEMATICAL GEOGRAPHY. 19 as regards the earth. It is now generally conceded that the sun, with all the planets, is moving through space with tremendous velocity, the direction at present being toward the constellation Hercules. The estimated velocity of the sun in its immense orbit is 1,382,000,000 miles per year. As the earth is carried along with the sun in its orbit, it is continually entering new realms of space. 13. The Earth.— The shape of the earth is that of a round ball or sphere slightly flattened at two opposite sides. Such a body is termed a spheroid. There are two kinds of spheroids — oblate and prolate; the former has the shape of an orange, the latter that of a lemon. The straight line that runs through the centre of a sphere or spheroid and terminates at its surface is called the diameter. If the sphere rotates — that is, spins like a top — the diameter on which it turns is called its axis. In the oblate spheroid the axis is the shorter diameter ; in the prolate spheroid the axis is the longer diam- eter. The shape of our earth is that of -an oblate spheroid. The polar diameter is 26.47 miles shorter than the equatorial diameter. 14. Proofs of the Rotundity of the Earth. — The earth is so large a sphere that its surface everywhere appears flat. The fol- lowing considerations will prove, however, that its form is nearly spherical : (1) Appearance of Approaching Objects. — If the earth were flat, as soon as an object appeared on the horizon we would see the upper and lower parts at the same time ; but if it were curved, the top parts would first be seen. Now, when a ship is coming into port we see first the topmasts, then the sails, and finally the hull ; hence the earth must be curved ; and since the appearance is the same Fig. 3.— Oblate Spheroid. Fig. 4.— Prolate Spheroid. 20 PHYSICAL GEOGRAPHY. from whatever direction the ship is approaching, we infer that the earth is evenly curved or spherical. This is shown in Fig. 5. If the rigging of a distant ship whose hull is below the horizon, as shown in Fig. 6, be examined with a suitable telescope, it will at once be apparent that a considerable distance exists between the ship and the water line on the hori- zon, since, if the glass be sharply focussed on the water line at the horizon, the ship will become indis- tinct, while, if the glass be sharply focussed on the distant ship, the water line at the horizon will become indistinct. (2) Circular Shape of the Horizon. — The horizon is the line which limits our view when nothing inter- venes. The fact that this is always a circle furnishes another proof that the earth is spherical. Fig. 5.— Curvature of the Earth's Surface. Fig. 6.— Telescopic Proof of Rotundity of Earth. The horizon would still be a circle if the earth were perfectly flat, for we would still see equally far in all directions ; but it would not everywhere be cir- MATHEMATICAL GEOGRAPHY. 21 cular, since to an observer near the edges some other shape would appear. It is on account of the spherical form of the earth that our field of view on a plain is so soon limited by the apparent meeting of the earth and sky. As we can see only in straight lines, objects continue visible until they reach such a distance as to sink below the horizon, so that a straight line from the eye will pass above them, meeting the sky far beyond, on which, as a background, the objects on the horizon are projected. (3) Shape of the Earth's Shadow. — We can obtain correct ideas of the shape of a body by the shape of the shadow it casts. Now, the shadow which the earth casts on the moon during an eclipse of the moon is always circular, and as only spherical bodies cast circular shadows in all positions, we infer that the earth is spherical. (4) Measurement. — The shape of the earth has been accurately ascertained by calculations based on the measurement of an arc of a meridian. We, therefore, not only know that the earth is oblately spheroidal, but also, approximately, the amount of its oblateness. (5) The Shape of the Great Circle of Illumination, or the line separating the portions of the earth's surface lighted by the sun's rays from those in the shadow, is another evidence of the rotundity of the earth. 15. The Dimensions of the Earth. — The equatorial diameter of the earth, or the shortest distance through at the equator, is, approximately, 7926 miles ; its polar diameter, or the length of its axis, is 7899 miles. The circumference is 24,899 miles. The entire surface is equal to nearly 197,000,000 square miles. The density of the earth is about 5§ ; that is, all its matter is five and two- third times heavier than an equal volume of water. 16. Imaginary Circles. — In order to locate places on the earth, as well as to represent portions of its surface on maps, we imagine the earth to be encircled by a number of curved lines called great and small circles. A great circle is one which would be formed on the earth's sur- face by a knife or plane passing through the earth's centre, hence dividing it into two equal parts. All great circles, therefore, divide the earth into hemispheres. 22 PHYSICAL GEOGRAPHY. The formation of a great circle on a sphere by cutting it into two equal parts is shown in Fig. 7. The shortest distance between any two places on the earth is along the arc of a great circle. All planes passing through the earth's centre form, approximately, great cir- cles on its surface. A small circle is one formed by a plane which does not cut the earth into two equal parts. The formation of a small circle by cutting a sphere into unequal parts is shown in Fig. 8. The great circles employed most frequently in geography are the equator and the meridian circles. The small circles are the parallels. Fig. 7.— Great Circle. Fig. 8.— Small Circle. If we divide the circumference of any circle, whether great or small, into three hundred and sixty equal parts, each part is called a degree. The one- sixtieth part of a degree is a minute ; one-sixtieth part of a minute is a second. The characters representing these divisions are used as follows: 34°, 12', 38"; which indicates thirty-four degrees, twelve minutes, and thirty-eight seconds. The Equator is that great circle of the earth which is equidis- tant from the poles. Meridian Circles are great circles of the earth which pass through both poles. The Meridian of any given place is that half of the meridian circle which passes through that place and both poles. A meridian of any place reaches from that place to both poles, and therefore is equal to one-half of a great circle, and, with the meridian directly MATHEMATICAL GEOGRAPHY. 23 opposite to it, forms a great circle called a meridian circle. There are as many meridian circles as there are places on the equator, or on any parallel. In large cities the meridian is generally assumed to pass through the principal observatory. Fig. 9. — Meridians and Parallels. Parallels are small circles which pass around the earth parallel to the equator. The meridians extend due north and south, and are everywhere of the same length ; the parallels extend due east and west, and decrease in length as they approach the poles. The Tropics are parallels which lie 23° 27' north and south of the equator : the northern tropic is called the Tropic of Cancer, the southern tropic is called the Tropic of Capricorn. The Polar Circles are parallels which lie 23° 27' from each pole. The polar circle in the Northern Hemisphere is called the Arctic Circle; that in the Southern Hemisphere, the Antarctic Circle. 17. Latitude is distance north or south from the equator toward the poles, measured along the meridians. It is reckoned in degrees. The meridian circles are divided into nearly equal parts by the parallels, and it is the number of these parts that occur on the meridian of any place between it and the equator, which determines the value of its latitude. If we conceive eighty-nine equidistant parallels drawn between the equator and either pole, they will divide all the meridians into ninety nearly equal parts ; the value of each of these parts will be one degree of latitude. Therefore, if the parallel run- ning through a place is distant from the equator forty-five of these parts, its 24 PHYSICAL GEOGRAPHY. latitude is 45°. If more than eighty-nine parallels be drawn, the value of each part will be less than one degree. Places north of the equator are in north latitude ; those south of it are in south latitude. Places near the poles are said to be in high latitudes; those near the equator, in low latitudes. Since the distance from the equator to the poles is one-fourth of an entire circle, and there are only 360° in any circle, 90° is the greatest value of latitude a place can have. Latitude 90° N., there- fore, corresponds to the north pole. To recapitulate : Latitude is measured on the meridians by the parallels. 18. Longitude is distance east or west of any given meridian. Places on the equator have their longitude measured along it ; everywhere else longitude is measured along the parallels. The meridian from which longitude is reckoned is called the Prime Meridian. Most nations take the meridians of their own capitals for their prime meridian. The English reckon from the meridian which runs through the observatory at Greenwich, and this is the meridian used for all calculations at sea. In the United States we reckon, in land surveys, from Washington. A place situ- ated east of the prime meridian, is in east longitude ; west of it, is in west longitude. Since there are only 180° in half a circle, the greatest value the longitude can have is 180° ; for a place 181° east of any meridian would not fall within the eastern half of the parallel on which it is situated, but in the western half; and its distance, computed from the prime meridian, would be 179° west. It is the meridians that divide the parallels into degrees ; there- fore, longitude is measured on the parallels by the meridians. 19. Value of Degrees of Latitude and Longitude. — As latitude is distance measured on the arc of a meridian, the value of one degree must be the -g-g-oth part of the circumference along that meridian, since there are only 360° in all. This makes the value of a single degree, approximately, equal to 69& miles. Near the poles the flattening of the earth causes the value of a degree slightly to exceed that of one near the equator. MATHEMATICAL GEOGRAPHY. 25 Length of 1° Latitude at Different Distances from the Equator. Latitude 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°. Length in miles . . . 68.70 68.72 68.78 68.83 68.98 69.11 69.22 69.31 69.36 69.39,. The value of a degree of longitude is subject to great variation. It is equal to the 3x0th part of the earth's circumference, provided the place be situated on the equator ; otherwise, it is the -g^th part of the parallel passing through the place whose longitude is taken ; and as the parallels decrease in size as we approach the poles, the value of a degree of longitude must likewise decrease as the latitude increases, until at either pole the longitude becomes equal to zero. The value of a single degree of longitude on the equator, or at latitude 0°, is equal to about 69^ miles. At latitude 45° it is equal to about 49 miles. 60° " " 35 " 80° " " 12 " 90° " " " Geographical Mile. — The ^r&^th of the equatorial circumference, or the one-sixtieth of a degree of longitude at the equator, is called a nautical or geo- graphical mile. The statute mile contains 1760 yards ; the geographical or nautical mile, 2028 yards. The nautical mile is sometimes called a knot. 20. Map Projections. — The term projection as applied to map- drawing means the method used for representing portions of the earth's surface on the plane of a sheet of paper. The projections in most common use are Mercator's, the stereo- graphic, and the conical projections. Of these the stereographic is best adapted to ordinary geographical maps where not more than one hemisphere is to be represented, and Mercator's to physical maps. All projections must be regarded as but approximations. Mercator's Projection represents the earth on a map in which all the parallels and meridians are straight lines. Mercator's charts are drawn by conceiving the earth to have the shape of a cylinder instead of that of a sphere, and to be unrolled from this cylinder so as to form a flat surface. The meridians, instead of meeting in points at the north and south poles, are drawn parallel to each other. This makes them as far apart in the polar regions as at the equator, and, consequently, any portion of the earth's 26 PHYSICAL GEOGRAPHY. surface represented on such a chart, if situated near the poles, will appear disproportionally large. In order to avoid the distortion in the shape of the land and water areas, the distance between successive parallels is increased as they approach the poles. The dimensions of the land or water, however, are made to appear much larger than they really are in the polar regions ; for example, Greenland is made • J*.-*" / BteM .-(- r Or % K ? 9 V i ?& s / ■i<- * S» -9. 1 » i ■■'•r-' % i ;^j c=. p. ^ i' ( .' V C / /' "X ~".. : > ' V -^ "^_i v ^. "Y V \ f l ?^4 1 \ f r u I / \ y '4 ^ P Fig. 10.— Tlie Earth on Mercator's Projection. to appear nearly as large as Africa. The immediate polar regions are never represented on such charts, the poles being supposed to be at an infinite distance. Mercator's charts are usually employed for physical maps, on account of the facility they afford for showing direction. Gall's Projection is a variety of Mercator's projection in which the paral- lels are more nearly equidistant from each other, thus destroying their facility for showing direction correctly, but giving less distortion of areas in high latitudes. The Stereographic Projection is that by which the earth's surface is represented as it would appear to an observer whose eye is directly on the surface, if he looked through the earth as through a globe of clear glass, and drew the details of the surface as they appeared projected on a transparent sheet of paper stretched in front of his eye across the middle of the earth. There may be an almost infinite number of such projections, according to the position of the observer. The two stereographic projections in most common use are the Equatorial and the Polar. MA THEMA TICAL GEOGRAPHY. 27 In an Equatorial Projection of the entire earth the equator passes through the middle of each hemisphere, and a meridian circle forms the borders. Fig. 11. — The Earth on an Equatorial Projection. In a Polar Projection of the entire earth the poles occupy the centres of each hemisphere, and the equator forms the borders. Fig. 12. — The Earth on a Polar Projection. In a Conical Projection the earth's surface is represented as if drawn on the frustum of a cone and afterward unrolled. This projection is suitable where only portions of the earth's surface, and not hemispheres, are to be represented. The cone is supposed to be 28 PHYSICAL GEOGRAPHY. placed so as to touch the earth at the central parallel of the country to be represented. In maps as ordinarily constructed, except in those on Mercator's projection, it is not true that in every por- tion the upper part is north, the lower part south, the right hand east, and the left hand west. In all maps due north and south lie along the meridians, and due east and west along the parallels. Since in most maps both parallels and meridians are curved lines, in all such maps, due north and south and due east and west will lie along the meridians and paral- lels, and not directly toward the top and bottom, or the right- and left-hand side. Fig. 13.— The Conical Projection. 21. The Hemispheres. — The equator divides the earth into a Northern and a Southern Hemisphere. The meridian of long. 20° W. from Greenwich is usually taken as the dividing-line between the Eastern and Western Hemispheres. $ 22. The Movements of the Earth; Rotation. — The earth turns around from west to east on its diameter or axis. This motion is called its rotation. To a person in a rapidly moving steam-car, the fences and other objects along the road will appear to be moving in the opposite direction : their motion is of course apparent, and is caused by the real motion of the car. Now, the motion of the sun and the other heavenly bodies, by which they appear to rise in the east and set in the west, is apparent, and is caused by the real motion of the earth on its axis; this motion must therefore be from west to east. The sun, the planets, and their satellites, so far as is known, also turn on their axes from west to east. The earth makes one complete rotation in about every twenty-four hours — accurately, 23 hours, 56 minutes, 4.09 seconds. The velocity of its rotation is such that any point on the equator will travel about 1042 miles every hour. The velocity, of course, diminishes at points distant from the equator, until at the poles it becomes nothing. 23. Proofs of the Rotation of the Earth.— That it is the MATHEMATICAL GEOGRAPHY. 29 earth that is rotating, or moving under the heavens, and not the heavens moving over the earth may be adduced as follows : (1) From Analogy. — By the use of the telescope we can see that the sun, moon, and planets are actually rotating or turning on their axes ; hence it is probable that the earth also is rotating. (2) Foucault's Pendulum. — In 1851, Foucault suspended a heavy iron ball from the dome of the Pantheon, in Paris, by a wire about 200 feet long, as shown in Fig. 14. The ball had, attached to its lower extrem- ity, a pin so placed as to just cut or scrape a ridge of sand placed on a circular platform on the floor below it. On starting the pendulum to swing in a true plane, by drawing the ball aside by a cotton thread, letting it come absolutely to rest, and then burning the thread, it was found that the floor seemed to move to the right, the ball cutting the ridge of sand at each vibra- tion in a different place ; or, in other words, the floor of the Pan- theon was visibly moving or turning under the plane in which the pendulum was vibrating. (3) Bodies falling from a great height are slightly deflected toward the east. Projectiles show a similar deviation. (4) The deviation of the winds and ocean currents, and the rota- tion of the wind in cyclones, demonstrate the rotation of the earth from west to east. 24. Change of Day and Night. — The earth receives its light and heat from the sun, and, being an opaque sphere, only one-half of its surface can be lighted at one time. The other half is in dark- ness, since it is turned from the sun toward portions of space where Fig. 14. -Foucault's Pendulum Experi- ment. 30 PHYSICAL GEOGRAPHY. it receives only the dim light of the fixed stars. The boundary-line, between the light and dark parts, forms, approximately, a great circle called the Great Circle of Illumination. Had the earth no motion either on its axis or in its orbit, that part of its surface turned toward the sun would have perpetual day, and the other part per- petual night; but by rotation, different portions of the surface are turned successively toward and away from the sun, and thus is occa- sioned the change of day and night. Since the earth turns completely from west to east, or through 360° in about 24 hours, it turns eastward 15° in every 60 minutes, or 1° every 4 minutes. Con- sequently, if the sun is just rising on a certain meridian, it is one hour before sunrise on the . meridian 15° to the west, and one hour after sunrise on the meridian 15° to the east. In the United States, in order to avoid the confusion arising from every place having its own time, the country is arbitrarily divided by the meridians into belts 15° apart. The time remains the same in all parts of each belt, changing by one hour at the boundaries. In travelling across the country toward the west, we set our watches back an hour at the boundaries, and forward an hour when travelling toward the east. v 25. The Revolution of the Earth. — The earth has also a motion around the sun, called its revolution. The revolution of the earth is from west to east; this is true also of all the planets and asteroids, and of all their satellites, except those of Uranus, and, probably, of Neptune. The earth makes a complete revolution in 365 days, 6 hours, 9 minutes, 9.6 seconds. This time forms what is called a sidereal year. The tropical year, or the time from one March equinox to the next, is somewhat shorter, or 365 days, 5 hours, 48 minutes, 49.7 seconds. The latter value is the one usually given for the length of the year. It is nearly 3651 days. It will be found th%t the sum of the days in all the months of an ordinary year is only equal to 365, while the true length is, approximately, one-quarter of a day greater. This deficiency, which in every four years amounts to an entire day, is met by adding one day to February in every fourth or leap year. The exact time of one revolution, however, is about 11 minutes less than 6 hours. These eleven extra minutes are taken from the future, and are paid by omitting leap year every hundredth year, except that every 400 years leap year is counted. In other words, 1900 was not a leap year, since 1900 is not exactly divisible by 400, but the year 2000 will be a leap year. MATHEMATICAL GEOGRAPHY. 31 The length of the orbit of the earth is about 577,000,000 miles. Its shape is that of an ellipse which differs but little from a circle. The sun is located at one focus of the ellipse, and, as this is not in the centre of the orbit, the earth must be nearer the sun at some parts of its revolution than at others. When the earth is in that part of its orhit which is nearest to the sun, it is said to be at its perihelion ; when in that part farthest from the sun, at its aphe- lion. The perihelion distance is about 90,259,000 miles; the aphelion distance, 93,750,000 miles. The earth reaches its perihelion about January 1st. The earth does not move with the same rapidity through all parts of its orbit, but travels more rapidly in perihelion than in aphelion. Its mean velocity is nearly 19 miles a second, which is about sixty times faster than the speed of a cannon-ball. The actual motion of the earth through space during its revolutions around the sun is proved by the annual parallax of the fixed stars, whereby each fixed star appears every year to be moving through the sky as if it were moving in a little orbit of 186,000,000 miles, the counterpart of the earth's orbit. This can only be explained by the actual motion of the earth in its orbit around the sun. 26. Laplace's Nebular Hypothesis. — The uniformity in the direction of rotation and revolution of the planets has led to a very plausible supposition as to the origin of the solar system, by the celebrated French astronomer, Laplace. This supposition, known as Laplace's Nebular Hypothesis, assumes : (1) That at some very remote time the sun, together with all its planets and their satellites, was collected in a single nebulous or cloud-like mass of intensely heated gas. (2) That this mass gradually collected under its own gravitation into an approximately globular mass, and began rotating. (3) On gradually cooling, the central mass or sun contracted, and, consequently, rotated more and more rapidly, flattening at the poles and bulging out at the equator, finally detaching a ring of nebulous matter, as in Saturn's rings. (4) That this ring would revolve around the central sun, and, subsequently, break and collect in a globular mass that would revolve around the central sun as a planet. These rings and planets would be successively thrown off from the central mass as it contracted, the planets similarly throwing off moons or satel- lites. The nebular hypothesis is not generally credited in all its details. Though probably true in the main, it appears to need modification. For example, Lock- yer insists that the original central mass, instead of being formed of glowing gas. consisted of a cloud of ice-cold meteoric dust. 32 PHYSICAL GEOGRAPHY. 27. The Plane of the Earth's Orbit is a perfectly flat surface that touches the earth's orbit at every point. It may be regarded as an imaginary plane of enormous extent on s^ ~^/\ which the earth moves in its journey around /. /.. \ the sun. /v^^"""*^"-/ --- -- \ ^8* Causes of the Change of Sea- Y ^ ^ ^ / X; v \\ sons. — The change of the earth's seasons \ ^J^x/ ^X^X^y is caused by the revolution of the earth, V / "~~--l_\i/ together with the following circumstances : \./ / (1) The inclination of the earth's axis to the plane of its orbit. The incli- Fig. 15. — Inclination of ... , , aao Q q/ . - \ « ^■ J . ^ t, ■■■ nation is equal to oo 66 . Axis to Orbit and Eclip- ^ tic. The ecliptic is the name given to a great circle whose plane coincides with the plane of the earth's orbit. Since the earth's axis is 90° distant from the equator, the plane of the ecliptic must be inclined to the plane of the equator 90° minus 66° 33', or 23° 27'. The mere revolution of the earth would be unable to produce a change of seasons, unless the earth's axis were inclined to the plane of its orbit. If, for example, the axis of the earth stood perpendicularly on the plane of its orbit, the sun's rays would so illumine the earth that the great circle of illumination would always be bounded by some meridian circle. The days and nights would then be of equal length, and the distribution of heat the same throughout the year. Under these circumstances there could be no change of seasons, because the sun's rays would always fall perpendicularly on the same part of the earth : i. e., on the equator. (2) The Constant Parallelism of the Earth's Axis. — during the earth's revolution its axis always points nearly to the same place in the heavens ; viz., to the north star. It is. therefore, always ap- proximately parallel to any former position. Unless the axis were constantly parallel to any former position, the present change of seasons would not occur. On account of the spheroidal form of the earth, only a small part of its surface can receive the vertical rays of the sun at the same time. This part may be regarded as nearly a point ; and since only one-half of the earth is lighted at any one time, the great circle of illumination must extend 90° in all directions from the point which receives the vertical rays. By rotation, all portions of the surface MATHEMATICAL GEOGRAPHY. 33 situated anywhere within the tropics in the same latitude are at some time during the day, so turned as to receive the verti- cal rays of the sun, and, consequently, the portion so illumined has the form of a ring or zone. Other things being equal, this zone contains the hottest por- tions of the surface, the heat gradually dimin- ishing as we pass toward either pole. On account of the inclination of its axis, the earth receives the vertical rays of the sun on new portions of its surface every day dur- ing its revolution ; and it is because different portions of the earth's surface are constantly being turned toward the sun that the change of seasons is to be attributed. As the earth changes its position in its orbit, the sun's rays fall vertically on different parts of the surface, so that during the year some part of the surface within 23° 27' on either side of the equator, receives the vertical rays. The astronomical year begins on the 20th of March, and we shall, therefore, first consider the position of the earth in its orbit at that time. An inspection of Fig. 16, will show that at this time the earth is so turned toward the sun that the vertical rays fall exactly on the equator^ The great circle of illumination, therefore, reaches to the poles, and the days and nights are of an equal length all over the earth. This time is called the March equinox Spring then begins in the Northern Hemisphere, and autumn in the Southern. This is shown more clearly in Fig. 17, which represents the relative positions of the illumined and non-illumined portions at that time. As the earth proceeds in its orbit, the inclination of the axis causes it to turn the Northern Hemisphere more and more toward the sun. The vertical rays, 3 Fig. 16.— The Orbit of the Earth, showing the Change of Seasons. 34 PHYSICAL GEOGRAPHY. therefore, fall on portions farther and farther north until, on the 21st of June, the vertical rays reach their farthest northern limit, and fall directly on the Tropic of Cancer, 23° 27' N., when the sun is said to be at its summer solstice. Since the portions receiving the vertical rays of the sun are now on the Tropic of Cancer, the light and heat must extend in the Northern Hemisphere to 23° 27' beyond the north pole, or to the Arctic Circle ; while in the Southern Hemi- sphere they must fall short of the south pole by the same number of degrees, or reach to the Antarctic Circle. The Northern Hemisphere then begins its summer, and the Southern, its winter. The relative positions of the illumined and non-illumined portions of the earth at the summer solstice are more clearly shown in Fig. 18. Here, as is shown, the great circle of illumination extends in the Northern Hemisphere as far over the pole as the Arctic Circle. After the 21st of June the Northern Hemisphere is turned less toward the sun, and the vertical rays continually approach the equator, all the movements of Fig. 17 .—The Earth at an Equinox. Fig. 18. -The Earth at the Summer Solstice. the preceding season being reversed, until on the 22d of September, the time of the September equinox, the equator again receives the vertical rays, the great circle of illumination again coinciding with the meridian circles. The earth has now moved from one equinox to another, and has traversed one-half of its orbit, and the Southern Hemisphere begins its spring, and the Northern, its autumn. From the 22d of September until the 20th of March, while the earth moves through the other half of its orbit, the same phenomena occur in the Southern Hemisphere that have already been noticed in the Northern. Immediately after the 22d of September the inclination of the axis causes the earth to be so turned toward the sun that its rays begin to fall south of the equator ; and, as the earth proceeds in its orbit, the Southern Hemisphere is turned more and more toward the sun, and the vertical rays fall farther -and farther toward the MATHEMATICAL GEOGRAPHY. 35 pole. This contiuues until the 21st of December, when the rays fall vertically on the Tropic of Capricorn, and the December solstice is reached. The great circle of illumination now extends beyond the south pole as far as the Antarctic Circle, but falls short of the north pole 23° 27', reaching only the Arctic Circle. Summer then commences in the Southern Hemisphere, and winter in the Northern. After the 21st of December the Southern Hemisphere is turned less and less toward the sun, and the part receiving the vertical rays approaches the equator, until on the 20th of March the equator again receives the vertical rays, and, with the March equinox, spring commences in the Northern Hemisphere, and with it a new astronomical year. The equinoxes and solstices, as a rule, occur on the dates named. Occasionally they occur a short time before or after said dates. 29. Mathematical Zones. — The Torrid Zone. — That belt of the earth's surface which lies between the tropics is called the Torrid Zone. At some time during the year every part of its surface receives the vertical rays of the sun. The Temperate Zones are included between the tropics and the polar cir- cles. The northern zone is called the North Temperate Zone, and the southern zone, the South Temperate Zone. The Polar Zones are included be- tween the polar circles and the poles. Fis< "--Mathematical Cli- The northern zone is called the North Frigid Zone, and the southern zone, the South Frigid Zone. These zones, which are separated by the parallels of latitude, are usually termed the astronomical or mathematical zones to distinguish them from others called physical zones, which are bounded by the lines of mean annual temper- ature. It will be noticed that both the distance of the tropics from the equator, and of the polar circles from the poles, is 23° 27', or the value of the inclination of the plane of the ecliptic to the plane of the equator. Apart from other causes, the winter in the Northern Hemisphere, outside the tropics, is less severe than at corresponding latitudes in the Southern Hemisphere, not only because the earth is nearest the sun during midwinter in the Northern Hemisphere, but also because the earth moves more rapidly when in perihelion than in aphelion. Consequently, the contrast of the seasons, so far as these causes 36 PHYSICAL GEOGRAPHY. are concerned, is greater in the Southern than in the Northern Hemisphere. As we shall hereafter see, this effect is to some extent modified by the greater quan- tity of water in the Southern Hemisphere. 30. Length of Day and Night. — Whenever more than half of either the Northern or the Southern Hemisphere is illumined, the great circle of illumination will divide the parallels unequally, and the length of the daylight in that hemisphere will exceed that of the night in proportion as the length of the illumined part, measured along any of the parallels, exceeds that of the non-illumined part. The length of daylight or darkness may exceed that of one com- plete rotation of the earth. The great circle of illumination may at times pass over either pole as far beyond as 23° 27' ; so that places situated within this limit may remain during many rotations exposed to the rays of the sun. The longest duration of daylight must occur at the poles, since the poles must continue to receive the sun's rays from the time they are first illumined at one equinox until the sun passes through a solstice and returns to the other equinox. Nowhere, outside the polar circles, will the length of daylight exceed one entire rotation of the earth. The length of the longest day at the equator, latitude 0°, is 12 hours. The length of the longest day at the poles, latitude 90°, is six months. 1 «)XKOO f SYLLABUS. There are three kinds of Geography : Mathematical, Political, and Physical. Commercial Geography is sometimes added to these. Physical Geography treats of land, water, air, plants, animals, and minerals. Geography deals mainly with the earth's present condition ; Geology deals with the earth's condition both in the past and present. The earth continues its motion around the sun in consequence of its inertia. Some of the distant stars are highly heated, and shine by their own light ; others are dark, being too cold to emit light. The solar system constitutes the sun and the bodies that revolve around it. The sun is about 1,330,000 times larger than the earth. The sun is a body heated to luminosity, and gives out or emits light and heat, like any other heated body. The surface of the sun that we ordinarily see is bounded by the photosphere. Beyond, and resting on the photosphere, is a gaseous envelope called the chromo- sphere. Above this is a highly tenuous layer called the corona. SYLLABUS. 37 The shape of the earth is that of an oblate spheroid whose equatorial diameter is about 26 miles longer than its polar diameter. That the earth is round and not flat is proved : (1) By the appearance of approaching or receding bodies ; (2) By the circular shape of the horizon ; (3) By the shape of the earth's shadow; (4) By actual measurement; (5) By the shape of the great circle of illumination. The earth's diameter is nearly 8000 miles, its circumference not quite 25,000 miles, and its area about 197,000,000 square miles. The imaginary circles employed in geography are the equator, the meridian circles, and the parallels. Latitude is measured on the meridians by the parallels. The greatest value the latitude of a place can have is 90° ; the greatest value of longitude is 180°. Longitude is measured on the equator or on the parallels by the meridians. Maps are drawn on different projections. Merca tor's, Gall's, and the Equa- torial and Polar projections are those in most general use. That the earth rotates on its axis from west to east is proved : (1) By Fou- cault's pendulum ; (2) By falling bodies ; and (3) By the deviation of the winds and the ocean-currents. It is rendered probable by analogy. In all maps north and south lie along the meridians, east and west along the parallels; when these are curved lines, the top and bottom of the map do not always represent north and south, nor the right and left, east and west. The change of seasons is caused by the inclination of the earth's axis to the plane of its orbit, and the constant parallelism of the axis to any former position when taken in connection with its revolution round the sun. The astronomical year begins March 20th. The equinoxes occur on the 20th of March and the 22d of September ; the solstices on the 21st of June and the 21st of December. The Torrid Zone is the hottest part of the earth because at some time during the year every part of its surface receives the vertical rays of the sun. ►OXKOO REVIEW QUESTIONS. The Solar System. How does the principle of inertia apply to the earth's motion around the sun ? What do you understand by the solar system? Describe the earth's position in the solar system. Which of the planets are between the earth and the sun ? Which are beyond the orbit of the earth ? What is a satellite ? Which of the planets have satellites ? How does the size of the sun compare with that of the earth ? Are any of the distant stars larger than our sun ? Distinguish between the sun's photosphere, chromosphere, and corona. 38 PHYSICAL GEOGRAPHY. In what part of space is the solar system ? Has our sun any motion through space? Enumerate the proofs of the rotundity of the earth. State accurately the length of the equatorial diameter of the earth; of its polar diameter; of its circumference. What is its area? How many times heavier is the earth than an equally large globe of water ? Imaginary Circles. Define great and small circles. Name the circles most commonly used in geography. What do you understand by latitude? How is latitude reckoned? Of what use is latitude in geography? Why can the value of the latitude never exceed 90°? Of what use are meridians and parallels in measuring latitude? What do you understand by longitude? How is longitude reckoned? Of what use is longitude in geography? Why can its value never exceed 180°? Of what use are meridians and parallels in measuring longitude ? Where is the value of a degree of latitude the greatest ? Of a degree of lon- gitude? Why? What effect has a Mercator's chart on the appearance of bodies of land or water in high northern or southern latitudes? How does Gall's projection differ from Mercator's projection? What is an equatorial projection? A polar projection? A conical projection? What is the position of the poles in an equatorial projection of a hemisphere? In a polar projection? Movements of the Earth. Prove that the earth turns on its axis from west to east. Explain the cause of the change of day and night. Define a sidereal year; a tropical year. Which value is generally taken for the length of the civil year? Describe Laplace's nebular hypothesis. Enumerate the causes which produce the change of seasons. On what days of the year will the sun's rays fall vertically on the equator 9 On the Tropic of Cancer? On the Tropic of Capricorn? THE LAND. Although water occupies the larger portion of the earth's sur- face, yet, when compared with the entire volume of the earth, its quantity is . comparatively insignificant ; for the mean depth of the ocean probably does not exceed two and one-third miles, and underneath this lies the cooled solid crust, with its heated interior. The crust and heated interior are composed of a variety of ele- mentary and compound substances. Elementary substances are those which have never been separated into components. Com- pound substances are those composed of two or more elementary substances combined under the influence of the chemical force. w 40 PHYSICAL GEOGRAPHY. SECTION I. THE INSIDE OF THE EARTR CHAPTER I. The Heated Interior. 31. The Earth Originally a Fluid Mass. — There is but little doubt that our earth was originally intensely heated throughout its entire mass. Its present spheroidal shape, which is that of all the heavenly bodies, is what it would have assumed on account of its original molten condition. The original molten condition of the earth is in accordance with Laplace's nebular hypothesis ; for, as we have seen, if this hypoth- esis is correct, the temperature of the earth at the time of its separa- tion from the nebulous sun, must have been sufficiently high not only to melt, but even to volatilize all its materials. 32. The Heated Interior of the Earth. — During the enormous time that has elapsed since its separation from the nebulous sun, the earth has been losing heat and cooling. But whether this cooling has continued sufficiently to permit the entire mass to become solid throughout, or whether it has been confined to a comparatively thin crust, is unknown. That the interior of our earth is still highly heated appears evident, however, from the following facts : (1) The deeper we penetrate the crust the higher the temperature becomes. The rate of increase of temperature varies in different localities, depending on how readily the different parts of the crust will permit heat from the interior to pass through it. On the aver- age, however, the heat increases about 1° F. for every 55 feet. This occurring in practically all parts of the earth, would seem to indi- cate that the entire inside of the earth is heated, and that the heat increases in intensity as we approach the centre. THE HEATED INTERIOR. 41 (2) In all latitudes prodigious quantities of melted rock or lava escape from the interior either through fissures in the crust or through the craters of volcanoes. This lava comes from inside the earth, where the heat, consequently, must be sufficient to melt rock. (3) Careful geological observations prove that the earth's surface is never at rest. It is gradually rising in some places and sinking in others. Moreover, there is abundant evidence to show that such movements have continued throughout all geological time. These gradual changes of level are apparently due to the warping of the earth's crust consequent on the gradual loss of heat, and it would appear that they can be explained only by the theory of a heated globe gradually cooling. 33. Condition of the Interior of the Earth. — We are ignorant of the present condition of the interior of the earth. Is it all highly heated ? Is the heat sufficiently great to melt all its ingredients, or is it in a molten condition only in part ? If the rate of increase of temperature with descent below the sur- face continues uniform, and if substances below the earth's surface melt at the same temperature as they do at its surface, temperatures would be reached, at depths of from 30 to 50 miles, at which all known materials would be fused. But we know nothing about the rate of increase, except for very small depths, the greatest distance man has penetrated the crust of the earth — a few miles at the most — being insignificant as compared with the 4000 miles of the earth's radius. Nor is the fusing point of substances below the earth's surface the same as at its surface. The enormous pressure to which such substances are subjected necessitate a greater heat to fuse or melt them. And this increase may be so marked at profound depths that it may be necessary to assume a greater thickness for the earth's crust than has generally been imagined, or indeed, as is the opinion of many, there may be no liquid interior at all. In the opinion of able mathematicians and physicists, our earth acts, under the attraction of the sun and moon, not like a yielding liquid globe ; i. e., a fused globe covered by a thin, solid crust, but 42 PHYSICAL GEOGRAPHY. rather as a globe that is rigid throughout. This has led them to reject the theory of a fused globe with a comparatively thin crust. These apparently hopelessly opposite conclusions of the astron- omer and geologist, concerning the condition of the earth's interior, may, nevertheless, says Russell, be harmonized. Perhaps the major- ity of leading geologists now believe that the earth is practically solid throughout. That its interior though solid, is nevertheless, potentially plastic; i. e., is sufficiently heated to become plastic and melt, provided the enormous pressure to which it is subjected is sufficiently decreased. Or, in other words, that the earth consists of a comparatively thin, rigid, cooled, and solid shell or crust, em- bracing a rigid, solid, but potentially plastic, heated interior. Some geologists have preferred to modify the ahove helief in the existence of a highly-heated and potentially plastic interior hy assuming that the earth is solid throughout, with a liquid or pasty layer between the solid crust and the solid interior. It is difficult, however, to see how an originally molten globe could have cooled in this manner. 34. Various Hypotheses of Condition of Interior. — Various hypotheses have been proposed to account for the heat of the inte- rior and the escape therefrom of melted rock or lava. Some of these are as follows: Chemical Hypothesis. — Davy and others regarded the heat as due to the action of water on unoxidized alkaline metallic substances, with which he assumed the earth's surface to be filled. This hypothesis is now discredited. Mechanical Hypothesis. — Mallet suggested a mechanical hypothesis in which the heat requisite for the melting of the rock is derived from the move- ments which occur when rocks are folded or faulted. That such friction might produce heat sufficient to melt rock would appear possible, but the hypothesis fails to explain, apart from a heated interior that is gradually cooling, the origin of the force which causes the folding or faulting. Aqueo -Igneous Hypothesis. — Shaler and others believe the accumulations of sediment, on sea-bottoms, would become highly heated by the heat from the interior by reason of the blanketing effect so exerted. While such an accumu- lation of heat might result in temperatures sufficiently high to account for erup- tions of hot mud, and for fusion of certain rocks in the presence of highly-heated steam, yet they would not be sufficiently high to account for true igneous fusion. THE HEATED INTERIOR. 43 35. Thickness of the Crust. — It is evident that we cannot assign any definite limit to the thickness of the earth's crust, mean- ing by the word crust the outer portions of the earth that have become solidified by cooling, because these portions most probably pass insensibly into those that have become solid through loss of heat and increased pressure. It seems probable, however, that the part solidified by cooling is small, compared with the entire bulk of the earth ; or, in other words, the potentially plastic or heated interior lies comparatively near the cooled crust. Effects of the Heated Interior. — In the remote geological past the cooling and consequent contraction of the molten earth produced a direct pressure on the interior. At the present time the cooled part of the earth or crust is a solid, rigid mass, not directly affected by the contraction of the heated mass within. When the heated interior cools it contracts and shrinks away from the solid crust, which adjusts itself to the new conditions by settling in the space left by the shrinking interior. This settling of the crust is accom- panied by flexures, bendings, and fractures, attended by changes of level of the earth's surface. Any fracture extending through the crust to the potentially plastic interior acts as a relief of pressure, thus permitting a portion of the interior at that place to assume the molten condition, and, under the pressure to which the interior is subjected, either to fill the line of fracture, or even to pass through and escape at the surface. The molten matter may escape either in fissure or sheet eruptions, or in crater or. volcanic eruptions. Moreover, both gradual and sudden changes of level may attend the cooling and shrinking of the interior. The gradual cooling of the heated, potentially plastic interior may, therefore, produce four different classes of effect: (1) Crater or Volcanic Eruptions ; (2) Fissure or Sheet Eruptions; (3) Gradual Changes of Level ; (4) Earthquakes. 44 PHYSICAL GEOGRAPHY. mm CHAPTER II. Volcanoes. 36. A Volcano is a mountain or other elevation, more or less conical near the top, provided with an opening or crater, through which, from time to _ time, vapors, ashes, and :\-v lava escape. The cra- ter may be either on the top or on the sides of the mountain. 37. Crater or Vol- canic Eruptions. — In crater or volcanic erup- tions the lava flows through chimneys and builds up lava cones at the crater or point of escape. The conical shape is due to the ejected material accu- mulating around the mouth of the crater in more or less concentric layers. Crater, or volcanic eruptions, are distinguished from fissure or sheet eruptions by the fact that in crater eruptions the lava escapes in streams, while in fissure eruptions it escapes in sheets. 38. Peculiarities of Craters. — The crater, as its name indicates, is cup- shaped. The rim is sometimes broken by the force of the eruption, as in Mount Vesuvius, where the eruption of 79 A. D. — the first on record — blew off the north- ern half of the crater. The material thus detached, together with the showers of ashes and streams of mud and lava, completely buried the cities of Hercula- neum and Pompeii, situated near its base. The crater is often of great size. Mauna Loa, on the island of Hawaii, has two craters — one on the summit, and the other on the mountain-side, about 4000 feet above the sea. The latter — Kilauea — is elliptical in shape, and about 7i miles in circumference ; its area is nearly 4 square miles, and its depth, from 600 to 1000 feet. Fig. 20.— An Eruption of Mount Vesuvius. VOLCANOES. 45 39. Calderas or Crater Rings are deep, circular depressions formed on volcanic mountains either — (1) By the blowing away of a portion of the cone; or, (2) By the caving-m of the crater floor, owing to the absence of supporting lava beneath. A caldera, now filled with water, is seen in Crater Lake, Oregon. 40. The Ejected Materials are mainly as follows : (1) Melted Rock, or Lava. — Lava varies, not only with the nature of the materials from which it was formed, but also with the conditions under which it has cooled, and the quantity of air or vapor entangled in it. Though usually of a dark gray, it occurs in various colors. Its texture varies from hard, compact rock to porous, spongy material that will float on water. When first emitted from the crater, ordinary lava flows about as fast as molten iron would on the same slope. On steep mountains, near the crater, the lava, when very hot, may flow faster than a horse can gallop ; but it soon cools and becomes covered with a crust that greatly retards the rapidity of its flow, until its motion can be determined only by repeated observations. At Kilauea, jets of very liquid lava are sometimes thrown out, which, while falling back into the crater, are drawn out by the wind into fine threads, thus pro- ducing what the natives call Pele's hair, after their mythical goddess. The volume of the ejected lava is often very great. Volcanic islands are usually formed entirely by lava streams. Hawaii and Iceland were, probably, formed entirely of lava emitted from numerous volcanic cones. (2) Ashes or Cinders consist of minute fragments of lava that are ejected violently from the crater ; at night they appear as showers of brilliant sparks. When they fall directly back on the mountain, they aid in rearing the cone. The ashes, when exceedingly fine, form what is called volcanic dust, clouds of which give rise to the erroneous idea that smoke issues from the crater. At the beginning of an eruption large fragments of rock are some- times violently thrown out of the crater. Volcanic dust is often carried by the winds to considerable distances from the crater. In warm, moist climates the layers of volcanic dust that cover the ground, in volcanic districts, decompose and form an excellent natural fertil- izer. When mixed with lake sediment, as is the case in some of the north- western parts of the United States it forms a wonderfully fertile soil. 46 PHYSICAL GEOGRAPHY. (3) Vapors or Gases. — The vapor of water, the principal vapor, often escapes in great quantities from the crater, especially at the beginning of the eruption. On cooling, it condenses and forms clouds, from which torrents of rain fall. These clouds, lighted by the glowing fires beneath, appear to be actually burning, and thus give rise to the erroneous belief that a volcano is a burning mountain. To the condensation of this vapor is, probably, to be ascribed the lightning which often plays around the summit of the volcano during an eruption. Besides the vapor of water, various gases escape, of which sulphurous acid is the most common. On Mount Erebus, a volcano in the Antarctic, the vapor is condensed into snow, which reaches the ground before melting. Vent holes called fumaroles are formed in the hardened crust over the recently ejected lava streams. Through them heated gas and vapors escape, from the glowing lava below, long after it has ceased to flow. Miniature craters are often formed around these opening's. When rain mingles with the ashes, torrents of mud are formed, which move rapidly down the slopes of the mountain, occasioning considerable damage. The rock that is formed by the hardening of volcanic mud is called tufa. 41. The inclination of the slopes of volcanic cones depends on the nature of the material of which they are formed. Where lava is the main ingredient, the cone is broad and fiat. The inclination of a lava Fig. 21.— Lava Cone. Inclination from n w t mo 3 o t0 10 o cone ranges from 3° to 10°, according to the liquidity of the lava. A very stiff lava will form a much steeper cone. Ashes and cinders form steeper cones, whose inclinations range from 30° to 45°. The lower slopes of the mountain are not so steep, and often extend for considerable distances. Etna, though only 10,834 feet high, has a base of nearly 40 miles in diameter. The sides of volcanic cones are often rent during the eruption, and the fissures filled with lava, which hardens and forms rocky ribs called dikes, as in Fig. 23. Sometimes the central cone becomes choked, and secondary or parasitic cones are formed. VOLCANOES. 47 Fig. 22. — Ash Cone. Inclination from 30 c to 45°. 42. The Cause of Volcanic Eruptions. — The settling of the solid crust, as it tends to fill the space left by the shrinking, heated interior, is attended by bend- ings, foldings, and fractures. Any of these fractures that extends to the potentially plastic interior acts as a relief of pressure at this point, and permits the inte- rior to become actually plas- tic, or even highly fluid. The pressure on this fluid rock may cause it to fill up the fissure, and even to escape therefrom through openings in the crust. These openings, when local, constitute the craters of volcanoes. Nearly all volcanic eruptions are attended by the escape of large quantities of steam. This fact has led some to regard the pressure of steam as the principal cause of the lava rising from the plastic interior. Were this true, the rocks form- ing dikes should be vesicular, or filled with small air spaces, instead of being, as they are, compact and dense. It would appear, however, that molten rocks rising from great depths, on coming in contact with water in the rocks nearer the surface, might become more fluid, and be aided by the steam pressure to reach the surface. In the opinion of some geologists, all volca- noes are situated in fissures that are filled, through fissure eruptions, with lava that has come up from great depths, and, that long after- ward, the surface water, percolating through masses of still liquid rock, pro- duce secondary eruptions through craters. It is for this reason that fissure eruptions are sometimes called primary eruptions, and volcanic eruptions, sec- ondary eruptions. The temperature required to melt dry rock is far greater than when water is present. It has been shown that, in the presence of highly heated water, rocks containing much silica will fuse at 800° F. The same rocks, if dry, might require 2500° F. It has, therefore, been asserted that in the presence of highly heated water, aided by the heat of the still glowing lava, the neighboring strata might be fused, and caused to escape from the surface as lava streams. Following the best authorities, however, we may regard the prime Fig. 23. — Volcanic Dikes and Parasitic Cones. 48 PHYSICAL GEOGRAPHY. cause of volcanic eruptions as the shrinkage or contraction of the heated interior. Cordier has shown that a radial contraction of the earth of but a single milli- meter (0.03937 inch) would he ample to supply matter for five hundred of the greatest known volcanic eruptions. 43. Volcanic Eruptions may be divided into two classes : explo- sive and non-explosive. (1) Explosive eruptions are attended by the formation of quan- tities of highly heated steam. They are usually preceded by earth- quake shocks ; then large quantities of ashes are thrown into the air, and this is followed by the escape of lava streams which flow down the mountain. On account of the great viscidity of some lavas, the evolved gases accumulate until considerable force is acquired. At Kilauea, liquid jets are thrown upward to the height of 40 feet. With very viscid lavas, like those of Vesuvius, bubbles of enormous size are suddenly formed, which burst with almost incredible force. Cases are on record in which it is estimated the ashes were projected 10,000 feet above the mouth of the crater. Volcanic Bombs.— Masses of plastic lava called volcanic bombs are sometimes thrown far upward in the air during the explosive eruptions of volcanoes. Acquiring a rotary motion, they assume a spherical form, and, hardening before they reach the ground, retain a spherical shape. Sometimes, striking the ground before they completely harden, they assume a flattened, oval shape. Spherical lava balls are also sometimes produced by a rolling motion in the advance portions of a lava stream. (2) Non-explosive eruptions occur where the lava is more fluid, and there is an absence of suddenly formed water vapor, or gas. When the crater is near the top of a very high mountain, the lava escapes quietly through a fissure, which opens in the moun- tain-side at some lower level, by reason of the pressure exerted by the column of liquid lava. Since a column of lava 500 feet high exerts a pressure of about 625 pounds to the square inch, when the mountain is high, the pressure against the sides of the crater may be sufficient to rend the solid rock. The eruptions of Vesuvius furnish examples of explosive eruptions ; those of Kilauea and Etna, of non-explosive eruptions. 44. Submarine Volcanoes. — Volcanoes are of common occur- VOLCANOES. 49 rence at the bottom of the ocean. These are called submarine vol- canoes. During eruptions their cones sometimes project above the water; but usually they soon disappear. 45. Active and Extinct Volcanoes. — Volcanoes may be clas- sified as active and extinct. Active Volcanoes are those which emit vapor, ashes, or lava from the crater. The tubes or passage-ways, through which the lava flows from great depths to the surface, are usually left filled with lava when the volcanoes cease to be active. Slowly cooling and hardening, they form what are called volcanic necks. In the old age of vol- v-, .^^'* Ig— -- ^ r --- ■ *." . * '■ ".'•* :■■■■:• ■ ..,- Fig. 24.— Volcanic Necks, New Mexico. canic mountains, when their outer layers have been gradually worn away or denuded, these volcanic necks or cones remain and form prominent features in the landscapes. Many such exist in New Mexico. The crater of an active volcano may at any time become permanently choked, and the volcano become extinct. Any volcano which has ceased to erupt during historical times is said to be extinct. It may, however, open at any time, after extended intervals of rest, when the volcano again becomes active. 4 50 PHYSICAL GEOGRAPHY. 46. The number of volcanoes is not accurately known. The best authorities estimate it at about 672, of which 270 are active. Of the latter, 175 are on islands, and 95 are on the coasts of the continents. 47. Regions of Volcanoes. — The principal volcanic regions of the earth are — (1) Along the Shores of the Pacific Ocean, which is encircled, with but few breaks, by an immense chain of volcanoes. On the Eastern Borders of the Pacific, in the Andean range, are the volcanic series of Chili, Bolivia, and Ecuador ; those of Central America and Mexico ; in the United States are some recently extinct craters in the Sierra Nevada and Cascade ranges and in Alaska; and, finally, connecting the system with Asia, the volcanic group of the Aleutian Islands. According to Russell, there are no recently active volcanoes from Central Mexico north to Southeastern Alaska, though there are many recently extinct craters, as well as hot springs and geysers. On the Western Borders volcanoes occur in the following districts : the Kamtchatkan Peninsula, Corea, with the submerged ranges of the Kurile Islands ; the Japan, the Loo Choo, and the Philippine Islands ; the Moluccas ; the Australasian Island Chain, terminating in New Zealand ; and, finally, nearly in a line with these, the vol- canoes of Erebus and Terror on the Antarctic continent. (2) In the Islands of the Pacific. — Volcanic activity is not wanting over the bed of the Pacific. The Sandwich Islands, the Society Group, the Marquesas, the Friendly Islands, the New Heb- rides, the Ladrones, and many others, are volcanic. (3) Scattered over the seas that divide the Northern and Southern Continents, or in their vicinity : In the Neighborhood of the Caribbean Sea. — This region includes the two groups of the Antilles in the Caribbean Sea, and the Galla- pagos Islands in the Pacific Ocean. In the Neighborhood of the Mediterranean and Bed Seas. — This region includes the volcanoes of the Mediterranean and its borders, VOLCANOES. 51 those of Italy, Sicily, the Grecian Archipelago, and of Spain, together with those near the Caspian and Red Seas. Between Asia and Australia. — This region includes the Sunda Islands, Sumatra, Java, Sumbawa, Flores, and Timor. In this region there are numerous active craters. In Java there are nearly 50 volcanoes, 28 of which are active, and there are nearly as many in Sumatra. There are 109 volcanoes in the small islands near Borneo. (4) In the Northern and Central Parts of the Atlantic Ocean. All the islands in the deep ocean, which do not form a part of the continent, are volcanic ; as the island of St. Helena, Ascension Island, the Cape Verdes, the Canaries, the Azores, and Iceland. Fig. 25. — Fuji Yama, a Comparatively Recent Volcano. The Cameroons Mountains, on the African coast near the Gulf of Guinea, together with some of the islands in the gulf, are volcanic 52 PHYSICAL GEOGRAPHY. (5) In the "Western and Central Parts of the Indian Ocean. Volcanoes are found in Madagascar and in the adjacent islands. They also occur farther south, in the island of St, Paul and in Ker- guelen Land, and in Mount Kilimandjaro, near the eastern coast of Africa. 48. Submarine Volcanoes.— From the difficulty in observing submarine volcanoes they are not so well known as the others. The following regions are well marked : In the Mediterranean Sea, near Sicily and Greece. In the Atlantic Ocean ; off the coast of Iceland ; near St. Michael, in the Azores ; and over the narrowest part of the ocean between Guinea and Brazil. In the Pacific Ocean ; near the Aleutian Islands, where two large moun- tain-masses have recently risen from the water ; and near the Japan Islands, where, about twenty-one centuries ago, according to native historians, Fuji Yama (Fig. 25), the highest mountain in Japan, rose from the sea in a single night. In the Indian Ocean, the island of St. Paul, in the deep ocean between Africa and Australia, exhibits signs of submarine activity. 49. Peculiarities of Distribution. — Nearly all volcanoes are found near the shores of continents or on islands. It was formerly believed that this distribution showed the cause of eruptions to be the access of sea-water to the interior. The general belief now 'is that this distribution is due to the fact that the earth's volcanoes lie along lines of original fractures in the earth's crust, and that these lines are not necessarily dependent on the present distribution of the land and water areas. Moreover, there exist lines of fracture, with either recently active or still active volcanoes, at con- siderable distances from the coast, such as the Great Basin District in the United States ; in parts of Mexico and Central America ; and in Thibet. In most regions the volcanoes lie along lines more or less straight. Lines joining such a series may be considered as originally huge fissures in the crust, the volcanoes occurring in their weakest places. Where one system of fissures crosses another, as in the Antilles, and in the Sunda Islands, points that are almost antipodal to each other, the volcanic activity is unusually great. 50. Life History of a Volcano (after Eussell).— The birth of a volcanic mountain is usually preceded by severe earthquake shocks resulting in the forma- tion of a fissure and a forcing of the potentially plastic heated interior into the fissure. This may result either in a fissure eruption, or in a local or crater eruption. The above phenomena attended the few volcanic mountains which VOLCANOES. 53 -In fissure eruptions the lava have been born within historic times, such as Monte Nuovo, near Naples, in 1538, and Jorullo, in Mexico, in 1759. The accumulation of lava and ashes around the crater usually results in the formation of a conical mountain, though the shape will, of course, depend on the conditions and character of the material ejected. As the volcano grows in height its activity becomes more and more marked, and the eruptions increase in number and severity. Then follow long periods of rest, during which the volcano appears to have passed into hopeless old age, when, suddenly, violent shocks again occur, and terrific explosions may blow off the top of the mountain, or rend vast fissures in its sides, and the vol- cano renews its youthful activity. These alternate periods of rest and activity may follow one another many times, but the eruptions become less and less marked, the lava-flows are replaced by sulphurous vapors and steam, until finally, even marked heat ceases, and the volcanic mountain is uo warmer than the surrounding country. Meanwhile erosion is taking place, and when the mountain ceases to be added to by fresh material from without, degradation and denudation occur. Deep gorges are cut in the mountain, and finally it is either entirely re- moved, or there remains only the hardened volcanic neck to mark the place of its birth. 51. Fissure or Sheet Eruptions, escapes through fissures in sheets, and spreads out in wide floods. True fissure eruptions have occurred only in the geological past. The lava in fissure erup- tions was more liquid than that issuing from most volcanoes. Fissure eruptions occur in three different forms : (1) Extensive vertical sheets filling great fissures in the rocks of nearly all geological forma- tions. On cooling, the mass forms what is called a dike. Dikes vary in width from a few inches to several yards. They are usually much harder than the rock through which they were forced, and, being less subject to erosion, often project considerably above the general surface. Fig. 26.— Basaltic Columns, Giant's Causeway. 54 PHYSICAL GEOGRAPHY. From their mode of formation, dikes are usually without traces of stratification, but, by cooling, a series of fractures are sometimes produced, giving to the dike a columnar appearance, the dike then consisting of basaltic columns. Fingal's Cave in Scotland and the Giant's Causeway in Ireland are noted examples. The intense heat of the fused rock causes the rocks forming the boundary walls of dikes to be markedly changed by the heat. Limestone is often con- verted into marble, bituminous coal into anthracite, and sometimes into coke. (2) Extensive horizontal sheets forced between parallel strata, or spread out over the bed of an ocean, or other body of water, or poured out on the surface of the land in great sheets. (3) Dome-shaped masses. Instead of escaping at the surface, the lava merely lifts the upper strata, and accumulates locally in dome- shaped masses called laccoliths or laccolites ; i. e., lake-like expansions of the lava stream. Sub-tuberant mountains are caused by the dome-shaped swellings produced in the overlying strata by the laccoliths. Some of the most important fissure eruptions are as follows : (1) The Columbian lava fields in the northwestern part of the United States. This region covers an area of about 250,000 square miles in Northern California, Washington, Idaho, and Oregon, as shown in Fig. 27. The deposits are from 3000 to 4000 feet thick. (2) The Deccan Trap, a region equal to about 200,000 square miles in area, in the western part of Hin- doostan. The deposits are from 200 to 600 feet thick. (3) The dikes and sheets of igne- ous rocks extending in North America for 1000 miles along the Atlantic slope from Nova Scotia to South Carolina. The area cov- ered was, probably, nearly equal to that of the Columbian lava, or the Deccan Trap, but much of this, however, is either buried beneath sedimentary deposits, or has been removed by erosion. The Palisades of the Hudson form a part of this lava sheet. At Jersey City its thickness is from 300 to 400 feet. Other extensive deposits occur in Abyssinia, Cornwalls, Wales, Scotland, and DA UTAH Fig. 27.— Columbian Lava Fields. EARTHQUAKES. 55 Ireland, as may be seen from a study of the map of the geological formations of the earth. 52. Gradual Elevations and Subsidences. — During the cool- ing of the heated interior, changes of level take place slowly, but continuously, by which large portions of the surface are raised or lowered from their former positions. The rate of these movements is so very slow — probably never exceeding a few feet in a century- — that they become apparent only after long intervals of time. Subsidences. — The following parts of the earth appear to be now sinking or subsiding ; viz., the northern parts of Norway and Sweden, the southwestern coasts of Greenland, the North American coast from Labrador to New Jersey. The bed of the Pacific over an area of some 6000 miles in length. Elevations. — The following parts appear to be slowly rising; viz., nearly all the islands and lands bordering on the Arctic shores of North America, Europe and Asia, Labrador, Hudson Bay, Newfoundland, and the Andes of South America. CHAPTER III. Earthquakes. 58. Earthquakes are shakings of the earth's crust, of degrees varying in intensity from scarcely perceptible tremors to violent agitations that overthrow buildings and open huge fissures in the ground. They may either accompany volcanic eruptions, or they may occur independently of such eruptions. An earthquake is sometimes called a seismic throb or shock. During severe earthquakes considerable areas are permanently raised or low- ered. During an earthquake in South America, in 1835, the entire coast-line of Chili and Patagonia was elevated from 2 to 10 feet above the ocean level. During an earthquake in 1819, near the mouth of the Indus, a tract some 2000 square miles in area sunk and was converted into a salt lagoon, while a much larger area was elevated some 10 feet. 54. Pacts Concerning 1 Earthquakes. — A careful study of earth- quakes appears to establish the following facts : 56 PHYSICAL GEOGRAPHY. (1) The place or origin of the shock is situated not far below the earth's surface, but is near the surface, probably, never deeper than thirty miles, and often much less. (2) The area of disturbance depends not only on the energy of the shock, but also on the depth of its origin below the surface : the deeper the origin, the greater the area of disturbance. (3) The shape of the origin is usually that of a line, often many miles in length. (4) The direction of the motion at the surface is nearly upward over the origin, and more inclined as the distance from the origin increases. (5) The shape of the area of disturbance depends on the nature of the materials through which the wave is moving. If these are of nearly uniform elasticity in all directions, the area is nearly cir- cular; if more elastic in some directions than in others, the area is irregular in shape. 55. Varieties of Earthquake Motion. — There are three varie- ties of earthquake motion : (1) Explosive. — These are attended by a violent motion directly upward. During such shocks the crust is broken, and bodies are thrown upward in the air. (2) Wave-like, or horizontally progressive, like waves in water. Here the area of disturbance is great. (3) Rotary, or those attended by a whirling motion of the crust. Humboldt mentions an earthquake that happened in Chili where the ground was so shifted that three great palm trees were twisted around one another like willow wands. There are two kinds of movement transmitted through the crust during earth- quakes : these are the earthquake motion proper, and the motion that produces the accompanying sounds. 56. The Velocity of Earthquake Motion varies according to the intensity of the shock and the nature of the material through which it is transmitted. No average result, therefore, can be given. Various observers have estimated it at from 8 to 30 miles per minute. 57. The Sounds Accompanying Earthquakes vary both in EARTHQUAKES. 57 kind and intensity. Sometimes they resemble the hissing noises heard when red-hot coals are thrown into water; sometimes they are rumbling, but more frequently they are of greater intensity, and are then comparable to discharges of artillery or peals of thunder. The confused roaring and rattling are, probably, caused by the different rates of transmission of the sound through the air and rocks. 58. Duration of the Shocks. — The earthquake shocks which cause the greatest damage are of but short duration, usually lasting but a few seconds or minutes. Though the violence of the shock is soon passed, disturbances may occur at intervals of days, weeks, or even years. During the earthquake in Calabria, in 1783, when nearly 100,000 persons per- ished, the destructive vibrations lasted scarcely two minutes, but the tremblings of the crust continued long afterward. During the earthquake at Lisbon, in 1755, when about the same number perished, the shock which caused the great- est damage continued but five or six seconds, while a series of terrible move- ments followed one another at intervals during the space of five minutes. 59. The Cause of Earthquakes. — As we have seen, the earth's surface is subject to many gradual changes of level, due to the cool- ing and contraction of the heated interior, and the consequent crush- ing, flexing, and fracture of the solid crust. If the yielding of the crust to the force causing these movements is constant, the motion will be gradual, and only perceived at the surface after long inter- vals of time. If, however, the crust resists the effort to move it, the force accumulates until, finally, the crust yields suddenly by fracture or crushing. This yielding is attended by ajar or concussion which shakes the earth. This concussion is propagated through the crust until it reaches the surface and moves outward from the point of exit. Earthquake shocks may also be produced by the slipping of fissured rocks, as they subsequently settle. If the theory of gradually accumulated strain be true, as it prob- ably is, the earth's crust must occasionally be in such a strained condition, that the slightest increase of force from within, or of diminished resistance from without, would disturb the conditions of equilibrium, and thus result in an earthquake. 58 PHYSICAL GEOGRAPHY. 60. Accumulated Strain Caused by Contraction consequent on cooling is well exhibited in the so-called "Prince Kupert's Drops," which are made by allowing melted glass to fall in drops through cold water. The sudden cooling of the outside produces forces which tend to compress the drop ; but, since these forces balance one another, no movement occurs until, by breaking off the long end of the drop, one set of forces is removed, when the others, no longer neu- tralized, tear the drop into many pieces. 61. Other Causes of Earthquakes. — Earthquakes may also be occasioned by — (1) The sudden formation and collapse of steam in subterranean regions. This is probably the cause of many of the slight shocks that occur in the neighborhood of active volcanic regions. (2) Shocks caused by falling masses. Those who deny the existence of a pasty interior, endeavor to explain the production of earthquakes by the shock caused by the gradual settling of the upturned strata in mountainous districts. There can be no doubt that even moderately severe shocks are caused by falling masses ; but such a force is utterly inadequate to produce severe earthquake shocks like that, for exam- ple, which destroyed Lisbon, when an area of nearly 7,500,000 square miles was shaken. 62. Periodicity of Earthquakes. — Although earthquakes may occur at any time, yet by a comparison of the times of occurrence of a great number it appears that they occur more frequently — (1) In winter than in summer ; (2) At night than during the day ; (3) During the new and full moon, when the attractive force of the sun and moon acts simultaneously on the same parts of the earth. Earthquake shocks are more frequent in winter and during the night, because the cooling, and consequent contraction, occur more rapidly at these times, and, therefore, the gradually accumulating force is more apt to acquire sufficient intensity to rend the solid crust. Earthquakes are more frequent during new and full moon, because the increased force on the earth's crust, caused by the position of the sun and moon at these times, is then added to the accumulated force produced by cooling. 63. Distribution of Earthquakes. — Earthquakes may occur in any part of the world, but are most frequent in volcanic districts. n R C T I c E A N NEW ZEALAND ANTARCTIC CWCLE MAP OF THE WORLD SHOWING THE DISTRIBUTION OF VOLCANOES AND REGION OF EARTHQUAKES. Z^L A R HO 120 100 3D EARTHQUAKES. 59 They are more frequent in mountainous than in flat countries. They are especially frequent in high mountains, while yet in the process of gradual elevation. According to Huxley, fairly pro- nounced earthquake shocks occur iu some part of the earth at least three times a week. There is, in many instances, an undoubted connection between volcanic erup- tions and earthquakes. Humboldt relates that during the earthquake at Kio- bamba, when some 40,000 persons perished, the volcano of Pasto ceased to emit its vapor at the exact time the earthquake began. The same is related of Vesu- vius at the time of the earthquake at Lisbon. 64. Phenomena of Earthquakes. — In order to give some idea of the phe- nomena by which severe earthquake shocks are attended, we append a brief description of the earthquake which destroyed the city of Lisbon, on the 1st of November, 1755. The earthquake was preceded by a sound like thunder, and almost immediately afterward a series of violent shocks threw down nearly every building in the city. The ground rose and fell like the waves of the sea ; huge chasms were opened, into which many of the buildings were precipitated. In the ocean a huge wave, over 50 feet high, was formed, which, retreating for a moment, left the bar dry, and then rushed toward the land with frightful force. This was repeated several times, and thousands perished from this cause alone. The neighboring mountains, though quite large, were shaken like reeds, and were rent and split in a wonderful manner. This earthquake was especially remarkable for the immense area over which the shock extended. It reached as far north as Sweden. Solid mountain- ranges, such as the Pyrenees and the Alps, were severely shaken. A deep fissure was opened in France. On the south, the earthquake waves crossed the Mediterranean and destroyed a number of villages in the Barbary States. On the west, the waves traversed the bed of the Atlantic, and caused unusually high tides in the West Indies. In North America the movements were felt as far west as the Great Lakes. Feebler oscillations of the ground occurred at intervals for several weeks after the main shock. REVIEW QUESTIONS. The Heated Interior. Why is it probable that the earth was originally molten throughout? Enumerate the proofs that the interior of the earth is still in a highly heated condition. What is meant by a potentially plastic interior ? What four classes of effects are produced in the crust by the heated interior ? Volcanoes. What are volcanoes ? In what respect do active volcanoes differ from those which are extinct? Explain in full the manner in which the shrinkage or contraction of the earth on cooling produces volcanic eruptions. 62 PHYSICAL GEOGRAPHY. Into what two classes may all volcanic eruptions be divided ? How are those of each class caused ? Give an example of each class. Under what five regions may all the volcanoes in the world be arranged ? Name some of the regions of submarine volcanoes. Give a brief statement of the life history of a volcano. What are dikes? How were they formed ? Distinguish between crater or volcanic eruptions and fissure or sheet erup- tions. In what three different forms do fissure or sheet eruptions occur? Describe the Columbian lava-fields. The Deccan Trap region. Enumerate some of the gradual changes of level which are now occurring in the crust of the earth. By what are these changes caused ? Earthquakes. What are earthquakes ? Name some facts that have been discovered about earthquakes. Name three kinds of earthquake motion. Which is the most dangerous? What is the main cause of earthquakes? To what other causes may they be attributed ? What facts have been discovered respecting their periodicity ? Give a short description of the earthquake which destroyed the city of Lisbon. In what parts of the earth are earthquake shocks most frequent? o-oJ^OO MAP QUESTIONS. Trace on the map the five principal volcanic districts of the earth. Does the eastern or the western border of the Indian Ocean contain the greater number of volcanoes ? Name the principal volcanic islands of the Atlantic Ocean. Of the Indian. Of the Pacific. In what part of the Atlantic Ocean are submarine eruptions especially frequent? Locate the following volcanoes: Hecla, Pico, Kilauea, Sarmiento, Llullayacu, Egmont, Cosiguina, Teneriffe, Antisana, Kilimandjaro, Demavend, Peshan, Osorno, Erebus, and Terror. Name the portions of the earth shaken by the earthquake of Lisbon. What noted volcanoes are found in the region of the earthquake of Lisbon? SECTION II. THE OUTSIDE OF THE EARTH. CHAPTER I. The Crust of the Earth. 65. Composition of the Crust. — The elementary substances are not equally distributed throughout the earth's crust. Many occur only in extremely small quantities, while others are found nearly everywhere. Although the deepest cutting through the earth's crust does not extend ver- tically more than about two miles below the level of the sea, yet the outcropping of the different formations enables us to study a depth of about sixteen miles of the earth's crust. Oxygen constitutes nearly one-half by weight of the known part of the crust. Silicon, which when combined with oxygen, forms silica or quartz, constitutes, either as sand, or combiued with various bases as silicates, one-fourth ; so that these two elements form at least three-fourths, by weight, of the entire crust. The following are also prominent ingredients : aluminium, which when combined with oxygen, forms alumina, the basis of clay, and magnesium, calcium, potassium, sodium, iron, and carbon. The above enumerated nine elements, according to Dana, form ^^ths, by weight, of the entire crust. Sulphur, hydrogen, chlorine, and nitrogen also occur frequently. The remaining elements are of comparatively rare occurrence. 66. Rocks. — In its geological sense, the word rock embraces various mixtures of substances called minerals, that occur naturally in sufficient masses to be properly considered as an essential part of the crust. This includes not only the hard strata, but also the softer beds of sand and clay. Minerals consist of fairly definite chemical compounds. Though 6?. 64 PHYSICAL GEOGRAPHY. rocks sometimes consist of a single mineral, they are usually formed by mechanical mixtures of two or more minerals. Some of the more important mineral constituents of the crust are felspar, quartz, mica, soapstone, limestone, and clay. Clays containing an excess of finely divided silica are called fuller's earth ; when mixed with a certain proportion of fine sand, they form what is called loam, and when containing a certain pro- portion of calcium, are called marl. Rocks may be divided into different classes : (1) According to their origin or the manner in which they were formed ; (2) Accord- ing to their condition or the way in which their mineral ingredients are arranged ; and (3) According to the presence or absence of organic remains. 67. According to their Origin, rocks may be divided into three distinct classes : igneous, aqueous, and metamorphic. (1) Igneous Rocks are those which have been in a molten state, and have solidified on cooling. They include two classes : (a) Plutonic rocks, or those which have slowly cooled helow the surface under great pressure, and have sofRufied as crystalline rocks; true granite is an example. (6) Volcanic rocks, or those which have erupted at the surface, and, having cooled rapidly and without pressure are, as a rule, devoid of crystalline form. Lava and basalt are examples. (2) Aqueous Rocks, or those deposited as sediment by water. When mineral matter settles in water, the coarser, heavier particles reach the bottom first, so that a sorting action occurs, which makes the different layers or strata vary in the size and density of their particles, and, to a great extent, in their composition. Aqueous rocks are sometimes called sedimentary rocks. (3) Metamorphic Rocks are those in which have occurred such marked chemical and mineral changes, produced by heat in the presence of moisture, together with pressure, that their original constitution has been more or less altered and their character disguised. Both sedimentary and igneous rocks are subject to these changes. THE CRUST OF THE EARTH. 65 The changes are in some cases so marked that it is difficult to distinguish between rocks of metamorphic and igneous origin. Many rocks which were for- merly believed to be of igneous origin, are now generally regarded as highly altered sedimentary strata. Even some granites are thought to be of meta- morphic origin. These changes, which are called metamorphisms, are caused by heat acting under pressure in the presence of moisture. Under these conditions a far less intense heat is required to remove all traces of stratification. Metamorphism appears to consist mainly in a rearrangement of the chemical constituents of the rocks. To the above are sometimes added an additional class; viz., jEolian rocks, or those formed from wind drifted materials. They are irregularly bedded. 68. According to their Condition, rocks may be divided into two classes : (1) Stratified Rocks, or those arranged in regular layers. Aqueous rocks are always stratified, and sometimes, though rarely, metamorphic rocks are stratified. (2) Unstratified Rocks, or those destitute of any arrangement in layers. They are of two kinds : (a) Igneous, or those which were never stratified. (6) Metamorphic, or those which were once stratified, but have lost their stratification by the action of heat. Unstratified rocks are sometimes called crystalline rocks, because they consist of crystalline particles. 69. Primitive Rocks. — The earth's earliest or primitive rocks were either those formed by the first cooling and hardening of the outside of the melted earth, or those thrown down as sediment in the primitive ocean. The crust, however, has been so repeatedly fractured and broken up and thrown down as sediment, that it is practically certain that all traces of the original crust have long ago disappeared. The lowest rocks we have yet reached were originally deposited as sediment in water, and have been so modified by in- tense heat that they present nearly all the appearances of rocks formed by the cooling of melted matter. In other words, almost all rocks are fragmental ; i. e., have been made out of the broken or worn fragments of older rocks. All beds of sand, gravel, stones, 5 66 PHYSICAL GEOGRAPHY. mud, clay, and earth are merely worn, pulverized, or weathered rock. 70. Fossils are the remains of plants or animals which have been buried in the earth by natural causes. Generally, the soft parts of the organism have disappeared, leaving only the harder parts. Sometimes the soft parts have been grad- ually removed, and replaced by mineral matter, such as lime or silica, thus produc- ing what are called petrifac- tions. At times the mere impression of the animal or plant is all that remains to tell of its former existence. When the remains of an animal or plant are exposed to the air, or buried in dry earth, they usually decompose and pass off almost en- tirely as gases ; but when buried under water, or in damp earth, their preserva- tion is more probable. Therefore, the species most likely to become fossilized are those living in, or near, water or marshes. 71. According- to the Presence or Absence of Fossil Remains, rocks may be divided into two classes : (1) Fossiliferons Rocks, or those which contain fossils. They are stratified, and are of aqueous origin. Metamorphic rocks, in rare instances, are found to contain fragments of fossils. (2) Non-fossiliferous Rocks, or those destitute of fossils. They include all igneous rocks and most of those that are metamorphic. 72. Paleontology is the science which treats of the plants and animals whose remains are now found only as fossils. Paleontology enables us to ascer- tain the earth's condition in pre-historic times, since by a careful examination of the fossils found in any rocks we discover what animals and plants lived on the earth while such rocks were being deposited. The earth's strata thus become the pages of a huge book ; and the fossils found in them, the writings concerning the old life of the world. By their careful study geologists have been enabled to find out much of the earth's past history. Fig. 28. — Plesiosaurus Macrocephalus (Buckland). 2 )>- 140 ICO 180 1G0 140 120 100 GEOLOGICAL FORMATIONS OF THE EARTH (AFTER BERGHAUS) =E3=E3=EEO=E 120 Longitude 140 East from 100 Greenwich ISO 140 120 100 80 REFERENCES ARCHAEIC AND GRANITES. _J PALAEOZOIC. MESOZOIC. | QUATERNARY. 1 - 1 SANDY AND DESERT WASTES. FORMATIONS. u ~-j ~ g~ West from 20 Greenwich Longitude 20 East from 40 Greenwich 60 THE CRUST OF THE EARTH. 67 73. Division of Geological Time. — A comparison of the various species of fossils found in the earth's crust discloses the following facts : (1) The fossils found in the lowest rocks bear but a slight resem- blance to the animals and plants now living on the earth. (2) The fossils found in the intermediate strata bear a resem- blance to existing species, though this resemblance is not so strongly- marked as in the upper strata. (3) The fossils found in the upper strata bear a decided resem- blance to existing species. It is on such a basis that the immense extent of geological time is divided into eras, the eras are subdivided into periods, the periods into epochs, and the epochs into ages. There are six geological eras : the Azoic, the Eozoic, the Palae- ozoic, the Mesozoic, the Cenozoic, and the Era of Man. As regards the enormous extent of geological time, Dana, reviewing the various estimates as to its duration, based on different physical phenomena, says: "The safe conclusion from all the geological and physical facts is that time is long, very long ; long enough for the development of all the earth's rocks, mountains, continents, and life. Geologists have no reason to feel ham- pered in their speculations, while the extreme results of calculation are 10,000,000 and 6.000,000,000 years." 74. The Azoic Era. — The word azoic means no life. The name was applied to that extremely extended time that existed from the formation of the earth until the first appearance of life. It included the time when the. temperature was so high as to have rendered the ' existence of life impossible. In certain formations, however, which were originally included in the Azoic Era, markings have been found that, in the opinion of some geologists, were due to the existence of a simple form of life ; therefore, toward the close of what has been included in the Azoic Era, life may have appeared. The Azoic Era included a long period during which our earth was a glowing star ; this period is, therefore, called the Astral Period, and a later period of the Azoic Era, whose boundaries have not yet been satisfactorily fixed, is called the Archaean Period. The Archaean Period was intended to include the most ancient 68 PHYSICAL GEOGRAPHY. rocks, the basement or fundamental rocks; i. e., igneous rocks that resulted from the cooling of the molten earth. The lowest known rocks of this period are fragmentary, having been apparently formed from sedimentary deposition of the original archaean rocks. They are highly metamorphosed. In North America the lowest of the Archaean rocks include the Laurentian, so called from their occurrence in extensive deposits in the neighborhood of the St. Lawrence River. 75. The Eozoic Era. — The word eozoic means the dawn of life. The name was applied to the time when life was first created. As already stated, this may have occurred toward the close of the Azoic Era. The Eozoic Era includes a period called by the United States Geological Survey the Algonkian Period. It comprises an extended series of sedimentary and metamorphosed rocks between the Archaean and the oldest of the Palaeozoic formations. They are represented in North America by the Huronian, so named from their occurrence in the neighborhood of Lake Huron. Most of the sedimentary rocks of the Eozoic Era are very highly metamor- phosed, so that remains of plants and animals are nearly obliterated. The occurrence in the rocks of graphite, a form of carhon, is regarded as strong evi- dence of plants, if not of animals. Algae or sea-weeds, among marine plants, and some of the lowest forms of the protozoa were, perhaps, abundant. 76. The Palaeozoic Era. — The word palaeozoic means ancient life. The Palaeozoic Era embraces the time of ancient life, during which the plants and animals bore comparatively little resemblance to those now living. The deposits of Palaeozoic Time are sometimes called the primary. The duration of the Palaeozoic Era was, prob- ably, equal to that of both the Mesozoic and Cenozoic Eras. Palaeozoic Time includes the following periods : (1) The Silurian Period, or the Period of Invertebrates ; (2) The Devonian Period, or the Period of Fishes ; (3) The Carboniferous Period, or the Period of Coal Plants. 77. The Silurian, or the Period of Invertebrates, is sometimes called the Period of Mollusks. Among plants, algce, or sea-weeds, THE CRUST OF THE EARTH. 69 are found ; among animals, protozoa, radiates, articulates, and mol- lusks, but no vertebrates. Hence the name, Period of Invertebrates. Mollusks were especially numerous. Vertebrates in the form of fishes appeared toward the close of this period. 78. The Devonian, or the Period of Fishes. — During this period all the sub-kingdoms of animals are found. The vertebrates, however, represented by fishes, were the dominant type of life, and from this fact the name has been given to the period. Land-plants are also found. Immense beds of limestone and red sandstone were deposited. The name Devonian is derived from the district of Devonshire, England, where the rocks abound. 79. The Carboniferous, or the Period of Coal-Plants. — Dur- ing this period the continents consisted mainly of flat, marshy areas, covered with luxur- iant vegetation, sub- ject, at loug inter- vals, to extensive inundations. The de- caying vegetation, decomposing under water, retained most of its solid constit- uent, carbon, and formed beds of coal. All the sub-kingdoms of animals were rep- resented, and reptiles also existed. The comparatively few land-plants of the preceding period now increased and formed a dense vegetation. Formation of Coal. — Every 100 parts of dry vegetable matter contains about 49 parts of carbon, 6 of hydrogen, and 45 of oxygen. Carbon is a solid ; hydrogen and oxygen are gases. It is from the carbon that coal is mainly Fig. 29. — Carboniferous Landscape, ration.) (A resto- 70 PHYSICAL GEOGRAPHY. formed. When the decomposition of the vegetable matter takes place in air, the carbon passes off with the hydrogen and oxygen as various gaseous compounds ; but, when covered by water, most of the carbon is retained, together with part of the oxygen and hydrogen. Every year our forests drop immense quantities of leaves, but no coal results, the deposit of one year being almost entirely removed before that of the next occurs. It has been computed that it would require a depth of eight feet of compact vegetable matter to form one foot of bituminous coal, and twelve feet of vege- table matter to form one foot of anthracite coal. Anthracite coal differs from bituminous mainly in the greater metamorphism to which it has been subjected ; it contains a greater proportion of carbon and less hydrogen and oxygen. The latter part of the Carboniferous Period is now called the Permian. 80. The Mesozoic Bra, the middle era of geological time, when the plants and animals began more closely to resemble those now living, includes the Triassic, the Jurassic, and the Cretaceous Periods. The deposits of the Mesozoic Era are sometimes called the Sec- ondary. The Mesozoic is sometimes called the Era of Reptiles. The era is characterized mainly by the prepon- derance of reptiles, many of which were very large, as, for example, the ple- siosaurus, an animal with a long, snake-like neck and a huge body, or the ichthyosaurus, with a head like a crocodile and short neck and large body. Both of these animals were furnished with fin-like paddles, and lived in the water. Huge pterodactyls, or bat-like saurians, flew in the air or paddled in the water. Mam- mals and birds also occur. 81. The Cenozoic Era, the era of geological time immediately preceding the present era, included the time during which animals Fig. 30. — Mastodon Giganteus. An Animal of the Mammalian Age. THE CRUST OF THE EARTH 71 and plants bore decided resemblance to those now living. The Cenozoic Era includes the Eocene, the Oligocene, the Miocene, and the Pliocene Epochs. It is sometimes called the Era of Mammals, because mammals, or animals that suckle their young, occurred in great numbers, and, being the highest type of life, gave the name to the age. The animals and plants of the Mammalian Era closely resembled existing species, though most of them were much larger ; as, for example, the dinotherium, a huge animal, with a trunk like an elephant, but with downward-turned tusks ; the palceotherium, and many others. 82. The Bra of Man, or the Quaternary, witnessed the intro- duction of the present animals and plants and the creation of man. It includes the Pleistocene Epoch, which in its turn embraces the pre-glacial, the glacial, and the post-glacial series. Table of Geological Divisions. Formations. Eras. Periods or Epochs. Quaternary ..... Man. Pleistocene Epoch. r Tertiary j I r i Cenozoic. \ I Pliocene Epoch. Miocene " Oligocene " Eocene " f - Secondary . . . . -j i I f Mesozoic. -j i I Cretaceous Period. Triassic Jurassic " ' Palaeozoic. -j I Eozoic. Azoic. j Permian Period. Carboniferous " Devonian Silurian Algonkian Archaean Astral 83. Order of Earth's Strata. — Where no disturbing causes existed, and the land remained under the seas, the rocks deposited 72 PHYSICAL GEOGRAPHY. during the preceding eras were thrown down in regular strata, one over the other. The Azoic were the lowest ; above them were the Eozoic, then the Palaeozoic, the Mesozoic, the Cenozoic, and, finally, those of the present time. Frequently, however, dislocations of the strata have disturbed the regular order, of arrangement. Some idea of this arrangement will be obtained from the table of geological divisions given on the preceding page. 84. Agencies Now Producing Changes in the Earth's Crust. — It is evident that geological time was characterized by extensive changes, both in the kind and luxuriance of life, and in the nature of its distribution. Changes are still occurring in the earth's crust, the agencies pro- ducing them differing in no respect, except in some instances in the intensity of their action, from those which were active in geological time. Many of these changes are so slow in their action that they require the great lapse of geological time to become apparent. Some, however, are sufficiently marked to have been readily observed within historical time. The most important agencies that are now in operation are as follows ; viz., the Heated Interior, Erosion or Denudation, includ- ing Weathering, Corrasion, and Transportation, Wind Corrasion and Transportation, Avalanches and Land-slides, Ocean Waves, and Man. 85. The Heated Interior of the earth, which, while gradually cooling, is still producing earthquakes and volcanoes, and is still gradually raising some portions of the earth's surface and lowering others. 86. The Erosion, Denudation, or gradual wearing away of the land uplifted above the sea-level. This action has taken place throughout the vast extent of geological time. By it, high moun- tains and portions of the continental masses, have been slowly carried piecemeal into the ocean. Erosion or denudation is still taking place at every exposed surface. Erosion is due to the action of three agencies ; viz., Weathering, Corrasion, and Transportation. 87. "Weathering is the mechanical or chemical disintegration THE CRUST OF THE EARTH 73 of rocks by a series of complex processes. Whenever a rock sur- face is exposed to the action of the weather ; i. e., to the action of heat, moisture, air, and the alternate freezing of water and melting of ice, its surface either gradually cracks, splits, or is broken up into small fragments, or rusts, or ^s corroded and eaten away. When these fragments remain in place they protect, to a great extent, the rock surfaces below them from further action, but in nearly all cases they are carried away or transported either by gravity, by the wind, or by running water, thus permitting the weathering action to con- tinue indefinitely. Some of the more important agencies that cause weathering are : (1) Heat and Cold. — The alternate expansion and contraction of the rocks attending variations in temperature, under the varying action of the sun's heat, tend to break them up. In general, these diurnal changes in temperature do not extend much further than a few feet below the surface, and the annual changes not further than about 50 or 60 feet. The effect of heat is most marked in tropical climates, hoth by drying up large tracts of land, thus rendering them deserts, and, by depriving them of their protective covering of plants, promoting and aiding chemical and mechan- ical action. The effect of cold is most marked in arctic regions, where the shrinkage suffices to break up the rocks, and this independently of the presence of moist- ure. This effect was especially observed by Kane in the wasting of the cliffs in North Greenland. (2) Alternate Freezing and Thawing. — In climates characterized by alternate freezing and thawing, water, soaking into porous rocks, or collecting in the crevices of impervious rocks, is a powerful agency in rending the rocks, since the water in freezing expands with a force sufficiently great to break the rocks into fragments. This cause is not so prominent in extremely high latitudes as it is in the less frigid regions. (3) Rusting, Corrosion, and Solution. — Either the air alone, or the air and its accompanying moisture, enter into chemical combi- nation with some of the substances in the rocks, thus gradually cor- roding or changing them into substances that are more readily 74 PHYSICAL GEOGRAPHY. carried away or dissolved by the percolating waters. The rain water which enters the ground is nearly pure water, and its solvent power for most mineral matters is very much less than when it con- tains certain foreign substances, such as carbonic acid or certain acids, derived from the soil through which it passes. Moreover, when heated, either by neighboring beds of recently ejected lava, or by reason of its sink- ing to great depths, its solvent, and, conse- quently, its corroding power, are very greatly increased. Water containing carbonic acid possesses marked powers of dis- solving limestone rocks. The surface drainage is apt to be exceedingly limited in limestone districts, the streams being either scanty or entirely absent. The surface water entering the ground, dissolves out the limestone and forms what are called sink-holes. The under- ground water, following the lines of easiest pas- sage, dissolves out the limestone, thus forming true underground streams, and, by eating out the rock, forms immense caverns. Fig. 31.— Natural Bridge, Virginia. Such caverns are found in the Mammoth Cave in Kentucky, and in the Luray Cave in Virginia. The water dripping from the roofs of the caves falls on the THE CRUST OF THE EARTH. 75 floor below, and, on the partial evaporation of the water and the loss of carbonic acid gas, deposits the lime as icicle-like pendants from the roof, called stalactites, and sharp hillocks or mounds, immediately beneath, called stalagmites. The meeting of these forms limestone pillars or columns. As the laud surface is eroded, the roof's of such caverus fall in, and the underground streams become surface streams. Occasionally, however, a part of the roof remains, thus form- ing what are called natural bridges. The Natural Bridge in Virginia is an example. 88. Corrasion is a term proposed by Russel for the cutting and consequent deepening and broadening of a stream channel by the material produced by weathering, etc., as it is carried or transported by the running water from a higher to a lower level. This action is called by some erosion, thus limiting it to the mechanical action of the running water, but erosion, strictly speaking, includes weathering, transportation, and corrasion. When the materials derived from weathering, etc., are thrown into a running stream, its wearing and cutting power are greatly increased by its load of mineral matter ; i. e., the silt, sand, gravel, boulders, etc., that are carried from place to place by the moving waters. The amount of the corrasion is dependent both on the character of the transported materials and on their density. Other things remaining the same, the corrasion of a stream Avill depend on the inclination of its channel and on the volume of its running water. A stream may both deepen its channel by vertical corrasion, and broaden it by lateral corrasion. A most remarkable instance of erosion, due both to marked cor- rasion, and to the weathering of the banks above the water level, is seen in the canons of the Colorado River, where the waters have eaten a channel through the hard limestones and granites that form the bed of the stream, until they now run through gorges whose walls ascend almost perpendicularly to the height of from 3000 to 6000 feet. A good idea "of this great depth may be obtained by walking along a straight street for about a mile (5280 feet), and then imagining the street set upright in the air. On looking down toward the starting-place, we would see it as it would appear at the bottom of a hole about 6000 feet deep. When fragments of hard rocks are caught in eddies or whirls in rocky beds, the corrasion results in the formation of elongated or 76 PHYSICAL GEOGRAPHY. pot-shaped holes, with smooth, steep sides and rounded bottoms, called, from their resemblance to ordinary iron pots, pot-holes. 89. "Wind Corrasion and Transportation, sometimes called "Wind Erosion. — It is not only moving water that corrades rocks. Bare rocky surfaces are worn away by strong winds, aided by the sharp particles of sand and other hard substances they carry with them. In some instances winds possess considerable power in transport- ing weathered and corraded materials when in the form of fine par- ticles or sand. On the borders of the Nile, in Egypt, the winds carry the sands of the desert and spread them over the narrow strips of fertile land bordering the river. Along many sea-coasts the winds are heaping up the sand in huge mounds called dunes or sand-hills. In Wyoming, on both sides of the Niobrara River, the wind has heaped up the sand over an area of about 20,000 square miles, thus forming a sandy desert. The sand is heaped up in round dome-shaped hills, the tops of some of which have been moulded by the whirling winds into shapes resembling craters. 90. Avalanches and Land-slides. — The action of weathering is most marked^-^i bare, rocky surfaces. Any cause, therefore, which tends to keep the surface bare, or which furnishes fresh sur- face to the action of the air, necessarily increases the rapidity of weathering. For example, the percolating waters may slowly under- mine or soften some of the deeper strata on the side of a mountain, thus causing the slipping of huge masses down the mountain, in what are called avalanches or land-slides. In this way the cutting down of the mountain is greatly accelerated. Weathering is more rapid in some kinds of rock than in others. Thus gran- ite, which consists of a mixture of felspar, mica, and quartz, is, in some varieties, readily broken up into a gravelly soil, owing to the comparatively rapid weather- ing of the felspar. 91. Transportation and Deposition. — The fine broken rock produced by the action of weathering is called rock waste, and con- stitutes the material from which soil is formed. Rock waste seldom remains in place over the rock surface from which it was derived. DISTRIBUTION OF THE LAND-AREAS. 77 By the continued action of the wind, and of running water, it is gradually carried either to lower levels or to the ocean, where it is deposited. The process by which it is carried from a higher to a lower level is called transportation. By the action of transportation lower river valleys are filled with materials obtained from higher re- gions, the beds of lakes are gradually filled with sediment, and sand- bars aro formed along sea-coasts. Deposits so formed are apt to again be carried or re-transported to lower levels, where they are re-deposited. 92. The Action of Ocean Waves, changing the outlines of coasts ; as may be seen in portions of the coasts of England and Scotland. 93. The Agency of Man is witnessed mainly in the destruction of the forests over extended areas. CHAPTER II. !Er3£i44,000,000i^ Distribution of the Land- Areas. 94. Geographic Effects of Light, Heat, and Moisture. — The peculiarities observed in the distribution of animal and vegetable life are caused largely by differences in the distribution of light, heat, and moisture. Since light, heat, and moist- ure are influenced by the interaction of land, water, and air, we must first study the distribution and grouping of these inorganic or dead forms before we can understand those that are living. 95. The Distribution of the Land. —Of the approximately 197,000,000 square miles that make up the earth's surface, about 144,000,000 are water and 53,000,000 land. The proportion is about as the square of 5 is to the square of 3. If, Fig. 32. -Relative Land- Water-Areas. and 78 PHYSICAL GEOGRAPHY. therefore, we erect a square on a side of five, its entire area will represent the relative water-area of the globe ; while a square whose side is three will represent the relative land-area. 96. The Distribution of the Land may be arranged under two heads : (1) The Horizontal Forms of the Land, or the different shapes produced in the land-areas at the coast lines, by the contact of land and water ; (2) The Vertical Forms of the Land, produced by the irregularities of the surface of the high lands and low lands. 97. The Horizontal Forms. — The land-areas are divided into continents and islands. The Eastern Hemisphere contains four continents : Europe, Asia, Africa, and Australia. The first three form one single mass, which is called the Eastern Continent. Though the word "continent" strictly refers to an extended area of land entirely surrounded by water, usage has sanctioned the application of the term to each of the grand divisions of the land. It is quite correct, therefore, to speak of the North American Continent, the Asiatic Continent, etc. The Western Hemisphere contains two continents: North and South America ; these constitute what is called the Western Continent The following are the extremities of the continents : [ In the Eastern Continent — Most northern point, Cape Chelyuskin, lat. 78° 16' N. Most southern point, Cape Agulhas, lat. 34° 51' S. Most eastern point, East Cape, long. 170° W. Most western point, Cape Verd, long. 17° 34' W. In the Western Continent — Most northern point, Point Barrow, lat. 72° N. i Most southern point, Cape Froward, lat. 53° 53' S. Most eastern point, Cape St. Eoque, long. 35° W. ; Most western point, Cape Prince of Wales, long. 168° W. 98. Peculiarities in the Distribution of the Land : (1) The continents extend farther to the north than to the south. (2) The land masses are crowded together near the north pole, which they surround in the shape of an irregular ring. DISTRIBUTION OF THE LAND-AREAS. 79 (3) The three main southern projections of the land — South Amer- ica, Africa, and Australia — are separated from one another by extensive oceans. 99. Land and Water Hemispheres. — The accumulation of the land in the north and its separation in the south lead to a curious result — nearly all the land is collected in one hemisphere, and nearly all the water in another hemisphere. If one point of a pair of compasses be placed on the north pole of a globe, and the other stretched out to reach to any point on the equator, they will describe on the surface of the globe a great circle, and consequently will divide the globe into hemispheres. If, while they are stretched this distance apart, one of the points be placed at about the city of London, a circle swept with the other point will divide the earth into land and water hemispheres. Such a great circle would pass through the Malay Peninsula and the coast of Peru. The Land Hemisphere contains all of North America, Europe, and Africa, and the greater part of South America and Asia. The Fig. 33.— Land and Water Hemispheres. Water Hemisphere contains the southern portions of South Amer- ica, the Malay Peninsula, and Australia. 100. Double Continents. — The six grand divisions or conti- nents may be divided into three pairs, called Double or Twin Continents. Each Double Continent consists of a northern and southern con- tinent, almost separated from each other, but connected by a narrow isthmus or island chain. The three double continents are North and South America, 80 PHYSICAL GEOGRAPHY. Europe and Africa, and Asia and Australia. There are, therefore, three northern and three southern continents. The northern continents lie almost entirely in temperate lati- tudes, while the southern lie mainly in the tropics. 101. Lines of Trend. — The study of any Mercator's projection will disclose the following peculiarities in the earth's outlines : There are two great systems of trends, or lines of direction, along which the shores of the continents, the mountain-ranges, the oceanic basins, and the island chains usually extend. These trends are north-easterly and north-westerly, aud intersect each other at nearly right angles. There are, however, some wide deviations from these directions. North-East Trends. — A straight ruler may be so placed along the south- eastern coast of Greenland and the south-eastern coast of North America that its edge will touch most of their shore lines. Its general direction will be north-east. It may be similarly placed along the south-eastern coast of South America, the north-western coast of Africa, and most of the western coast of Europe ; along the south-eastern coast of Africa ; the south-eastern coast of Hindoostan ; and along the eastern coast of Asia, without its general direction differing- much from north-east. North-West Trends. — A straight ruler may be so placed as to touch most of the western shores of North America and part of the western coast of South America; most of the western coast of Greenland, or the north-eastern coast of North America, and part of the western coast of Africa. All these courses are sensibly north-west. If placed with one end at the mouth of the Mackenzie River, and the other on the south-western extremity of Lake Michigan, it will cut nearly all the great lakes in Central British America. The direction of the island chains of the Pacific Ocean is plainly characterized by these two trends, many of the sepa- rate islands being elongated in the direction of the trend of their chain. 102. Continental Contrasts. — The main prolongation of the Western Continent extends in the line of the north-western trend, while that of the Eastern Continent extends in the line of the north-eastern trend. The axes of the continents, or their lines of general direction, therefore, intersect each other nearly at right angles. The Western Continent extends for considerable distances both DISTRIBUTION OF THE LAND-AREAS. 81 north and south of the equator, while the Eastern Continent lies mainly north of the equator. The Western Continent, therefore, is characterized by a diversity of climates ; the Eastern Continent, by a similarity. The distribution of vegetable and animal life in each continent is necessarily affected by the peculiarities of its climate. 103. Shore Lines. — When land bordering on the ocean is up- lifted, the part of the sea bottom which is raised above the water level forms a coast plain, whose shore lines are comparatively smooth or unbroken. The water near the coast is fairly shallow, and har- bors are few. But, when a subsidence occurs, the low land is covered by the water, until the mountains or highlands face the coasts. The shore lines are then irregular and complicated, the water near the coast is deep, and harbors are more numerous. All shore lines are subject to changes in outline by subsequent elevations or subsidences. Shore lines are also subject to changes by the action of the waves, ocean currents, or rivers. For example, sand-bars or reefs may form off shore, shutting off long, narrow bodies of water, called lagoons. The winds blow sand from the beach, and form huge irregularly shaped sand-drifts or dunes. More- over, the rivers and tidal currents still further modify the sand-bars by cutting them away in some places and re-depositing them in other places ; or, the rivers alone, deposit vast quantities of min- eral matter, either at their mouths or along the adjoining coast. Where the sea encroaches on the land the coastal plain may be cut back, thus forming a high bluff or cliff, washed by the sea at high tides. 104. Changes in the Outlines of the Continents. — The grad- ual elevations and subsidences that are taking place in the earth's crust, whereby parts of the coast lines are submerged, or parts of the floor of the ocean sufficiently raised to become dry land, are causing changes in the continental outlines. These changes, how- ever, are so small in amount that they require long periods of time to become noticeable. The coast or shore lines of the northern continents are very irreg- ular, the shores being deeply indented with gulfs and bays, while 82 PHYSICAL GEOGRAPHY. those of the southern continents are comparatively simple and unbroken. The continents are most deeply indented near the regions where the pairs of northern and southern continents are nearly separated from each other. The continents differ greatly from one another in the extent of their indentations. Europe is the most indented of all the conti- nents. The area of her peninsulas, compared with that of her entire area, is as 1 to 4. Asia comes next in this respect, the pro- portion being 1 to 5?, while in North America it is but 1 to 14. The following table gives, in the first column, the area of each of the conti- nents ; in the second, the length of coast line ; and in the third, the number of square miles of area to one mile of coast line : Square miles of Continents. Area. Coast line. surface for one mile of coast. 17.500,000 sq. miles. 35,000 miles. 500 Africa 12,000,000 " 16,000 " 750 North America . 8,400,000 " 22,800 " 368 South America . 6,500,000 " 14,500 " 449 Europe .... 3,700,000 " 19,500 " 190 Australia .... 3,000,000 " 10,000 " 300 Europe has, in proportion to its area, About three times as much coast line as Asia. About four times as much as Africa. About twice as much as North America. More than twice as much as South America. Europe therefore is the most, and Africa the least, deeply indented of the continents. The extent and character of the coast line of a country has a marked influence on its progress in civilization. When harbors are numerous and safe, and water communication is possible with the interior of the continent, as in Europe and North America, progress is marked, but where the continent is shut in, has few harbors, and comparatively limited water communication, as in Africa and Aus- tralia, progress is restricted. ISLANDS. 83 CHAPTER III. Islands. 105. Relative Continental and Insular Areas. — Of the 53,000,000 square miles of laud, nearly 3,000,000, or about one- seventeenth, is composed of islands. 106. Varieties of Islands. — As regards their situation, islands are either continental or oceanic. Continental Islands are those that lie near the shores of the continents. They rest on shallow ocean beds, called continental shelves, that lie near the continents and that form parts of the submerged continents. Continental islands are, therefore, to be regarded as projections of the submerged continents. They have, usually, the same trends as the shores of the neighboring main- land. The British Isles and the Asiatic and Australian Island chains rest on the continental shelves of their adjoining continents. Continental islands, as a rule, are larger than oceanic islands ; this is caused by the shallower water in which they are situated. Papua and Borneo have each an area of about 250,000 square miles ; either of these islands is more than twice as large as the combined areas of Great Britain and Ireland. Many of the smaller continental islands, such as the numerous fiord islands off the coasts of Norway, British Columbia, Patagonia, and Maine ; the delta islands so common at the mouths of certain rivers ; the low sand-bars and mud flat islands of certain coasts, and the small, high, rocky islands of other coasts, are of comparatively recent formation. 107. American Continental Island Chains. (1) The Arctic Archipelago comprises the large group of islands north of the Dominion of Canada. It consists of detached portions of the neighboring continent. (2) The Islands in the Gulf of St. Lawrence and its neigh- borhood are apparently the northern prolongations of the Appala- chian mountain-system. They rest on or near the North American continental shelf. 84 PHYSICAL GEOGRAPHY. (3) The Bahamas lie off the south-eastern coast of Florida, to which they belong by position and structure. Their general trend is north-west. (4) The "West Indies form a curved range, which connects the peninsula of Yucatan with the coast-mountains of Venezuela. Here both trends appear, though the north-western pre- dominates. They rest on or near the continental shelves of the two continents. (5) The Aleutian Islands form another curved range, which connects the Alaskan Peninsula with Kamtchatka ; their general trend is north- east. They are connected with the elevations of the North American Continent. (6) The Islands "West of the Dominion of Canada and Alaska. — These are fiord islands, and are the summits of sub- merged northern prolongations of the Pacific coast ranges. (7) The Islands of the Patagonian Archipelago are also fiord islands or the summits of submerged prolongations of the Andes of Chili. 108. Asiatic Continental Island Chains consist of a series of curved ranges extending along the coast, and resting on or near the Asiatic continental shelf. They intersect each other nearly at right angles. (1) The Kurile Islands are a prolongation of the Kamtchatkan range. (2) The Islands of Japan extend in a curve from Saghalien to Corea. (3) The Loo Choo Islands extend in a curve from the islands of Japan to the island of Formosa. (4) The Philippines form two diverging chains, which merge on Fig. 34.— West India Island Chain. 1, Cuba ; 2, Hayti ; 3, Jamaica ; 4, Porto Rico ; 5, Caribbee Islands ; 6, Bahamas. ISLANDS. 85 the south into the Australasian Island chain. The eastern chain extends to the southern extremity of Celebes, and the western to that of Borneo. The Asiatic chains beloug to a submerged mountain-range extending from Kamtchatka to the Sunda Islands. Their general direction is parallel to the elevations of the coast. 109. The Australasian Island Chain. The Australasian Island chain is composed of a number of islands extending along curved trends over a length of nearly 6000 miles, from Sumatra to New Zea- 14 2 346 1> land. They rest on or near the Asiatic or Australasian continental shelves. The islands extend along three curved lines, whose general direction is north-west. The Australasian chain was prob- ably connected with the Asiatic Con- tinent during recent geological time, and separated from it by subsidence. Its numerous volcanoes and coral formations prove that subsidence is still taking place. 110. Peculiarity of Dis- tribution. — The following peculiarity is noticed in the distribution of continental islands : Each of the continents has an island, or a group of islands, near its south-eastern extremity. For example, North America has the Bahamas and the West Indies ; Greenland has Iceland ; South America has the Falkland Islands ; Africa has Madagascar ; Asia has the East Indies ; and Australia has Tasmania. 111. Oceanic Islands are those situated far away from the con- tinents. They occur either in vast chains, which generally extend along one or the other of the two lines of trend, or as isolated groups. Fig. 35. — Australasian Island Chain. 1, Sumatra ; 2, Java ; 3, Sumbawa ; 4, Flores ; 5, Timor ; 6, Borneo ; 7, Celebes ; 8, Gilolo ; 9, Ceram ; 10, Papua ; 11, Louisiade Archipelago ; 12, New Caledonia ; 13, New Zealand ; 14, Ad- miralty Islands; 15, Solomon's Archipelago; 16, Santa Cruz ; 17, New Hebrides. 86 PHYSICAL GEOGRAPHY. Oceanic Island Chains. The following are the most important : (1) The Polynesian Chain ; (2) The Chain of the Sandwich Islands ; (3) The Tongan or New Zealand Chain. The Polynesian Chain consists of a series of parallel chains, extending from the Paumotu and the Tahitian Islands to the Caro- lines, the Ralick, and the Radack groups. Their gen- eral direction is north-west; the total length of the chain is about 5500 miles. The Chain of the Sand- wich Islands extends in a north-westerly direction. Its length is about 2000 miles. The New Zealand Chain extends north-east as far as the Tonga Islands, cutting the Australasian chain at right angles. 112. Isolated Oceanic Islands are mainly of two kinds : the Volcanic and the Coral. As a rule, the Volcanic islands are high, while the Coral islands seldom rise more than twelve feet above the water. Volcanic Islands are not confined to isolated groups, but occur also in long chains. The Polynesian, Sandwich, and New Zealand Chains contain numerous volcanic peaks. But the high, isolated oceanic islands are almost always of volcanic origin, and, consisting of the summits of submarine volcanoes, are usually small. Some of the Canary and Sandwich Islands, which are of this class, rise nearly 14,000 feet above the sea. 113. Coral Islands, or Atolls, though of a great variety of shapes, agree in one particular : S s;>. tf^V. Fig. 36.— Polynesian Island Chain. 1, Marquesas ; 2, Paumotu ; 3, Tahitian ; 4, Rurutu group ; 5, Hervey group : 6, Samoan, or Navigator's ; 7, Vakaafo group ; 8, Vaitupu ; 9, Kingsmill ; 10, Ralick ; 11, Radack ; 12, Caro- lines; 13, Sandwich. ISLANDS. 87 They consist of a low, narrow rim of coral rock, enclosing a body of water called a lagoon. Fig. 38.— Coral. Fig. 37. — A Coral Island. 114. Mode of Formation of Coral Islands. — The reef form- ing the island is of limestone, derived from countless skeletons of minute polyps that once lived beneath the surface of the waters. The skeletons, how- ever, are not separate. The polyp propagates its species by a kind of budding ; that is, a new polyp grows out of the body of the old. In this way the skeletons of countless millions of polyps are united in one mass, and assume a great variety of shapes. One of the most common species of reef-forming corals, the madrepora, is shown in Fig. 38. Many other forms exist. The delicate coral structures, together with shells from mollusks and calcareous plants, are ground into frag- ments by the action of the waves ; by the infiltration of water con- taining lime in solution, they become compacted into hard lime- stone, on which new coral formations grow. The growth of the coral mass is directed upward, and ceases when low-water mark is reached, because exposure to a tropical sun kills the polyps. But the action of the waves continues, and the broken fragments are gradually thrown up above the general level of the water. In this way a reef is formed, whose height, limited by the force of the waves, seldom exceeds twelve feet. On the bare rock, which has thus emerged, a soil is soon formed and a scanty vegetation appears, planted by the hardy seeds scattered over it by the winds and waves. The coral island never affords a very comfortable residence for man. The palm tree is almost the only valuable vegetable species ; the animals are few and small, and the arable soil is limited. Moreover, the island is subject to occa- sional inundations by huge waves from the ocean. 88 PHYSICAL GEOGRAPHY. 115. Distribution of Coral Islands. — According to Dana, the reef forming coral polyp is found only in regions where the winter temperature of the waters is never lower than 68° Fahr. Some varieties, however, will grow in colder water. Coral islands are confined to tropical waters where the depth does not greatly exceed 100 feet, and which are protected from cold ocean-currents, from the influence of fresh river waters, and from muddy bottoms ; and which are remote from active volcanoes, whose occasional submarine action causes the death of the coral polyp. Though some coral polyps grow in quiet water, the greater part thrive best when exposed to the breakers. The growth, therefore, is more rapid on the side toward the ocean than on the side toward the island. Coral islands are most abundant in the Pacific Ocean. The following groups contain numerous coral islands : the Paumotus, the Carolines, the Radack, the Ealick, the Kingsinill groups, the Tahitian, Samoan, and Fijii Islands, and New Caledonia. In the Indian Ocean are the Laccadives and the Maldives, and in the Atlantic Ocean, the West Indies and the Bermudas. 116. Varieties of Coral Formations. — There are four varieties of coral formations : (1) Fringing Reefs, or narrow ribbons of coral rock, lying near the shore of an ordinary island. (2) Barrier Reefs, which are broader than Fringing Reefs, and lie at a greater distance from the shore, but do not extend entirely around the island. A barrier reef off the coast of New Caledonia has a length of 400 miles. One extends along the north-eastern shore of Australia for over 1000 miles. Barrier reefs are not continuous, but often have breaks in them through which vessels can readily pass. (3) Encircling Reefs are barrier reefs extending entirely around the island. As a rule, encircling reefs are farther from the shore of the island than barrier reefs. Tahiti, of the Society Islands, is an example of an encircling reef. (4) Atolls. — This name is given to reefs that encircle lagoons or bodies of water entirely free from islands. The varieties of reefs just enumerated mark four successive steps or stages in the progress of formation of the coral island. ISLANDS. 89 When a more careful study of the habits of the reef-forming coral polyp disclosed its inability to live in the ocean at greater depths than 100 or 120 feet, the opinion, which formerly prevailed, of coral islands rising from profound depths, had to be abandoned. This belief had its foundation in the fact that a sounding-line almost invariably showed depths of thousands of feet of water near the shore of a coral island, and yet brought up coral rock. In no case, however, did the rock contain living polyps. A hypothesis of Darwin, which appears well sustained by the extensive observations of Dana and others, explains the great depth of coral formations. 117. Darwin's Hypothesis of Coral Islands. — According to this naturalist, the coral formation begins near the shore of an island that is slowly sinking. If the growth of the reef upward equals the sinking of the island, the thickness of the reef is limited only by the time during which the operation continues. In Fig. 39 is shown, in plan and section, an island with elevations, A and B, and river, a. The coral island begins as a fringing reef, somewhere off the coast of an ordinary island at c, c, c, when the conditions are favorable. The coral reef must gradually extend around the island, since its growth toward the ocean is soon limited by the increasing depth, and toward the shore of the island, by the muddy waters near the surf and the absence of the breakers. Meanwhile, as the island is sinking, the channel separating the reef from the coast increases in breadth. A barrier reef is thus formed, which at last com- pletely surrounds the island, and becomes an encircling reef. The higher portions of land, which are still above the waters, form islands in the central lagoon. Op- posite the mouth of the river a, the growth is prevented by the fresh water, and a break in the reef is thus produced. These breaks are sometimes sufficient to permit a ship to enter the lagoon. At last all traces of the old island disappear, and its situation is marked by a clear lagoon, surrounded by a narrow rim of coral which follows, approximately, the old coast line. Parts of the coral region of the Pacific appear to have ceased subsiding, as the wooded condition of the islands shows. In some cases subsequent elevation has occurred. Fig. 39.— Growth of a Coral Island. 90 PHYSICAL GEOGRAPHY. 118. Other Hypotheses of Coral Formations. — Recent obser- vations show that the formation of some coral reefs, especially of many that lie along the continental shores, cannot be explained by Darwin's hypothesis. For example, Louis Agassiz has shown that the southern part of Florida is of coral formation, and has been deposited by means of successive concentric barrier reefs, that have been converted into continuous land by the accumulation of sand and silt between adjoining reefs. LeConte similarly attributes this Florida formation to the deposit of a submarine shell bank, by the action of the Gulf Stream. When the bank was sufficiently near the surface, the coral polyps began to grow, especially near the edge where the food supply was sufficient. Alexander Agassiz has pro- posed a similar hypothesis, but regards the Gulf Stream as bringing food for the coral polyps rather than shells, sand, and silt for the formation of submarine flats. Figure 40 is a map of Florida with its reefs and keys. Figure 41 is a sec- tion along the line A- A. In Figure 40 the line a, a, shows what was at one time the limit of the southern coast of Florida, b, b, b, b, is the present limit of the southern coast, c, c, c, c, are the keys, which are low islands. d, d, is the growing coral reef, e, e, are the Everglades, dotted with islands, called hummocks. Between c, c, and d, d, is the ship channel. Outside the growing coral reef d, d, are the pro- found depths of the Gulf Stream, G. S. The growth of the reef-formations is explained hy LeConte as follows (Fig. 41) : a was at one time the limit of the southern coast of Florida, b is the present southern coast, which at one time was a coral reef like d. Upon 6, a line of coral islands gradually formed, connecting it with the old southern coast, a. The ship channel hetween a and b gradually filled up and formed the Everglades e, e. Mean- while, another reef formed in the position of the present keys c, the ship channel Fig. 40. -Florida Reefs and Keys (LeConte). BELIEF FORMS OF THE LAND. 91 Fig. 41.- -Everglades, Reefs, and Keys of Florida (LeConte). being between 6 and c. Tbis reef has now grown to be a line of coral islands, and the ship channel, between b and c, converted into shoals and mud flats /• The present ship channel is be- tween c and d. In course of time the southern coast will extend to the present line of keys c, and the shoal water /, will become anotherE verglades. Outside the present keys c, an- other coral reef d, is growing, to wbich the coast will ultimately extend, and which will mark the limit of the formation, owing to the deep waters of the Gulf Stream, immediately beyond it. formation. Murray proposes a similar hypothesis, which he endeavors to apply to all coral formations. He supposes the accumulation of shallow shell and sand banks, and the growth thereon of the coral polyps, where the water is not too deep. Growth is limited to the edge of the banks, where the food supply is sufficiently great, thus leaving the central lagoon. According to this hypothesis, coral for- mation would afford an evidence of rest, or even of elevation, rather than of subsi- dence. This hypothesis does not appear to be able to explain the formation of the numerous coral islands of the Pacific and the deep ocean generally. For these, Darwin's hypothesis is almost universally adopted. But that views simi- lar to those outlined by the Agassizs, LeConte, Murray, and others, account for the formation of Florida, the reefs of Yucatan, the Bahamas, the sunken coasts of Cuba, the coasts of Brazil, and possibly the coast of Australia is now generally credited. The dotted lines show the successive steps of its CHAPTER IV. Relief Forms of the Land. 119. By the Forms of Relief of the Land is meant the eleva- tion of the land above the mean level of the sea. The highest land in the world is Mount Everest, of the Himalayas; it is 29,000 feet above the level of the sea. The greatest depression is the Dead Sea in Palestine, which is about 1312 feet below the level of the sea. The sum of these is somewhat less than six miles. An elevation of six miles is insignificant when compared with the 92 PHYSICAL GEOGRAPHY. size of the earth. If represented on an ordinary terrestrial globe, it would be scarcely discernible, since it would project above the surface only about the y^ooth of the diameter. The highest eleva- tions of the earth are proportionally much smaller than the wrinkles on the skin of an orange. If, as in Fig. 42, a sphere be drawn to represent the size of the earth, its radius will be equal to about 4000 miles. If, now, the line A B, be drawn equal to the radius, it will represent a height of 4000 miles. One-half this height would be 2000 miles ; one- half of this 1000, and successive halves 500 and 250 miles. An elevation of 250 miles would not, therefore, be very marked. Although the irregularities of the sur- face are comparatively insignificant, yet they powerfully affect the distribution of heat and moisture, and, consequently, that of animal and vegetable life. An elevation of about 350 feet reduces the temperature of the air 1° Fahr. — an effect equal to a difference of about 70 miles of latitude. High mountains, therefore, though under the tropics, may support on their higher slopes a life similar to that of the temper- ate and the polar regions. 120. The Relief Forms of the Land are divided into two classes : Low Lands and High Lands. — The dividing line between low lands and high lands is usually taken at 1000 feet, which is the mean or average elevation of the land. This distinction, however, is not always observed. Low Lands are divided into plains and hills. High Lands are divided into plateaus and mountains. If the surface is comparatively flat or level, it is called a plain when its elevation above the sea is less than 1000 feet, and a plateau when its elevation is 1000 feet, or over. Even an irregular, hilly surface, when extensive, is sometimes, though inaccurately, called a plateau, even when considerably under 1000 feet in elevation. Fig. 42. —Relative Height of Mountains. RELIEF FORMS OF THE LAND. 93 If the surface is diversified, the elevations are called hills when less than 1000 feet high ; and mountains when 1000 feet or over. The low lands of the Eastern Continent lie mainly in the northern part ; those of the Western Continent lie mainly in the central parts. 121. Plains owe their comparatively level surface either to the long-continued erosion of high lands; to the gradual settling of sedimentary matter, or to extended lava floods. 122. Classes of Plains. — Plains may be divided into classes according to their origin, as alluvial plains, or those formed by sedi- mentary matters. Such plains are lacustrine, jluviatile, or marine, as formed respectively by the sediment of lakes, rivers, or seas. Plains may also be divided into classes, either according to their position or according to the materials that form them. According to their position we have — (1) Coastal Plains or low lands lying between mountain-ranges and the coast. These plains slope gently to the water. They are usually due to a gradual uplift of part of the ocean's bed. They vary in width from narrow tracts to many miles. Where the coastal plains are broad, cities are situated, both at the mouths of the larger rivers, and at the head of navigation ; i. e., on the inner margin of the coastal plain, at the foot of the mountain-ranges, as shown in Fig. 43. Wilmington, Charleston, Savannah, Raleigh, and Augusta are examples. (2) Inland Plains or regions where the strata are composed of nearly horizontal layers. The great plain of north-eastern Siberia Fig. 43. — Portion of Atlantic Coastal Plain. 94 PHYSICAL GEOGRAPHY. between lat. 50° and 60° N. is an example. As regards their age, such plains are usually young, as is evidenced by their rivers not having cut deep valleys. (3) Worn-down Mountain Low Lands. Old Plains. Peneplains. — Interior plains, however, sometimes owe their almost level surface to excessive denudation. These plains were formerly mountains or plateaus, but have been so denuded by rivers as to assume the form of only moderately elevated, gently roll- ing low lands, called plains of denuda- tion, or peneplains ; i. e., almost plains. The plains of Central Eussia furnish an example. According to the materials that form them, plains are — (1) River- made Plains. — Where many streams descend the slopes of a mountain the accumulation of mineral matter brought down, unite at the base of the mountain to form a broad, gently sloping plain. Such plains are found filling the lower part of the val- ley between the opposite slopes of two parallel mountain-ranges, an example of which is shown in the plain of the San Joaquin River in California, between the Sierra Nevada Moun- tains and the Coast Range. (2) Dust Plains. — The winds may bring fine dust particles from distant regions and form level accumulations called dust plains. An example is found in the basin of the Hoang- Ho in China. Such deposits are sometimes called loess deposits, from a German word meaning loose. (3) Lava Plains, or accumulations of lava. As we have seen, these are at times very extensive, covering hundreds of thousands <

""* m / ^S\ \yiMih^ ~^fc\ S \ V^v^J^Sacramcnto* ^. „ x^ J oJsV^j ° ^vN* 1 ) f\S^n FrancMspcr ¥\ -$- \ ^^" SS\~ •A K w-v j- \&( j >V*l % O £ N V ■ V^A O^v J\ K^ O \ \V o \ \P \X Fig. 50. — Orographic Chart of South America. (Light portions, mountains ; shaded, plains.) 1, The Andes Mountains ; 2, Plateau of Quito ; 3, Plateau of Bolivia; 4, Aconcagua; 5, Plateau of Guiana; 6, Plateau of Brazil; 7, The Orinoco; 8, The Amazon ; 9, The Rio de la Plata. RELIEF FORMS OF THE CONTINENTS. 105 4000 feet of the supposed height of Sorata. Some authorities claim that several peaks in Bolivia reach an elevation of nearly 25,000 feet. The Andes Mountains terminate abruptly in the precipitous ele- vations of Cape Horn. Numerous table-lands are included between the parallel ranges: the most important are — the plateau of Quito, 9543 feet; the plateau of Pasco, in North Peru, 11,000 feet ; the plateau of Bolivia, from 12,000 to 14,000 feet. From most of these higher plateaus, volcanic peaks arise. 139. The Secondary Mountain -systems of South America are the plateaus of Brazil and Guiana. They both lie on the eastern border. The Plateau of Brazil is a table-land whose average height is about 2500 feet. Narrow chains or ridges sepa- rate the river-valleys. The plateau of Brazil - T; -"- : ■ ._■ ■ ]''" ; L v ° i- '-'.-^Jgj^-. forms the watershed be- tween the tributaries of the Amazon and the La Plata. Along the Atlan- tic a nearly continuous range descends in steep ter- races to the ocean. The av- erage altitude is more than double that of the western portion of the plateau. The highest peaks are somewhat over 8000 feet high. The Plateau of Gui- ana, smaller than the Plateau of Brazil, but ahout equally elevated, forms the watershed between the tributaries of the Orinoco and the Amazon. 140. The Great Low Plain of South America lies between the predominant and the secondary mountain-systems. It is mainly of alluvial origin, but slightly elevated, and is much more level than the great plain of North America. Fig. 51.— Amazon River Scenery. 106 PHYSICAL GEOGRAPHY. This plain is drained by the three principal river-systems of the continent, by which it is divided into three parts : the Llanos of the Orinoco, the Selvas of the Amazon, and the Pampas of the Rio de la Plata. The Llanos are grassy plains which, during the rainy season, resemble our prairies, but during the dry weather are deserts. The Selvas, or forest plains, are covered by an uninterrupted luxuriant forest. The vegetation here is so dense that in some places the broad rivers form the only ready means of crossing the country. Near the river-banks are vast stretches of swampy ground. The Pampas are grassy plains which in some respects resemble the Llanos. A coast plain lies between the Andes and the Pacific. It is widest near the Andes of Chili, where in some places it is 100 miles in 26,000 18,000 12,000 6,000 VZ597?>i&Z2y/APJv?. >ss;sss;;sJZZZZ; Fig. 52. — Section of South America from East to West. 1, Volcano Arequipa ; 2, Lake Titicaca ; 3, Nevada de Sorata ; 4, Central Plain ; 5, Moun- tains of Brazil. breadth. Between the parallels of 27° and 23° the plain is an abso- lute desert, called the desert of Atacama. Here rain never falls and vegetation is entirely absent. 141. Approximate Dimensions of South America. Area of continent, about 6,500,000 square miles. Greatest breadth from east to west, 3230 miles. Greatest length from north to south, 4800 miles. Coast line, 14,500 miles. Culminating point, Aconcagua, 23,910 feet. 142. Contrasts of the Americas. — In both North and South America the predominant system lies in the west, the secondary systems in the east, and the low plains in the centre. They differ in the following respects : In North America the predominant system is a broad plateau, hav- ing high mountain-ranges ; the principal secondary system is narrow and is formed of parallel ranges ; the low plains are characterized by BELIEF FORMS OF THE CONTINENTS. 107 undulations, and contain many deep depressions occupied by extensive lake-systems. In South America the predominant system is narrow; the secondary systems are broad; the low plain is extremely flat, and contains but jew depressions, consequently it has no extensive lake-systems. III. EUROPE. 143. Surface Structure. — The Predominant Mountain-system is in the southern part of the continent. The Secondary Systems are in the north and east. The Great Low Plain lies between the Predominant and Second- ary Systems. A line drawn from the Sea of Azov to the mouth of the Ehine River, divides Europe into two distinct physical regions. The country north of this line is sometimes called Low Europe, and that south of it, High Europe. The great low plain lies on the north, and the predominant mountain-system on the south. 144. The Predominant Mountain-system of Europe is com- posed of a highly complex series of mountain-systems extending along the northern shores of the Mediterranean in a curve, from the Straits of Gibraltar to the shores of Asia Minor. The system is highest in the centre, where the Alps form the culminating point of the system. The average elevation of the Alps ranges from 10,000 to 12,000 feet. The highest peak, Mont Blanc, 15,787 feet, is the culminating point of the European Continent. The Matterhorn and Monte Rosa are but little inferior in height. On the south-west the system is con- tinued to the Atlantic by the Cevennes and adjoining ranges in France, and the Pyrenees and Cantabrian in the northern part of the Spanish Peninsula. The Pyrenees are an elevated range, with peaks over 11,000 feet high. On the east the system extends in two curves to the Black Sea by the Carpathian and Transylvanian Mountains on the north, and the Dinaric Alps and the Balkan Mountains on the south. 145. Divisions of Predominant System. — The predominant mountain-system of Europe may be conveniently regarded as con- 108 PHYSICAL GEOGRAPHY. sisting of a central body or axis, the Alps, with six projections or limbs — three on the north, and three on the south. The three divisions on the north include — The Western Division, or the mountains of France, includ- ing the mountains lying west of the valleys of the Rhine and the Rhone ; The Central Division, or the mountains of Germany, situated Fig. 53.— Orographic Chart of Europe. (Light portions, mountains; shaded portions, plains.) 1, The Alps ; 2, Mont Blanc ; 3, Pyrenees ; 4 Cantabrian ; 5, Sierra Estrella ; 6, Sierra Nevada; 7, Mountains of Castile; 8, Appennines; 9, Dinaric Alps; 10, Balkan; 11, Pin- dus ; 12, Taurus ; 13, Caucasus ; 14, Cevennes ; 15, Plateau of Auvergne ; 16, Vosges ; 17, Black Forest; 18, Jura; 19, Hartz ; 20, Bohemian Plateau; 21, Carpathians; 22, Hun- garian Forest ; 23, Transylvanian Mountains ; 24, Kiolen Mountains ; 25, Urals. between the Western Division and the upper valleys of the Oder and the Danube; BELIEF FORMS OF THE CONTINENTS. 109 The Eastern Division, or the mountains of Austria-Hungary, situ- ated between the Central Division and the Black Sea. These divisions contain a highly complicated system of minor elevations. Their complexity is due to the frequent intersection of the north-eastern and north-western trends. Basin-shaped plateaus, like the Bohemian and Transyl- vanian, are thus formed. The Western Division includes most of the mountains of France, as the Cevennes, the mountains of Auvergne, and the Vosges Mountains. The Central Division includes the Jura Mountains in Switzerland, the Swiss and the Bavarian Plateaus, the Black Forest Mountains, the Hartz Mountains, and the Bohemian Plateau. The Eastern Division includes most of the mountains of Austria, as the Carpathians, the Hungarian Forest, and the Transylvanian Mountains. 146. The Secondary Mountain-systems of Europe comprise the system of the Scandinavian Peninsula, the Ural Mountains, and the Caucasus Mountains. The System of the Scandinavian Peninsula includes the elevations of Norway and Swe- den. With the excep- tion of the Kiolen Mountains in the north, the system does not embrace distinct moun- tain-ridges, but consists mainly of a series of broad plateaus that de- scend abruptly on the west in numerous deep- ly-cut fiord valleys that were formed by gla- ciers or slowly moving masses of ice, and which were subsequently par- tially submerged. Through these fiord valleys the sea penetrates nearly to the heart of the plateau. The more gradual eastern slopes are occupied by numerous small lakes. The System of the Urals is composed of a moderately elevated Fig. 54.— Fiord on Norway Coast. 110 PHYSICAL GEOGRAPHY. range extending from the Arctic Ocean on the north to the plains of the Caspian on the south. The elevated island of Nova Zem- bla may be considered as forming a part of its northern prolon- gation. The Caucasus Mountains extend from Europe into Asia. They contain peaks exceeding in elevation those of the Alps. The high- est of these peaks are found in Asia. 147. The Great Low Plain of Europe lies between the pre- dominant and secondary mountain-systems, and stretches north- eastward from the Atlantic to the Arctic. It is remarkably level, and is highest in the middle, where the Valdai Hills form the prin- cipal watershed of Europe. Westward the plain is continued under the North Sea to the British Isles, where a few inconsiderable ele- vations occur. South of the Alps the large plain of the Po River stretches across the northern part of Italy. Europe projects on the south in three mountainous peninsulas : The Iberian Peninsula, including Spain and Portugal ; The Italian Peninsula ; The Turco- Grecian Peninsula. The Iberian Peninsula. — The principal mountains are the Sierra Estrella, the mountains of Castile, and the Sierra Nevada. The Pyrenees separate the Peninsula from France. The Cantabrian Mountains extend along the northern coast. The Italian Peninsula contains the Apennines, extending mainly in the direction of the north-western trend. The Turco-Grecian Peninsula. — The Dinaric Alps extend along the coast of the Adriatic; the Balkan Mountains extend from east to west, through Turkey; and the Pindus from north to south, through Turkey and Greece. 148. Approximate Dimensions of Europe. Area of continent, 3,700,000 square miles. Coast line, 19,500 miles. Greatest breadth from north to south, 2400 miles. Greatest length from north-east to south-west, 3370 miles. Culminating point, Mont Blanc, 15,787 feet. RELIEF FORMS OF THE CONTINENTS. Ill IV. ASIA. 149. Surface Structure. — The Predominant Mountain-system J is in the south. The Secondary Systems surround the Predominant System. Fig. 55.— Orographic Chart of Asia. (Light portions, mountains ; shaded por- tions, plains.) 1, Himalaya Mountains ; 2, Karakorum ; 3, Kuen-lun ; 4, Belor ; 5, Thian Shan ; 6, Altai ; 7, Great Kinghan ; 8, Yablonoi ; 9, Nanling ; 10, Peling ; 11, Vindhya ; 12, Ghauts ; 13, Hindoo-Koosh ; 14, Elburz; 15, Suliman ; 16, Zagros; 17, Taurus; 18, Caucasus; 19, Asiatic Island Chain. The Great Low Plain lies on the north and west, between the mountain-systems of Asia and the secondary system of the Urals. Europe and Asia are sometimes considered as geographically united in one grand division, called Eurasia. 150. The mountain-systems of Asia are nearly all connected in one huge mass, which extends in the line of the north-east trend, from the Arctic to the Indian Ocean. Though in reality one vast 112 PHYSICAL GEOGRAPHY. system, yet they are most conveniently arranged in one predominant and several secondary systems. The Predominant System is the plateau of Thibet, the loftiest table-land in the world. It is between 15,000 and 16,000 feet high, and is crossed by three huge, nearly parallel mountain-ranges : the Himalayas on the south, the Kuen-lun on the north, and between them the Karakorum. The Himalayas, the loftiest mountains in the world, rise abruptly '^SSi from the plains of .-]% Northern Hindostan. "> Like the Alps, their axis is curved, but in the opposite direction. The breadth of the system varies from 100 to 200 miles ; the length is about 1500 miles. The highest point is Mount Everest, 29,000 feet above the sea; it is the culminating point of the Asiatic Continent and of the world. Kun- chinjunga and Dhawalaghiri are scarcely inferior in height. 151. The Secondary Systems lie on all sides of the predomi- nant system, though mainly on the north and east of the predomi- nant system. The Asiatic Continent projects on the south in the three mountainous peninsulas of Arabia, Hindostan, and Indo- China. On the north and east of the plateau of Thibet is an extended region called the plateau of Gobi, considerably lower than the sur- rounding country. The Kuen-lun and Great Kinghan Mountains bound it on the south and east, and the Altai Mountains on the north. Fig. 56.— Himalaya Mountains. BELIEF FORMS OF THE CONTINENTS. 113 On the west lie the Thian Shan and Altai, which by their open val- leys afford ready communication with the low plains on the west. The plateau of Gobi varies in height from 2000 to 4000 feet. The greatest depression is in the west, and is occupied by Lake Lop and the Tarim Eiver. A small part of the region near the mountain-slopes is moderately fertile ; the remainder is mainly desert. The Altai Mountains are but little known, but some of their peaks exceed 12,000 feet. They are continued eastward by the Yablonoi Mountains. East of the plateau of Gobi a range extends north-easterly through Mantchuria. On the south and west of Thibet lie the plateaus of Iran, Armenia, and Asia Minor. The Plateau of Iran includes Persia, Afghanistan, and Beloochis- tan. It is a basin-shaped region from 3000 to 5000 feet high. The Elburz and Hlndoo-Koosh Mountains form its borders on the north, the Suliman, on the east, and the Zagros, on the south and west. The Suliman Mountains rise abruptly from the plains of the Indus. Across these mountains occurs the only practicable inland route between Western Asia and the Indies. The Plateaus of Armenia and Asia Minor lie west of the Plateau of Iran. Armenia is 8000 feet high, and bears elevated mountains : Mount Ararat, 16,900 feet, is an example. On the west, the penin- sula of Asia Minor, or Anatolia, extends between the Black and Mediterranean Seas, and is traversed by the Taurus Mountains. The Caucasus Mountains lie north of the plateau of Armenia. They are an elevated range extending between the Black and Cas- pian Seas, and forming part of the boundary-line between Europe and Asia. Mount Elburz, the "Watch-Tower," the culminating peak, is 18,493 feet high. The Arabian Plateau occupies the entire peninsula of Arabia. It is separated from the plateau of Iran by the Persian Gulf and the valleys of the Tigris and the Euphrates. The Plateau of Decca,n occupies the lower part of the peninsula of Hindostan. It is crossed on the north by the Vindhya' Moun- tains, and along the coasts by the Eastern and Western Ghauts. The Peninsula of Indo-China is traversed by a number of moun- tain-ranges which diverge from the eastern extremity of the Hima- 8 114 PHYSICAL GEOGRAPHY. layas. The Nanling and the Peling mountains extend from east to west through China. 152. The Great Low Plain is a continuation of the European plain. It extends from the Arctic Ocean south-westerly to the Caspian and Black Seas. It is hilly on the east, but level on the west. South of the 60th parallel it is comparatively fertile- Near the shores of the Arctic are the gloomy and inhospitable Tundras. The Tundras are vast regions which in summer are covered with occasional moss-beds, huge shallow lakes, and almost innumerable swamps, and in winter with thick ice. The tundras are caused as follows : The rivers that flow over the immense plain of Asia rise in the warmer regions on the south. The thawing of their upper courses while their lower courses are still ice-bound, permits large quantities of drift ice to accumulate at tbeir mouths, which, damming up the water, causes it to overflow the adjoining country. Depressions of the Caspian and the Sea of Aral. — Two" re- markable depressions occur in the basins of the Caspian and the Sea of Aral, and that of the Dead Sea. These are all considerably below the level of the ocean. The waters of the Caspian and the Sea of Aral were probably once connected in a great inland sea. The Smaller Asiatic Plains are drained by several river-systems. These are the Plain of Mantchuria, drained by the Amoor ; the Plain of China, drained by the Hoang-Ho and the Yang-tse-Kiang ; 5 I*4 7* Fig. 57.— Section of Asia from North to South. 1, Cape Comorin ; 2, Deccan ; 3, Plain of India ; 4, Himalayas ; 5, Everest ; 6, Kuen- lun; 7, Karakorum; 8, Thibet ; 9, Upper Tartary ; 10, Ararat; 11, Elburz; 12, Thian Shan ; 13, Altai ; 14, Mountains of Kamtehatka ; 15, Arctic Ocean, Mouth of Yenesei. the Plain of India, drained by the Indus, the Ganges, the Brahma- pootra, and the Irrawaddy ; and the Plain of Persia, drained by the Tigris and the Euphrates. 153. Approximate Dimensions of Asia. Area of continent, 17,500,000 square miles. Coast line, 35,000 miles. Greatest length from north-east to south-west, 7500 miles. RELIEF FORMS OF THE CONTINENTS. 115 Greatest breadth from north to south, 5166 miles. Culminating point, Mount Everest, 29,000 feet. 154. Comparison of the Relief Forms of Europe and Asia. ! — In both Europe and Asia the chief elevations are in the south and the great low plains in the north,. Asia, like Europe, extends ; toward the south iu three great peninsulas : Arabia, Hindostan, and j Indo-China. V. AFRICA. 155. Surface Structure. — Nearly the entire continent of Africa ! is a moderately elevated plateau. It, therefore, has no great low | plains ; but the interior is lower than the mar- . ginal mountain-systems, ! and, in this respect, the true continental type, high borders and a low interior, is preserved. 156. The Predomi- ' nant Mountain -system is i in the east. The Secondary Sys- tems are in the south, west, and north. The great interior de- pression is in the middle, and is surrounded by the predominant and second- ary systems. A narrow, low plain extends along most of the I coast of Africa. This j plain is broadest on the | north-west, between the I plateau of the Sahara and the Atlas Mountain-system. 157. The Predominant Mountain-system extends along the Fig. 58.— Orographic Chart of Africa. (Light portions represent mountains ; shaded por- tions, plains.) 1, Abyssinian Plateau ; 2, 3, Kenia and Kilimand- jaro ; 4, Lupata ; 5, Drag-on ; 6, Nieuveldt ; 7, Mo- cambe ; 8, Crystal ; 9, Cameroons ; 10, Kong ; 11, Atlas ; 12, Lake Tchad ; 13, Madagascar. 116 PHYSICAL GEOGRAPHY. entire eastern shore, from the Mediterranean Sea to the southern extremity of the continent. It is highest near the centre, in the plateaus of Abyssinia and Kaffa. The culminating point is, probably, to be found in the volcanic peaks of Kenia and Kilimandjaro, whose estimated heights are about 19,000 feet. In the Abyssinian plateau, on the north, the average elevation is from 6000 to 8000 feet. From the plateau, in detached groups, arise peaks, the highest of which are over 15,000 feet. From the Abyssinian plateau the system is continued northward to the Mediterranean by a succession of mountains which stretch along the western shores of the Red Sea. Some of the peaks are from 6000 to 9000 feet high. South of the plateau of Kaffa the system is con- tinued by the Lupata and Dragon Mountains to the southern extrem- ity of the continent. The Zambesi and Limpopo Rivers discharge their waters into the Indian Ocean through deep breaks in the system. 158. Secondary Systems. — On the south the Nieuveldt and Snow Mountains stretch from east to west, with peaks of over 10,000 feet. Table Mountain is on the south. On the west the Mocambe and Crystal Mountains extend from the extreme south to the Gulf of Guinea. Near the northern end of this range, but separate from it, are the volcanic peaks of the Came- roons Mountains, 13,000 feet high. The Kong Mountains extend along the northern shores of the Gulf of Guinea in a general east-and-west direction. Some of the peaks are snow-capped. In the extreme north of Africa are the Atlas Mountains, which rise from the summit of a moderately ele- vated plateau. Some of the peaks are 13,000 feet high. 159. The Great Interior Depression north of the equator is di- vided into two distinct regions. A straight line extending from Cape Guardafui to the northern shores of the Gulf of Guinea marks the boundary. The mountain-systems north of this line have a general east-and-west direction ; those south of it a north-and-south direction. The Plateau of the Sahara occupies the northern part of the inte- rior depression. Its general elevation is about ] 500 feet, though here and there plateaus of from 4000 to 5000 feet occur, and even BELIEF FORMS OF THE CONTINENTS. 117 short mountain-ranges with peaks of 6000 feet. The main portion of the region is covered with vast sand-fields, with occasional rocky masses, and is, for the greater part, an almost absolute desert. Near Long. 14° E. from Greenwich, in the district of Fezzan, the plateau is divided from north to south by a broad valley, in which occur many remarkable de- pressions, some of which are several hundred feet below the level of the Mediterranean. Here fer- tile spots, called oases, are common. South of the Sahara is the Soudan, a re- markably well-watered "and fertile region. Lake Tchad occupies the greatest depression. The interior, which lies south of this, is but little known. It is, probably, a moderately elevated plateau. Extensive lake basins — Albert Nyanza, Victoria Nyanza, and Tan- ganyika — lie near the predominant mountain-system. 160. Approximate Dimensions of Africa. Area of continent, 12,000,000 square miles. Coast line, 16,000 miles. Greatest breadth from east to west, 4800 miles. Greatest length from north to south, 5000 miles. Culminating point, Mount Kenia, or Kilimandjaro, about 19,000 feet. Fig. 59. -Desert of Sahara. VI. AUSTRALIA. 161. Surface Structure. — The Predominant Mountain-system is in the east. The Secondary Systems are in the west and north-west. The Great Low Plain lies between the predominant and second- ary systems, and slopes gently to the southern coast. 118 PHYSICAL GEOGRAPHY. The Predominant System extends aloug the entire eastern shore, from Torres Straits to the southern extremity of Tasmania. It is, for the most part, composed of broad plateaus. The system is highest in the south-east, where the name Australian Alps is given to the range. Mount Kosciusko, 7000 feet, probably forms the culminating point of the Aus- tralian Continent. The system descends abruptly on the east, and by gentle slopes on the west to the low plains of the interior. 162. The Secondary Sys- tems, on the west and north- west, are of but moderate ele- vation. Fig. 60.— Orographic Chart of Australia. (White portions, mountains ; shaded portions, plains.) 1, Australian Alps; 2, Kosciusko; 3, 4, 5, Secondary Systems ; 6, Murray River. 163. The Great Low Plain lies in the interior. Accurate informa- tion as to its peculiarities is yet wanting. A moderate elevation on the north connects the eastern and western systems. The south-eastern portion, which is the best known, is well watered and remarkably fertile. Basin-shaped valleys are found in the west. The lower parts are occupied by lakes EyTe, Torrens, and Gairdner. 164. Approximate Dimensions of Australia. Area of continent, 3,000,000 square miles. Coast line, 10,000 miles. Greatest length from east to west, 2400 miles. Greatest breadth from north to south, 2000 miles. Culminating point, Mount Kosciusko, 7000 feet. 165. Contrasts of Africa and Australia. — In the north, the African Continent resembles Europe and Asia in the arrangement of its forms 01 relief. In the south, it resembles the Americas. As a whole, the African Continent resembles Australia more closely than any other. In both Africa and Australia the predominant system is in the east, and extends along the entire coast. In each the sec- RELIEF FORMS OF THE CONTINENTS. 119 ondary systems are in the west and north. But Africa terminates in a plateau which descends abruptly to the sea, while Australia is ter- Fig. 61. — Australian Scenery. minated by a great low plain which descends by long, gentle slopes from the interior. SYLLABUS. Eocks consist of various mixtures of substances called minerals. Rock -masses are divided, according to their origin, into igneous, aqueous, and metamorphic, to which are sometimes added Molian. According to their condition, rocks are divided into stratified and unst ratified ; according to the presence or absence of organic remains, into fossiliferous- and non-fossiliferous. Unstratified rocks are sometimes called crystalline. Aqueous rocks are sometimes called sedimentary. Aqueous rocks are stratified. Igneous rocks are unstratified. Metamorphic rocks were originally stratified, but lost their stratification through metamor- phism. iEolian rocks are roughly stratified. 120 PHYSICAL GEOGRAPHY. The earth's oldest or primitive rocks were either formed by the first cooling of the outside of the melted earth, or were thrown down as sediment in the primitive ocean. It is doubtful whether these rocks have ever been reached. Aqueous rocks may contain fossils. Igneous rocks never contain fossils. Metamorphic rocks, in rare instances, may contain fragments of fossils. Geological time is divided into eras ; the eras are subdivided into periods ; the periods into epochs ; and the epochs into ages. There are six geological eras ; viz., the Azoic, the Eozoic, the Palaeozoic, the Mesozoic, the Cenozoic, and the Era of Man. The Azoic Era includes the Astral Period and the Archaean Period. The Eozoic Era includes the Algonkian Period. The Palaeozoic Era includes the Silurian Period, or the Period of Invertebrates, the Devonian Period, or the Period of Fishes, and the Carboniferous Period, or the Period of Coal Plants. The most important agencies now producing change in the earth's crust are the Heated Interior, Erosion or Denudation, including Weathering, Corrasion, and Transportation ; Wind Corrasion and Transportation ; Avalanches and Land- slides; Ocean Waves, and Man. There is more water than land surface on the earth, in proportion of 25 : 9, or as 5 a : 3*. The land-masses surround the north pole in the shape of an irregular ring. Nearly all the land-areas are collected in one hemisphere, and the water-areas in another hemisphere. The northern continents are almost entirely in the temperate latitudes ; the southern continents are mainly in the tropics. The land-masses may be divided into three doublets, consisting of pairs of northern and southern continents, almost or entirely separated from each other. There are two great systems of trends or lines of direction. These trends are north-east and north-west. The northern continents are characterized by deeply indented coast lines ; the southern are comparatively simple and unbroken. Europe is the most, and Africa the least, deeply indented of the continents. One-seventeenth of the land-area is composed of islands. Islands are either continental or oceanic. The smaller continental islands include fiord islands, delta islands, low mud- flats and sand-bar islands, and small, high, rocky islands. There are four successive stages in the formation of a coral island or atoll : 1. The fringing reef ; 2. The barrier reef; 3. The encircling reef; 4. The coral island or atoll. The greatest elevations and depressions in the earth's surface are small when compared with its size. Low lands are either plains or hills. High lands are either plateaus or moun- tains. Alluvial plains are lacustrine, fluviatile, or marine. Plains are divided, ac- SYLLABUS. 121 cording to their position, into coastal, inland, and worn-down mountain low lands, old plains or peneplains. They are divided, according to the materials that form them, into river-made plains, dust plains, and lava plains. Plateaus, according to their position, are marginal or intermont ; according to their age, they are young or old. Mountains may be divided into three classes : 1. Mountains by flexure ; 2. Mountains by fracture; 3. Mountains by injectiou of lava between strata. Valleys are either longitudinal or transverse. All continents have high borders aud a low interior. The highest border faces the deepest ocean. North and South America resemble each other in the arrangement of their relief forms. Their predominant systems are in the west ; their secondary sys- tems are in the east ; their great low plains are between the predominant and secondary systems. The predominant system of North America is the Cordillera of the Eocky Mountains. The secondary systems are — the Appalachian system, the plateau of Labrador, the Height of Land, and the Arctic plateau. The predominant system of South America is tbe Cordillera of the Andes. The secondary systems are — the plateaus of Guiana and Brazil. The great low plains are — the Llanos of the Orinoco, the Selvas of the Amazon, and the Pampas of the La Plata. Europe and Asia resemble each other. Their predominant systems are in the south ; their great low plains are north of their predominant systems. The pre- dominant system of Europe is the Alps. The secondary systems of Europe are — the mountains of the Scandinavian Peninsula, the Ural Mountains, and the Caucasus Mountains. The predominant mountain-system of Asia is the plateau of Thibet. The secondary systems of Asia are— the plateau of Gobi, the Thian-Shan and Altai Mountains, the plateau of Indo-China, the plateau of Deccan, the plateau of Iran, the plateau of Asia Minor, and the plateau of Arabia. The predominant mountain-system of Africa includes the mountains of the eastern coast. The secondary systems include the Nieuveldt and Snow Mountains in the south, the. Mocambe, Crystal, Cameroons, and Kong Mountains in the west, and the Atlas Mountains in the north. The predominant mountain-system of Australia includes the mountains of the eastern coast. The secondary system includes the mountains found in the west and north. Africa and Australia resemble each other. Their predominant systems are in the east; their secondary systems are in the west and north; their depressed areas are between the two. «»;<« 122 PHYSICAL GEOGRAPHY. REVIEW QUESTIONS. What two elementary substances form the greater part by weight of the earth's crust ? Into what classes may rocks be divided according to their origin ? According to their condition ? According to the presence or absence of fossils ? What is paleontology ? Into what two classes may igneous rocks be divided ? Define iEolian rocks ; fragmental rocks; sedimentary rocks; crystalline rocks. What do you under- stand by the term primitive rocks ? Define Azoic Era, Eozoic Era, Palaeozoic Era, Mesozoic Era, and Cenozoic Era. Explain the nature of the changes in the earth's surface caused by the atmosphere ; by water ; by the agency of man. Describe in full the process of weathering. Define erosion, denudation, corra- sion, wind erosion, transportation. What is meant by the load of a river? How are dunes formed ? What must be the sides of two squares whose areas represent the relative land- and water-areas of the earth ? What are the actual areas in square miles? How would you draw a circle around the earth which will divide it into land and water hemispheres ? What do you understand by lines of trend ? Define lagoons ; dunes. Describe the American island chains ; the Asiatic chains. Describe the Australasian island chain ; the Polynesian chain. Which are the higher, volcanic islands or coral islands ? Why ? Name the principal steps or stages in the progress of formation of a coral island. State Darwin's hypothesis for the formation of coral islands. State any other hypotheses that have been proposed for coral formation. To what do plains owe their level surface ? Distinguish between lacustrine, fluviatile, and marine plains. What single term will include all these plains? Distinguish between coastal and inland plains. Describe the formation of peneplains. Distinguish between marginal and intermont plateaus. Define mountain-system ; axis of mountain-system. What is Orology ? Explain the manner in which mountains are formed. Into what classes may mountains be divided according to their origin? Define block mountains. What is meant by a dissected block mountain ? Embayed mountain? Distinguish between a longitudinal and a transverse valley. Give a short account of the surface structure, or the arrangement of the high and low lands, of North America. Of South America. Of Europe. Of Asia. Of Africa. Of Australia. Which of these resemble each other ? In what respect do they all resemble one another? SYLLABUS. 123 Name the culminating points of each of the continents. Name the predomi- nant and secondary mountain-systems of each of the continents. How many times larger is Asia than Australia? Than Europe? Africa? North America? South America? North America. Describe the Cordillera of the Kocky Mountains. Where is the Great Basin ? By what mountains is it surrounded ? Name the principal mountains of the Appalachian system. What rivers drain the great low plain of North America ? South America. Describe the Cordillera of the Andes. Name the principal plateaus of the Andes. Where is the plateau of Guiana ? Of Brazil ? What three large river-systems drain the great low plain of South America ? Europe. Describe the predominant mountain-system of Europe. Describe the chain of the Alps. What river-systems divide its northern slope into three divisions? Name the principal mountains of each division. What three peninsulas project southward from the southern slopes of the pre- dominant mountain-system? Name the principal mountains of each peninsula. Asia. Describe the predominant mountain-system of Asia. What mountains form the northern boundary of the plateau of Gobi ? The southern boundary ? What mountains form the boundaries of the plateau of Iran ? Is Arabia a plateau or a plain? Describe the great low plain of Asia. Africa. Describe the predominant mountain-system of Africa. Where are the Mocambe Mountains ? The Crystal Mountains, the Cameroons, the Atlas, the Kong, the Lupata, and the Dragon? Australia. Describe the predominant mountain-system of Australia. Describe the secondary mountain-system. The great low plain. PART III. THE WATER. 3>*tt°° CHAPTER I. Physical Properties of Water. 166. Composition. — Water is formed by the chemical combina- tion of oxygen and hydrogen, in the proportion, by weight, of eight parts of oxygen to one part of hydrogen ; or, by volume, of one part of oxygen to two parts of hydrogen. 167. Properties. — Pure water is a colorless, transparent, taste- less, and inodorous liquid. It freezes at 32° Fahr., and, under the ordinary pressure of the atmosphere, boils at 212° Fahr. Water exists in three states: solid, liquid, and gaseous. Under ordinary cir- cumstances it freezes at 32°. It evaporates, or passes off from the surface as vapor, at all temperatures, even at 32° ; but it is only at the boiling-point that the vapor escapes from below the surface as well as at the surface of the liquid. Heated in open vessels, under the ordinary pressure of the atmosphere, the temperature of boiling water cannot be raised higher than 212°, any increase of heat only causing it to boil more rapidly. Heated in closed vessels, which prevent the escape of steam, and permit the pressure to accumulate, its temperature can be raised very high. Conversely, on high mountains, where the pressure of the atmosphere is lower than at the level of the sea, water boils at temperatures lower than 212° Fahr. 168. Maximum Density of Water. — A pint of cold water is heavier than a pint of warm water, because, as water is cooled, it contracts and grows denser. The coldest pint of water, however, is not the heaviest. The heaviest pint of water is water at the tem- perature of 39.2° Fahr. This temperature, therefore, is called the temperature of the maximum density of water. If water at this temperature be heated, it becomes lighter, or expands ; if water at 126 PHYSICAL GEOGRAPHY. this temperature be cooled, it also becomes lighter or expands until ice is formed, which floats on the water. When at the temperature of its maximum density, water is 7.2° warmer than the freezing- point. 169. Effect of the Maximum Density of Water on its Freezing. — If cooling water continued to contract until freezing began, the ice first formed would sink to the bottom, and, this continuing, the entire mass would soon become solid. In this manner great bodies of fresh water, in times of cold, might freeze throughout, when not even the heat of a tropical sun could entirely melt them. But for this curious physical property of water, at least three-fourths of the globe would be incapable of sustaining its present life. The entire floor of the ocean, both in the tropics, and in the temperate and tbe polar regions, is covered with a layer of cold, salt water, at nearly the tem- perature of its maximum density. In the tropics the surface-water is warmer and lighter than this dense layer, and in the polar regions it is colder and lighter. Ground or Anchor Ice. — In very cold climates, ice, called ground or anchor ice, sometimes forms around sharp-pointed rocks on the bottom of rivers. When the ice deposit is of sufficient size its buoyancy brings it to the surface, along with the entangled rocks. The ice floating with the river current thus transports the rock or other mineral matter often for considerable distances. 170. Specific Heat of "Water. — Another remarkable physical property of water — its specific heat — enables it to play an important part in the economy of the earth. The specific heat of a body is the quantity of heat-energy required to produce a definite increase of temperature in a given weight of that body. Water has a very high specific heat ; that is, a given quantity of water requires more heat-energy to warm it, and gives out more heat-energy on cooling, than an equal quantity of any other common substance. The quantity of heat required to raise a pound of ice-cold water to 212°, would heat a pound of ice-cold iron to a bright-red heat, or to about 1600° Fahr. ; or, conversely, a pound of boiling water, cooling to the freezing-point, would give out as much heat as a pound of red-hot iron cooling to 32° Fahr. PHYSICAL PROPERTIES OF WATER. 127 The enormous capacity of water for heat is of great value to the life of the earth. The oceanic waters are vast reservoirs of heat, storing heat in summer and giving it out in winter. The great specific heat of water prevents it from either heating or cooling rapidly. Large bodies of water, therefore, in any region, prevent great extremes of heat and cold. 171. Heat Absorbed or Emitted during Change of State. — During the conversion of a solid into a liquid, or a liquid into a vapor, a large quantity of heat-energy is absorbed. This heat- energy does not increase the temperature of the body, and, therefore, cannot be detected by the thermometer. The heat-energy is then in the condition of stored or potential energy, sometimes called latent heat. When water-vapor condenses into a liquid, or when liquid water freezes, the stored heat-energy again becomes sensible as heat. In freezing, water gives out heat, and raises the mean temperature of the surrounding air. In melting, ice takes in heat, and lowers the mean temperature of the surrounding air. 172. Stored Heat-energy of Ice-cold Water. — Water has a higher latent heat than any other common substance. In order to heat a pound of water 1° Fahr. an amount of heat called a heat- unit, or a pound-degree, is required. Before one pound of ice at 32° Fahr. can melt and form one pound of water at 32° Fahr., it must take in 1J/.2 heat-units ; and yet a thermometer, plunged in the water from melting ice, will indicate the same temperature as when entirely surrounded by lumps of the unmelted material. The great latent heat of ice-cold water has an important influence on the freez- ing of large bodies of water, since, after the surface-layers have reached the tem- perature of the freezing-point, they have still 142 heat-units to lose before they can solidify. Again, when ice reaches a temperature of 32° Fahr., it has still 142 heat-units to absorb before it can melt. Were it not for this fact, destructive floods would often result from the rapid melting of the winter's accumulation of snow and ice. 173. Stored Heat-energy of "Water-vapor. — Before one pound of water can pass off as vapor, it must take in sufficient heat to raise 128 PHYSICAL GEOGRAPHY. nearly 1000 pounds of water 1° Fahr. The vapor which then escapes is still at the same temperature as the water from which it came. The 1000 heat-units, or pound-degrees of heat, have been rendered latent, and have no influence on the thermometer. When the vapor in the air is condensed as rain, snow, hail, dew, fog, cloud, or mist, the stored heat-energy again becomes sensible. Much of the vapor which is formed in the equatorial regions is car- ried by the winds to high latitudes, where, on condensing, it gives out its heat and moderates the intense cold which would otherwise exist. 174. Solvent Powers. — Water is one of the best solvents of all common substances. During the constant washings of the continents by the rains, the surfaces are cleansed from decaying animal and vegetable matters, which are partly dissolved and are thus carried in solution by the rivers into the ocean. The atmospheric waters in a similar way cleanse the air of many of its impurities. 175. "Water is the Main Food of Animals and Plants. — By far the greater part of the bodies of animals and plants is composed of water. Without large quantities of water no vigorous life can be sustained in any locality. Deserts are caused by the absence of water. x>>&.c CHAPTER II. Drainage. 176. Drainage. — The water which falls from the atmosphere as rain, hail, or snow, either sinks through the porous strata and is drained under ground, or runs directly off the surface. Thus result two kinds of drainage — Subterranean and Surface. The rapidity of drainage is greater in open, porous strata, such as are formed of rock waste, than in hard rocks, such as granites. The freezing of the soil may cause all the drainage to take place at the DRAINAGE. 129 surface. Where the surface soil is underlaid by readily soluble for- mations, such as limestone, crevices in the rocks are enlarged by solution, and the surface-waters may sink so rapidly into the ground as either to decrease greatly the volume of the surface streams, or to obliterate them entirely. 177. Subterranean Drainage. — The water which sinks through the porous strata continues descending until it meets impervious layers, when it either runs along their surface, bursting out as springs at some lower level, where the layers outcrop, or it collects in sub- terranean reservoirs. The origin of all springs is to be traced to subterranean drainage. Underground streams sometimes attain considerable size. In portions of the Swiss Jura, streams burst from the sides of hills in sufficient volume to turn the wheels of moderately large mills. In a few instances, the subterranean streams can be navigated for considerable distances, as in the Mammoth Cave of Ken- tucky, or in the Grotto of Adelsberg, near Trieste. 178. Surface Drainage. — The water which is drained directly from the surface, either runs down the slopes in rivulets and rills, which, uniting with larger streams, is poured directly into the ocean, or it collects in the depressions of basin-shaped valleys, where, having no connection with the ocean, it can be discharged by evaporation only. Thus arise two kinds of surface drainage — oceanic and inland. 179. Wells. — The water that has sunk into the slopes of hills slowly descends to the lower land of the valleys, where it either escapes in springs, or collects in the strata near the surface. In nearly all parts of the valley, water may be obtained by sinking wells at depths varying from 10 to 30 feet. 180. Springs are the outpourings of the subterranean waters, They may occur as — (1) Hillside Springs. — On hill-sides, where a bed of sand, gravel, Fig. 62.— Hill-side Spring. 130 PHYSICAL GEOGRAPHY. or other porous material, rests on an impervious bed of clay or hard rock. Thus iu the hill- side shown in Fig. 62 ? the upper portion of the hill is formed of a bed of sand, S, resting on a bed of clay, C, or other impervious layer. Rain falling on the surface sinks down through the porous sand until it reaches the impervious layer, and either runs down the surface by its weight, or is forced down by the pressure of the water in the higher portions of the porous sand, until it escapes as a spring at A. A somewhat simi- lar spring is shown in Fig. 63. Here a broken, fractured rock rests on an inclined impervious rock. Springs of this class are generally small. (2) Fissure springs, where the water issues from fissures either in faulted rocks, or at other breaks in the rocks. Where such fissures penetrate the strata to great depths they may form the points of dis- charge for the escape of the subterranean water from basins of Fig. 64.-Fissure Spring. considerable area. Consequently, fissure springs are usually large. If, as in Fig. 64, a pervious, water-logged stratum, A A, is situated between two layers of impervious rock, C C and D D, and faulted as shown, A Hill-side Spring. DRAINAGE. 131 the fissure at F F, will determine the location of a spring ; for, the water contained within the porous strata, prevented from either rising or sinking by the impervious rock, will descend, until it reaches the fissure, and immediately rise through the fissure to the surface and emerge at S, as a spring. (3) Artesian Springs or Wells. — In basin-shaped regions, where porous, water-logged strata are contained between parallel, imper- vious strata, if the upper impervious strata are broken naturally by fissures, somewhat in the manner of fissure springs, or are purposely bored by man, the waters will escape through the openings by reason of the pressure of the water in the higher parts of the reservoir. They are usually called artesian springs or wells, and differ from ordinary wells in that their waters are discharged by the pressure of the water in the reservoir, thus rendering pumping unnecessary. They are, therefore, true springs. Fig. 65.— Artesian Spring. If, for example, as in Fig. 65, A A, represents a pervious, water- logged stratum, lying between two impervious strata, and a boring be made at C B, the water will issue in a jet, but not so high as the line A C, on account of friction and air resistance. Artesian wells have been sunk to great depths, and it is a significant fact that the temperature of the issuing waters is always proportional to the depth, show- ing, for every 55 feet of descent, a nearly constant increase of 1° above the tem- perature of ordinary springs— viz., about 60° Fahr. In the case of the artesian well of Grenelle, Paris, the successful boring of which was accomplished only after many years of the most discouraging labor, and which reached a depth of 132 PHYSICAL GEOGRAPHY. nearly 1800 feet, the temperature of the water was 82° Fahr. A well at Neusalz- werk, Prussia, has penetrated 2200 feet; its temperature is 91° Fahr. 181. Classification of Springs. — Springs are conveniently arranged in different classes according to peculiarities in the size, shape, and depth of their reservoirs, and the nature of the mineral substances composing the strata over which the waters flow, or in which they collect. The Reservoirs of springs are the places where the waters that sink into the ground collect. Reservoirs are usually merely porous strata, such as beds of sand or gravel, which lie between impervious layers of clay or hard rock. The water collects in the spaces be- tween the particles of sand or gravel. 182. Size of Reservoir. — When the reservoir is large and the rainfall sufficiently great, the spring is constant; when small, the spring is temporary. Constant Springs are those which flow continually, and which are but little affected in the volume of their discharge even by long-continued droughts. Temporary Springs are those which flow only for a short time after wet weather, drying up on the appearance of even moderate droughts. Since the volume of water discharged by some springs is very considerable, we must infer that their reservoirs are of great size. Very large springs probably receive the drainage from many square miles of surface. Springs are common on the shores of the ocean. Their waters are fresh because the outflow of the fresh water from the land into the sea prevents the inflow of the salt water. This is the case even on coral islands, where the height of the land is but ten or twelve feet above the sea. A comparatively shallow well, on such islands, usually yields fresh water, derived, of course, from the rainfall. 183. Depth of Reservoir. — According to the distance the reser- voir is situated below the surface of the earth, springs are divided into Cold, and Hot or Thermal. 184. Cold Springs are those whose temperature does not DRAINAGE. 133 exceed 60° Fahr. Their waters are sometimes much colder than this. Very cold springs owe their low temperatures to the sources whence they draw their supplies. In mountainous districts these can generally be traced to the melting of huge snow-fields, or masses of ice called glaciers. The temperature in such cases is nearly that of ordinary ice-water. The reservoirs of all springs the temperature of whose waters ranges from 50° to 60° are usually near the surface. They are cooler than surface waters, because they are shielded from the The temperature of springs of this kind is but slightly affected by changes in the temperature of the outer air. Since the reservoirs of ordinary springs are shielded from the hot air in summer, and from the cold air in winter, their waters are colder than river-water in summer, and warmer than river-water in winter. Their waters average, in their temperature, that of the strata over which they flow in their subterranean courses. The mean annual temperature of the strata over which the waters flow may be ascertained by plunging a thermometer into the water as it comes out of the spring. 185. Hot or Thermal Springs range in temperature from 60° Fahr. to the boiling-point. In geysers the temperature of the water, far down in the tube, is considerably above the boiling-point at the surface. Hot springs which occur in the neighborhood of active volcanoes owe their high temperature to beds of recently ejected lava situated in the vicinity of their reservoirs. Hot springs, however, are common in regions distant from volcanic disturbance. In such cases their high temperature must be attrib- uted to the distance the water sinks below the earth's surface, the heat being derived from the interior. Hot springs are common in regions where the strata are disturbed, and fractures are numerous, since their waters are apt to come up from great depths. Some hot springs may owe their high temperature to the oxidation of sulphides, or to some other chemical action. 134 PHYSICAL GEOGRAPHY. 186. Geysers are boiling springs which, at intervals more or less regular, shoot out huge columns of water with great violence. The jets of water sometimes reach a height of more than 200 feet. The geyser issues from the summit of a conical hil- lock of silicious material de- posited by the water. A broad, shallow basin usually surmounts the hillock and forms the mouth of a deep, funnel-shaped tube. The sides of both tube and basin are lined with a smooth in- crustation of silica. In the Great Geyser of Iceland, the basin is 52 feet wide and the tube 75 feet deep. Both the tube and basin are the work of the spring, being deposited from the silica contained in the highly heated waters. It is only when the tube has reached a certain depth that the spring becomes a true geyser. When the depth becomes too great, the geyser eruptions cease, the waters forcing their way through the walls of the tube to some lower level. Hence, in all geyser regions, numerous deserted geyser-tubes and simple ther- mal springs occur. The waters of some geyser regions are calcareous. In such cases the tube of the geyser is, of course, formed of limestone. 187. Bunsen's Theory of Geysers. — Bunsen explains the cause of geyser eruptions as follows : The heat of the volcanic strata, through which the geyser- tube extends, causes the water which fills it to become highly heated. The water at the bottom of the tube, having to sustain the pressure of that above it, gradu- ally acquires a temperature far above the boiling-point at the surface. The tem- perature of the water in the tube will, therefore, decrease from the bottom to the surface. If now, when the tube is filled, the water, near the middle, is brought to its boiliug temperature, the steam thus formed momentarily lifts the water in the upper part of the tube, when the water in the lower part, released from its press- ure, bursts into steam and forcibly ejects the contents of the tube. Bunsen succeeded in lowering a thermometer into the tube of the Great Geyser in Iceland just before an eruption. At the depth of 72 feet he found the Fig. 66.— Geyser in Eruption. DRAINAGE. 135 temperature of the water to be 261° Fahr., or 49° above the ordinary boiling- point. 188. Geyser Regions. — There are three extensive geyser regions : (1) In Iceland, in the south-western part of the island, where over one hundred geysers occur in a limited area. (2) In New Zealand, about the centre of the northern island, where, near the active volcano Tongariro, over one thousand mud springs, hot springs, and geysers burst from the ground. (3) In Yellowstone National Park, in Wyoming, where numerous large geysers and hot springs occur, mostly near the head-waters of the Madison and Yellowstone Rivers, at heights in some places as great as 8000 feet above the sea-level. Here the boiling-point of the water at the surface of the geysers, owing to the diminished atmos- pheric pressure, is as low as about 200° Fahr. A small geyser region is found in California, near San Francisco. 189. Nature of the Mineral Substances forming the Reser- voir. — The subterra- nean waters dissolve various mineral mat- ters either from the strata over which they flow, or from their res- ervoirs ; this is espe- cially true of thermal springs, owing to the greater solvent powers of the heated waters. The waters of mineral springs usually contain a number of mineral substances. Mineral springs are divided into various classes according to the predomina- ting material; viz., (1) Calcareous springs are those whose waters contain lime in solution. Fig. 67.— Calcareous Deposits, Yellowstone National Park. 136 PHYSICAL GEOGRAPHY. Thermal waters, charged with carbonic acid, usually contain large quantities of lime, which they have dissolved from subterranean strata. On reaching the sur- face the waters cool and part with some of their carbonic acid, and deposit layer after layer of hard limestone, called travertine. In this way immense quantities of lime-stone are brought to the surface from great depths, leaving huge subter- ranean caverns. In portions of Tuscany, Italy, beds of travertine occur more than 250 feet thick. Besides the stalactitic and stalagmitic deposits of lime-stone in caverns, the highly charged calcareous waters descending the surface slopes of the hills in por- tions of the Yellowstone Park have deposited the lime in a series of parapets at different heights, forming basins, filled with water which drips or flows to lower levels. Such deposits are shown in Fig. 67. (2) Silicious springs are those whose waters contain silica. (3) Sulphurous springs are those whose waters contain sulphuretted hydrogen and various metallic sulphides or sulphates. Sulphurous springs are found in Baden ; near Vienna ; and in Virginia. (4) Chalybeate springs are those whose waters contain iron. (5) Salt springs or brines are those whose waters contain common salt. The springs of Halle, in the Alps of Salzburg, yield 15,000 tons of salt an- nually. The artesian well of Neusalzwerk, Prussia, yields about 28,000 tons annually. In the United States the springs of Salina and Syracuse are among the most important. The water in the springs of Salina is ten times Salter than ocean-water. The salt is obtained from these springs by the evaporation of the water. (6) Acidulous springs are those whose waters contain large quan- tities of carbonic acid gas, as the Seltzer springs in Germany, and those of Vichy in France. CHAPTER III. Rivers. 190. Definitions. — The water that issues from the ground as. springs ; that which is derived from the melting of ice or snow ; or that which drains directly from the surface after rainfall, runs down RIVERS. 137 the slopes of the land and collects in the depressions formed by the intersection of the slopes, forming rills or rivulets, which, at last, combine in larger streams called rivers. The source of a river is the place where it rises ; the mouth, the place where it empties ; the channel, the depression through which it flows. Rivers usually rise in mountains, where the rainfall is greater than elsewhere, and where vast beds of snow and ice occur. Since the downward motion of a river is caused by the inclination of its chan- nel from the source to the mouth, a correct idea of the general inclination of any- country can he obtained by a careful study of a map in which the directions of the rivers are represented. In studying the various river-systems the student should endeavor to obtain in this way clear ideas of the general directions of the continental slopes. The River-system is the main stream, with all its tributaries and branches. The Basin is the entire area of land which drains into the river- system. The Water-shed is the ridge or elevation which separates two op- posite slopes. The streams flow T in opposite directions from the water-shed. The Velocity of a river depends on the inclination or pitch of the channel, and the volume or depth of the water. 191. River Courses or Tracts. — A river, from its source to its mouth, may be divided into three courses or tracts : (1) The upper or mountain course or tract ; (2) The middle or valley course or tract ; (3) The lower or plains course or tract. 192. The Upper or Mountain Course of a river is the part which is situated in the mountainous or hilly country near its source. Here the river has a high velocity, and its channel is characterized by sharp , sudden turns, alternating with long, straight courses, and is comparatively free from fine debris. In the upper course erosion occurs almost entirely along the bottom of the channel, so that the river runs between steep, and sometimes almost vertical, banks. In this way river-valleys are formed. In the upper and middle courses rapids and waterfalls occur. 138 PHYSICAL GEOGRAPHY. a a \ 'A V 1 l \ 2 2 _ 3 4 3 d Fig. 68.— Erosion of Waterfall. Rapids and Waterfalls. — During the erosion of the channel, where harder rocks occur in the bed of the stream, the softer strata, imme- diately adjoining them down stream, are rapidly worn away, and the obstruction becomes at last the head of a waterfall. The height grows rapidly from the increased force of the falling water, and con- tinues until stopped by some similar obstruction below. Thus, suppose a a, Fig. 68, is the bed of a river, the direction of flow of which is shown by the arrow. The softer rock being worn away more rapidly, the bed reaches the level, 1, 1. A fall, and consequent increase in the velocity of the river, soon causes the level of the bed to reach 2, 2, 3, 3, and 4, 4, successively. At the same time the falling water eats away the vertical wall of the precipice, causing the waterfall to move up stream. The water then cuts the precipice away in steps, at 5, 6, 7, thus changing the fall into cascades. These are finally worn away, as shown at 8, changing the cascades to rapids, when, finally, the fall disappears entirely, or the erosion of the hard rock is completed. Waterfalls, therefore, near the middle course of a river, indicate a young stream. The waterfalls which occur near the source of a river in the mountainous region have a different origin. Here a lofty precipice may be cut out from a single block. Unlike the waterfalls just described, they indicate a very old channel ; i. e., are the final result of long-continued erosion. When the water falls perpendicularly — that is, when it does not slip or slide — it forms a waterfall or cataract; in all other cases of swift descent it forms rapids. The grandest falls in the world, those of the Niagara, are 160 feet high. Though greatly inferior to many others in height, yet the volume of their water is so great that they surpass all others in grandeur. The Victoria Falls of the Zambezi in Africa nearly equal in volume those of the Niagara. Their height is 360 feet. The highest falls in the world are those of the Yosemite, in California. Two projecting ledges break the sheet into three falls, whose total height exceeds 2000 feet. One of the highest falls in Europe is the Staubbach or Dust-brook, in the valley of the Lauterbriinnen in Switzerland. The water makes one sheer fall of 959 feet, and is lost in a sheet of mist before it reaches the ground. RIVERS. 139 Waterfalls may also be due to the draining of a lake. Many waterfalls were caused toward the close of the glacial epoch during the withdrawal of the ice sheet. 193. The Middle or Valley Course extends from where the river emerges from the mountainous or hilly dis- tricts to the low plains near the mouth. The channel is wide, and con- tains coarse gravel and small boulders. The ero- sion of the bottom of the channel is insignificant, but at the sides, especially during freshets, the river undermines its banks, and thus widens its valley. Here the river is divided into two distinct portions : the river-channel proper and the alluvial flats or flood-grounds. 194. The Lower or Plains Course extends from the middle course to the mouth. The fall is slight, and the velocity small. The channel is filled with fine silt or gravel, except in swifter portions, where coarse gravel may occur. 195. Changes in River-courses. — During floods, when the velocity and eroding power are greatly increased, extensive changes often occur in river- courses. After the floods have subsided the water is found running through new channels, its old ones being either completely filled with deposits of mud, or occupied by slender streams. Along the Mississippi these partially deserted channels are called bayous, and, in places, widen out into large lakes (see Fig. 70). The Eed Itiver appears to have formerly emptied into the Mexican Gulf through a separate channel. In the basins of the Amazon, t'he Ganges, and the Po, the old deserted channels are numerous on both banks of the streams. The Falls of Niagara. 140 PHYSICAL GEOGRAPHY. 196. River Mouths. — A wide, open river mouth is called an Estuary; the accumulation of mud or saud which occurs in the mouths of certain rivers is called a Delta. 197. Inundations. — Duriug certain seasons of the year, the amount of water drained into the river-channel is greater than it can discharge; it then overflows its banks, and inundates the sur- rounding country. Inundations of rivers are caused — (1) By excessive rainfall ; (2) By periodical rains ; (3) By the melting of ice and snow. In the tropics, where the rainfall is more or less periodical, the inundations of the rivers are also periodical. The melting of the ice and snow, which occurs regularly at the beginning of the warm weather, also causes periodical inunda- tions. The Nile rises annually on account of the periodical rainfall of its upper sources ; the Mississippi semi-annually, once from the melting of snow, and once from the winter rainfall. Where both the area of the river-basin and the rainfall in inches are known, experience permits of a calculation, by means of which the probable time and extent of rise of water in a river can be approximately predicted. In times of heavy rainfall, the Weather Bureau of the United States is enabled to predict the probable rise of the important rivers. Influence of the Destruction of the Forests on Inundations. — When the forests are removed from a large portion of a river-basin, the rains are no longer absorbed quietly by the ground, but drain rapidly off its surface into the river-channels, and thus in a short time the entire precipitation is poured into the main channel, causing an overflow. It is from this cause that the disastrous effects of otherwise harmless storms are produced. The inundations are most intensified by this cause in the early spring, when the ice and snow begin to melt. The destructive effects of the floods are increased by masses of floating ice, which, becoming gorged in shallow places in the stream, back up the waters above. The increased frequency of inundations in certain parts of the United States is, to a great extent, to be attributed to the rapid destruction of the forests. 198. The Quantity of Water Discharged by a River depends principally — (1) On the size of its basin ; (2) On the amount of its rainfall. 199. Age of Rivers. — Like most physical features of the earth, RIVERS. 141 river-systems pass through various stages of development. They manifest youth, maturity, and old age. (1) Immature or Young River-systems. — Such are the rivers that drain a recently uplifted district. The main rivers occupy the trough of the district, and the brauches drain the slopes of the ridges. Depressions in the troughs are first filled by the main stream, thus forming lake-systems. The presence, therefore, of extended lakes in any river-system is a sign of the youth of the basin it drains. Examples of such youthful or immature drainage is seen in the lake-system of the St. Lawrence and of the great African lakes. In matured rivers the lakes are absent and no falls remain, since no marked inequalities exist in the river bed. In young rivers waterfalls are common. (2) Matured River-systems. — As the age of the river-system increases, erosion cuts away the land at the outlet of its lakes, thus draining the lake-basins ; the waterfalls eat back their precipices until the falls have disappeared. The river has then developed its tributary streams, receives its full load of sediment, and has adjusted its grade or inclination so as to give it a velocity just sufficient to carry its silt to its outlet. The river is then said to be graded, and has reached maturity. (3) Old Rivers. — As erosion continues, the river at last reaches old age. Under the combined influence of weathering and erosion the hills have nearly disappeared, the river has reduced its drainage basin to a peneplain, the load in the river decreases, becomes finer and finer, and the stream moves slowly to the outlet. No matter what may be the age of a river, so far as its develop- ment is concerned, it may be caused to begin its work anew, if its basin be uplifted further above the sea-level. In this case the river is said to be rejuvenated or revived. 200. Changes Produced by Depression or Elevation of River-basins. — Changes in level of the surface of the river-basin result in marked changes in the character of its drainage, and con- sequently in its river-system. (1) Depression of River-basin; Dismembered Rivers. — Where a 142 PHYSICAL GEOGRAPHY. river-system suffers a depression of its basin near its mouth at the ocean, an estuary or bay is formed, and the tributary rivers dis- charge into the ocean as separate systems. This is the case with the rivers of Virginia and Maryland, that now empty into Chesapeake Bay. Such rivers are said to be dismembered. (2) Elevation of River-basins. — A number of primarily inde- pendent streams, on their way to the sea, across the newly formed surface of a coastal plain, formed by the uplift of a shallow ocean bed, may unite in a single stream. Such rivers are called engrafted rivers. It is in this way that some of the largest river-systems in the world have been formed. The Mississippi has received as engrafted streams the White, the Arkansas, the Red, and the Tennessee. Rivers. So, also, have many of the great rivers of Bra- zil been engrafted on the mighty Amazon. 201. The Transporting Power of a Eiver increases as the sixth power of its velocity. Doubling its velocity, therefore, enables it to transport particles sixty-four times heavier. The transporting power will vary with the inclination of the channel ; with an increase in the volume of its water; and with the size of the parti- cles of its load — the smaller the size of the particles, the greater the transporting power of the river. The transporting power of a river is often greatly aided by ice. s«3>@^Oo CHAPTER IV. The Work of Rivers. 202. "Work of Rivers. — Rivers are ceaselessly at work carrying from the upper to the lower courses in the channel, or to its banks, the rock waste derived from weathering; or the worn, eroded mate- rial derived from the wear of the rock. Valleys are thus formed miles in width and hundreds of feet in depth. The material trans- ported by the river consists of boulders, gravel, sand, and fine mud. When in a finely divided state this material is called detritus or silt. This material constitutes the river's load. 120..—.-. . 140 1G0 180 160 140 120 100 ^\ T ARCTIC CIRCLE "~? A HYDROGRAPHICAL MAP SHOWING THE OCEANIC BASOS, AREAS AND RIVER SYSTEMS OF THE EARTH N T B O 120 Longitude 140 East from 160 Greenwich 180 140 120 100 120 est 40 from 20 Greenwich Longitude 20 East 40 from 60 Greenwich 100 120 THE WORK OF RIVERS. 143 The amount of silt transported by rivers is almost incredible. According to the careful estimates of Humphreys and Abbot, the silt brought down every year by the Mississippi and thrown into the Mexican Gulf, if collected in one place, would cover a field one square mile in area to the depth of 268 feet. 203. Invisible Load of Rivers. — Besides the rivers' visible load of waste, silt, or detritus, there is also an invisible load consisting of dissolved mineral matter. According to Kussell, the Hudson carries daily to the sea 183 tons of dissolved mineral matter. The Mississippi discharges yearly into the Gulf of Mexico 113,000,000 tons of dissolved mineral matter. 204. Transporting Power and Deposition of Rivers Influ- enced by Changes of Level in the Earth's Surface. — Since the transporting power of a river depends on its velocity, any change in level that affects the velocity necessarily influences its transporting power. Since repeated changes of level have occurred in practi- cally all portions of the earth's surface, this influence on the trans- porting power and deposition of rivers must have been very marked. 205. Deposition of Silt. — Since the eroded mineral matter is heavier than water, it will settle in all parts of the river course. It will, however, remain for a more or less extended time in those places only where the velocity of the river is comparatively slight. These places are as follows : (1) In the channel of the river ; (2) On the banks, over the alluvial flats or flood-grounds ; (3) At the mouth ; (4) Along the coast near the mouth. 206. In the Channel. — In rivers that traverse great plains, the inclination near the mouth is slight, and the diminished velocity allows the material to accumulate in the channel, thus raising the general level of the stream. When the rivers traverse settled districts, the inhabitants are compelled to erect huge river-walls to prevent the flooding of the adjacent lands ; and, in some places, the channel has been filled to such an extent that the ordinary level of the river is higher than that of the plains along its banks. The levees or banks of the Mississippi are of this nature. On the level plain of Lombardy the surface of the Po, in some places, is higher than the tops of 144 PHYSICAL GEOGRAPHY. the neighboring houses. When floods occur in such districts, the breaking of a levee, or river-wall, is attended by much loss. 207. Alluvial Cones and Fans. — Where a river suddenly passes from a precipitous gorge on a mountain and enters a valley, the sudden and excessive change in grade causes most of its load to be deposited in the channel at the foot of the gorge. The con- ical deposit thus formed is called, from its shape, an alluvial cone or fan. Such piles are frequently several thousands of feet high, with bases of from three to five miles in radius. Alluvial fans and cones are found especially in arid regions, where the rainfall occurs only during limited seasons. 208. Rafts.— Drift timber, thrown into the stream by the undermining of the banks, is common in rivers that traverse wooded districts. Portions of such timber, becoming imbedded in shallow parts of the channel, form obstructions which prevent the passage of floating timber during subse- quent floods. The impedi- ment so formed checks the velocity of the stream, and mud deposits occur between the trees. Such accumula- tions are called rafts. The raft of the Eed River, previ- ous to its removal, was thir- teen miles in length. 209. On the Alluvial Flats or Flood-grounds. — The low flat plains on the sides of the river, which are formed by the erosion of the banks in the mid- dle and lower courses, are covered by the water, when the river overflows its banks. In the shallow water over these parts, the velocity of the water is slight, and the silt is deposited, thus forming rich alluvial plains. Fig. 70.— Alluvial Flats of the Mississippi. (Showing deserted courses and fluviatile islands and lakes.) THE WORK OF RIVERS. 145 In large rivers the flood-grounds often attain considerable size. In the Mis- sissippi at Vicksburg the width of the alluvial plain is over 60 miles. In the lower courses of a river, the velocity being slight, com- paratively small obstacles suffice to turn the waters from their course. The river-channel is, therefore, characterized by wide bends or curves, called meanders. At the bend of a river the main current is directed against one of the banks, where rapid erosion takes place, the eroded material accumulating lower down the river, in the bed of the stream, where the velocity is slight. The river is thus continually dam- ming up portions of its old channel and cutting new ones. The rapid excavation of these portions of the alluvial plain is favored by the loose materials which compose it. Sometimes the river cuts a new channel across the narrow neck of a bend, part of its waters running through the old channel and part through the new. In this way fluviatile islands are formed. One of the channels is sometimes separated from the other by a deposition of mud or sand. The water fills the old channel by soaking through the soil, and thus fluviatile lakes are formed. Numerous fluviatile lakes occur near the banks of the Lower Mississippi and the Red River. 210. Ferrel's Law Applied to River Courses. — Under the influence of the earth's rotation, a river, in the Northern Hemisphere, flowing north or south, tends to corrode its right bank more than its left, since, if flowing poleward, its tendency is to be deflected toward the east, and, if flowing toward the equator, to be deflected toward the west ; similarly, in the Southern Hemisphere, rivers tend to cor- rode their left banks more than their right. 211. River Terraces. — The river, after filling its valley with silt in the form of a broad, alluvial plain, may change its action and begin cutting away its plain. In this way, as the river successively deepens its channel, portions of the old plain are left as terraces. The changes in the river's action are due to successive changes in level, to changes in the amount of silt or waste in the stream, or to changes in the depth of the lower river course or channel. 212. At the Mouth. — Delta Formations. — When rivers empty into parts of the ocean devoid of marked currents, or into lakes, the silt or detritus is deposited very much in the shape of alluvial fans or cones. Such deposits are called deltas, from their general 10 146 PHYSICAL GEOGRAPHY. resemblance to the shape of the Greek letter Delta {A). When strong currents exist, the material is carried beyond the river mouth and deposited in bars or flats along the shores. Deltas are divided by Russell into two classes : (1) Deltas formed by streams of high grade and, consequently, great velocity. Here the load consists of mixed detritus of various sizes, which, on reaching the ocean or lake is sorted, the coarser and heavier particles falling at once to the bottom, the finer sand and silt being carried further out, while the very fine silt is carried still fur- ther away and forms a mud sheet over a fairly extended area. Three marked regions are thus produced. (2) Deltas formed by streams of low grade and, consequently, slight velocity, such as the Mississippi, the Yukon, the Mackenzie, the Nile, and the Ganges. Here the load consists of fine silt, and the three divisions of the preceding class are wanting, the fine silt settling in broad deposits with somewhat indefinite borders, forming shoals and new land. Here the river discharges through numerous branches, each branch tending to build up a delta at its mouth. In low-grade deltas the main stream discharges through a number of separate branches, or distributaries. The Delta of the Mississippi is the largest in the Western Continent. Its entire area is about 12,300 square miles, although hut two-thirds of it are perma- nently above the water, the remainder heing a sea- marsh. It begins a little below the mouth of the Eed Eiver. The stream cuts through the delta in one main channel but, near the extreme end of the delta, forms several mouths. On all sides of the main stream, numerous smaller streams force their way into the Gulf of Mexico through the soft material. Fig. 71.— Delta of the Mississippi (Dana). The Delta of the Nile, at its outlet into the Medi- terranean, occupies an area of nearly 9000 square miles, A large portion of the THE WORK OF RIVERS. 147 sediment of the river is deposited over the flood-grounds during inundations. The fertility of the land is largely dependent on these deposits. The Delta of the Ganges and the Brahmapootra, in the Bay of Bengal, is considerably larger than the Delta of the Nile. Between the Hoogly and the main branch of the Ganges, numerous streams force their way between countless islands, called the Sunderbunds, inhabited by tigers and crocodiles. The Po, the Rhone, the Rhine, and the Danube in Europe, the Tigris, the Euphrates, the Yang-tse- Kiang and Hoang-Ho in Asia, and the Senegal and the Zambezi in Africa, have extensive deltas. 213. Drowned Rivers. — The sinking or depression of coastal plains, traversed by rivers, permits the sea to enter their valleys, thus changing them into bays and estuaries, bordered by numerous islands, where the higher lands rise above the waters. Such rivers are common on the Atlantic and St. Lawrence slopes in North America. The estuaries of the Chesapeake and Delaware Bays are examples. The St. Lawrence formerly discharged at a point eastward of the present loca- tion of Nova, Scotia. The Hudson formerly discharged some 70 miles east of Long Island. The rugged coast of Maine, with its numerous islands, capes, bays, etc., is due to the partial depression of a rough surface below the sea level. Here the rough surface is partially due to hills and ridges whose origin is to be traced to glacial deposition. Golden Gate, Puget Sound and its numerous islands, and much of the coast northward to Alaska, have been formed, in a similar manner, by subsidence of river-systems, traversing a region modified by glacial action. 214. Along the Coast, near the Mouth. — Fluvio-marine Fig. 72.- -Fluvio-marine For- mations. 148 PHYSICAL GEOGRAPHY. Formations are deposits of silt and sand that form along the coast near and opposite the mouths of rivers, under the combined action of the river-current and the tides of the ocean. A bar or sand-spit is formed at some little distance from the mouth of the river, where the outflowing river-current and the inflowing tide tend to neutralize each other. An example of this formation is shown in Fig. 72, where sand- bars or spits have formed along the coasts of North Carolina off the mouths of the drowned rivers, thus separating the sounds from the ocean. The shallow waters of the sounds, being thus protected from the scouring action of the waves, are slowly filled up by the silt from the rivers, and may, in course of time, be gradually changed into swamps, and the coast-line be moved further seaward. 215. Development of a River-system. — The drainage sys- tems of any section of country undergo profound changes during the erosion of their valleys. In a newly emerged surface, such as that shown in Fig. 73, from Scott, a slope, whose escarp- Fig. 73.— First Stage (DeLaparent). Fig. 74.— Second Stage (DeLaparent). ments are indicated by the shaded lines, is drained by five streams, a, e, e, f, and g. Should one of these streams, say c, possess the advantage of somewhat more water than the others, it will more rapidly erode and deepen its channel, and thus its tributaries will acquire a greater fall ; side branches then form and tap the neigh- DRAINAGE SYSTEMS. 149 boring streams e and a, which then find, through the invading stream, a readier escape for their waters above the point of tapping. In Fig. 74, c has in this manner captured or beheaded the neighbor- ing streams a and e. Some of these streams, such as e', may be changed in direction, flowing into the capturing stream. In this case h becomes a wind-gap. The point where the stream cuts through the ridge becomes a water-gap, as at m. °>*c CHAPTER V. Drainage Systems. 216. Continental Drainage is dependent on the position of the mountain-systems and the direction of their slopes. The mountain- ridges or peaks, or the high plateaus, form the water-sheds. In some cases, from the slopes of a single peak or plateau, the water drains in different directions into distinct river-systems, and empties into different oceans. 217. North America. — The central plain of North America is drained by four large river-systems: the Mackenzie into the Arctic Ocean ; the Saskatchewan and the Nelson into Hudson Bay ; the St. Lawrence into the Gulf of St. Lawrence ; and the Mississippi into the Gulf of Mexico. The basin of the Mississippi occupies the long slopes of the Rocky Mountains and the Appalachians. The Missouri and the Ohio are the principal tributaries of the Mississippi. An uplift of the coastal plain bordering on the Mexi- can Gulf has resulted in engrafting on the Mississippi system the Red, the Tennessee, the White, and the Arkansas. Numerous streams descend the eastern slopes of the Appalachian Mountains into the Atlantic. Owing to the position of the predominant system, the streams which empty into the Pacific are comparatively small. The prin- cipal large streams are the Yukon, the Columbia, and the Colorado. 150 PHYSICAL GEOGRAPHY. There are several remarkable isolated water-sheds or drainage-centres in North America. These are — (1) In the central part of the Eocky Mountains, where the land drains in different directions into the systems of the Mississippi, the Columbia, and the Colorado Eivers. (2) In the northern part of the Eocky Mountains, where the drainage is received by the systems of the Yukon, the Mackenzie, and the Saskatchewan Eivers. 218. South America resembles North America in its drainage systems. The long, gentle slopes of the Andes, and those of the systems of Brazil and of Guiana, are occupied at their intersections by the three great river-systems of the continent : that of the Ori- noco, in the north ; that of the Amazon, near the centre ; and that of the La Plata, in the south. Nearly the entire continent is drained by these rivers and their tributaries into the basin of the Atlantic. The Pacific receives no considerable streams. Only impetuous mountain-torrents are found. The Magdalena, which drains north, corresponds to the Mackenzie ; the Ori- noco and the Amazon, which drain east, to the Nelson and the St. Lawrence ; and the La Plata, which drains south, to the Mississippi. 219. Europe forms an exception to the other continents as regards its drainage. Though some of its large rivers rise in its predominant mountain-system, yet the majority rise in the incon- siderable elevations of the Valdai Hills. The Alps are drained by four large rivers — the Rhone, the Rhine, the Danube, and the Po. These all have large deltas. The Great Low Plain of Europe is drained toward the north and west by the Petchora and Dwina into the Arctic ; by the Dana, the Niemen, the Vistula, and the Oder into the Baltic ; and by the Elbe and the Weser into the North Sea. It is drained toward the soiith and east by the Ural and the Volga into the inland basin of the Caspian ; and by the Don, the Dnieper, and the Dniester into the Sea of Azov and the Black Sea. All the peninsulas have streams traversing them. The Seine, the Loire, and the Garonne from France, and the Douro, the Tagus, and the Gaudiana from DRAINAGE SYSTEMS. 151 Spain and Portugal, empty into the Atlantic. The Ebro from Spain, and the Po from Italy, empty into the Mediterranean. 220. Asia possesses the most extensive inland drainage of all the continents. The plateaus are surrounded by lofty mountains ; the rainfall is scant over the plateaus, and the waters can find no out- let to the sea except by overflowing mountain barriers. The outer slopes, however, are drained by some of the largest rivers in the world. The Great Northern Plain drains into the Arctic, mainly through the Lena, the Yenisei, and the Obe. The Eastern Slopes drain into the Pacific through the Amoor, the Hoang-Ho, the Yang-tse-Kiang, and the Cambodia. The Southern Slopes drain into the Indian Ocean through the Irrawaddy, the Brahmapootra, the Ganges, the Indus, the Tigris, and the Euphrates. The principal drainage-centre in Asia is the Plateau of Thibet, from which de- scend the Hoang-Ho, the Yang-tse-Kiang, the Cambodia, the Irrawaddy, the Ganges, the Brahmapootra, and the Indus. 221. Africa, being low in the interior, with high mountain-walls on her borders, is characterized, like the Americas, by the union of her smaller river-systems into a few large streams, which drain nearly the entire continent. These embrace the Nile, emptying into the Mediterranean ; the Zambezi, into the Indian Ocean ; and the Orange, the Congo, the Niger, and the Senegal, into the Atlantic. 222. Australia. — The Murray, which drains the south-eastern part of the continent into the Indian Ocean, is the only considerable stream. 223. Principal Oceanic Systems. — A careful study of the river- basins of the different oceans discloses the following fact : The Atlantic and Arctic Oceans receive the waters of nearly all the large river-systems of the world. The cause of this is as follows : The predominant systems being situated nearest the deepest ocean, the long, gentle slopes descend toward the smaller, shallower oceans (the Atlantic and the Arctic), which thus receive the greatest drainage. 152 PHYSICAL GEOGRAPHY. CHAPTER VI. Lakes. 224. Lakes are bodies of water that occupy depressions in the general drainage level of the land. They form part of either the oceanic or the inland drainage systems. The waters of lakes empty- ing into the ocean are fresh; those having no connection with the ocean are salt. Lakes are but transitory features of the earth's surface. The rivers connected with lake systems tend both to fill the lake-beds by the deposit of sediment at the river inlets into the lake, and to erode or cut down their outlets, and thus discharge the lake waters. Sometimes the lakes disappear because they are filled up by sediment deposited by the rivers or blown in by the winds. As the lakes gradually fill in this way, they are changed into swamps and, finally, into level, fertile plains. More frequently, however, the river cuts down the outlet and thus destroys the lake by discharging its waters. The depressions to which lakes owe their formation are due to a variety of causes, which serve as an excellent basis for their division into different classes. (1) Lakes of New Land-areas. — In comparatively level country, such as is frequently found in land recently raised above the sea- level, the small inequalities in the land are filled with the surface water in the form of shallow ponds or lakes. Such lakes are numer- ous in the United States in Florida, along the coast of the Gulf of Mexico, and in parts of the South Atlantic coast, all of these being recently elevated land areas. Since these shallow lakes are apt to become swamps they are sometimes called marsh lakes. (2) Lakes in the Lower Course of a River, especially in the Delta District. — Here the level nature of the surface is favorable to the formation of shallow lakes and swamps. Where the deltas are formed by streams of low grade, the natural levees formed along the branching outlets, or distributaries, are broken through during floods, and their low areas left as shallow lakes. Lakes Pontchartrain and Borgne, in Louisiana, are of this type. Changes in river courses, LAKES. 153 as we have seen, are apt to form ox-bow lakes in the abandoned meanders. (3) Lagoons or Sea-shore Lakes. — The coves or bays so numerous on certain sea-coasts may be shut off from the ocean by the forma- tion of sand-bars, and thus converted into lakes. Such lakes are numerous along the Atlantic shores of the United States. (4) Glacial Lakes. — A glaciated district ; i. e., a district once oc- cupied by a glacier, is characterized by numerous lakes. Of such an origin are the lakes in the northern part of the United States and the Dominion of Canada. These lakes generally occupy long, narrow basins. They include the lakes of Wisconsin and Minne- sota, the finger lakes of New York, the Adirondack lakes, and the abundant lakes of New England. Such, too, are the many lakes of Sweden. The basins of glacial lakes have been cut or grooved in the hard rock by the movement of the glacial masses with their imbedded rocks ; or, they have been formed during the retreat of the ice by deposits laid across streams, thus damming them up and forming lake-basins of their channels ; or, the general drainage surface has been deranged by irregular deposits derived in various ways by the glacier or its streams. (5) Lake-systems in Regions of Depression and Elevation. — When river-systems traverse regions in which more or less sudden changes of level may occur, as in growing mountain districts, the river valley may be depressed in one place and elevated in another further down stream. In such cases the depressed part is occupied by a lake that discharges into the river below. Such a cause has produced many of the larger lakes in the Alps. (6) Lake-basins due to Earthquakes. — The permanent changes of level sometimes caused by earthquakes, resulting in an uneven sur- face, frequently give rise to lakes by a collection of the surface waters in the lowest depressions. The lakes of the sunken country in south-eastern Missouri are of this character. (7) L/ake-basins due to Landslides and Lava Streams. — When a landslide, or a lava stream, deposits material across a valley, the stream draining the valley fills up the depression, forming a lake which is discharged over its lowest margin. 154 PHYSICAL GEOGRAPHY. (8) Playa Lakes. — Where rivers flow through a level country in interior basins, where the rainfall is scanty, and large lakes are absent, the water, during times of flood, spreads out in a temporary lake. Silt is deposited on this area, forming an extremely level plain. Such a lake district is called a playa. Black Rock Desert in Nevada is an example. (9) Climatic Changes Indicated by Deserted Lake-basins. — Many regions, now arid, were formerly well watered. In the Sahara desert, the existence of former large rivers is proved by the numer- ous wadies or dry river valleys. Great Salt Lake in Utah was formerly a much larger lake, called in honor of the explorer of its original outlines, Lake Bonne- ville. Another such lake, Lahontan, for- merly occupied the Fig. 75,-Lakes Bonneville and Lahontan. wegtern pftrt of Nevada# Humboldt Lake now occupies a portion of its former area. Fig. 76.— Depth of Lakes. 225. Lake-systems with Oceanic Drainage. — North Amer- ica contains the most extensive lake-system in the world. This LAKES. 155 Fig. 77.— A Mountain Lake, 8000 feet high. region surrounds Hudson Bay, and drains into the Arctic through the Mackenzie ; into Hudson Bay through the Saskatchewan ; or into the Atlantic through the St. Law- rence. To it belong the Great Lakes — Supe- rior, Michigan, Huron, Erie, and Ontario — embracing a combined area of nearly 100,000 square miles — and the numerous lakes of Brit- ish America. The Great Lakes occupy one of the most remarkable series of depressions in the general land-surface of the world. Lake Superior, though some 600 feet above the level of the ocean, reaches, in its greatest depths, far below the ocean's surface, being over 400 feet below the general level of the Atlantic. Athabasca, Great Slave, and Great Bear Lakes drain into the Arctic through the Mackenzie ; Lake Winnepeg, into Hudson Bay through the Nelson ; and the Great Lakes, into the Atlantic through the St. Lawrence. Europe contains two extensive systems of fresh-water lakes. The larger region is in Low Europe, and surrounds the Baltic Sea and its branches. The smaller region is in the Alps in High Europe. Africa contains an extensive system of lakes west of the predomi- nant system. Victoria Nyanza and Albert Nyanza, which drain into the Nile; Lake Tanganyika, which drains into the Living- stone or the Congo ; and Lake Eyassa, which drains into the Zam- bezi, are the principal lakes. The remaining continents contain but few large fresh-water lakes. In South America we find Lake Maracaybo, with brackish water from its vicinity to the sea ; and in Asia, Lake Baikal. North America. — The largest inland drainage-system is in the Great Basin, containing Great Salt, Walker, Pyramid, and Owen Lakes. 156 PHYSICAL GEOGRAPHY. South America. — The largest region of inland drainage includes the plateau of Bolivia. Europe and Asia contain a vast region of inland drainage extend- ing from the Valdai Hills eastward to the Great Kinghan Mountains, embracing most of the Asiatic plateaus. The region contains Lake Elton in Russia, and the Caspian and Aral Seas. The combined area of the last two is 175,000 square miles. Africa contains Lake Tchad in the Soudan and Lake Ngami in Southern Africa. Australia contains Lakes Eyre, Torrens, and Gairdner. 226. Lake-systems "with Inland Drainage are intimately con- nected with inland river-systems. The water of such lakes is salt. 227. The Dead Sea in Syria is remarkable for the quantity of its saline ingre- dients. In every one hundred pounds of its waters there are over twenty -six pounds, or more than one-fourth, of various saline ingredients. A still Salter lake is Lake Van in Eastern Turkey. It contains 33 per cent, of saline ingredients. 228. Salt Lakes are characteristic of regions with scanty rain- fall and great evaporation. They are formed in two ways : (1) By the isolation of a part of the ocean, such as by the forma- tion of a sand-bar, or by an elevation of a part of the sea-bottom. (2) By the continued concentration of river water in lakes with no outlet. The lakes then lose fresh water only by evaporation, and, since all river water contains saline substances, the lakes must be continually increasing in saltiness. In either case the rainfall must be scanty, since, otherwise, the level of the lake would rise until the lake discharged itself over the lowest point, and the water would become fresh. Precipitation of Dissolved Saline Substances. — When a body of salty water begins to deposit its various dissolved salts, the least soluble is thrown down first; for example, in the case of salt lakes formed by the shutting off of parts of the ocean, the gypsum, or sulphate of lime, is first thrown down, and afterward common salt, which is far more soluble than gypsum. 229. Marshes, Swamps, Morasses, and Bogs. — Where the drainage is so incomplete that the water cannot readily run off, swamps, marshes, morasses, and bogs are formed. Generally, the first stage is to form a very shallow marsh-lake. Then vegetable matter LAKES. 157 begins to collect, and the swamp is converted into a marsh. When vegetable matter is kept moist it does not decay as completely as it would if exposed to dry air. Instead of merely passing off as gaseous products, it is slowly changed into a blackish slime which accumulates and forms a swamp. But in addition to this action, which is common to all vegetable matter, there are several varieties of a plant called the sphagnum, a kind of water moss which greatly aids swamp formation. This plant has long, thread-like roots which die as they sink in the water, but continue growing at their upper extremity. The dead rootlets accumulating, at last fill up the lake and convert it into a morass or bog. Sometimes only the surface of a lake will be covered with a closely entwined mass of the sphagnum plant, thus making a quagmire. Lives are often lost owing to the treacherous nature of the covering. Peat bogs are formed in a similar way by the collection of vege- tation in damp places. Peat appears to require for its best forma- tion both cold and moisture, though it is sometimes found in warm climates. By means of the above processes lake basins are converted into alluvial or lacustrine plains, traversed by the streams that formerly fed the lakes. These streams eventually cut their way through the alluvial plains, forming terraced valleys and completely removing all traces of the former lakes. 280. Some Characteristics of Fresh-water Lakes. — Fresh- water lakes have a comparatively short life. They are but tempo- rary features of the surface drainage systems. There are two causes which tend to obliterate them : (1) The filling up of their basins by sedimentation from their inflowing streams. (2) The outflowing streams tending to cut down their outlets and thus gradually drain the lake. 231. Utility of Lakes. — By offering extended basins into which the rivers, when swollen, can disgorge themselves, lakes greatly diminish the destructive effects of inundations in their outlet streams, often checking them entirely, They afford extended surfaces for evaporation, and, acting as settling reservoirs collecting the finer sediment of the rivers, their beds form fertile plains when deserted by their waters. 158 PHYSICAL GEOGRAPHY. SYLLABUS. Water is formed by the union of oxygen and hydrogen. It is a solid at and below 32° Fahr., and a liquid from 32° to 212° Fabr. It passes off as vapor at all temperatures. Large bodies of water moderate the extremes of temperature, because water takes in more heat while warming, and gives out more heat in cooling, than any other common substance. After a pound of water has been cooled to 32° Fahr., it has still 142 heat-units to lose before it can freeze. After a pound of ice has been warmed to 32° Fahr., it has still 142 heat-units to gain before it can melt. Therefore, both freezing and melting are gradual processes. The rains cleanse the surface of the earth and purify the atmosphere. Water is necessary for the existence of life. It forms the main food of both animals and plants. The atmospheric waters are drained into the ocean either by surface or by subterranean drainage. Springs are: (1) Hillside springs; (2) Fissure springs; (3) Artesian springs or wells. According to the size of their reservoirs, springs are either constant or tempo- rary. When their reservoirs are superficial, springs are cold ; when deep-seated, they are hot or thermal. Springs whose waters are moderately cold have their reservoirs near the sur- face. Their lower temperature is due to their waters being shielded from the sun. Hot or thermal springs owe their high temperature to the heat they receive from the interior of the earth. Geysers are boiling springs, which, at irregular intervals, discharge huge col- umns of water with great violence. The most extensive geyser regions are those of Iceland, New Zealand, and Wyoming. Calcareous springs contain lime ; silicious, silica ; sulphurous, sulphuretted hydrogen and metallic sulphides or sulphates ; chalybeate, iron ; brines, common salt ; acidulous, carbonic acid. Eivers are fed both by surface and subterranean drainage. The main stream with all its tributaries and branches is called the river-system. The territory drained into the river-system is called the river-basin. The ridge or elevation separating opposite slopes is called the water-shed. In the upper or mountain courses of rivers erosion occurs mainly on the bottom of the channel ; in the middle or valley courses, at the sides. Inundations are caused by excessive rain and by the melting of ice and snow. The destruction of forests, by increasing the rapidity of surface drainage, increases the violence of floods. Lakes along river-courses decrease their violence by allowing the torrents to discharge or spread out their waters. Eiver-systems are : (1) Immature; (2) Mature; (3) Old. SYLLABUS. 159 In the lower courses of rivers rich alluvial plains are formed by the erosion of the banks and the subsequent deposition of the silt during inundations. The depression of a river-basin may cause the tributary rivers of a system to discharge into the ocean as separate systems. Such rivers are said to be dismem- bered. The elevation of a river-basin may cause several independent river- systems to unite in a single system. Such rivers are said to be engrafted. Eivers are ceaselessly at work transporting boulders, gravel, sand, mud, and silt from their upper to their lower courses. The boulders, gravel, sand, and fine mud transported by a river from its upper to its lower courses constitute its load. The dissolved mineral matter constitutes its i7ivisible load. When a river suddenly passes from a steep mountain gorge and enters a valley, it deposits its load at the foot of the gorge in an alluvial fan or cone. The earth's rotation causes rivers in the Northern Hemisphere, if flowing north or south, to tend to corrode their right banks more than their left, and in the Southern Hemisphere, to tend to corrode the left banks more than the right. Silt may accumulate : (1) In the channel; (2) Along the banks; (3) At the mouth; and (4) Along the coast near the mouth. When rivers empty into lakes, or into the ocean, where tides and currents are absent or feeble, the eroded material, or silt, accumulates at the mouths of the rivers in masses termed deltas. The following rivers have extensive delta-formations, viz., the Mississippi, Ehine, Rhone, Po, Danube, Hoang-Ho, Yang-tse-Kiang, Ganges, Brahmapootra, Indus, Nile, Tigris, Euphrates, and the Zambesi. The central plain of North America is drained north into the Arctic Ocean through the Mackenzie ; east, into the Atlantic through the Nelson and the St. Lawrence ; and south, into the Gulf of Mexico through the Mississippi. Numerous streams drain the eastern slopes of the Appalachian Mountains into the Atlantic. The central plain of South America is drained north into the Caribbean Sea through the Magdalena; east, into the Atlantic through the Orinoco and the Amazon ; and south, into the Atlantic through the Eio de la Plata. The rivers draining the predominant mountain-system of Europe are the Ehone, Ehine, Danube, and Po; those draining the great low plain rise either in the Valdai Hills or on the northern slopes of the predominant mountain-system. Asia possesses the most extended system of inland drainage of the continents. Africa is drained by the union of her smaller rivers into a few large streams. In Australia, the only large stream is the Murray, which drains the south- eastern part of the continent. The Atlantic and the Arctic Oceans drain about three-fourths of the conti- nental waters. Lakes are: (1) Lakes of new land-areas ; (2) Delta lakes; (3) Lagoon or sea- shore lakes ; (4) Glacial lakes ; (5) Lakes of regions of depression or elevation ; (6) Earthquake lakes ; (7) Lakes due to land-slides and lava-streams ; (8) Playa lakes. The largest systems of fresh-water lakes occur in North America and Africa. 160 PHYSICAL GEOGRAPHY. The Great Lakes of North America occupy remarkable depressions in the con- tinent. Salt lakes are formed : (1) By the isolation of a part of the ocean ; (2) By the continued concentration of river-water in lakes without an outlet. Marshes, swamps, morasses, and bogs are due to incomplete drainage. REVIEW QUESTIONS, What effect has the temperature of the maximum density of water on the freezing of large bodies of fresh water? What is the composition of water ? Enumerate some of the physical properties which enable water to play so important a part in the economy of the earth. How do large bodies of water moderate the extremes of heat and cold? Why are freezing and melting necessarily gradual processes? State some of the useful results produced by rain. Explain the cause of deserts. Define subterranean drainage. Surface drainage. Explain the origin of fissure springs ; of hillside springs ; of artesian springs. What is the probable cause of the high temperature of hot springs ? How may the probable depth of the reservoir of an artesian spring be ascer- tained from the temperature of its waters ? What are geysers? Explain the cause of their eruption. What is the origin of the tube and basin of a geyser ? Name the three largest geyser regions of the world. What is travertine ? How is it formed ? How are the precipices of waterfalls caused? In what courses of a river are they most common? Name the highest waterfall in the world. Name the grandest. Distinguish between an estuary and a delta. How does the destruction of forests increase the severity of inundations? Upon what does the quantity of water in a river depend? What do you understand by an immature river-system ? By a matured river- system ? By an old river-system ? State the characteristics of each. Distinguish between the visible and the invisible load of a river. In what different portions of a stream may the silt or detritus be deposited ? Into what two classes are deltas divided by Russell ? Define dismembered river. Engrafted river. Drowned river. Captured or beheaded river ; wind-gap ; water-gap. Explain the formation of an alluvial fan or cone. What are rafts ? How are they caused ? Explain the formation of fluviatile islands and lakes. SYLLABUS. 161 State Ferrel's law as applied to river courses. Name some of the most extensive delta-formations in North America. In Europe. In Asia. In Africa. What is the probable origin of the swamp-lands of the Atlantic seaboard ? In what respects do the drainage systems of North America resemble those of South America? How were the depressions now occupied by lakes formed ? Explain the causes of the saltness of some inland waters. Name the principal systems of inland drainage of the world. Describe the formation of marshes, swamps, morasses, bogs, and peat bogs. Give a short life history of a fresh- water lake. ^O^CHO MAP QUESTIONS. Which two oceans drain the largest proportion of the areas of the continents? Name the important rivers which drain into the Atlantic from North America. From South America. From Europe. From Africa. Name the important rivers which drain into the Pacific from North America. From Asia. Name the important rivers which drain into the Indian Ocean from Africa. From Asia. From Australia. Name the important systems of inland drainage in North America. In South America. In Europe and Asia. In Africa. In Australia. Name an important steppe lake and river in each of the continents. Name the large rivers which drain the predominant mountain-system of Asia. Of Europe. Of Africa. Of North America. Of South America. Of Australia. Describe the fresh-water lake-region of North America. Of South America. Of Europe. Of Africa. Name the Atlantic rivers which have large deltas. The Pacific rivers. The Indian rivers. 11 162 PHYSICAL GEOGRAPHY. SECTION II. OCEANIC WATERS. CHAPTER I. The Ocean. 232. Composition. — The water of the ocean contains a number of various saline ingredients, which give it a bitter taste and render it heavier than fresh water in the proportion of 1.027 to 1. Every hundred pounds of ocean-water contains about three and one-third pounds of various saline ingredients. Chloride of sodium, or common salt, chloride and sulphate of magnesium, sulphate and carbonate of lime, chloride of potassium, and bromide of magne- sium, are the principal saline ingredients. 233. Origin of the Saltness of the Ocean. — The rivers are constantly dis- solving from their channels large quantities of mineral matters, and pouring them into the ocean. In this way immense quantities of mineral ingredients have been dissolved out from the crust, and thrown into the ocean. The ocean is saltest at about the latitude of the tropics, where the evaporation exceeds the rainfall ; where the rainfull exceeds the evaporation, the water is slightly fresher than at the equator. In inland seas, like the Mediterranean or the Eed Sea, which, though con- nected with the ocean, yet lose much more of their waters by evaporation than by outflow, the proportion of salt is slightly greater than in the ocean. In such cases a current generally flows into the sea from the ocean. In colder latitudes, inland seas, like the Baltic, receiving the waters of large rivers, contain rather less salt than the open sea, and as a rule, a current flows from them into the ocean. 234. Color. — Though transparent and colorless in small quanti- ties, yet in large masses the color of sea-water is a deep blue. The same is true of fresh water. Over limited portions of the ocean the waters are sometimes of a reddish or a greenish hue, from the pres- ence of numerous minute organisms. Sometimes a pale light or 'phosphorescence, visible only at night and due to the presence of animalculse, appears where the air comes THE OCEAN. 163 into contact with the water, as in the wake of a vessel, or on the crests of the waves. 235. Temperature. — The temperature of the surface water varies from about 80° F. in the tropics, to nearly 27° F. (its freezing-point), in the polar regions. Between these extremes, the temperature varies irregularly with the latitude, owing to the ocean currents. Iu the deep ocean, the temperature of the water on the floor of the ocean is very nearly constant ; viz., near the temperature of the maximum density of ocean-water, which is nearly the same as its freezing-point ; for, ocean-water, unlike fresh water, continues to contract and grow denser as it is cooled, until near its freezing- point. Unlike the land, the ocean is not subject to sudden changes in temperature, either during the day or year. The daily range is not much greater than about 3° F., and the annual range is not much greater than 15° F. The nearly constant low temperature of the deep ocean water in all latitudes is readily understood. As the water in the polar region reaches the temperature of its greatest density, it sinks to the bottom and spreads over the floor of the ocean in ali latitudes, so that, except where stirred by ocean currents, the entire bottom of the ocean is covered by a layer of dense, heavy water, whose tempera- ture is nearly constant. The upper limit of this line of invariable temperature varies with the latitude. Near the equator, where the waters are heated to great depths, it is found at about 10,000 feet below the surface. Toward the poles, it comes nearer the surface, reaching it at about Lat. 60°, from which point it again sinks, being found at Lat. 70° at about 4500 feet below the surface. The salts dissolved in ocean-water lower its freezing-point. Ordinary ocean- water freezes at about 27° F. Where the ocean is less salt its freezing-point is higher ; when it is more salt its freezing-point is lower. 236. Ocean Ice. — Ice formed from ocean-water is comparatively fresh, nearly all the salt being separated as the ice forms. The salt thus thrown out is dissolved by the water below the ice, which, be- coming salter, has its freezing-point lowered. The water below the ice may, therefore, have a temperature lower than that at which the surface water freezes, and yet remain liquid. Ice formed from ocean-water expands a little on freezing and, therefore, floats. The ice which forms on the ocean in the polar 164 PHYSICAL GEOGRAPHY. regions collects in what are called, when in extensive unbroken areas, ice-fields or ice-sheets. This ice may form to the depth of from 3 to 7 '■ Fig. 78. — Ice-pack (John M. Justice). feet in one winter. Detached masses floating about are called ice-floes. Ice-floes, when drifted or blown together, or against the shore, become piled up in irregular masses called ice-packs. The ice that forms along the shore in a narrow, nearly level shelf, is called the ice-foot. 237. Shape of the Ocean's Bed. — The oceanic waters occupy a vast sunken area whose mean depth is nearly two and a half miles. The true bed of the ocean is assumed as beginning at the 100-fathom line, where the water is 600 feet deep. Anything shallower than this is taken as the submerged continental border, or, as it is gen- erally called, the continental shelf. A continental shelf, 75 or 100 miles in breadth, extends along the eastern coast of North America, from Newfoundland to Florida, and around the shore of the Mexican Gulf and part of Central America. The British Isles and the Asiatic and Australasian Island chains rest on continental shelves that lie on the border of Asia and Aus- THE OCEAN. 165 tralia. The continental shelf on the Pacific borders is compara- tively narrow. In cold climates these continental shelves form the world's best fishing-grounds. Beyond the limits of the continental shelves, that is, on the true continental borders, the waters suddenly deepen, and the true oceanic basin begins. The floor of the ocean is by no means uniformly level or flat, although it is far from being so diversified as the surface of the land. Careful soundings show that it has vast plateaus and plains, and that it extends for great distances in gentle undulations and slopes called floors. The reason for this is evident. Erosion, so common on the land, is practically absent in the deep ocean, which, constantly receiving sediment from the rivers, or from the remains of countless animals, is gradually having its smaller irregularities filled up. It has, however, submerged mountain-ranges, the summits of which form island chains. Moreover, there are many places where there are deep and abrupt slopes ; these are called cauldrons, when the depressions are wide and deep with sloping sides ; furrows, when they are nar- row with deep sides ; and troughs, when they are narrow and shallow. Any relatively wide elevation is called a bank, and any horizontal bank is called a shelf. A shoal is any place where the water is less than 5 fathoms (30 feet) in depth. The western half of the Pacific and Atlantic, and the eastern half of the Indian Ocean, contain nearly all the very deep depressions. A general idea of the varying depths of the ocean may be obtained from a study of the map of the Oceanic Areas. 238. Depth of the Ocean. — The depth of the ocean is deter- mined by means of the sounding-line. The sounding-line for deep- sea measurements consists of a thin steel wire, attached to a sounding instrument, weighted by a heavy iron ball B, arranged so as to detach the ball on its striking the bottom. The ball surrounds a brass tube T, called the water-bottle, which is open while descending, but is closed automatically as the line is hauled in. The arrangement of the parts is shown in Fig. 79. The water-bottle is employed for obtaining specimens of deep-sea water. Recent soundings give the greatest depth of the Atlantic as sit- 11 166 PHYSICAL GEOGRAPHY. T uated about 100 miles north of Porto Rico, where the water is 27,366 feet deep. The greatest depth in the Pacific is 31,614 feet; it is found near Guam Island, north-east of the Philippines. In order to examine the character of the ocean beds,, dredges, drawn by wire ropes, are employed. To these ropes, nets are sometimes at- tached, in order to secure chance speci- mens of deep-sea life. 239. The Oceanic Areas.— The ocean forms one continuous body of water, but for purposes of description and study it is usually divided into five smaller bodies: the Pacific, At- lantic, Indian, Arctic, and Antarctic Oceans. The last two are separated from the preceding by the polar cir- cles; the others are separated mainly by the continents. As the continents do not extend to the Antarctic Circle, the meridians of Cape Horn, Cape of Good Hope, and South Cape in Tas- mania, are taken as the ocean boun- daries south of these points, as shown in the map of the oceanic areas and river-systems. The following table gives the relative size of the oceanic areas : The Pacific occupies about \ the entire water-area. " Atlantic " " i " Indian " "I " " " Antarctic " " ^7 " " " Arctic " " & 240. Articulation of Land and Water. — The indentations of the oceans, or the lines of junction between the water and the land, may be arranged under four heads : (1) Inland or Mediterranean Seas, or those surrounded by a nearly continuous or unbroken land-border ; as the Gulf of Mexico, Hudson Bay, the Baltic, and the Mediterranean, in the "ZT Fig. 79.— Deep-sea Sounding Instruments. THE OCEAN. 167 Atlantic ; the Red Sea and the Persian Gulf, in the Indian ; and the Gulf of California, in the Pacific. (2) Border Seas, or those isolated from the rest of the ocean by peninsulas and island chains ; as the Caribbean Sea, the Gulf of St. Lawrence, and the North Sea in the Atlantic ; and Bering Sea, the Sea of Okhotsk, the Sea of Japan, and the North and South China Seas, in the Pacific. (3) Gulfs and Bays, or broad expansions of water extending but a short distance into the land ; as the Gulf of Guinea and the Bay of Biscay, in the Atlantic ; and the Bay of Bengal and the Arabian Sea, in the Indian. (4) Fiords, or deep inlets, with high rocky headlands, extending far into the land. The Atlantic Ocean is characterized by inland seas ; the Pacific, by border seas ; the Indian, by gulfs and bays. 241. Ooze Deposits. — Foraminiferal Land.— The reef-forming coral polyps are not the only animalculse, the accumulation of whose bodies after death, adds to the land- masses of the earth. Deep-sea soundings show that over extended areas the floor of the ocean is evenly covered with a creamy layer of mud or ooze, which, like the deposits of the coral animalculse, is composed principally of carbonate of lime. This ooze consists almost entirely of microscopic skeletons of a group of animalculse known as Foramin- ifera, from the great number of per- forations or openings in their hard parts. These animalculse are so small that 1,000,000 are equal in bulk to only one cubic inch. As shown in Fig. 80, they are highly magnified. They appear to live in the layers of water near the surface, and after death, to fall gradually to the bottom of the sea. Soundings show the presence of their remains over very extended areas. Fig. 80.— Foraminifera. 168 PHYSICAL GEOGRAPHY. Many of the very deep parts of the ocean's bed are covered, not with fora- miuiferal deposits, hut with a layer of red mud composed of finely-divided clay. Its origin is probably as follows : In very deep parts of the oceau, before the fora- miniferal deposits reach the bottom their lime is dissolved, and the undissolved parts form the deposits of fine red mud. cj^c CHAPTER II. Oceanic Movements. 242. The Oceanic Movements may be arranged under three heads : waves, tides, and currents. "Waves are swinging motions of the water, caused by the friction of the wind on the surface. Their height and velocity depend on the force of the ^ = - wind, and the depth of the jHBlrM?f|j|jlfflgil : M^^Sttm basin in which they occur. The stronger the wind, and the deeper the ocean, the higher the waves and the greater their velocity ; great waves formed in the deep ocean by powerful winds are frequently called seas. Height of Waves. — Scoresby measured waves in the North At- lantic 43 feet above the level of the trough. Waves have been re- ported in the South Atlantic, off the Cape of Good Hope, between 50 and 60 feet high. Navigators have occasionally reported higher waves, but the accuracy of their measure- ments may perhaps be doubted. In the open sea, with only a moderate wind, the height of ordinary waves is about 6 feet. The distance between two successive wave-crests varies from 10 to 20 times their height. Waves 4 feet high have their successive crests about 40 feet apart ; those 33 feet high, about 500 feet apart. Fig. 81.— Ocean Waves. 120 Longitude 140 East from 160 Greenwich 180 140 120 100 80 Longi T I C REFERENCES ~1 WARM CURRENTS ~~] COLD CURRENTS =~=5 SEA WEED c '# West 40 from 20 Greenwich Longitude 20 East 40 from 60 Greenwich OCEANIC MOVEMENTS. 169 243. No Progressive Motion of Water in Waves. — In wave motion, the water seems to be moving in the direction in which the wave is advancing, but this is only apparent ; light objects, floating on the water, rise and fall, but do not move forward with the wave. When the wind is high, the top of the wave is pushed forward and breaks, forming white caps. In shallow water, however, the water really advances. The forward motion of the wave is then retarded, so that the following waves reach it, thus increasing its height. The motion at the bottom is lessened, and the top curls over and breaks, producing what are called breakers. On gently sloping shores, the water which runs down the beach, after it has been thrown upon it by the breakers, forms, at a little distance from the shore, the dreaded undertow of our bathing-resorts. 244. Force of the Waves. — When high, and moving in the direction of the wind, the waves dash against any obstacle, such as a line of coast, with great force, and may thus cut it away and change the coast-line. This action occurs only on exposed, shelving coasts. The wave-motion is, in general, very feeble at 40 feet below the surface. The eroding action of the ocean waves is, therefore, far inferior to that of the continental waters. 245. Earthquake Waves. — Earthquakes, occurring on the floor of the ocean, set immense masses of water in motion, and produce low waves that move with a great velocity. On reaching shallow water the height of the wave increases often to 50 or 60 feet, so that the waves may cause great damage on low coasts. Such waves are often improperly called tidal waves. An earthquake in Southern Peru, in 1868, sent such a wave westward across the Pacific to New Zealand and Australia, north-westward to the Sandwich Islands, and north to the coast of Oregon. 246. Tides are periodical risings and fallings of the water that succeed each other with great regularity about every six hours. Unlike waves, whose motion is confined mainly to the surface, tides affect the waters of the ocean from the top to the bottom. The rising of the water is called flood tide; the falling, ebb tide. When the waters reach their highest and lowest points, they are 170 PHYSICAL GEOGRAPHY. stationary for a few minutes. These points are called respectively high water and low water. High water occurs at any place twice in every 24 hours and 51 minutes. 247. Theory of the Tides. — If the earth were uniformly cov- ered with a layer of water, as shown in Fig. 82, where the depth of the water is purposely exaggerated for clearness, the passage of the moon over any place, such as at A, would cause the water to lose its globular form, and become ,--- -^ raised or bulged at A and B, and flattened or lowered at D and E. In other words, the water would become deeper at A and B, at ! the parts of the earth nearest and furthest from the moon, and shallower in all places at right angles to these parts, such, for exam- Fig 82 — Lunar Tide P* e ' as at ® ano ' ^' ^ r > * u °^ ner words, the passage of the moon over any part of the waters would cause high water on the parts of the earth directly under the moon, and low water at all parts 90° from the parts of high water. This deepening and shallowing of the water is caused by the attraction of the moon. As the moon passes over J., the water is drawn toward it from all sides, thus deepening the water at A, and rendering it shallower at D and E. The cause of the deepening of the water at B, on the side furthest from the moon, is as follows : The solid earth being, as a whole, nearer the moon than the water at B, but further from it than the water at A, must take up a position, under the attraction of the moon, nearly midway between A and B, thus leaving a protuberance at B, nearly equal to that at A. High tides then, produced by the moon, occur at those points of the earth's surface which are cut by a straight line passing through the centres of the earth and the moon. Low tides are formed in all places 90° from these points. Had the earth no rotation, the tidal waves formed by the moon's attraction would slowly follow the moon in its motion around the earth. But, since by the earth's rotation, different parts of the sur- OCEANIC MOVEMENTS. 171 face are rapidly brought under the moon, its tidal waves move rap- idly from one part of the oceau to another. Had the moon no motion around the earth, there would be two high tides and two low tides in, exactly, every 24 hours. While, however, the earth is making one complete rotation, the moon, in its motion around the earth, has so far changed its position that the earth rotates, on the average, for some 51 minutes longer, before the same point again comes directly under the moon ; hence, 24 hours and 51 minutes are necessary to complete the tides. Since the uniformity of the water surface is broken by the ele- vations of the land, the progress of the tidal wave is greatly affected by the size, shape, and depth of the oceanic basins, and the position of the continents. Owing to the obstructions offered by the conti- nents, and by inequalities in the bed of the ocean, a considerable retardation of the tidal wave is effected, so that high tide may not occur at a place until long after the moon has passed over it. 248. Solar Tides. — The sun also produces a system of tidal waves, but owing to its greater distance from the earth, the tides thus produced are much smaller than those of the moon. The tide pro- duced by the moon is about 2i times greater than that produced by the sun. The tidal wave moves, in general, from east to west, or in a direction opposite to that of the rotation of the earth. The motion of so large a mass of water, thus opposed to the earth's rotation, must gradually diminish the axial velocity, and, eventually, entirely stop the rotation of the earth ; in this way an increase in the length of day and night should be produced ; so far, however, no increase has been de- tected, although astronomical observations extend backward for long periods. The increased axial velocity, produced by the contraction of the globe, probably balances the retarding influence of the tides. In the deep ocean, and near the mouths of rivers, the duration of the flood and ebb are about equal ; but in most rivers, at some distance from the mouth, the ebb is longer than the flood. The cause is to be found in the fact that the outflowing river current meets and temporarily neutralizes the inflowing flood tide, thus diminishing its duration, and afterward, adding its motion to the ebb, makes the difference between the two still greater. The tidal wave often ascends a stream to a much greater elevation above the level of its mouth than the height of the tide at the river's mouth. In large rivers, like the Amazon, the tidal wave, as it advances up the river, rises nearly 100 feet above the sea-leVel. 172 PHYSICAL GEOGRAPHY. When the tidal wave strikes the shore of a continent it is reflected, and moves back again into the ocean, meeting and modifying the incoming tidal wave. If the two waves move simultaneously in the same direction the incoming tide will be increased ; if they move in opposite directions, the tide will be decreased. It is for this reason that the tides are, at times, unusually high in some places, and almost entirely absent in others. Then, again, some places, such as the eastern coasts of Scotland and England, receive their tides by two separate waves that reach them by different routes from the north and the south ; in some places a tide of double height occurs ; other places are almost without tides ; while in still other places, four separate and distinct periods of high water occur in every twenty-four hours and fifty-one minutes. Some of the proofs of the connection between the tides and the attraction of the moon and sun are as follows : (1) The interval between corresponding high tides at any place is the same as the interval between two successive passages of the moon over that place : 24 hours, 51 minutes. (2) The tides are higher when the moon is nearer the earth. (3) The tides are higher when the sun and moon are simultane- ously acting to cause high tides in the same places. Phases of the Moon. — An New inspection of Fig. 83, in which Moon.,, . ° ' the sun is assumed to be on the Quarter. right-hand side of the prolonga- tion of the line, 6 a, will show that during new and full moon, the earth, moon, and sun are all in the same straight line, but that, during the first and last quarters, they are at right angles. The portions of the earth and moon turned toward the sun are illu- mined, the shaded portions are in the darkness. To an observer on the earth, the moon, at a, appears new, since the dark part is turned toward him ; at 6, however, it must appear full, since the illumined portions are toward him. At c and d, the positions of the quarters, only one half of the illumined half, or one quarter, is seen. 249. Spring and Neap Tides. — When the sun and moon act simultaneously, on the same hemisphere, as shown in Fig. 84, the tidal wave is higher than usual. The flood tides are then highest Quarter. \^ Fig. 83. — Cause of the Phases of the Moon. OCEANIC MOVEMENTS. 173 and the ebb tides lowest. These are called spring tides. They occur twice during every revolution of the moon — once at full and once at new moon. The highest spring tides occur a short time before the March and the September equinoxes, when the sun is over the equator. When, however, the sun and moon are 90° apart, or in quadra- ture, each produces a tide on the portion of the earth directly under it, diminishing that produced by the other body. High tide, then, occurs under the moon, while the high tide caused by the sun becomes, by comparison, a low tide. Such tides are called neap Neap Tides, flood and ebb moderate. Spring Tides, flood and ebb excessive. Fig. 84.— Position of the Earth, Moon, and Sun during Spring and Neap Tides. tides. During their prevalence, the flood is not very high, nor the ebb very low. They occur twice during each revolution of the moon, but are lowest about the time of the June and December solstices. The average relative height of the spring tide to that of the neap tide is about as 7 to 4. 250. Birthplace of the Tidal "Wave. — Although a tidal wave is formed in all parts of the ocean where the moon is overhead, yet the "Cradle of the Tides" may properly be located in the great southern area of the Pacific Ocean. Here the combined attraction of the sun and moon originate a wave, which would travel around 174 PHYSICAL GEOGRAPHY. the earth due east and west, with its crests north and south ; but, meeting the channels of the oceans, it is forced up them toward the north. Its progress is accelerated in the deep basins, and retarded in the shallow ones. On striking the coasts of the continents, deflected or secondary waves move off in different directions, thus producing great complexity in the form of the parent wave. 251. Co-tidal Lines. — The progress of the tidal wave, in each of the oceans, is best understood by tracing on a map lines connect- ing all places which receive the tidal wave at the same time. These are called co-tidal lines. The distance between two consecutive lines represents the time, in hours, required for the progress of the tidal wave. Where the wave travels rapidly, the co-tidal lines are far apart ; when its progress is retarded, they are crowded together. Since it is possible to take the height of the tide only on the coasts of islands and continents, the tracks of the co-tidal lines in the deep ocean must be, to a considerable extent, conjectural. 252. The Pacific Ocean. — Twice every day a tidal wave starts in the south-eastern part of the Pacific Ocean, west of South Amer- ica, somewhere between the two heavy lines marked xn on the chart. It advances rapidly toward the north-west in the deep valley of this ocean, reaching Kamtchatka in about 6 hours. Toward the west, its progress is retarded by the shallower water and by the numerous islands, so that it only reaches New Zealand in about 6 hours and enters the Indian Ocean in about 12 hours. 253. The Indian Ocean. — The 12-hour-old tidal wave from the Pacific, meets and moves along with a wave started in this ocean by the moon, and advances in the direction indicated by the co-tidal lines entering the Atlantic Ocean about 12 hours afterward. 254. The Atlantic Ocean. — The tidal wave from the Indian joins two other waves, one formed by the moon in the Atlantic, and the other a deflected wave that has backed into the Atlantic from the Pacific. The tidal wave thus formed advances rapidly up the deep valley of the Atlantic, reaching Newfoundland 12 hours after- ward, or 36 hours after it started in the Pacific. It then advances rather less rapidly toward the north-east, reaching the Loffoden OCEANIC MOVEMENTS. 175 176 PHYSICAL GEOGRAPHY. Islands 12 hours afterward, or 48 hours after leaving its starting- place in the Pacific. 255. Tides in Inland Seas and Lakes are very small and, consequently, difiicult to detect. In the Mediterranean Sea the tides on the coasts average about 18 inches. Observations on Lake Michigan at Chicago show that the tides have a range of 1.5 inches for the neap tide, and 3 inches for the spring tide. Movements in lake waters, due to the prolonged action of strong winds, are far more pronounced. A gale from the north has caused the waters of Lake Michigan to rise to the height of seven feet at Chicago. The waters of Lake Erie have been caused to fall between seven and nine feet below the normal level by a storm from the west, unusually high water being caused at the eastern end of the lake. 256. Height of Tidal "Wave.— The tides are lowest in mid- ocean, where they range from two to three feet. Off the coasts of the continents, especially when forced up narrow, shelving bays, deep gulfs, or broad river mouths, they attain great heights. The cause of these unusual heights is evident. When the progress of the tidal wave is retarded, either by the contraction of the channel or by other causes, the following part of the wave overtakes the advanced part, and thus, what the wave loses in speed it gains in height, from the heaping up of the advancing waters. Where the co-tidal lines, therefore, are crowded together on the chart, high tides are indicated ; for example, the Arabian Sea and Bay of Bengal, the North and South China Seas, the eastern coasts of Patagonia, the Bay of Fundy, the English Channel, and the Irish Sea have very high tides. Near the heads of the Persian Gulf and the China Sea, the tides sometimes rise about 36 feet. At the mouth of the Severn, the spring tides rise from 45 to 48 feet ; on the southern coast of the English Channel, 50 feet ; and in the Bay of Fundy, near the head, the spring tides, aided by favoring winds, sometimes reach 70 feet, and, occasionally, even 100 feet. 257. Other Tidal Phenomena. The Bore or Eager. — On entering the estuary of a river, the volume of whose discharge is considerable, the onward progress of the tidal wave is checked ; but, piling up its waters, the incoming tide at last overcomes the resistance of the stream, and advances rapidly in several huge waves. The OCEAN CURRENTS. Ill tides of the Hoogly, the Elbe, the Weser, the Yang-tse-Kiang, and the Amazon are examples. In the latter river the wave is said to rise from 30 to 50 feet. Races and Whirlpools. — When considerable differences of level are caused by the tides, in parts of the ocean separated by narrow channels, the waters, in their effort to regain their equilibrium, move with great velocity, producing what are called races. At times several races meet each other obliquely, thus produc- ing whirlpools. Near the Channel Islands, and off the northern coasts of Scot- land, races are numerous. The Maelstrom, off the coasts of Norway, is an instance of a whirlpool, though the motion of the waters is not exactly a whirl- ing one. The main phenomenon is a rapid motion of the waters, alternately backward and forward, caused by the conflict of tidal currents off the Loffoden Islands. CHAPTER III. Ocean Currents. 258. Constant Ocean Currents. — Besides tidal currents, the waters of the ocean are disturbed to great depths by currents moving with considerable regularity to and from the equatorial and polar regions, thus producing a constant interchange of their waters. These movements are called constant currents; their motion, unlike that of waves, is a real, onward movement of the water. It would appear that currents sweep the floor of the ocean at all depths, for everywhere the composition of the water is practically the same. Constant ocean currents resemble rivers, but are immensely broader and deeper. As a rule, their temperature differs considerably from that of the waters through which they flow. They are not confined to the surface, but exist, also, at great depths. They are called under- or counter-currents, when they flow in a direction opposite to that of the surface currents. 259. Origin of Constant Ocean Currents. — The principal cause of constant ocean currents is the difference in the density of the waters of the equatorial and polar regions arising from their difference of temperature. As the waters of the polar regions lose their heat, they grow dense, and, sinking below the warmer, lighter waters, spread, as an 12 178 PHYSICAL GEOGRAPHY. under-current, over the entire floor of the ocean until they reach the equatorial regions, as is shown by the fact that a layer of water is found in all parts of the deep ocean at nearly the same temperature ; viz., near the temperature of its greatest density. At the same time, the warmer, lighter water of the equatorial regions, flows, in all longitudes, as surface currents toward the polar regions. In this manner, a constant interchange is effected between the equatorial and the polar waters, which, for the greater part, takes place along the bottom from the poles to the equatorial regions, and along the surface from the equatorial to the polar regions. Where the ocean is deep enough, the colder, denser water will be underneath and the warmer, lighter water on the surface. But in shallow water the cold currents come to the surface, displacing the warmer currents and deflecting them to deeper parts of the ocean. Were the earth uniformly covered with water and had no rotation, there would thus be established north and south currents in all longi- tudes. Were the earth uniformly covered with water and rotating, the waters flowing toward the equator would be deflected, both from the north and south polar regions, to the westward, and, unit- ing at the equator, would form a westward equatorial current sweeping around the earth. But this westward flowing water, would, in all longitudes, be constantly turning north and south as a surface current, finally moving eastward in the polar regions and joining the deep-seated polar currents to the equator. If now the continental masses be considered, it will be seen that they oppose the westward and eastward movements of the equatorial and polar currents. On reaching the shores of the continents, the equatorial currents are deflected in north and south branches. Their rate of flow is increased, on reaching the border of the ocean, on account of the smaller depth. By reason of the rotation of the earth the currents that are moving toward the poles are deflected toward the east, and those that are moving toward the equator are deflected toward the west. Besides the main interchange between the equa- torial and the polar regions, there is a smaller interchange effected between the poles and the eastward running waters. OCEAN CURRENTS. 179 Ocean- currents that are practically limited to the surface are called drifts; deeper currents are called streams. The action of the winds tends to move the surface currents in the direction in "which the winds are blowing. This action is by some authorities regarded as the principal cause of ocean-currents. The winds are undoubtedly a partial cause, but cannot be regarded as the principal cause. Both ocean-currents and winds have their origin in the same causes; viz., differences of density caused by differ- ences of temperature. Apart from the greater deflecting effect produced on the ocean-currents by the interposition of the continents, and the greater disturbing eflfect produced on the aerial currents by the local differences of temperature of the land- and water-areas, the general directions of the ocean-currents and winds are the same. The winds, therefore, cannot but affect the ocean-currents, and are an important, indeed, perhaps, the chief factor, in producing the drift cur- rents. This is seen by the direction of the drifts being sometimes reversed by the high winds of storms, as well as by the fact, that in the Indian Ocean, the direction of the drifts follows the direction of the monsoons. The winds, how- ever, cannot produce the deep-seated polar currents, or ensure that thorough mixing of the water at all depths, which is shown to exist by the fact that ocean water at all depths has practically the same composition, and that no matter from how great a depth the water has been taken, it is found to contain atmo- spheric air in solution. 260. General Features of Constant Currents. — The following motions of the surface currents are com- mon to the three central oceans : (1) A movement of the equatorial waters a, a, Fig. 86, from east to west ; (2) Their deflection into northern and southern branches (b and c), on reaching the western borders of the ocean ; (3) A movement of the waters be- yond the equator from west to east (d, e) ; (4) A separation of these latter currents into two branches (/, g and ^, *), one continuing toward the poles, and the other toward the equator, where they join with the equatorial currents, thus com- pleting a circuit in the shape of a vast ellipse ; -*>©4c REVIEW QUESTIONS, Of what use is the atmosphere in the economy of the earth ? Define meteorology. Describe the construction of a barometer ; of a thermometer. Define hypsometry ; thermograph ; barograph ; climate. Why are the vertical rays of the sun warmer than the oblique rays? What is the characteristic climate of the tropics? Of the temperate regions? Of the polar regions? In what different ways does the atmosphere receive its heat from the sun ? Define heat equator. In what parts of the Eastern Hemisphere is the greatest mean annual tempera- ture found ? In what parts of the Western Hemisphere ? What influence is produced on the climate of high latitudes by a preponder- ance of moderately elevated land masses? On the climate of the tropics? Why should the temperature of the atmosphere decrease with the altitude? Name all the modifiers of climate which prevent the mathematical climatic zones from coinciding with the physical climatic zones. 220 PHYSICAL GEOGRAPHY. What is the origin of winds ? Name the currents of which the atmospheric circulation principally consists. Explain Ferrel's law as to the action of the rotation of the earth on the direc- tion of the equatorial and polar currents. Name the principal wind zones of the earth. Explain, in full, the origin of land and sea breezes. In what respect do mon- soons resemble land and sea breezes? Name the principal monsoon regions of the earth. Define tropical cyclones. Where do they originate ? In what direction does the wind rotate in the Northern Hemisphere ? In the Southern Hemisphere '? In what direction does the storm progress in each hemisphere ? Explain the cause of the rotation of the wind. What are hurricanes? Typhoons? Define extra-tropical cyclones. In what respects do extra-tropical and tropical cyclones resemble each other? In what respects do they differ? Name some hot desert winds caused by the passage of extra-tropical cyclones. Name some cold winds so caused. Define cold wave ; warm wave. Name the important facts which have been discovered respecting the north- easters and other severe storms of the United States. MAP QUESTIONS, Trace on the map of isothermal lines the areas of greatest heat in the Eastern Hemisphere. In the Western Hemisphere. Show from the map of the isothermal lines wherein the physical torrid zone differs in position from the mathematical torrid zone. In which hemisphere do the isothermal lines deviate more from the parallels of latitude, in the northern or the southern ? Trace on the map of the isothermal lines the limits of drift ice. What are the mean summer and winter temperatures of Sitka ? Of Quebec ? What causes exist to render the climate of Sitka so much warmer than that of Quebec, notwithstanding the difference of their latitudes? What are the mean summer and winter temperatures of Mexico, Madras, Singa- pore, Berlin, London, Philadelphia, Algiers, Melbourne, and Eio Janeiro ? What instances can you find on the map of the increase in the mean annual temperature of places through the influence of ocean currents? Of winds? Of rainfall ? Of the decrease ? Trace on the map of the winds the boundaries of the various wind zones. Point out the limits of the monsoon regions of the world. What cold wind blows over Texas ? Describe the route a vessel would take in sailing from America to Europe. From New York to San Francisco. From America to Australia. PRECIPITATION OF MOISTURE. 221 SECTION II. MOISTURE OF THE ATMOSPHERE. CHAPTER I. Precipitation of Moisture. 319. Evaporation. — From every water surface, and even from all masses of ice and snow, water vapor is constantly passing off into the atmosphere at all temperatures. This giving off of vapor from the surface of water is called evaporation. It is evapora- tion which dries the wet earth, when the moisture is unable either to run off by surface drainage, or to soak through porous strata. Water vapor is invisible, and is about three-fifths as heavy as air. Consequently, a cubic foot of moist air is lighter than a cubic foot of dry air at the same temperature. Water vapor diffuses readily through the air, and is borne by the winds to all parts of the earth. About one-half, by weight, of the vapor of the atmosphere is within a little over a mile above the mean sea level. 320. The Rapidity of Evaporation is influenced by the follow- ing circumstances : (1) The Temperature of the Atmosphere. — The capacity of the air for absorbing moisture increases with the temperature. Warm air can hold more vapor than cold air. (2) The Extent of Surface Exposed. — Evaporation takes place only from the surface ; therefore, the greater the surface, the greater the evaporation. (3) The Quantity of Vapor Already in the Air. — Dry air absorbs moisture more rapidly than moist air. All evaporation ceases when the air is completely saturated. (4) The Renewal. of the Air. — During calm weather, the air in 222 PHYSICAL GEOGRAPHY. contact with a water surface becomes more nearly saturated, and so lessens evaporation. Gentle breezes, by renewing the air, increase the rapidity of evaporation. (5) Pressure on the Surface. — A diminished atmospheric pressure increases the rapidity of evaporation. The yearly evaporation at latitude 13° ~N., at Madras, is 91.2 inches ; at Boston, latitude 42° N., it is 39.1 inches ; at Great Salt Lake, 41° N., it is 80 inches. 321. The Dew Point. — When the air contains as much vapor as it is capable of holding, it is said to be at its dew point. The quantity of moisture necessary to saturate a given quantity of air and bring it to the dew point varies with the temperature. Cold air requires less moisture to saturate it than air which is warmer, and, therefore, may feel damper than warm air, which may contain more vapor. We thus distinguish between the actual humidity, or the amount of vapor actually present in a given volume of air, and the relative humidity, or the relation between the amount present and that required to saturate the air at the given temperature. The humidity of the air is determined by means of an instrument called a hygrometer. Weight in grains of aqueous vapor in one cubic foot of saturated air at different temperatures. Temperature, Fahr. Weight in Grains. Approximate Values. 0° 0.545 0.6 10° 0.841 0.9 20° 1.298 1.3 30° 1.969 2.0 40° 2.862 2.9 50° 4.089 4.1 60° 5.756 5.8 70° 7.992 8.0 80° 10.949 11.0 90° 14.810 15.0 100° 19.790 20.0 No matter how much aqueous vapor a given quantity of air con- tains, if its temperature be lowered, it will grow relatively moister until, if the fall of temperature be sufficient, its dew point is reached. Consequently, in warm weather, the air toward evening, while cool- ing from the highest temperature of the day, is growing relatively PRECIPITATION OF MOISTURE. 223 moister, until finally it may reach its dew point ; i. e., become saturated with water vapor, and as soon as the temperature falls below the dew point, a deposition of moisture will begin, either in the liquid or the solid state. As a rule, the quantity of moisture in the air decreases from the equator toward the poles, and from the coasts of a continent toward the interior. It also varies with the character of the prevalent winds and the time of the year. 322. Precipitations. — The invisible vapor may become visible, and be precipitated from the atmosphere either as dew, frost, mist, fog, haze, cloud, rain, sleet, hail, or snow. These are called precipi- tations. Law of Precipitations. — In order that any form of precipitation may occur, the air must be cooled below the temperature of its dew point. 323. Distribution of Precipitations. — The quantity of moisture in the air depends on its temperature and its vicinity to the sea. The amount of precipitation regularly decreases as we pass from the equator to the poles, and from the coasts of the continents toward the interior. 324. Dew. — If, during a warm day, when the air contains con- siderable water vapor, a dry glass be filled with cold water, the outside of the glass will soon become covered with small drops of water, derived entirely from the air. The moist air which comes in contact with the cool sides of the glass has its temperature lowered below the dew point, and deposits as vapor the moisture it no longer can retain. The dew which is deposited during certain seasons of the year on plants and other objects on the earth has a similar origin. Objects on the earth cool more rapidly than the surrounding air, which deposits its moisture on them whenever they lower its temperature below the dew point. When the objects are colder than 32° Fahr., the dew is deposited as hoar-frost. Some recent experiments of Aitken appear to show that part of the dew deposited on the ground, or on vegetation, conies from the ground and from the 224 PHYSICAL GEOGRAPHY. plants. Both the ground and vegetation are constantly giving off water, which readily evaporates during day time, but collects during cold nights when the air is nearly saturated, and evaporation has nearly ceased. Dew falls or is deposited more copiously on some objects than on others. This is because some objects radiate or give off their heat more rapidly than others, and thus, becoming cooler, condense more of the moisture of the air. More dew is deposited during a clear night than during a cloudy one, because objects cool more rapidly when the sky is clear than when it is cloudy ; thick clothing keeps the body warm, not because the clothes give any heat to the body, but because they are non- conductors, and prevent the escape of heat from the body. In like manner the clouds, acting as blankets to the earth, prevent its losing heat rapidly. More dew falls or is deposited during a still night than during a windy one. The air must remain long enough in contact with cold objects to enable them to lower its temperature and collect its moisture. Powerful winds prevent this, while gentle breezes favor the depo- sition, by bringing fresh masses of air into contact with the cold objects. In the tropics, during seasons when the sky is clear, the dew is so copious that it resembles a gentle rain. In the deposition of dew, the moisture is derived from a comparatively tbin stratum of air in the immediate neighborhood of the cool object. All other kinds of precipitations are produced by the cooling of a large mass of air. 325. Fogs and Clouds. — Whenever the temperature of a large mass of air is reduced below its dew point, its moisture begins to collect in minute drops, which diminish the transparency of the air, and form fogs, mists, or haze when near the surface, and clouds when in the upper regions. Fogs and clouds are the same in their origin and composition, and differ only in their elevation. Haze is a name applied to the condition of the air when there is a general, but comparatively small, loss of its transparency, due to the pressure of very minute drops of water. The presence of fine particles of mineral dust, or fine smoke particles, also produces a haze. Some PRECIPITATION OF MOISTURE. 225 recent laboratory experiments by Aitken have led him to believe that the formation of clouds is dependent on, or at least greatly aided by, the presence of dust-particles in the air. The readi- ness with which both clouds and rain form in the higher regions of the air, where the dust is practically absent, does not appear to support this belief. Clouds and fogs are formed of minute drops of water, a sub- stance about eight hundred times heavier than air. Notwithstand- ing their relatively greater weight, they are prevented from settling rapidly by the resistance of the air. This is rendered possible by the minute size of the drops, which are much smaller than the rela- tively heavier dust-particles, which are wafted about by the winds. Whenever the drops exceed a certain size, they fall as rain or snow. When at temperatures above 32° Fahr., the minute drops of water in clouds vary in size from 40 1 00 to 10 1 00 of an inch in diameter. Clouds or fogs result when- ever a mass of air is cooled below the temperature of its dew point, which occurs when two bodies of air of different temperatures are rapidly min- gled, especially if, as is usu- ally the case, the warmer of the two bodies is the moister. Clouds or fogs dis- appear on the approach of a dry, warm wind. Clouds are higher in the tropics than in the polar regions, and, as a rule, are higher during the day than during the night. Off the banks of Newfoundland, the warm, moist air of the Gulf Stream is cooled by the cold, moist air of the Labrador ocean current. Hence result the dense fogs so common over this part of the ocean. 15 Fig. 100.— Primary Forms of Clouds. v Cirrus, v v Cumulus. 226 PHYSICAL GEOGRAPHY. 326. Classification of Clouds. — Clouds assume such a variety of shapes that it is difficult to classify them. Meteorologists, how- ever, have recognized the existence of four primary forms : the Cir- rus, the Cumulus, the Nimbus, and the Stratus. The Cirrus Cloud consists of fleecy, feathery masses of con- densed vapor, floating in the higher regions of the atmosphere, The name cirrus is given from the resemblance the cloud bears to a lock of hair. These clouds are called by sailors cats' tails or mares' tails. From their elevation, the moisture is, probably, in the condition of ice-particles. Halos, or circular bands of light around the sun, are caused by light passing through the ice-particles of cirrus clouds. The Cumulus, or Heap Cloud, is a denser cloud than the cirrus, and is formed ^_ in the lower regions .-. ifffiilS^^^^BM Wv^^^^^H^^^ft °f the air, where the quantity of vapor is great. Cumulus clouds generally consist of rounded masses, in the shape of irregular heaps, with moderately flat bases. They are caused by ascending currents of air, which have their moisture condensed by the cold produced by expansion. Cumulus clouds occur Their height seldom exceeds Fig. 101. — Primary Forms of Clouds. v Nimbus. v- v- Stratus. during the hottest part of the day, two miles. The Nimbus, or Storm Cloud, is any cloud from which rain is falling. Any of the various forms of clouds may collect and form a nimbus cloud. The nimbus is not considered as a distinct form of cloud by some meteorologists. PRECIPITATION OF MOISTURE. 227 The Stratus, or Layer Cloud, forms in long, horizontal sheets or bauds. These clouds are most common in the early morning and evening, when the ascending currents are weak. They are caused by the gradual settling of the other forms of clouds. The stratus cloud sometimes falls to the surface of the earth, and becomes a fog. 327. Secondary Forms of Clouds. — The cirrus, stratus, and cumulus clouds assume a variety of shapes, producing various sec- ondary forms. The Cirro- Cumulus, the Cirro- Stratus, and the Cumulo-Stratus are the most prominent second- _ ary forms of clouds. The first two are modifications of the cirrus cloud ; the third, of the cumulus. The Cirro-Cumulus is a cirrus cloud, arranged in little rounded masses, shaped something like cumuli. They are some- times called " wool sacks," and indicate dry weather. The Cirro-Stratus is a cirrus cloud which has settled in bands or layers. The bands are not con- tinuous, but are arranged in blotches or bars, and often give to the sky the speckled appearance of a mackerel's back, producing the so-called mackerel sky. The appearance of a mackerel sky indicates — (1) That the moisture of the upper strata of air is condensing ; (2) That it is growing dense enough to arrange itself in layers. Therefore, a mackerel sky generally indicates approaching rain. m Fig. 102. — Secondary Forms of Clouds. v- Cirro-Cumulus. v- v Cirro-Stratus. v >r v Cumulo-Stratus. 228 PHYSICAL GEOGRAPHY. The Cumulo-Stratus is the form produced by the heaping together of a mountain-like mass of cumulus clouds ; the base par- takes of the nature of the stratus cloud, but the top clearly resem- bles cumulus clouds. These clouds differ but little from the nimbus or storm cloud. Like all other natural objects, clouds take their colors from the color of the light by which they are illumined. Consequently, by day they are white in the sunlight, and grayish or dark in the shadow. At dawn and sunset, they take in the characteristic sun- Fig. 103. — Actual Photograph of Clouds (John M. Justice). rise and sunset colors. In Fig. 103 an actual photograph of clouds is shown in which many of the different forms of clouds are to be found. 328. Rain. — When, during the formation of a cloud, the condensa- tion of moisture continues, the drops of which the clouds are composed increase in size, and, uniting, fall to the earth as rain. As the rain- drops fall through the clouds they grow larger by the addition of other drops which unite with them. Moreover, they may grow larger by the condensation of moisture on their surfaces as they fall PRECIPITATION OF MOISTURE. 229 through very moist air. Raindrops, therefore, are larger when the clouds are thick. They are, in general, larger in the tropics than in the polar regions, and during the day than at night. In fine rain the size of the drops varies from -^ to -^o °f an mcn i n diameter. In heavier rains they reach a diameter of y 1 ^- of an inch and over. To produce rain, it is necessary that the temperature of a large mass of air be reduced considerably below its dew point. There are several ways in which this cooling may be effected : (1) By a Change of Latitude. — A warm, moisture-laden wind may blow into a cold region. The equatorial currents of air deposit their moisture in the temperate and polar zones on account of the chilling experienced as they recede from the equator. (2) By a Change of Altitude. — An ascending current of air carries the moisture of the lower regions into the upper regions, where the rapid expansion of both air and vapor, under the dimin- ished pressure, chills the air and condenses its moisture. It is mainly in this manner that the rains of the tropical region are caused. The rains of mountainous districts have a similar cause. A moist wind, reaching a mountain-range, is forced by the wind back of it to ascend the slopes. Contact with the cold, upper slopes, as well as the cold produced by expansion due to elevation, cause condensation of the vapor as rain. (3) The Mingling of Masses of Cold and Warm Air. — By this means heavy clouds and a moderate rainfall may be produced; but the precipitation can never be considerable, because the cooler air will be warmed by the mixing, and, therefore, will have its capacity for moisture increased instead of diminished. In all of the methods above mentioned there is an actual motion of the moisture-laden air. Rainfall is, therefore, usually an attend- ant on cyclonic and thunder-storms, and on other violent movements of the air. Sleet consists of frozen rain. The word is also sometimes applied to a mingling of rain with fine particles of snow or hail. 329. Distribution of the Rainfall. — The distribution of rain may be considered both in regard to its periodicity and its quantity. 230 PHYSICAL GEOGRAPHY. The distribution of the rain is dependent upon the direction of the wind. Each wind zone has a characteristic rainfall. The following simple principles determine the rainfall in any par- ticular wind zone : (1) The equatorial currents are rain-bearing, because they are moist ; while on their way to the poles, their temperature and con- sequent capacity for moisture are constantly decreasing. (2) The polar currents are dry, because they are constantly increas- ing in temperature as they approach the equator ; hence, they take in, rather than give out, moisture. The polar currents, when they have reached the zones of the trade winds, may hring abundant rains, provided they have previously crossed an ocean. They then discharge the moisture with which they are saturated, either by ascending or by blowing against the elevations of the continent. 330. Periodical Rain Zones include the zone of calms and the zones of the trades. The Zone of Calms. — In the zone of calms it rains nearly every day. In the early morning the sky is cloudless ; but about the middle of the day, as the heat increases, the ascending currents, rising higher, begin to condense their moisture; cumulus clouds form, and, increasing rapidly, soon cover the sky, when torrents of rain descend during the afternoon or early evening, accompanied by thunder and lightning. After a few hours the rain ceases, and the sky again becomes clear. In this zone it seldom rains late at night. The annual fall in this zone has been estimated at about 100 inches. 331. The Zones of the Trades. — Since the trades are generally dry winds, it is only when their temperature is considerably de- creased that they can cause rain. In the zone of the trades, except in mountainous districts, and on the windward coasts of a continent, the rainfall occurs during the greatest heat of the season, when the sun is directly overhead and the ascending currents are powerful. Hence, it rains during a few months in summer, when immense quantities of water fall ; the remainder of the year is dry. Copious dews, however, occur at night. The precipitation is not continuous throughout the entire summer. Since the PRECIPITATION OF MOISTURE. 231 rain falls only when the sun is nearly overhead, a brief interval of dry weather occurs in regions near the equator, thus dividing the season into two parts ; one, during the passage of the sun over the zenith ; the other, on his return to the zenith from the adjacent tropic. The tropical cyclones that blow at times bver portions of the zones of the trades bring with them extremely heavy rains. Over the ocean, during most of the year, there is no rain in the zone of the trades, although the actual humidity of the air is quite high. 332. The Monsoon Region of the Northern Indian Ocean. — During the prevalence of the winter monsoon, the north-east trade winds bathe the eastern shores of Hindostan in copious rains, while the western shores, shielded by the ranges of the Ghauts, are dry. During the summer monsoon, the south-west winds bathe the western shores and the southern slopes of the Himalayas in heavy rains, while the eastern shores are dry. This monsoon also brings rains to the western coasts of the peninsula of Indo-China. 333. Non-Periodical Rain Zones include the zones of the pre- vailing westerly winds and the zone of the polar winds. Rainfall in the Zones of the Prevailing- Westerly Winds. — The air in these zones is subject to frequent cyclones of the extra- tropical type which are attended by plentiful rainfall. These may occur at any season of the year. Since extra-tropical cyclones are most sevei-e in winter, some parts of the zones have their greatest rainfall at this season of the year. In other regions, especially over the heated continents, the over-heated areas, producing local storms, cause the rainfall to be greater in summer. The annual rainfall in these zones varies in different parts from 30 to 80 inches, or even more. Rainfall in the Zone of the Polar Winds.— In these zones the winters are dry, because the dry, cold polar currents then prevail ; but during the summer the equatorial currents sometimes prevail, and bring with them dense clouds and fogs, accompanied by driz- zling rains. The snows occur mainly in spring and autumn. The annual precipitation, including both snow- and rainfall, is somewhat less than 15 inches, and in some regions it is less than 10 inches. When, as in this case, the snowfall is included, the amount of water the melted snow would produce is estimated. 232 PHYSICAL GEOGRAPHY. Vertical Section. All the rain zones, together with their wind zones, follow the sun in his annual movements north and south of the equator. 334. Quantity of Rain. — The quantity of rain which falls in a given time on any area is determined by means of an instrument called a rain-gauge or pluviometer. The rain-gauge as employed by the United States Weather Bureau is in the form of a cylindrical vessel with a horizontal base, surmounted by a funnel-shaped vessel A, called the receiver. Connected with the receiver is a vertical glass tube of such smaller diam- eter that the rain falling into A mounts in C just ten times higher than the actual fall, thus facilitating its exact measurement. B is an overflow attachment. The rain-gauge is placed in an exposed position, where it is free from eddies or whirls. If, during any given time, the water in C is one inch deep, then during that time the rainfall over the area equals one-tenth of an inch. In speaking of the rainfall of a country, the moisture which may fall as snow is always included. An inch of rain over a surface a square yard in area equals in weight 46! pounds ; on the surface of an acre it is nearly equal in weight to 100 tons. The animal rainfall is distributed, as regards quantity, as follows: Irrespective of the elevations of the surface, more rainfalls in the tropics than in the temperate regions, and more in the temperate than in the polar regions. The quantity thus decreases with approximate regularity from the equator toward the poles. This is caused by a similar decrease in the heat and evaporation. While the amount of rain that falls decreases from the equator to the poles, the number of cloudy or rainy days increases, being greater near the polar than in the equatorial regions. More rain falls on the coasts of a continent than in the interior, because the winds are moister near the ocean. That coast of a continent which first receives the prevailing wind has the greatest rainfall. More rain falls in the Northern Hemisphere than in the Southern. Fig. 104.— Rain- gauge. PRECIPITATION OF MOISTURE. 233 This is due to th'e greater extent of the land-area of the Northern Hemisphere. Mountains receive a heavier rainfall than the plains below, because the moist winds, in order to cross the mountains, are forced to ascend their slopes, and thus pass into a colder region of the atmosphere. Therefore, the sources of rivers are generally found in mountainous districts. Mountains are among the most important causes of rain. When the mountains are high, the winds may reach the farther slopes dry and vaporless. The tropical Andes of South America afford an excellent example of this. Plateaus, though higher than plains, receive, as a rule, less rain, because they are usually surrounded by mountain-chains which rob the winds of their moisture. Moreover, the air over a plateau is warmer than at a corresponding height in the atmosphere over the plains, and therefore fails to condense the moisture. The rainfall of the Western Continent, both in the tropical and temperate regions, is greater than that of the Eastern Continent; thus, in the tropics of the Western Continent, 115 inches of rain fall yearly, while the same portions of the Eastern Continent receive but 77 inches. In the temperate zones in America the annual rain- fall is 39 inches, while in Europe it is but 34 inches. The mean annual rainfall at Philadelphia is 46.93 inches. The preceding principles find ample illustration in the following table : Table of Annual Rainfall foe Wettest and Driest Years (mainly- Wettest. Inches. Cherra Pongee, India . 591.46 San Luis de Maranhao, Brazil (Johnston) . . 280.00 Paramaribo, Guiana (Johnston) 229.20 Kutching (Straits Set- tlements) 188.28 Colombo, Ceylon . . . 139.77 Bombay 122.00 after Tripp). Tropics. Driest. Wettest. Driest. Inches. Inches. Inches- 283.00 Cayenne,Guiana (John 116.27 Jubbulpore .... 94.80 28.80 94.40 52.00 . 93.30 43.50 88.40 18.40 159.73 75.50 20.70 60.55 . 40.81 19.01 33.90 234 PHYSICAL GEOGRAPHY. Temperate Zones. Wettest. Inches. New Orleans 110.6 Genoa 108.4 Boston 67.7 Cincinnati 65.2 Fort Leavenworth . . 59.6 New Haven 58.1 Lisbon 51.0 Geneva 49.5 Driest. Inches. 41.9 28.2 27.2 25.5 15.9 30.7 13.5 20.7 Wettest. Driest. Inches. Inches. Marseilles 43.0 10.6 Edinburgh 39.0 13.7 San Francisco .... 38.8 11.9 London 35.3 17.3 Sacramento 34.9 8.4 Naples 32.6 16.0 St. Petersburg .... 29.3 12.1 Paris 27.7 8.3 Fig. 105 shows a chart of the mean annual rainfall of the earth, with the value of the rainfall given in inches. Fig. 105. — Chart of Mean Annual Rainfall. 335. Cause of Deserts. — Deserts are caused entirely by the ab- sence of moisture. Their soil, though usually finely pulverized, or sand-like, does not differ from that of other areas, save in the ab- sence of vegetable mould. Thus, neither the nature of its tempera- ture nor the character of its soil is the cause of the desert of Sahara, since a vigorous vegetation always follows the appearance PRECIPITATION OF MOISTURE. 235 of water on the successful boring of an artesian well. It is prob- ably true, that deserts once formed, tend to perpetuate themselves by the influence their naked surfaces exert on the rainfall. 336. Rainless Districts. — In some parts of the world rain falls so seldom, or in such limited quantity and at such long intervals, that they are known as rainless districts. The most extensive rain- less districts are found in the Eastern Continent. Desert Belt of the Eastern Continent. — From the western shores of Northern Africa, eastward to the Great Kinghan Moun- tains in Asia, extends an almost uninterrupted belt of desert lands. This belt includes the great Desert of the Sahara, the Arabian and Persian Deserts, and the Desert of Mongolia. The aridity is more nearly absolute in the western part of Sahara, and in the desert of Arabia, where rain seldom falls. Toward the east, in Persia and Mongolia, scanty rains occur. T»he cause of this immense desert tract is to be found in the dry trade-winds, which blow over most of the region. Having previously crossed the vast conti- nent of Asia as upper currents, they arrive at the deserts dry and vaporless. Even that portion of the region which receives the winds from the Mediterranean has but a very scanty rainfall, because any clouds that form are usually soon dissipated by the hot air of the desert. Persia and Mongolia owe their deserts to their high mountain bor- ders, which rob the clouds of their moisture. The high system of the Himalayas effectually prevents any of the moisture of the south- west winds from penetrating the plateau of Mongolia. Arid tracts occur in the Kalahari desert, in Africa, and near the Tropic of Capricorn, in Australia. Deserts of trie Western Continent. — In North' America the location of the high mountain-ranges near the Pacific prevents the passage over them of the prevalent westerly winds with their burden of vapor from the Pacific. Their western slopes have an ample supply of rain, but the land to the east is very dry. This is especially true in the United States, where in Southern California. Arizona, New Mexico, Colorado, and neighboring districts, agri- 236 PHYSICAL GEOGRAPHY. culture practically depends on irrigation. Further to the north the rainfall is not so scanty. In South America, on the western slopes of the Andes, between the parallels of 27° and 23° S., is found the Desert of Atacama. Here rain seldom falls, although the ground is occasionally refreshed by mists and dews. The cause of the absence of rain is to be traced to the high Andes, which condense all the moisture of the trades on . their eastern slopes, the winds thus arriving at the western slopes dry and vaporless. It is questionable whether any desert region is absolutely rainless. Even in the Sahara, and in Arizona and Lower California, there is at times a scanty rainfall. -o^o* CHAPTER II. Hail, Snow, and Glaciers. 337. Hail falls when considerable differences of temperature exist between higher and lower strata of very moist air, and the moisture is suddenly condensed in the presence of great cold. Usu- ally, several layers or bands of dark, grayish clouds precede these storms. Hail falls most frequently in summer, near the close of an excessively warm day, as a not uncommon accompani- ment of the thunder-storms of the United States. m _^- mm ^ lm , Structure of the Hailstone. /" T=^ sr*M~ 3k —If a large hailstone be placed /^~w^./' — v\^^ Sk' " '~v%\ ik on a k°t surface and left until one- F/^ W-.-«" ^flfl M'W'ifrW^^m^m na ^ * s me ^ e( ^i its structure may It i V ' C^^JfJI W readily be examined. Concentric v / \^^jJ^^J J§/ I w\ "( M fin layers, similar to those of an onion, I /^C^ " ' rntm Jm? w iU ^ e noticed, arranged around ^Uls ^mi//' ^^^^Mr-i\--iiimw a central nucleus, sometimes of ^^||j|§§|fil^ ^^^^lw ice and sometimes of snow. Hail- Fig. 106.— Structure of a Hailstone. stones are more or less oblately spheroidal in shape. Their gen- eral weight varies from a few grains to several ounces, but they have been known to weigh several pounds. HAIL, SNOW, AND GLACIERS. 237 Origin of Hail. — The cause of hail is not fully understood, and several theories have been framed to account for it. Perhaps the least objectionable theory regards hail as the result of a convec- tional disturbance of the air, which results in the wind rotating, as in a cyclone, only the axis of the whirl is horizontal instead of vertical. Drops of frozen rain, or sometimes pellets of snow, caught in the whirl, are covered succes- sively with layers of snow and ice, as they are successively carried into colder or warmer regions. Thunder and lightning are the invariable attendants of hailstorms, and some authorities have attributed the formation of the stones to successive electrical attractions and repulsions of snowflakes between a snow and a rain cloud. There does not, however, appear to be any evidence of this. 338. Snow. — When the moisture of the air is condensed at any temperature below 32° Fahr., the vapor crystallizes as it condenses, and snowflakes are formed. Snowflakes grow, as they fall, by condensing additional moisture from the air. They are larger in mild than iu cold weather. They assume quite a variety of forms, but are built up by various groupings of minute rhombo- hedrons of ice. The star-shape is the most common. If the temperature of the air near the surface is much warmer than 32° Fahr., any snow that is formed in the upper regions will melt before reaching the ground. Hence, in the temperate zones, as a rule, snow reaches the sur- face only in winter, while in the tropics it never reaches the surface, except near the summits of lofty mountains. It is a mistake to suppose that the fall of snow is greater in regions near the poles than elsewhere ; for in high latitudes there is comparatively little moisture in the air. The snowfall is heaviest in the cool temperate regions. 339. Effect of Snow-covered Areas on the Temperature of the Air. —A snow-covered area produces a marked effect in chilling the air over it. Since snow is a non-conductor of heat, it gains practically no heat from the Fig. 107.— Snow-crystals. 238 PHYSICAL GEOGRAPHY. earth, and, since it is a good radiator and poor absorber of beat, it rapidly loses its heat and becomes cold. A comparatively thin layer of snow, therefore, say a foot or so, once formed, tends to remain throughout the winter. Such snow-fields tend to retard early vegetation. This retarding of the drainage of the winter snowfall is of great value in regions of scanty rainfall, such as exist in the Western United States, since it renders easier the storage of the winter fall for purposes of irrigation. 340. Snow Line. — Regions of Perpetual Snow. — The snow which falls on mountains slowly moves down the slopes by the weight of the snow above. The distance it will move before completely melting depends on a number of circumstances. The lower limit of permanent snow is called the snow line. Above it are the regions of perpetual snow, where the ground is covered with snow through- out the year. The height of the snow line depends : (1) On the Amount of the Snowfall. — The greater the. fall, the farther down the mountain the snow will move before completely melting. (2) On the Temperature of the Valley. — The warmer the valley the higher the snow line. The snow line is, therefore, highest in the tropical regions, and lowest near the poles. (3) On the Inclination' of the Mountain Slope. — The steeper the slope, the more rapidly the snow will move, and the farther it will go before melting ; therefore, the lower the snow line. According to Guyot, the snow line, with some exceptions, is about three miles above the sea in the tropics ; rather less than two miles in the temperate lati- tudes ; and less than a mile near the northern extremities of the continents ; while still farther north, on the polar islands, the snow line is but a few hundred feet above the sea. SNOW LINE. Europe.— Norway, Lat. 70° N 3,400 feet. " 60° N 5,500 " " Alp's, Lat. 46° N. (south side) 9,200 " " " " " (north side) 8,800 " Asia.— Altai Mountains, Lat. 50° N 7,000 " Himalayas, Lat. 31° N 17,000 " Africa.— Kilimandjaro, Lat. 3° S 16,000 " North America. — Eocky Mountains, Lat. 43° N. . . . 12,467 " South America. — Andes, Ecuador, Lat. 1° S 15,800 " " " " 54° S 3,700 " HAIL, SNOW, AND GLACIERS. 239 The snow line is generally lower in a moist than in a dry atmosphere, he- cause of the greater fall of snow from moist air. As a rule, that slope of a range which is exposed to the prevalent wind has a lower snow line than the opposite slope. The position the slope occupies in relation to the vertical rays of the sun also exerts an influence on the height of the snow line. 341. Glaciers are masses of snow and ice, which move slowly down the higher mountain valleys. Their upper parts are formed of soft snow ; their lower portions of clear, hard ice. Their origin is as follows : The weight of the huge snow-fields, which form above the snow line, forces the mass slowly down the slopes to regions where the waste exceeds the supply. The pressure, due to the weight of the layers, especially where the mass is forced through a contraction in the valley, squeezes out the confined air, to which snow, in great part, owes its white color, and the lower part of the glacier thus becomes changed into a compact mass of clear ice. The alternate thawing and freezing to which the mass is subjected also contribute to the change from snow to ice. In the lower parts of the glacier the ice is marvellously clear. In large masses its color is of a deep azure blue. In the middle portions of the glacier the ice is coarse and white. The higher region of but partially changed snow is called the neve region. Here the snow occurs in coarse white grains. The process of formation is a continuous one. The neve region is supplied by fresh falls of snow, which replace those forced down the slopes. 342. Types of Glaciers. — Russell divides glaciers into three types; namely, (1) Alpine Glaciers, or those which occur in highest mountain regions, such as the glaciers of the Alps, the Himalayas, the Rocky Mountains, the Mountains of Scandinavia, etc. Alpine glaciers have their origin on the summits and flanks of lofty mountains, and form ice streams that descend their many valleys. (2) Piedmont Glaciers, or those formed by the union of the Alpine glaciers on the plains adjacent to the mountain defiles. These gla- ciers may be compared, in the ice drainage of the land, to lakes in the water drainage. The Malaspina glacier at the southern base of Mt. St. Elias and neighboring mountains in Alaska, is an example of a Piedmont 240 PHYSICAL GEOGRAPHY. glacier. It covers an area of some 1500 square miles and varies in thickness from 1000 to 1500 feet. (3) Continental Glaciers, or glaciers of vast extent that cover large parts of continents. The glaciers of Greenland and of the Antarctic regions are of this class. 343. Drainage of Snow and Ice. — Glaciers resemble rivers, since they receive the drainage of their basins through the solid material which flows into them ; their motion, however, is much slower. Several glaciers often unite and flow on as a single stream ; but their solid condition prevents the intermingling which occurs in rivers, and the separate streams may generally be distinctly traced throughout the remainder of their course. Like rivers, the top and middle portions move more rapidly than the sides or bottom, owing to the diminished friction. The motion of the glacier is not necessarily dependent on the natural slope of the land. In the cases of continental glaciers the ice mass moves up slopes, as well as down, the direction of motion being determined by the pitch of the upper surface of the ice. Some glaciers, however, are devoid of motion, being, in this respect, like motionless or stagnant water. 344. Peculiarities of Glaciers. — The surface of the glacier is often comparatively smooth ; but when irregularities occur, either in the direction of the valley or in the slope of its bed, the glacier is broken into deep fissures, called crevasses. Crevasses are most nu- merous on the sides, from which they extend either obliquely up the stream, or directly across, in deep transverse fissures. The former are generally due to a bend in the valley, one side being compressed and the other extended ; the latter, to steep and abrupt descents in the bed. Crevasses, therefore, are rapids in the ice stream. Crevasses vary in breadth from mere crevices, that a knife-blade can scarcely penetrate, to yawning chasms over 100 feet in width. The depth of tbe wider crevasses is generally profound. Their vertical walls afford a convenient oppor- tunity for studying many peculiarities of formation. Looking down the walls of the crevasses, the ice appears of a deep azure blue. The surface ice is a dirty white. The crevasses gradually disappear below the cause of disturbance, the frac- tures mending by regelation ; i. e., a property which fragments of moist ice have of becoming firmly cemented together when their surfaces are brought into contact under pressure. HAIL, SNOW, AND GLACISES. 241 The water derived from the melting of the ice issues from a cav- ernous arch at the end of the glacier. The volume of the issuing stream, which is often considerable, is, of course, dependent on the temperature, and is greater during the warm months of the year. Many rivers, such as the Rhone and the Rhine, in Europe, and the Ganges, in Asia, have their origin in these glacier streams. The distance the glacier extends below the snow line depends on the mass and velocity of the ice and the rapidity with which it melts. When the winter snows are light, and the following summer unusually warm, the end of the glacier retreats up the mountain. On the contrary, heavy snowfalls in winter, followed by a cool summer, permit the end of the glacier to advance far into the valley below. 345. "Work of Glaciers. — All along the borders of the valleys, stones and dirt roll down the declivities, and, accumulating on the surface of the moving mass, are carried with it to a lower level. These accumulations of dirt and stones are called moraines; they are named according to their position as follows : (1) Lateral Moraines, or those which collect along the sides of the glacier. (2) Medial Moraines, or those situated in some cen- tral portion of the glacier, and resulting from the junc- tion of two or more lateral moraines, where two or more glaciers have united in a single stream. (3) Terminal Moraines, or the crescent-shaped ridges formed at the end of Alpine glaciers on the melting of their extremities. 16 Fig. 108.— The Moraine of the Mer de Glace. 242 PHYSICAL GEOGRAPHY. The terminal moraines formed at the margin of piedmont and continental glaciers are called frontal moraines. The material transported by glaciers is known as glacial drift. Only a portion of the debris carried forward by the glacier is deposited in the terminal or frontal moraine. Most of the drift is arranged beneath the ice as a ground moraine. This includes the substances frozen in the ice mass and pushed along with it over its bed. 346. Glacial Deposits and Sediments. — The mineral matter carried forward and deposited by glaciers may be divided into two classes : (1) Glacial Deposits, or those made directly from the ice mass, such as moraines, drumlins, etc. (2) Glacial Sediments, or those deposited from streams of water escaping from the glacial ice. 347. Erosion. — Such immense masses of ice must deepen con- siderably the valleys through which they move. Where they have deserted their former valleys, evidences of their previous existence are to be found not only in the long lines of unstratified rocks, boul- ders, and mud, left by their moraines, but especially in the deep grooves, or scratches, cut in the bottom or sides of the valleys by rocks imbedded in the moving ice mass. These scratches are par- allel, and show the direction of the motion. The water which issues from the terminal cave is deeply charged with a fine sediment, the result of erosion. This sediment is exceedingly fertile, and, spread out by the rivers on the flood-grounds, becomes a source of agricultural wealth. 348. Geographical Distribution of Glaciers. — Some of the principal glacial regions of the world are as follows: Europe. — In the Swiss Alps, where there are two regions — those of Mt. Blanc and those of Monte Rosa. The Mer de Glace (sea of ice), one of the best known of the European glaciers, descends from the slopes of Mt. Blanc. It is formed by the confluence of three glaciers — du Geant, de Lechaud, and de Talefre. Glaciers also occur in the Bernese Alps, in the Pyrenees, in the Austrian Alps, and in the Caucasus Mountains. HAIL, SNOW, AND GLACIERS. 243 North America. — In Alaska, in the mountain-ranges on its south- ern borders, adjacent to. the Pacific, especially arouud Mount Logan, 19,500 feet, and St. Elias, 18,025 feet, or the many adjacent peaks, there are extended glaciers of the Alpine type. The Taku glacier, which dis- charges into Taku In- let, and the Muir gla- cier, which discharges into Glacier Bay, are tide water glaciers, and form numerous ice- bergs. The Malaspina glacier, at the southern base of Mt. St. Elias and neighboring peaks, has an area of, approx- imately, 1500 square miles. To the west lies the Bering glacier. Both of these are, in their lower parts, of the Piedmont type. Gla- ciers also occur in the south-western parts of the Dominion of Can- ada and in the north-western part of the United States. Greenland. — A vast ice sheet or continental glacier, with an area of about 600,000 square miles, covers nearly all Greenland. It is the largest existing glacial mass in the Northern Hemisphere. The ice flows outward in all directions in glacial streams, many of which reach the ocean and form numerous icebergs. Humboldt Glacier, in Lat. 79° 31' N., is one of the most important. It is some 40 miles in length, and from 200 to 300 feet above the sea-level. Vast snow-fields, with numerous glaciers, are also found on Grinnell Land, and on the islands west of Baffin's Bay and in Davis' Strait. Besides the above, glaciers occur in South America, in Tierra del Fuego, and in parts of the Andes ; in Asia, in the Himalayas ; and in New Zealand. Extensive glacial regions also occur in the Ant- arctic regions. Fig. 109.— The Mer de Glace (Zeller). BAIL, SNOW, AND GLACIERS. 245 349. Icebergs. — When the glacier extends into the sea, the base is undermined by the warmer waters of the ocean, and great frag- ments are broken off by the waves, forming floating mountains of ice, called icebergs. Icebergs are particularly numerous in the North Atlantic, into which they descend from the extensive Arctic glacial region. 350. Varieties of Icebergs. — Where the surface of the ice mass above the water is exposed to the air, the ice is of dazzling white- 110.— Taku Glacier. ness ; below the water, where it is protected from the air, it has the characteristic deep-blue color of glacial ice. In a region where icebergs are being separated from the glacial mass three types of bergs are seen in bright sunlight : (1) Dazzling White Bergs, or those where the lighter white ice projects only above the surface. (2) Blue Bergs, or where unequal melting has caused the inver- sion of the bergs, thus bringing the blue ice above the surface. This change of position is due to the bergs becoming bottom buoyant. 246 PHYSICAL GEOGRAPHY. (3) Dirt-bedecked Bergs, due, possibly, either to fragments of bottom ice or to parts of the berg that formed the sides of crevasses. The ice floes of the polar seas have their origin in the snow which falls into the cold water, remaining partially dissolved and subse- quently freezing, thus adding to the thickness of the ice formed. 351. Glacial Epoch. — At the beginning of the Quaternary Period, the climate of northern latitudes grew colder, probably owing to an increase in the elevation of the Northern Continents. At the same time the air became moister, so that the accumulation of snow was favored. The glaciers of the mountainous district, therefore, moved down their valleys, and spread gradually over the Northern Conti- nents, covering them with an ice sheet or continental glacier. This epoch is known as the glacial epoch. In North America, where it was best marked, this ice sheet extended generally north of the line marked on the map shown in Fig. Ill ; it existed also in some of the high mountainous districts in the south. Alaska appears to have escaped this ice sheet. When the ice sheet finally melted, it left evidences of its former presence by extensive erosion in some regions, and by the depo- sition of eroded materials in others. Over vast areas are found thick layers of stones, gravel, sand, and clay, both unstratified and roughly stratified, deposited either by the ice alone, or by the ice aided by the water derived from its melting. These deposits are known as glacial drift. 352. Varieties of Glacial Drift Deposits. — Drift deposits occur in various forms : (1) Moraine Deposits. — These assume forms that depend on the character of the moraine, whether terminal, lateral, medial, ground, etc. These are unstratified. (2) Till Sheets, or deposits of tough clay, containing more or less sand, stones, and boulders of various sizes, devoid of any regular arrangement. The till is believed to have been deposited by the ground moraine of the ice sheet, and to have been formed largely from the bed rock on which it rests. HAIL, SNOW, AND GLACIERS. 247 Till deposits cover large areas in most of the northern parts of the United States that were covered by the ice sheet of the glacial epoch. In some regions, such as New England, the boulders and large stones are so numerous that farming is practically impossible. In other states, such as Ohio, and most of the states as far west as the Dakotas, the till is not so rocky, and is, consequently, more fer- tile. Till sheets are usually unstratified. The accumulations of till and other drift deposits often assumed characteristic shapes called drumlins, eskers, and harries. Drumlins are low, rounded, oblong hills, probably formed under the ice sheet. They correspond to saud-bars or mud-flats in rivers, or Fig. 112. — Drumlin. to sand-hills or dunes on the land. They are very common in New England, in New York, and in other states west to the Dakotas. Eskers are long, tortuous ridges of gravel or sand, deposited by the streams that flowed through winding tunnels under the ice. Karnes are rounded hillocks piled up by the underground waters. Outside the glaciers, when the streams descended valleys, they formed deposits called valley trains. 353. Characteristic Drainage. — The drainage of a drift-covered region possesses all the characteristics of immaturity, and lake- basins are, consequently, numerous. Before the drainage systems were fully established on the new surface, the irregularities were occupied by lakes. A drift-covered region is, therefore, a lake region. The finer drift materials, settling in these lakes, form extremely fertile plains on the disappearance of the water. 354. Glacial Groovings or Scratches. — Where the drift does 248 PHYSICAL GEOGRAPHY. not cover the ground, evidence of the former presence of the ice is found in parallel cuttings, scratches, or groovings in the surface of hard rocks, due to the action of stones embedded in the ice mass. These scratches, as shown in Fig. 113, are fre- quently crossed by other marks, showing a change in the direction Fig. 113.— Glacial Scratches. ~ ,-, n -, oi the now, due, probably, to a change in the level or inclination of the surface of the ice mass. 355. Boulders, or erratic blocks, that have been transported by the ice and left, on its melting, in far-distant re- gions, are very common. Some single specimens of these weigh several thou- sand tons. 356. Fiords. — Certain varieties of shore lines are called fiords. Fiords are deep inlets separating lofty headlands. These inlets often extend from 50 to 100 miles into the inte- rior, while numerous high rocky islands lie off the shore. The coast of Norway presents a typical example of such a shore line. The origin of fiords appears to be as follows : Deep valleys were cut by glacial action during a period of elevation. Through subsequent subsidence, and the Fig. 114.— Boulders. HAIL, SNOW, AND GLACIERS. 249 disappearance of the glacier, the sea penetrated far into the valleys, leaving the summits of the mountains as numerous islands. The depths of the fiord valleys may be judged by the fact that some of them on the American coast, between Maine and Labrador, are shown by soundings to extend more than 3000 feet below the level of the sea. Since the bottom of these channels is prob- ably covered with drift, their real depth is much greater than that indicated by soundings. Dana names the following as fiord coasts — Maine, Labrador, Newfoundland, Greenland, British Columbia, Alaska, Norway, Western South America south of 41° S. Lat., Tasmania, and South Australia — and calls attention to the fact that they are all in glacial latitudes, as indicating their origin. Fiords are, therefore, another proof of the existence of ancient glacial systems. 357. Extensive Disturbances in River Channels and Drain- age Areas were caused either by the attending changes of level or by the presence of the ice sheet. The direction of flow of many streams was reversed, and the outlets of many lakes changed. Lake Winnipeg, for example, on the melting of the ice mass, instead of emptying into the Hudson Bay, as at present, emptied into the Mississippi River. Temporary lake basins, some of which were of enormous area, were formed by the damming up of the drainage area by the retreating ice mass. Lake Agassiz is an example. This lake, formed during the retreat of the ice sheet, was nearly 700 miles long from north to south, and, probably, 250 miles broad. Its surface was, probably, 700 feet above the present level of Lake Winnipeg. The waters of this lake discharged south, through the now extinct Warren River, into the Mississippi. Its boundaries are shown in Fig. 111. Other examples of prehistoric lakes are found in Lakes Bonne- ville, 19,750 square miles in area, and Lahontan, 8422 square miles in area. They were formed by increased rainfall in districts where arid conditions now prevail. They appear to have had their origin in the changes of climate that produced the ice sheet of the glacial epoch. Their locations are shown in Fig. 111. 250 PHYSICAL GEOGRAPHY. CHAPTER III. Electrical and Optical Phenomena. 358. Elementary Electrical Principles. — Although we are ignorant of the real nature of electricity, we are well acquainted with the principles which control its action. An electrical flow or current is always produced by the action of the force that sets electricity in motion, called electromotive force (usually contracted E. M. F.). In order to set electricity in motion a conducting path or circuit must be provided. E. M. F. is meas- ured in units called volts. When a body, such as a piece of glass, is rubbed against a bit of cat skin, both the glass and the skin are charged with electricity, that is, are electrified, or have an E. M. F. developed in them by friction. If the glass be touched by a knuckle of the hand, an elec- tric spark passes, and the glass is discharged, or a current of electricity is momentarily produced. Generally speaking, when an E. M. F. is properly applied to a circuit, an electric discharge is sent through the circuit. 359. Electric Sources. — There are various ways in which E- M. F.'s and, consequently, electric discharges or currents, may be produced. Any device for producing E. M. F. is called an electric source. Energy is required to produce an electric current, and, therefore, is expended in all sources that are connected with working circuits. Some of the more important electric sources are voltaic batteries, dynamo-electric machines, frictional electric machines, and thermo- electric piles. In voltaic batteries, chemical energy is transformed or converted into electric energy. In dynamo-electric and frictional electric machines, mechanical energy is converted into electric energy. In the thermo-electric piles, heat energy is converted into electric energy. In every electric source the electricity is assumed to flow out of the source at one place, and to return to the source, after passing ELECTRICAL AND OPTICAL PHENOMENA. 251 through the electric circuit, at another place. These places are called poles, the positive (+) pole being the pole from which the electricity leaves the source, and the negative (— ) pole, the pole at which it returns to the source. 360. Conductors and Non-conductors. — All electric circuits offer a measurable resistance to the passage of an electric discharge. Electric resistance is measured in units called ohms. The resistance of a circuit depends both on the nature of the materials of which it is composed and on their lengths and areas of cross-section. The longer the circuit, and the smaller its area of cross-section, the greater its resistance. Some substances, such as most metals, charcoal, acids, aque- ous solutions, and various animal and vegetable substances, are good conductors of electricity ; others, on the contrary, such as gums, resins, glass, silk, and dry air are poor conductors, or are, rela- tively, non-conductors. Consequently, when it is desired that a cir- cuit should possess a small resistance, it is made as short as possible, of good conducting material, such as copper wire, and of as heavy a cross-section as may be required. Wlien a circuit is necessarily of high resistance, a high E. M. F. is necessary in order to enable a large current to pass through it. The rate at which electricity passes through a circuit is called the electric current, and is meas- ured in units called amperes. An ampere is the current which would pass through a circuit whose resistance is one ohm under an E. M. F. of one volt. 361. Atmospheric Electricity. — Electric excitement, or an electric charge, is always present in the atmosphere. The electricity of the air is usually positive, although it often changes rapidly to negative on the approach of clouds or fogs. It is feeblest within a few feet of the surface, and increases with the elevation above the general surface of the earth. Origin of Atmospheric Electricity. — The exact causes of the electricity of the atmosphere are unknown. It has been ascribed to a variety of circumstances, the chief of which are evaporation and condensation ; unequal heating of the earth by the sun's rays ; combustion ; animal and vegetable life ; and the friction of winds against each other, or against the earth's surface. 252 PHYSICAL GEOGRAPHY. 362. Lightning and Thunder.— Lightning occurs when the electric charge of a cloud discharges to the earth or to a neighboring cloud. The discharge is attended by a vivid spark, called lightning. The destructive effects of lightning are due to the discharge which occurs between the clouds and the earth. The heat of the dis- charge vaporizes the rain- drops, and enormously expands the air, produc- ing, on their subsequent condensation and cooling, a partial vacuum, which is further increased by the momentary pushing aside of the air by the discharge. The surround- ing air rushing violently into this vacuum produces the sound called thunder. Fig. 115. — Lightning Flash (John M. Justice.) The electromotive force of the lightning flash is enormously higher than that produced by artificial means, and must be equal to many millions of volts. This high potential is due to the enormous decrease in the surface of a single rain-drop as compared with the combined surfaces of the thousands of smaller drops which have coalesced to form it. 363. Varieties of Lightning. — There are six varieties of light- ning : zig-zag or chain, sheet, heat, globular, volcanic, and multiple or ribbon lightning. Zig-zag 1 Lightning 1 probably owes its forked shape to the resistance of the air. The zig-zags are not angular, but curved, as may be seen from an examina- tion of the photograph of lightning flashes as shown in Fig. 115. The air-parti- ELECTRICAL AND OPTICAL PHENOMENA. 253" cles, being crowded together in the path of the spark, the lightning darts to one side, where the air is not so dense. Sheet Lightning 1 usually accompanies thunder-storms, and appears as an ex- panded flash, which illumines the clouds. Heat Lightning - , or light- ning without thunder, is gen- erally seen near the horizon during hot weather. It is, probably, caused by the re- flection of lightning from a storm below the horizon. Globular Lightning. — On rare occasions the light- ning appears in the form of a globe of light, which re- mains stationary in the air or moves slowly through it. Its cause is unknown. • Volcanic Lightning.— During volcanic eruptions flashes of lightning often oc- cur near the craters. Volcanic lightning is probably caused by the rapid condensation of water vapor emitted with the ashes and lava. Multiple or Ribbon Lightning is a rare form, in which the discharge takes the shape of a number of parallel discharges, giving the flash the appearance of a ribbon. Fig. 116 shows a photograph of such a discharge in which there are fourteen distinct discharges separated by dark spaces. The cause is unknown. 364. Lightning Rods, invented by Franklin, protect the build- ings on which they are placed by quietly discharging the electricity from the overhanging cloud. They generally ensure this by an opposite electric discharge passing from the earth up the rod, and neutralizing the charge of the cloud. When struck by a flash, the rods safely conduct the discharge to the ground. Unless they are placed in good metallic connection with the earth, and with all con- ductors near them, they are sources of danger rather than of protec- tion. Fig. 116.— Multiple or Ribbon Lightning. (Photographed by John M. Justice.) 254 PHYSICAL GEOGRAPHY. 365. St. Elmo's Fire. — When the atmosphere is highly charged with electricity, faint tongues of fire are often seen on the extremities of tall bodies, such as the masts of ships, steeples, etc., due to an elec- tric discharge, known as the brush-discharge. This phe- nomenon is known as St. Elmo's fire, and is harmless. 366. The Aurora Bore- alis, or northern light, is a phenomenon of great beauty, occurring in the sky of high latitudes in both the Northern and Southern Hemispheres. It appears in a variety of forms; at- times huge pillars of fire move rapidly across the heavens, or the northern sky is lighted as by a drift- ing storm of luminous snow. The most common appearance, however, is that of an arch of fire, from which streamers flash toward the zenith. Auroras are most frequent in high latitudes, though not in the immediate vicinity of the poles. Auroras are caused by the passage of electricity through the rare air of the upper regions. The proofs are as follows : During the continuance of an aurora, the telegraph wires show the presence of an unusual electrical disturbance, and the magnetic needle is subject to frequent oscillations; the same phenomena may be produced by the passage of an electrical current through rarefied gases, as in Geissler tubes — different colors arising from its passage through different gases. Moreover, the close accordance of the periods of greatest frequency of auroras with similar periods of sun spots would seem to suggest their depend- ence on the magnetic conditions of the sun and the earth. 367. Magnetism. — Magnets are bodies which have the power of attracting particles of iron, or the opposite poles of other magnets. Fig. 117.— St. Elmo's Fire. ELECTRICAL AND OPTICAL PHENOMENA. 255 7 sin All magnets possess an atmosphere of influence surrounding them, called the magnetic field. The magnetic field is traversed by magnetic flux, or lines of magnetic force, which come out of the magnet at one point and enter it at another, thus forming a mag- netic circuit. The points where the lines come out and enter the magnet are called poles ; the former being the positive or north pole, and the latter, the negative or south pole. The general direction of these lines may be shown by sprinkling iron filings over a sheet of paper held horizontally over a magnet and then gently tapping the paper. The filings will come to rest in the general direction of the magnetic lines, as shown in Fig. 118. Magnets are either natural or arti- ficial. Lodestone, a species of iron ore composed of oxygen and iron, is naturally magnetic. Pieces of hard- ened iron or steel may be magnetized by rubbing them with a lodestone, thus producing what are known as artificial magnets, often called perma- nent magnets, because they retain their magnetism. By passing elec- tric currents around a piece of soft iron there is produced what is called an electro-magnet. Such a magnet loses its magnetism as soon as the electric current ceases to pass, and is, therefore, sometimes called a temporary magnet. All magnet- izable substances become magnetized when they are brought into a magnetic field. If a magnetized bar or needle be suspended at its centre of grav- ity so as to move freely in a horizontal plane, it will, after a few oscillations, come to rest with one of its ends pointing nearly to the geographical north pole of the earth. This end of the magnet is Magnetic Flux. Z5Q PHYSICAL GEOGRAPHY. Fig. 119.— The Magnetic Needle. called its north pole, the opposite end, its south pole, and the magnet itself, a magnetic needle. 368. Magnetic Attractions and Repulsions. — If a magnet is brought near a magnetic needle, as in Fig. 119, attraction or repulsion will ensue — repulsion, when the poles are of the same name; attraction, when they are of oppo- site names. Thus, when a north pole is approached to a north pole, or a south pole to a south pole, they repel each other ; but when a north pole is approached to a south pole, or a south pole to a north pole, they attract each other. If the approach- ing magnet is powerful, it will de- flect the magnetic needle, although several feet distant from it; and if placed permanently in this posi- tion, the magnetic needle will no longer point to the north, hut will turn toward the disturbing magnet. 369. The Magnetic Properties of the Earth. — The magnetic needle points to the north for the same reason that the opposite poles of magnets point to each other when they are sufficiently near. The entire earth acts as a huge magnet, with its poles in the neighborhood of its geographical poles, and the needle points toward these poles on account of their magnetic attraction. The earth, like all magnets, possesses a magnetic field. Lines of magnetic force come out at its north magnetic pole, pass around its surface through the air, and enter the earth at its south magnetic pole. A magnetic needle, placed in the earth's field, if free to move, will come to rest with the earth's lines of force entering at its south pole and passing out of its north pole. That pole of the needle which points to the geographical north is, therefore, of opposite magnetic polarity to the earth's polarity in the Northern Hemisphere. 370. Origin of the Earth's Magnetism. — The exact cause of the earth's magnetism is unknown. Currents of electricity circu- lating through any conductor render it magnetic. Electric currents are generated in nearly all substances when they are unequally heated. The earth appears to owe its magnetism to the circulation around it of currents of electricity, produced, most probably, by ELECTRICAL AND OPTICAL PHENOMENA. 257 the unequal heating of different portions of its surface by the sun's rays. These currents would follow the sun in its apparent motion from east to west. Since the earth's magnetism appears to have its remote cause in the sun's heat, variations in the earth's temperature should be followed by corresponding variations in the intensity of its magnetism. This is found to be the case. Magnetic storms, or unusual variations in the earth's magnetism, have been noticed to correspond with outbursts of solar activity, as manifested by the unusual occurrence of sun spots. 371. Magnetic Declination.— Since the earth's magnetic poles do not coincide with its geographical poles, the magnetic needle does not, except in a few localities, point to the true geographical Fig. 120.— Declination Chart. (West Declination is represented by the continuous lines ; East Declination by the dotted lines ; the Agones by the heavy lines.) north, but to the east or to the west of it. This deviation from the true north is called the declination or variation. Its value is east or west, accordingly as the needle points to the east or the west of the true or geographical north. The amount of this variation differs in different parts of the earth. 17 258 PHYSICAL GEOGRAPHY. 372. Isogonal Lines. — Lines connecting places which have the same declination are called isogonal or isogonic lines. Lines con- necting places where the needle points to the true north, are called agones, or lines of no declination. The direction of the isogonal lines is shown in the declination chart, the figures near the lines giving the value of the declination in degrees. The agone in each hemisphere is marked 0. In the New World it enters South America near Eio Janeiro, curves around the Antilles, passes near Washington, D. C, through the western part of Hudson Bay, and enters the magnetic pole near Boothia Felix. The agone, in the Old World, passes through the west of Aus- tralia, near the western coasts of Hindostan, through Persia, the eastern part of the Caspian Sea, and through the White Sea, in Europe. The oval curves in Eastern Asia seem to indicate a secondary magnetic pole. In nearly all of Europe, in the whole of Africa and Arabia, in eastern North and South Amer- ica, and in nearly ail of the Atlantic and Indian Oceans, the declination is west. It is also west along part of the eastern shores of Asia, around the secondary magnetic pole. In the other parts of the world the declination is east. 373. The Inclination or Dip of the Needle. — The lines of force of the earth's magnetic field are, in most places, inclined to the earth's surface. The position of the needle is, therefore, hori- zontal in but few localities. In most places, one end of the needle is inclined to the earth. This is called the inclination or dip of the needle. In the Northern Hemisphere, it is the north pole, and in the Southern, the south pole, that is inclined. 374. Magnetic Equator; Isoclinal Lines. — The nearer we approach either magnetic pole the greater is the angle of dip. At either pole, the needle points vertically downward ; midway between the poles, or at the magnetic equator, the needle is horizontal. Lines connecting places which have the same angle of dip are called isoclinal lines. They correspond in a remarkable manner with the isothermal lines. This seems to show the dependence of the intensity of magnetism on the dis- tribution of the sun's heat. 375. Variations in the Earth's Magnetism. — The intensity of the earth's magnetism is by no means constant, but varies at different hours of the day, at different days in the year, and at different cycles, or long intervals of time. Magnetic variations are, therefore, either diurnal, annual, or secular. Besides these there are ELECTRICAL AND OPTICAL PHENOMENA. 259 irregular variations, which attend so-called magnetic storms that occur during periods of unusual solar activity, as indicated by the prevalence of an unusual number of sun spots. During the variations of the earth's magnetism, the value of the declination and inclination of the needle varies. Since the value of the declination varies the position of the earth's magnetic poles must change. In the secular varia- tions, as shown by a study of declination charts for many years, the magnetic needle, at a certain place, pointed more and more to the east, following the change of the poles. After a long period it became stationary, and then began to move slowly to the west. Thus, in 1580, the magnetic declination in London was 11° 18' E. The needle was then slowly moving eastward. In 1663, the declination became zero, the needle pointing due north and south. It then began moving west, reaching its greatest declination, 24° 30' W., in 1818. It has since been moving to the east, being, in 1901, about 15° 32' W. 376. Optical Phenomena are caused by changes in the direc- tion, intensity, or composition of sunlight during its passage through the atmosphere. Sunlight, when passed through a prism, is dispersed or separated into a great number of different colored lights. The following seven groups of colors are prominent: violet, indigo, blue, green, yellow, orange, and red. These are called the 'prismatic colors, or, collect- ively, a spectrum. They differ in the case with which they are refracted, or turned out of their course, in passing from one medium to another of different density. The above prismatic colors seen in the spectrum are named in the order of their refrangibility, begin- ning with violet, the most refrangible, and ending with red, the least refrangible. 377. Rainbows are arches of the prismatic colors, caused by the dispersion of the light during its passage through the falling drops of rain. The rays entering the drops are reflected from the interior surfaces farthest from the sun, and emerge separated into the pris- matic colors. Rainbows are seen when the observer stands with his back toward the sun. They are largest when the sun is nearly set- ting. A secondary bow sometimes occurs outside the primary, with the order of its colors reversed. It is caused by the light which is twice reflected from the back of the drops. 260 PHYSICAL GEOGRAPHY. 378. The Sunset Tints of the Sky. — As the sun sets, its disk is of an orange or red color, because most of its blue rays have been scattered, and only the orange and the red rays reach the eye. Hence the horizon is colored by orange and red tints as the sun sinks. Some of the sunset colors are due to refraction of the light through the denser and moister lower layers of air. The sunset glow of rose and purple, seen shortly after sunset, is due to the presence of dust particles in the air. 379. The Blue Color of the Sky is due to the fine dust par- ticles suspended in the air, to minute drops of water, or to minute ice crystals. The blue color is purer in clear weather, and is deeper in the higher regions of the air. This, probably, is because the suspended particles are finer in the higher regions and in clear weather. 380. Halos and Ooronae are rings of prismatic colors surround- ing the sun and moon. Halos are caused by the presence in the air of small crystals of ice or snow. Parhelia, or mock suns, and Paraselenae, or mock moons (bright spots which somewhat resemble suns and moons), are fre- quently seen where the com- plicated circles of halos in- tersect each other. Coronae are circles of light, seen most frequently around the moon. They are caused by the pres- ence of a small quantity of condensed vapor in the air. They usually indicate changes in the weather. 381. The Mirage is a general term applied to the appearance which objects present when viewed by means of rays of light that have passed through strata of air, which gradually increase or de- Fig. 121.— Halo. ELECTRICAL AND OPTICAL PHENOMENA. 261 crease in density. In this way the objects appear either inverted or erect, but always out of their true position. Sometimes the objects appear repeated, one image being seen above the other. The mirage occurs both over water and land. It is caused by the turn- ing of the rays of light out of their original direction. The Mirage of the Desert occurs over hot, arid surfaces, whenever the strata of air increase rapidly in density from the surface upward. The rays of light from distant objects, such as trees, are reflected from one of the lower layers of air, and, entering the eye of the ob- server, appear to come from inverted objects, which seem to be sur- rounded by a sheet of water. The images of real trees are seen, but out of their true situation, so that when the observer reaches the place he finds neither trees nor water. The mirage frequently occurs on the sea. Vessels that are too far below the horizon to be directly seen, become visible by refraction. This phenomenon is called looming. The vessels are seen both erect and inverted, and sometimes appear suspended in the clouds. Islands, too distant to be directly seen, sometimes become visible from the same cause. SYLLABUS, The rapidity of evaporation increases : (1) With the temperature of the atmosphere ; (2) With the extent of surface exposed ; (3) With a decrease in the quantity of vapor already in the air ; (4) With the renewal of the air ; and (5) With a decrease of the pressure on the surface. When air can hold no more moisture in an invisible state, it is said to he satu- rated or at its dew point. Whenever the air is lowered below the temperature of its dew point, its moist- ure is deposited as dew, frost, fog, cloud, mist, haze, snow, hail, sleet, or rain. More dew is deposited on clear nights, when the wind is moderate, than on cloudy nights, when the wind is high. In fogs, mists, and clouds the vapor is condensed as minute drops of water. The formation of clouds is aided by the presence of dust particles in the atmosphere. The primary forms of clouds are the cirrus, the cumulus, the nimbus, and the stratus. The secondary forms of clouds are the cirro-stratus, the cirro-cumulus, and the cumulo-stratus. 262 PHYSICAL GEOGRAPHY. Bain falls whenever the temperature of a mass of air is lowered considerably below the temperature of its dew point. This reduction of temperature may occur: (1) By a change of latitude; (2) By a change of altitude; (3) By the intermingling of moist cold air and moist warm air. As a rule, the equatorial currents bring rain, the polar currents, drought. In the zone of calms, it rains nearly every day during the hottest part of the day, or in the afternoon, when the ascending currents are strongest. In the zone of the trades, it rains during the summer. In the zone of the prevailing westerly winds, cyclones are frequent, and are accompanied by a plentiful rainfall. In the zone of the polar winds, the winters are clear ; snows and drizzling rains occur in spring and autumn. An inch of rain on the surface of a square yard is equal in weight to 46.75 pounds ; an inch on the surface of an acre, to a weight of about 100 tons. The quantity of rain decreases from the equator to the poles, and from the coasts of the continents toward the interior. More rain falls on mountains than on plains ; more on plains than on plateaus ; more in the Northern than in the Southern Hemisphere. The rainfall of the Western Continent is greater than that of the Eastern. In the tropics of the Western Continent, the annual rainfall is 115 inches ; in the Eastern Continent, only 77 inches. Deserts are caused by the absence of rain. The desert belt of the Eastern Continent extends from the western shores of Northern Africa eastward to the Great Kinghan Mountains in Asia. It includes the Sahara, the Arabian and Persian Deserts, and the Desert of Mongolia. In the Western Continent, the western slopes of the Pacific mountain-ranges have an ample supply of rain. The western slopes of the Andes, between Lat. 27° and 23° S., are almost entirely devoid of rain. Hail falls when the moisture is suddenly condensed by cold. Snow falls when the vapor is condensed at temperatures at or below 32° Fahr., under conditions favorable to gradual crystallization while condensing. Sleet is frozen rain. The snow line is the lower limit above the sea where snow remains throughout the year. The height of the snow line depends : (1) On the amount of the snowfall ; (2) On the temperature of the valley ; (3) On the inclination of the slopes. Glaciers are immense masses of ice, formed by the snow which accumulates on the slopes of mountains above the snow line. Glaciers move slowly by gravity down the mountain slopes, bearing with them accumulations of dirt and stones, called moraines. Moraines are : (1) Lateral ; (2) Medial ; (3) Terminal ; (4) Ground ; (5) Frontal. Glaciers are found in the following mountains : The Alps, the Pyre- nees, the Caucasus, the Scandinavian, the Himalayas, in some of the mountains of the northern, north-western, and western parts of North America, in the Andes, in the mountains of New Zealand, and elsewhere. Glaciers are of three types : (1) Alpine ; (2) Piedmont ; (3) Continental. SYLLABUS. 263 When glaciers descend into the sea, the waters undermine them, and detach huge masses, called icebergs, which float away to great distances. As regards their appearance, icebergs may be : (1) Dazzling white ; (2) Blue ; (3) Dirt-bedecked. At the beginning of the Quaternary Period, a change occurred in the climate of the earth, by which all the northern continents were covered with glaciers. The glacial drift occurs as: (1) Moraine deposits; (2) Till sheets. These deposits assume various forms, such as drumlins, eskers, kames, and valley trains. The following are fiord coasts: Maine, Labrador, Newfoundland, Greenland, British Columbia, Alaska, Norway, Western South America south of 41° S. Lat., Tasmania, and South Australia. The unit of electromotive force is called a volt ; the unit of current is called an ampere; the unit of resistance is called an ohm. Comparing the flow of elec- tricity to that of a current of water in a pipe, the volt corresponds to the pres- sure causing the flow, the ohm to the friction or other resistance opposing the flow, and the ampere to the quantity of the flow per second. Lightning results when the electricity of a cloud discharges to the earth or to a neighboring cloud. There are six forms of lightning : zigzag, sheet, heat, globular, ribbon, and volcanic. When the air contains an unusually great quantity of electricity, faintly luminous balls called St. Elmo's fire are seen on the extremities of tall objects. Auroras are caused by the passage of electricity through the rare air of the upper regions of the atmosphere. The earth acts like a huge magnet. It possesses a magnetic field, and has hues of magnetic force entering its south magnetic pole in the Northern Hemi- sphere, and coming out of its north magnetic pole in the Southern Hemisphere. The magnetic needle points to the north, from the action of the magnetic poles of the earth. The cause of the earth's magnetism is not certainly known. It is probably due to electrical currents which circulate around it. Magnetic storms, or unusual variations in the earth's magnetism, correspond with outbursts of solar activity, as manifested by the unusual appearance of sun spots. The deviation of the magnetic needle from the true north is called its decli- nation ; the deviation from a horizontal position, its inclination. Both declination and inclination are subject to diurnal, annual, and secular variations. Isogonal lines connect places which have the same declination. Isoclinal lines connect places which have the same inclination. Isoclinal lines are nearly coincident with the isothermal lines. Optical phenomena are caused by changes in the direction, intensity, or com- position of sunlight during its passage through the atmosphere. Rainbows are caused by the action of light on falling raindrops. Halos are caused by snow crystals; Coronae, by minute particles of water in the air; Mirage, by the rays of light being turned out of their original direction. 264 PHYSICAL GEOGRAPHY: REVIEW QUESTIONS. What do you understand by evaporation ? Name the circumstances upon which the rapidity of evaporation depends. Define dew point. What condition is necessary in order that the invisible moisture of the atmo- sphere may become visible in any form of precipitation ? Under what circumstances is dew deposited ? Why is more dew deposited on a clear night than on a cloudy one ? Why is more dew deposited on a still night tban on a windy one? Under what circumstances are fogs, or mists, produced ? How do fogs or mists differ from clouds ? What influence on the formation of clouds is believed to be produced by the presence of dust particles in the atmosphere? What is the condition of the particles of water which form the clouds ; are they minute drops, or hollow vesicles? Describe the appearance of the cirrus cloud. How does its height compare with that of other clouds ? During what parts of the day are stratus clouds most common ? To what do they owe their banded appearance ? Describe the cumulus cloud. During what part of the day is it most common ? Name three conditions under which rain may be caused. By which are the heaviest rains generally produced ? Are the equatorial currents likely to bring rain or drought? Why? The polar currents? Why? Name the periodical rain zones. When does it rain in the zone of calms? In the zone of the trade-winds? Why? Describe the rainfall in the zone of the prevailing westerly winds. In the zone of the polar winds. Describe the construction and operation of a rain-gauge or pluviometer. Why should more rain fall on a mountain than on the lowlands at its base ? Why should more rain fall on the coasts of a continent than in the interior ? Compare the mean annual rainfall of the tropics of the Eastern and Western Continents. Of the temperate regions of America and Europe. Name the rainless districts of the Eastern Continent. Of the Western Con- tinent. What is the cause of the almost total absence of rain in these districts ? Under what circumstances is hail produced ? Define snow line. Upon what does the height of the snow line depend? At what height above the sea-level is it found in the tropics? In the temperate regions ? In the polar zones ? How are glaciers formed ? Name three varieties of glaciers. In what respects do glaciers resemble rivers ? SYLLABUS. 265 Name some rivers which take their origin in the melting of glacial ice. Define lateral, medial, terminal, frontal, and ground moraines. Explain the manner in which fiord valleys were formed. What is the prob- able origin of lakes in glacial districts ? Name some of the European mountain-systems which contain extended gla- cier regions. Name two Asiatic mountain-ranges which contain such regions. How are icebergs formed ? Is the ice of which icebergs are composed salt or fresh ? What are ice floes? State their origin. Into what two general classes may glacial deposits be'divided ? Define drumlin, esker, kame, and valley train. Name some of the evidences of the former existence of glaciers in any country. Describe the Glacial Epoch. Explain the origin of Lake Agassiz ; Lakes Bonneville and Lahontan. How is an electrical flow or current produced ? Define volt ; ohm ; ampere ; circuit. Under what circumstances does lightning occur ? What is the cause of the accompanying thunder ? Name six varieties of lightning. By what are auroras caused? Why does the magnetic needle point to the north ? What is believed to be the cause of the earth's magnetism? Define isogonal lines ; isoclinal lines. With what lines are the isoclinal lines nearly coincident? Explain the phenomena of the rainbow. What is the cause of the sunset tints of the sky ? Of the blue color of the sky? What are halos and coronae ? By what are they caused ? -~oXKc MAP QUESTIONS, Trace on the map of the winds the portions of the world included in the zone of calms. Trace in a similar manner the portions included in the zones of the trades, and in the zones of the prevailing westerly winds. Why should the eastern shores of tropical South America be moist, and the western dry ? To what peculiarity of position does Northern Africa owe its scanty rainfall ? Trace on the declination chart the agone, or line of no declination, in the Western Hemisphere. Trace the line of no declination in the Eastern Hemi- sphere. What smaller line of no declination exists in this hemisphere ? Notice that in the Western Hemisphere the isogonal lines all meet in a point near Hudson Bay ; what does this meeting indicate ? PART V. PLANT LIFE, ANIMAL LIFE, AND MINERALS. The variety and luxuriance of life on the earth are far greater than is at first apparent. Besides the larger animals and plants, myriads of microscopic forms inhabit the land, the water, and the air. From the burning sands of tropical deserts to the eternal snows of the poles, widely differing forms of plants and animals occur, each peculiarly fitted for its particular conditions of growth. An organic form differs in many respects from one that is inor- ganic. An organic form, such as an animal or a plant, has its origin in a germ ; grows from nourishment taken into its structure ; has a regular development in growth by successive stages, from birth to maturity, when it reproduces its kind and passes on to decay and death. 266 PLANT GEOGRAPHY. 267 An inorganic form, such as a crystal, grows by accretions or additions from outside its body, does not reproduce its kind, has no regular development or growth, being perfect from its first existence, and has necessarily neither decay nor death. SECTION I. PLANT LIFE. CHAPTER I. Plant Geography. 382. Living Matter. — All life, whether vegetable or animal, consists of various groupings of cells. Cells are globular masses, formed of a peculiar jelly-like matter called protoplasm, composed of various complex combinations of carbon, oxygen, and sulphur, called proteids. At its beginning, all life consists of a minute germ cell, filled with more or less transparent protoplasm, and containing a darker, opaque spot called the nucleus. Examined by a sufficiently powerful glass, all living protoplasm is seen to be in constant mo- tion, currents passing through the different parts, in somewhat definite directions. In all the higher forms of life, as the germ cell develops, it multi- plies, and various organs appear, peculiar to the form of life from which the germ cell was derived. All living bodies contain organs, and living matter is, therefore, sometimes called organic matter, to distinguish it from non-living, or inorganic matter. Science has not yet disclosed the nature of the change whereby non-living matter is converted into living protoplasm. To produce living matter the inter- vention of already living matter is, so far as is known, absolutely necessary. 383. Intermediate Position of Plants. — Protoplasm forms an essential part of both plants and animals. Plants alone, however, possess the power of manufacturing protoplasm directly from inor- ; 268 PHYSICAL GEOGRAPHY. ganic or non-living matter. Plants prepare food for animals, who consequently, are dependent on plants for their existence. Both plants and animals are consumers of the proteid compounds. Plants alone are producers. In the scale of existence, therefore, plants occupy a position intermediate between minerals and animals. 384. Photosynthesis. — Only plants containing green coloring- matter, or chlorophyll, possess the power of preparing their own food- supply directly from the materials obtainable from the soil or from the air. Plants devoid of chlorophyll, such as mushrooms, toadstools, and fungi generally, are unable to do this, and are dependent on other plants for their existence. By far the greater number of plants are included in the first class. When an ordinary green plant takes in the raw material which forms its food : i. e., mainly carbon dioxide and water, a recombina- tion of the elements of these substances usually takes place in the leaves under the influence of the sunlight. This process is called photosynthesis. As a result, various carbohydrates, or combinations of carbon and hydrogen, are formed, the oxygen of the carbon dioxide being liberated. The proteids are subsequently formed from the car- bohydrates, by combination in different proportions with nitrogen, sul- phur, and other elements derived from the soil. The food thus manu- factured by the plant is either carried directly to parts of the plant where work is being done, or the excess is stored in some part. Most plants produce more food than they need, and it is on this extra food that animals live. 385. Plant Geography treats of the distribution of plant life over the earth. The plants of any section of country, taken collec- tively, are called its flora. Plant geography differs essentially from botany. Botany arranges plants into classes, according to peculiarities in their organs of growth and reproduction. Plant geography considers plants only in reference to their appearance, by which they give a distinct character to the vegetation of a country, or to their general usefulness to man. In this limited view, all the minuter differences in structure or organization are passed over, the general form being the main geographical element of a plant, and the element with which physical geography is principally interested. PLANT GEOGRAPHY. 269 386. Soil. — In various ways the, rock masses are broken up into more or less finely divided waste, which forms the basis of the soil in which plants grow. Usually, there is added to the rock waste a vege- table humus or mould, containing the decaying remains of animals or plants. This vegetable mould is gradually accumulated during many years. The principal agencies by which the solid rock is broken up or disintegrated are weathering, erosion, corrosion, and glacial action. Some soils are formed in place, while others are transported, often for considerable distances, from their origin. Soils may be considered : (1) In regard to their chemical composition ; (2) In regard to their physical properties ; that is, to the extent to which they absorb and retain their water supply. Coulter divides soils into the following classes : (1) Rock, that is, uncrumbled rock, on which certain plants only can exist ; (2) Sand, which possesses a very small water capacity unless it rests on a bed of clay, in which case its water capacity is great ; (3) Lime soils ; (4) Clay, which possesses a great water capacity ; (5) Humus or vegetable mould, which is rich in decaying animal and vegetable matter ; (6) Salt or alkaline soils. From a mere geographical standpoint, the nature of the soil is far from being the most important element in the distribution of vegetation, for, even when a soil is absent, if the other requisites of light, heat, and moisture are present, the simpler vegetable forms soon appear, and slowly prepare, even on a bare, rocky surface, a soil which is able to sustain species, which develop more and more highly with the development of the soil. 387. Conditions Requisite for Plant Growth. — Plants require for their growth certain conditions of light, heat, and moisture ; and as the requisite amount of each of these varies with different spe- cies of plants, we find in every climatic zone a characteristic flora. The soil must contain those mineral ingredients which form a part 270 PHYSICAL GEOGRAPHY. of the structure of the plant, and must contain them in a condition in which they can be readily assimilated by the plant. In considering the conditions requisite for plant growth, reference must be had not only to the actual temperature existing in any region, but also to its distribution throughout different parts of the year. The temperature limits under which plants can exist vary from 0° to 122° F. Certain plants can grow even in the waters of hot springs. In studying the temperature effect on plants, we must consider not only the temperature of the air, but also the tempera- ture of the soil in which the plants grow, and in this connection the soil cover must also be considered. A cover of snow or leaves has the effect of reducing the extremes of temperature. In considering the quantity of water which falls as rain, or the quantity present in any soil, reference must also be had to the water level in the soil. In some soils the water level is only a short distance below the surface ; while in others, such as sandy soils, it is often a considerable distance below the surface. The water in the soil consists either of free water, which can be drained away, or of water which adheres to the soil. 388. Moisture, Heat, and Light are the prime essentials of vegetation, and it is on their distribution that the distribution of vegetation is principally dependent. 389. Plant Groups, or Societies. — Plants may be divided into groups, or societies, according to the quantity of water they require for their vigorous growth. These plant groups or societies are as follows : (1) Water-plants, or such as grow only in water or in very wet soils. An example of a water-plant is to be found in the sargassum, a species of sea-weed that accumulates in the sargasso seas in mid- ocean. The buoyancy of this weed is increased by air contained in ball-shaped floats, as shown in Fig. 122. (2) Drought plants, or those which can exist only in very dry soil with dry air ; these include plants capable of withstanding occa- PLANT GEOGRAPHY. 271 sional, periodical, or constant droughts. The various species of cactus are examples of drought plants. (3) Intermediate plants, or those which occupy a middle ground, requiring neither an excess of moisture nor of dryness. (4) Salt plants, or those whose existence depends not so much on the quantity of water present, as on the fact that this water contains certain saline or salty matters. Such plants are found on the shores of the ocean, or in the neighborhood of salt or alkaline springs. 390. Distribution of Vege- tation. — The influence of heat and moisture is noticed as we pass from the equator to the poles, or from the base to the summit of a tropical mountain. Thus arise a horizontal and a vertical distribution of vegetation. The greatest luxuriance of vegetation is found in the equa- torial regions, where both heat and moisture are most abundant. Here a greater variety of species occurs, and the individual plants are large and brilliantly colored both in their leaves and flowers. As we pass toward the poles, the number of the species diminishes ; trees disappear, being replaced by shrubs and herbs, and these, in turn, by lichens and mosses, until, amid the snows of the polar lati- tudes, even the simplest forms of vegetable life are often wanting. 391. There are various ways in which the vegetation of the earth may be studied : (1) According 1 to Meyen, we may divide the earth's surface into zones ac- cording to the latitude, and the mountainous elevations into zones according to the altitude. Since the distribution of heat is not dependent on the latitude or the altitude alone, we may advantageously modify this plan, and divide the zones hy the isotherms, as has been suggested by Dove. Fig. 122.— Sargaseum. 272 PHYSICAL GEOGRAPHY. (2) According - to Schouw, we may divide the earth's surface into regions characterized by groups of peculiar floras, and separated by natural barriers. The great number of the regions required to give thoroughness to Schouw's system renders its use inadvisable. (3) According to Humboldt and others, we may divide the earth's surface into zones, according to the appearance of the plants inhabiting them. Here, plants of entirely different species are grouped by their mere outward resem- blances into what are called forms. The first method is the one most suitable for our purposes. We shall follow, in the main, Dove's modification, as adapted by A. E. Johnston, and divide the sur- face of the earth into zones, according to the isotherms or lines of mean annual tem- perature. This system is based on the fact that the character of the vegeta- tion is dependent mainly on the tem- perature, which, in its turn, regulates the quantity of moisture. 392. Horizontal Zones of Vegetation. — The earth's hor- izontal zones of vegetation are as follows : (1) The Tropical Zone. Fig. 123.— Date Palm. (2) The Sub-tropical Zones. (3) The Warm Tem- perate Zones. (4) The Cold Tem- perate Zones. (5) The Sub-arctic Zone, or Zone of Conifers. (6) The Polar Zone, or Zone of Alpine shrubs, mosses, and lichens. Fig. 124.— Avenue of Royal Palms. PLANT GEOGRAPHY. 273 393. The Tropical Zone, or the zone of palms, bananas, spices, and aromatic plants, lies on each side of the equator, between the isotherms of 73° Fahr. It includes most of the land within the tropics of both hemispheres. The excessive heat and moisture of this zone produce an especial luxuriance in the vegetation. Trees attain enormous size, the foliage is bright, the flowers brilliant, and the number of species great. The forests are characterized by a great variety of trees, and are almost impenetrable from the numerous parasitic plants with which the trees are covered, and the gigantic, rope-like climbers, or lianas, that twine among them. Palms, bananas, tree-like grasses, and orchids are among the charac- teristic plants. Orchids are curious plants, inhabiting damp forests. They attach themselves to trees and rocks, drawing nearly all their nourishment from the air. As a class, they are noted for their vivid coloring and the curious forms of their flowers. The well-known vanilla bean is obtained from an orchid. The grasses of tem- perate latitudes are repre- sented in the tropical zone by the bamboo, which often attains the height of 60 feet. The banyan-tree, a spe- cies of fig-tree, Fig. 125, is found in the East Indies. From a colossal trunk nu- merous air-branches are sent out, which, descending to the ground, take root, and in their turn send out other branches, and in this way an extended area is cov- ered. A single tree has been known sufficiently large to give shade to 7000 men at the same time. Fi S- 125.— Banyan-tree. The Llanos of the Orinoco lie in the tropical zone. During the dry season they are almost entirely devoid of vegetation ; but during the wet season they are covered with grasses. The Indian Archipelago affords an excellent illustration of the wonderful 18 274 PHYSICAL GEOGRAPHY. luxuriance of the vegetation of the tropics. Here the gigantic Rafflesia bears flowers three feet in diameter ! In the northern and southern portions of the tropical zone, where the mean annual temperature ranges from 79° to 73° Fahr., the vegetation, though similar to that of the equatorial regions, begins to lose its density aud luxuriance. The forests contain less under- growth and fewer parasitic plants. Tree-like ferns and figs are especially" abundant. 394. The Sub-tropical Zones, or the Zones of Laurels and Myrtles, extend in each hemisphere, from the isotherm of 73° to 68° Fahr. Here, the heat of summer, though sufficient to ripen most of the tropical fruits, is not so intense as in the tropical zone. The winters are mild, and scarcely arrest the vegetation. The palms and bananas of the preceding zones are common, but the charac- teristic vegetation is found in the abundance of trees with thick, shining leaves, such as the laurels, magnolias, and myrtles. 395. The Warm Temperate Zones, or the Zones of Ever- green Trees, or trees which do not shed their leaves, extend in each hemisphere, from the isotherm of 68° to 55° Fahr. In this zone, trees with thick, shining leaves occur, mingled with oaks, beeches, and others similar to those found in our own forests. No palms occur, but in their place we find a number of glossy-leaved, ever- green trees and handsome evergreen shrubs. In those portions of this zone which are in the neighborhood of the Mediter- ranean, the bay, myrtle, laurel, fig, and the olive are characteristic. The cork oaks, chestnuts, and pomegranates are frequent. The vine, said to be a native of this zone, attains here its greatest growth, the stem often reaching a thickness of half a foot. In America, oaks, pines, and tulip-trees occur. The southern warm temperate zone includes portions of New Zea- land and Australia, and in South America the Pampas of the Kio de la Plata, a region of grass-covered plains. 396. The Cold Temperate Zone, or the Zone of Deciduous Trees, or those which drop their leaves in autumn, extends in the Northern Hemisphere, from the isotherm of 55° to 41° Fahr. For- ests of deciduous trees are the main characteristics of this zone ; oaks, birches, beeches, chestnuts, walnuts, maples, elms, larches x sycamores, PLANT GEOGRAPHY. 275 and alders are among the most common of the deciduous trees. Mosses and lichens frequently cover the trunks of the trees, and a rich and varied undergrowth occurs ; the holly, clematis, wild rose, honeysuckle, and rhododendron are examples. Extensive meadows, covered with grasses, are found in this zone. The deciduous character of the trees, and the almost total absence of ever- greens, produce a marked contrast between winter and summer. During winter, the foliage almost entirely disappears, and snow covers the ground for long periods. This zone is essentially one of extensive forests. In connection with the warm temper- ate zone of the North- ern Hemisphere, it has always contained the most highly civilized races of Fig. 126.— Fir-trees, Yellow- stone Park. men, and is especially rich in the number and luxuriance of its food- plants. 397. The Sub-arctic Zone, or the Zone of the Cone-bearing Trees, extends, in the Northern Hemisphere, from the isotherm of 41° Fahr. to regions where the mean annual temperature for the month of September is 36.5° Fig. 127.— The Pines, Byberry, Phila. 276 PHYSICAL GEOGRAPHY. Fahr. In this zone both forests and grassy meadows abound. The forests are especially characterized by cone-bearing trees, with evergreen, shining, needle- shaped leaves, such as the pine, spruce, hemlock, cedar, and fir. In the northern portions of the zone beeches and alders are found, and willows, when the soil is moist. The meadows are covered with grasses and flowers, and afford abundant pasturage. The northern limit of trees is marked on the map of the plant regions. 398. The Polar Zone, or the Zone of Alpine Shrubs, Mosses, Lichens, and Saxifrages, extends from the limits of the sub-arctic zone to the pole. In this zone no trees occur except those of a stunted growth. Alpine shrubs of tortuous, compact growth, such as the Alpine rhododendra, the dwarf birch, willow, and alder, occur. Sedges and grasses are found. The pastures of the preceding zones are absent; in their place we find extended areas covered with lichens. The northern plains of Siberia are covered with extensive marshes, called the Tundras, where the ground, during most of the year, is frozen to great depths. The short summers suffice to thaw the surface only, when a few mosses and lichens appear. Near the extreme northern limits of the polar zone, beyond the isotherm of 41° Fahr. for the month of July, such plants only are found as can thrive during the brief Arctic summer of from four to six weeks. Shrubs are entirely absent ; lichens and mosses occur, together with stunted Alpine herbs. In Spitzbergen, lichens and mosses are found, the former being especially numerous. 399. The Vertical Distribution of Vegetation. — It is difficult to make a satisfactory systematic distribution of vegetation into vertical zones, since the tem- perature and moisture, on which such an arrangement must be based, are subject to very considerable variations. Thus, the position of the mountain-ranges as regards the prevalent wind, the direction of the mountain slopes, and the extent of the elevated plateaus, all exert so powerful an influence on the mean annual temperature and the rainfall, that in the same mountain-range, opposite slopes, or even different parts of the same slope, afford marked climatic contrasts. In ranges that are widely separated the differences are still greater. The following chart indicates the characteristic flora at similar elevations in the different continents. PLANT GEOGRAPHY. 277 (1) Between the level of the sea and. 5000 feet above the sea-level the vegetation is, in general, similar to that of the tropical and sub-tropical zones. Palms, bananas, tree-ferns, barley, potatoes, sugar-cane, rice, and cotton are found in the lower parts. (2) Between 5000 and 10,000 feet, the vegetation is, in general, similar to that of the warm temperate zones. In America, the birch and cedar occur in the Fig. 128. — Vertical Distribution of Vegetation (Black). lower portions of the region, and Peruvian bark and the cinchona trees, so useful in medicine, in the upper portions. In Africa and Europe, the pine, birch, and oak occur ; and in Asia, the oak ; here also the vine is cultivated. (3) Between 10,000 and 15,000 feet, the vegetation, in general, is that of the cold temperate zones. Deciduous trees occur ; rye, wheat, barley, and oats are cultivated. (4) Between 15,000 and 20,000 feet, the flora corresponds, in general, to that of the polar and arctic zones. A few rhododendrons and birches occur on the warmer Asiatic slopes, and occasionally, crops of barley are cultivated. The greater part of this zone is covered with perpetual snow. In the descriptions here given, it will be noticed that the correspondence of the vertical and horizontal zones is but of a general character. 400. Plant Regions. — In some localities a few plants occur in such vast numbers over extended areas as to give a characteristic appearance to the country they cover. A brief mention will be made of such regions, especially as they illustrate the influence of the presence or absence of moisture on the vegetation. 401. Forests occur wherever the moisture is abundantly and regularly distributed throughout the year. As a rule, forests are limited to those portions of the world where the rain either falls at 278 PHYSICAL GEOGRAPHY. all times of the year, or is abundant during the season in which the trees are growing, as in the zones of the prevailing westerly winds. Dense forests also occur in the tropics, where moisture is abundant, and some forms occur even in fairly arid districts. Forests may be divided into the following classes : (1) Evergreen Foliage Forests. — These occur mainly in the trop- ical regions, though they are sometimes found in the warmer parts of the temperate zones. They require excessive moisture and heat, and have been aptly styled " rainy forests." The soil is rich and black from the long accumulation of vegetable humus. Tree-ferns, palms, bamboos, with varied lianas or climbers, are characteristic. Aerial or parasitic plants find lodgement on the trees and climbers. These forests constitute the true " evergreen " trees ; the foliage is evergreen because ever-growing, leaves being almost constantly formed and shed. (2) Deciduous Forests, or those whose trees shed their leaves at regular periods. Such forests characterize the temperate zones. The leaves assume vivid coloring a short time before falling. In deciduous forests the undergrowth varies inversely with the thick- ness of the tree growth and the corresponding density of the shade. In beech forests the shade is so dense that the undergrowth is scanty. In oak forests the shade is not so dense, and the under- growth is more abundant. Deciduous forests are usually mixed, containing many varieties of trees, such as maple, oak, elm, beech, walnut, chestnut, hickory, etc. (3) Coniferous Forests are sometimes called evergreen forests. They are not evergreen because ever-growing, but because they do not shed all their narrow, needle-shaped leaves at one time as do the deciduous forests. Such forests include among other trees the pine, spruce, cedar, fir, and hemlock. Where the forests consist mainly of pines, the shade is not so dense as those in which the fir or hemlock prevails. These trees are capable of existing with a comparatively limited water-supply, the small surface of the needle-shaped leaves with thick walls preventing excessive loss of water. (4) Leafless Forests are characteristic of such tropical regions as itude 40 "West from 20 Greenwich Longitude 20 Ea^t from 40 Greenwich GO C REFERENCES 1 ^Ts| TROPICAL ZONE 1 1 COLO TEMPERATE ZONES I SUB ARCTIC ZONE mam SUB TROPICAL ZONES "KvARMTEMPERATEZONEs f | POLAR ZONE A. C E A N PLANT GEOGRAPHY. 279 Java and the arid lands lying near the Gulf of California, both in the United States and Mexico. Here the soil is bare and dry, and the trees possess no leaves. In some regions, for example, near the Gulf of California, varieties of the great cactus repre- sent the tree forms of vegetation. (5) Swamp Forests include willows, alders, birches, etc. Conifers, such as hemlocks, pines, junipers, cypress, and the larch, also occur. 402. Steppes. — When the moisture is scanty and not well distributed throughout the year, but the rainfall is periodical, and long droughts occur in the intervals between the rainy seasons, the forests are replaced by areas called steppes, which, during the wet seasons, are covered with grasses, shrubs, or herbs ; but during the dry seasons are almost destitute of vegetation. Steppes are found in the Llanos and Pampas of South America, in the Great Plains of North America, in the grassy steppes of Australia, Russia, and Asia, in the German heaths, and in the African savannas. Salt steppes, which sometimes occur, are characterized by the absence of vegetation, caused not so much by absence of moisture as by the presence of saline substances. The steppe of the Great Salt Lake region in the United States forms an example. As we have already seen, all such regions are characterized by a limited water-supply. 403. Meadows and Prairies.— Meadows and prairies, like steppes, are covered with tall grasses, but the vegetation is more permanent, the droughts being only occasional. They are found, therefore, in the temperate zones, in the regions of constant rains. An extended prairie region is found in the valley of the Missis- sippi, on both sides of the stream. 404. Deserts are regions characterized by an almost entire absence of vegetation ; they are found mainly in the zones of the trade winds, and are to be ascribed entirely to the absence of moist- ure. Their bare surfaces are subject to great and sudden changes of temperature, being, as a rule, excessively warm during the day, and often quite cool at night. These changes are due to the readi- ness with which a bare surface receives and parts with heat. 280 PHYSICAL GEOGRAPHY. Deserts occur, however, in parts of the world outside the zones of the trade winds, and may be divided into the following classes : (1) Tropical Deserts. — Here the water-supply is a minimum and the heat a maximum, vegetation is almost entirely absent, and the soil is bare. (2) Cactus Deserts. — Such deserts are characteristic of parts of the South-western United States, including Texas, New Mexico, Ari- zona, and Southern California. The spines, bristles, and thorny walls of these plants protect them from attacks of animals. (3) Salt and Alkaline Deserts. — These are due to the presence » :~C -. ' . •-- Fig. 129. — American Desert. of large quantities of alkali in the soil. Death Valley in Southern California and the region in the neighborhood of the Dead Sea in Arabia are examples. 405. Swamp Moors are meadow-like stretches of swampy ground. They occur especially on the borders of reed swamps, on the side nearer the land. They are characterized by sedges and coarse grasses. In some cases trees and shrubs, such as alders, willows, and birches, nourish, thus making a swamp thicket. 406. Sphagnum Moors, so called from the almost exclusive occurrence of the common peat or bog moss called the sphagnum. PLANT GEOGRAPHY. 281 These mosses continue their upward growth after the lower parts have died. They grow rapidly, and make such inroads on the waters of shallow lakes as to transform the lake into a marsh or quaking bog. A peculiarity of such moors is found in the antiseptic character of their water, which prevents the growth of those plant forms known as bacteria, which aid in producing decay. For this reason the bodies of animals submerged in such water do not decay. CHAPTER II. Cultivated Plants. 407. Plants appear to have been originally confined, by condi- tions of soil or climate, to certain localities, from which they have gradually spread to other localities where conditions exist favorable to their growth. In many instances, however, plants furnishing materials for food, clothing, or other staples for the human family, have been purposely transplanted and widely diffused by man. In these cases their successful cultivation is limited to regions where suitable climate and soil either existed naturally, or have been arti- ficially produced. 408. Distribution of the Cereals. — The cereals include barley, rye, oats, wheat, maize or Indian corn, and buckwheat. The cereals and the potato form the most important food-plants of the temperate zones. Barley, thought to be a native of Tartary, can be grown farther north than any other grain ; it is found in Lapland, as far as 70° N. Lat. ; it is largely used in brewing malt liquors. Rye is found in Norway as far north as Lat. 67° N. It is the most common grain in Russia, Germany, and in portions of France. Oats is, probably, a native of the Caucasus ; its northern limit is about 65° N. Lat. in Norway. Wheat is, probably, a native of Tartary. It is the most impor- tant of the cereals, and has a wide vertical and horizontal distribu- 282 PHYSICAL GEOGRAPHY. tion. Its northern limit is 64° N. Lat. in Norway. It is the food grain of nearly all the civilized nations of the earth. The northern states of the United States, and the southern parts of Canada, are the principal producers. Russia is also a large producer. Maize, or Indian Corn, a native of America, is extensively cul- tivated from the southern part of Chili to high latitudes in North America. It forms one of the most valuable food crops in the United States. Its northern European limit is, perhaps, near the iso- therm of 65° Fahr. Buckwheat, a native of the colder portions of the Chinese Empire, is extensively cultivated on the plateaus of Central Asia, and, generally, in the cool temper- ate regions of the rest of the world. Buckwheat is especially valuable on account of the ability it possesses of thriving in sandy or moory soils, where other simi- lar food-plants will not succeed. Potatoes. — The native country of this important food product appears to have been either Chili or Peru. Though cultivated in both the tropical and temperate regions, it is to be regarded as a food-plant of the temperate zones. It pos- sesses a very remarkable range, being cultivated from the extremity of Africa to Lapland : the requisite temperature for its culture in the tropical regions being found on mountain-slopes. 409. The Pood-plants of the Tropical Regions are rice, dates, cocoanuts, bananas and plantains, cassava, bread-fruit, sago, yams, etc. Rice is the seed of a variety of grass that grows in tropical swamp lands. It is a native of China. It is cultivated in China, Central America, Africa, Egypt, Nubia, Persia, the Americas, and the West Maize, or Indian Corn. CULTIVATED PLANTS. 283 Indies. It requires considerable heat and an abundance of moisture. Rice forms the main food of a large proportion of the world's popu- lation. Dates form an important article of food in North Africa, both for man and beast. Dates are obtained from the date-palm, a native of a strip of land on the southern slopes of the Atlas Mountains, where the tree occurs so plentifully as to give to the country the name of Beled-el-Jerid, or the Land of Dates. Different varieties of the date are found in the Saharan oases and in other parts of the world. Cocoanuts are the product of the cocoa-palm, which is valuable for its food, timber, foliage, and fibres. The cocoa-palm is a native of Southern Asia, but is cultivated throughout the tropical regions of Ceylon, Sumatra, Java, and the islands of Poly- nesia. Bananas and Plan- tains are thought to be natives of Southern Asia. They are ex- tensively cultivated throughout the tropical zones, both north and south of the equator. Since their fruit is very nutritious, and the yield of a given area great, they form an exceed- ingly important staple of food. Cassava, a nutritive, starchy material, is obtained from the root of the manioc, a tropical shrub. In some species the fleshy root is several feet long, and is nearly as thick as a man's arm. Cassava meal is obtained by 131. — Banana-tree. 284 PHYSICAL GEOGRAPHY. pounding or beating the root and heating it, in order to expel a poisonous principle which it contains. Tapioca is prepared from cassava. The manioc is a native of Brazil, but is cultivated in Western Africa, in Congo, and in Guinea. Bread-fruit is the pulpy fruit of a tree which grows only in the tropics. The fruit, when baked, resembles bread in taste. The tree yields fruit during most of the year; it is said to be a native of the South Sea Islands, though it is now quite com- mon in the Friend- ly and Society groups,and in many of the neighboring islands. Sago is a starchy substance, obtained from the pith of several species of palm trees which grow in the Moluc- cas. A single tree often yields from 600 to 800 pounds of sago. Yams are the large tubers of a number of plants resembling sweet potatoes. They are cultivated in the Southern United States, in Africa, in South America, and in the West Indies. 410. Sugar-cane is a native of India, but is now extensively cultivated throughout the tropical and warm temperate zones of both hemispheres, in the West Indies, Southern United States, Guinea, Brazil, Mauritius, Bourbon, Bengal, Siam, China, Java, and the neighboring islands. Beets are cultivated in large quantities in Europe and in Cali- fornia for the production of sugar. Fig. 132.— Bread-fruit. CULTIVATED PLANTS. 285 411. Fruits of the Tropical and Warm Temperate Zones. — Besides those already enumerated, we find the following : oranges, lemons, limes, citrons, pineapples, mangoes, figs ; and, in the cooler portions, cherries, peaches, apri- cots, and pomegranates. 412. Distribution of Plants yielding Bev- erages. — The principal plants yielding bever- ages by infusion are tea, coffee, and cocoa. Tea consists of the dried leaves of a number of evergreen shrubs, natives of China. Tea is cultivated in China, Japan, and Assam, the principal tea-producing countries of the world. It is cultivated also in Java, Southern India, Ceylon, Australia, Natal, and Brazil. Tea is cultivated in Japan as far as Lat. 39° N. Tea was introduced into Europe by the Dutch, in 1610. Coffee is the berry of a tree found native in Abyssinia. The tree attains a height of from 15 to 20 feet ; it has shining green leaves, and bears white flowers, which are followed by reddish-brown berries, each of which contains two grains of coffee. Brazil is the greatest coffee-producing country of the world. Coffee is grown also in the northern and north-western parts of South America, in the "West Indies, in Java, Sumatra, Ceylon, Mauritius, Southern Arabia, and on the west coast of Africa. Cocoa. — The cocoa-tree is cultivated in Cen- tral America, Guiana, Chili, India, Japan, and in several islands in the Indian Ocean. The tree attains a height of about 20 feet. Chocolate is prepared from the seed of the cocoa-tree. 413. Spices, such as pepper, cloves, nutmeg, and cinnamon, are cul- tivated mainly within the tropics. Vanilla, used in flavoring, is limited to this region. Fig. 133.— Sugar- cane. 286 PHYSICAL GEOGRAPHY. Pepper. — Black pepper is the dried seed of a climbing shrub that grows wild in Western Hindostan. It is raised in the West Indies, in Siam, in the Philippines, and in the Malay Peninsula. Red or Cayenne pepper is grown in Guiana and the East. Cloves are the dried flower-buds of an evergreen tree, a native of the Moluccas. It is grown extensively in the island of Amboyna, in the Malay Archipelago, in Zanzibar, in Guiana, and in the West Indies. Fig. 134.— Tea-plant. Fig. 135.— Coffee. Nutmegs are the seed of a tree that grows in the Banda Islands and in the Moluccas. The tree is now grown in tropical America, in Mauritius, and in Madagascar. The nutmeg is covered by several layers of vegetable matter, one of which is the mace of commerce. Cinnamon is the inner bark of a small tree growing wild in Ceylon. It is cultivated in Ceylon and many other tropical countries. Vanilla is obtained from the dried, fragrant pods of a plant grown mainly in Mexico, Central America, and Brazil. 414. The Principal Narcotics used in different parts of the world are opium, prepared from a species of poppy ; the betel-plant, a native of Hindostan, the leaves of which are chewed, together CULTIVATED PLANTS. 287 with the areca-nut ; hasheesh, a narcotic used in India ; and tobacco, the dried leaves of a plant grown extensively in Mexico, Cuba, Brazil, and in the United States. 415. Plants Valuable for giving Materials for Clothing- are cotton, hemp, and flax. Cotton is a native of India. The principal cotton-fields of the Avorld are in the Southern United States, India, China, Africa, Egypt, Australia, Southern Europe, South America, and the West Indies. Cotton is among the most valuable of the vegetable sta- ples. Its culture and manufacture give employment to many people, and have been an important factor in the development of the human race. Hemp and Flax are cultivated in the temperate regions of Rus- sia and throughout Great Britain and the United States. The Philippine Islands produce an especially valuable variety. The plants producing medicines, and products employed in the arts or manufactures, are : The Cinchona-tree, found on the upper slopes of the tropical Andes. Quinine is obtained from the bark of the tree. Gum Arabic, obtained from the East Indies, Egypt, and Africa. Indigo, a blue dye, obtained from the indigo-bearing plants. Brazil-wood, Nicaragua-wood, . and Log- wood yield reddish dyes. Quercitron and Black Oak yield a yellow dye. Turpentine and Rosin are products of the pine tree. Caoutchouc, or India-rubber, is the juice of a tropical tree. Olive Oil is derived from the fruit of the olive-tree, cultivated on the borders of the Mediterranean. Cocoanut, Palm, Flaxseed, and Cotton-seed Oils are ob- tained respectively from the cocoanut, the palm, and the seeds of flax and cotton. «*«^c CHAPTER VI. The Insular Possessions of the United States. 526. Enumeration. — The recently acquired insular possessions of the United States are the Philippine Islands, the Hawaiian or Sandwich Islands, the island of Porto Rico, the island of Guam, in the Marianne Group, and the island of Tutuila, in the Samoan Group, and Wake Island, in the mid-Pacific Ocean. 527. The Philippines. — The Philippine Islands lie off the east- ern coast of Asia in the Pacific Ocean, approximately, between THE INSULAR POSSESSIONS OF THE UNITED STATES. 359 Longitude East from Greenwich 360 PHYSICAL GEOGRAPHY. Long. 116° and 127° E. from Greenwich, and Lat. 5° and 20° N. They are, consequently, situated entirely within the tropics. They are south-east of China and east of Anam, from which they are separated by the South China Sea. They comprise about 2000 islands, with a combined land area estimated at 140,000 square miles. Luzon, the largest island, lies near the northern end of the group, and has an area about equal to that of the State of Ohio. Mindanao, the next largest island, has an area somewhat less. Samar, Pan ay, Palawan, and Mindoro have each an area, approxi- mately, equal to the State of Connecticut. Leyte, Negros, Cebu, Masbate, and Bohol are other fairly large islands. The coast line of the principal islands is, approximately, 11,000 miles. Manila, situated on Manila Bay, in the island of Luzon, is the principal city of the Philippine Islands. Fig. 163.— Native Houses, Philippine Islands. 528. Agricultural Products. — Animals. — The principal agri- cultural products are rice, corn, hemp, sugar, tobacco, cocoanuts, THE INSULAR POSSESSIONS OF THE UNITED STATES. 361 and cacao. Yams, bananas, and pineapples are also raised. Only about one-ninth of the area is under cultivation. The rice and corn are principally produced in Luzon and Mindoro. Hemp is produced in southern Luzon, Mindoro, and Mindanao. Tobacco is raised in nearly all the islands, but the best and largest crops are those of Luzon. Sugar is raised in most of the islands south of Luzon. Cocoanuts are grown 'in southern Luzon. Cacao is raised in the southern islands. Cattle, sheep, and goats have been intro- duced into the archipelago. Pigs and chickens are common. The carabao or water buffalo is the principal beast of burden. The native houses are constructed largely of bamboo, which forms the framework. The floors are covered with split bamboo poles Fig. 164. — Bamboo Lumber-yard in the Philippines. placed with the rounded side upward. The lowest floor, which is entered by means of a bamboo ladder, is several feet above the ground. 529. Minerals and Forests. — Undeveloped deposits of coal or lignite, petroleum, copper, iron, and lead are said to exist. Gold is mined to some extent in Luzon. 362 PHYSICAL GEOGRAPHY. Dense forests cover large areas of all the islands, and in them are found many species of trees which furnish valuable hard woods. Sapan wood, from which a dye is extracted, known as "false crim- son," to distinguish it from cochineal, cedar, bancal, and ebony are some of the woods. The bamboo, a tree-like variety of grass that often attains a height of 50 feet or over, is native to different parts of the archipelago. It is noted for" the wonderful rapidity of its growth. The bamboo poles constitute the principal woods of the native lumber yards. Not only does this wood enter largely into the construction of the native houses, but it is employed also for the masts of small ships, in the construction of the native boats, and for agricultural and house- hold implements, etc. There are numerous excellent harbors in the archipelago. 530. Climate.— The climate is hot, especially in the southern portion of the group. The excessive moisture often renders the heat very oppressive. The south-west monsoon, which begins in May, brings heavy rains, which continue until October. The heaviest rainfall occurs during July and August. During the prevalence of the north-east monsoon, which occurs between October and May, the weather is comparatively dry. The annual rainfall, in some parts of the islands, is as high as 114 inches. Luzon, and the islands in its vicinity, are subject to severe typhoons. The region is one of active volcanoes, and is subject to severe earthquake shocks. Mayon, near the southern end of Luzon, 8925 feet high, is one of the most famous volcanoes in the Fig. 165. — Volcano of Mayon, Philippine Islands. THE INSULAR POSSESSIONS OF THE UNITED STATES. 363 Fig. 166. — Rice Fields of Hawaii. archipelago. It is almost constantly in action. Taal, on the island of Luzon, and Apo, on Mindanao, are also important volcanoes. 531. The Hawaiian or Sandwich Islands are situated in the Pacific Ocean, approximately between Long. 154° W. and 161° W., and from 19° to 22° KLat. The princi- pal islands lie south of the Tropic of Cancer, and are, therefore, situated within the tropics. Hawaii, at the southern end of the group, is the largest island. Maui, Oahu, Kauai, Molokai, and Lanai are next in importance. The total land area is about 6740 square miles. The principal city is Honolulu, on the island of Oahu. Its situation, in relation to the sailing routes between the principal ports of the Pacific, has given it the name of " The Cross- roads of the Pacific." 532. Surface Structure. — The Hawaiian Islands are of volcanic origin, and are, therefore, mountainous. Hawaii is formed mainly of three great volcanic cones: Mt. Kea, 13,805 feet, now extinct; Mt. Hualalai, in eruption in 1801, 8275 feet, and Mauna Loa, 13,675 feet, in frequent eruption. Kilauea, a well-known active volcano, is a crater on the eastern slopes of Mauna Loa. 533. Climate. — Rainfall. — On the coasts tropical climates exist. The mean annual temperature of Honolulu for the daytime is 80° F. On the mountain slopes, the usual decrease in temperature with increased elevation occurs. The rainfall on the slopes toward the prevailing wind is copious, especially at fairly considerable elevations. At altitudes of 900 feet the annual rainfall is 116 inches, and at 2800 feet, 179 inches. 534. Productions. — The soil is of volcanic origin. The princi- pal agricultural products are sugar and coffee. Tropical fruits, such 364 PHYSICAL GEOGRAPHY. as bananas and pine-ap- ples, are grown in large quantities ; oranges, limes, lemons, and rice are also grown. The sugar crop is especially valuable. The rice fields are worked almost entirely by the Chinese. Extensive forest areas exist. 535. Porto Rico is the most eastern and the smallest of the Greater Antilles of the West Indies. It is situated about 60 miles east of the island of Hayti, be- tween Long. 65° 15' W. and 67° 15' W. from Greenwich, and 17° 50' and 18° 45' N. Lat. It is, therefore, a tropical island. Its approximate area is 3670 miles. It is a mountainous country, high in the centre and low near the coasts. SYLLABUS. 365 536. Climate and Products. — The heavy rainfall has eroded the mountains so that, from the ocean, they present a deeply cut, ser- rated appearance. The soil is extremely fertile. The principal agricultural products are sugar, coffee, tobacco, cotton, maize, and rice. There is also pro- duced a great variety of tropical fruits, such as bananas, pineapples, mangoes, grape fruit, oranges, and lemons. Valuable forests, containing a great variety of trees, cover much of the mountainous country. The island has valuable mineral deposits. 537. Guam, the principal island of the Ladrone or Marianne Group, was ceded by Spain to the United States, December 10, 1898. Guam is situated in the Pacific Ocean, north-east of the Philippines, about 5200 miles from San Francisco, and 900 miles from Manila ; it has a length of about 32 miles and a circumference of 100 miles. 538. Tutuila, one of the Samoan Islands, is situated in the tropical Pacific Ocean, south of the equator, some 4000 miles from San Francisco, 2200 miles from Hawaii, and 4200 miles from Manila. It has an area of about 54 square miles, and contains the magnifi- cent harbor of Pago-Pago, sufficiently large to more than accommo- date the entire navy of the United States. 539. "Wake Island, a lonely island in the mid-Pacific, is 1300 miles east of Guam, and 2000 miles west of Honolulu. It has a good harbor. SYLLABUS. The United States of America, exclusive of Alaska and its insular possession, covers an area of 3,026,500 square miles. The coast line is comparatively simple and unbroken. The principal indenta- tions on the east are Long Island Sound, Delaware and Chesapeake Bays, and Albemarle and Pamlico Sounds ; on the west, the Gulf of Georgia, the Bay of San Francisco, and Puget Sound. The slope of the Atlantic shores is gradual, whereas that of the Pacific shores is abrupt. 366 PHYSICAL GEOGRAPHY. On the Atlantic coast, the islands north of Cape Cod are, for the most part, rocky ; while those on the south are generally low and sandy. Two distinct mountain-systems traverse the United States: (1) The Pacific Highlands on the west, which form a part of the Cordillera of the Eocky Moun- tains ; (2) The Atlantic Highlands, or the Appalachian system on the cast. The Pacific Highlands includes the Eocky Mountains on the east and the Pacific Mountain chain ; i. e., the Cascade Mountains in Oregon and Washington and the Sierra Nevada Mountains in California. Some of the highest peaks of the Eocky Mountains are Long's Peak, Pike's Peak, Blanca Peak, Spanish Peak, and Fremont's Peak. Some of the highest peaks of the Pacific Mountain chain are Mt. Bainier, Mt. Shasta, Mt. Whitney, and Mt. Brewer. The culminating point of the North American Continent is Mt. McKinley, in Alaska. The Atlantic Highlands, or the Appalachian System, include the White Mountains, Green Mountains, the Adirondacks, the Catskill Mountains, the Alleghanies, the Blue Eidge, and the Cumberland Mountains. There are two great low plains in the United States, the Atlantic Coast Plain, and the Central Plain, or the Plain of the Mississippi Valley. The principal rivers draining into the Atlantic are the Penobscot, Merrimac, Connecticut, Hudson, Delaware, Susquehanna, Potomac, Eoanoke, Cape Fear, Santee, Savannah, Altamaha, and St. John's. The principal rivers draining into the Mexican Gulf are the Chattahooche, the Alabama, the Mississippi, the Sabine, the Trinity, the Brazos, the Colorado, and the Eio Grande. The principal Pacific rivers are the Columbia, the Sacramento, the San Joaquin, and the Colorado. The Great Lakes, Superior, Michigan, Huron, Erie, and Ontario, form the largest system of fresh-water lakes in the world. The rivers and lakes of the Great Basin have no outlet to the ocean, and therefore form true steppe systems. Soda Valley in southern California and Death Valley in south-eastern Califor- nia are below the mean level of the sea. The United States extend between the isotherms of 40° and 77° Fahr., and therefore lie in the north temperate and in the torrid physical zones. A marked contrast exists between the temperature of the eastern and the western coasts of the northern half of the country. The eastern coasts are far colder than the western. The greater warmth of the western coasts is to be attributed to the influence of warm ocean currents, westerly winds, and heavy rainfalls. From observations dating back as far as 1738 the climate of the United States since that time does not appear to have undergone any decided change. The United States, exclusive of Alaska and its insular possessions, lies in the zone of the variable winds. Westerly winds, therefore, predominate. SYLLABUS. 367 The rainfall is heaviest on the coasts, especially on the southern and western. On the Pacific coast the rainfall is heaviest in the north. On the Atlantic coast it is heaviest on its south. East of the Mississippi it may rain at any time of the year, the rainfall being dependent on the prevalence of the extra-tropical cyclone. West of the Missis- sippi the rainfall is irregular; in parts of the Pacific Coast the rain is most frequent in winter. On the Pacific seaboard in Washington and Oregon the annual rainfall is 75 inches. On the borders of the Gulf States it is 65 inches. On the Atlantic coast it varies from 40 to 50 inches. Between the eastern slopes of the Pacific range and the Rocky Mountains the rainfall is so scanty that agriculture is impossible without irrigation. The Weather Bureau was established for the observation of the meteorological conditions of the country. Nearly all the great storms of the United States are extra-tropical cyclones that cross the country in a generally easterly direction. When once started, it is comparatively easy to predict coming changes in the weather. Weather and storm signals consist of flags designed to indicate the probable weather and temperature of the coming day and information concerning a coming storm. There are four characteristic plant regions in the United States : the Forest, the Prairie, the Steppe, and the Pacific. The Prairie Eegion lies west of the Mississippi Valley to the Plateau of the Great Plains. It is covered with grasses and flowering herbs. The Steppe Eegion lies between the western limits of the Great Plains to the Sierra Nevada and Cascade Ranges. It has a scanty vegetation, mainly the cactus and wild sage, and in some places is practically a desert. The Pacific Region lies between the western limits of the Steppe Region and the Pacific Ocean. In Washington and Oregon and on the mountain slopes gen- erally, dense forests occur. The Forest Region lies mainly east of the Mississippi ; the characteristic trees are the pine, spruce, hemlock, fir, juniper, beech, maple, birch, alder, oak, and poplar. The principal large animals of the United States are those which have been domesticated, as the horse, ox, cow, sheep, mule, goat, and dog. Among wild animals are the black bear, panther, deer, grizzly bear, wolf, and manatee, or sea-cow. The principal agricultural productions are wheat, corn, rye, oats, barley, buck- wheat, hay, hops, flax, tobacco, rice, cotton, and sugar. Named in the order of their money values the principal mineral products of the United States are pig iron, bituminous and soft coal, copper, anthracite or hard coal, gold, silver, petroleum, building stone, natural gas, and lead. The principal metals named in the order of their money values are iron, copper, gold, silver, lead, zinc, aluminium, and mercury. 368 PHYSICAL GEOGRAPHY. The Territory of Alaska has an area of 580,107 square miles, and is nearly one-fifth that of the area of the rest of the continental domain of the United States. Bering Sea on the west, and the Gulf of Alaska on the south, are the prin- cipal indentations of the coast. Bristol Bay, Kuskokwim Bay, Norton Sound, and Kotzebue Sound are among the most important of the smaller indentations. The principal islands are St. Lawrence Island, Nunivak, and the Pribyloff Islands in Bering Sea, the Aleutian Islands, Kodiak Islands, and the Alex- ander Archipelago, including Baranoff, Chichagof, Kupreanof, and Prince of Wales Islands. The northern portions of Alaska are low and flat, and are covered by tundras or frozen moor-lands. The rest of the country is generally mountainous, and is traversed by prolongations of the Pacific Mountain system of North America. Mts. McKinley and St. Elias are the principal peaks. The principal river of Alaska is the Yukon, which is some 2000 miles long, and is one of the largest rivers of the North American Continent. The Kusko- kwim is the only other important river. The Yukon has a delta mouth — the Kuskokwim, an estuary. The climate of Alaska is cold and wet, though under the combined influences of the Japan Current, the rains, and the warm south-westerly winds, the climate is less severe than at corresponding latitudes in the interior or on the Atlantic coast. Dense growths of grasses abound during the brief summer. Forests of cedar and spruce cover portions of the lower mountain slopes and river valleys. The chief animals are the polar and brown bears, the mink, black or silver fox, the moose, and the caribou. The whale is found in the waters off the north- ern shores ; the seal and the sea-otter are sources of wealth by reason of their valuable furs. The walrus is killed by the natives for its flesh, oil, and tusks. Salmon, halibut, cod, and herring are the principal food fish. Deposits of coal, silver, gold, copper, iron, and lead occur in different parts of the country. The inhabitants consist of various elements, the principal of which are the Esquimaux, the Indians, the Aleuts, the Creoles, and the people of the archi- pelagoes of the southern and south-eastern coast, together with a constantly increasing white population attracted by the extensive gold deposits. Sitka, on Baranoff Island, is the principal settlement. The recently acquired insular possessions of the United States are the Philip- pine Islands, the Hawaiian or Sandwich Islands, Porto Eico, Guam, in the Mari- anne group, Tutuila, in the Samoan group, and Wake Island. The Philippine Islands are south-east of China and east of Anam. Luzon, Mindanao, Samar, Pauay, Palawan, Mindoro, Leyte, Negros, Cebu, Masbate, and Bohol are the principal islands. The area of all the islands is about 140,000 square miles. The principal agricultural products are rice, corn, hemp, sugar, tobacco, cocoa- SYLLABUS. 369 nuts, and cacao. Undeveloped deposits of coal or lignite, petroleum, copper, iron, and lead are said to exist. Dense forests which furnish valuable hard woods cover large areas in all the islands. The bamboo is common. The Philippines lie in the tropics. The climate is therefore hot, especially in the southern portion. The south-west monsoon brings heavy rains from May to October. The annual rainfall in some parts of the islands is 114 inches. Severe typhoons occur in Luzon. Severe earthquakes and active volcanoes characterize the Philippines. The Hawaiian or Sandwich Islands are situated near the middle of the North Pacific, south of the Tropic of Cancer. They possess, therefore, a tropical climate. Hawaii, Maui, Oahu, Kauai, Molokai, and Lanai are the principal islands. The area of all the islands is about 6740 square miles. Honolulu, on Oahu, is the principal city. The Hawaiian Islands are of volcanic origin. On the coasts the climate is tropical. On mountain slopes that face the prevailing winds the annual rainfall in places readies 179 inches. The principal agricultural products are sugar and coffee. Tropical fruits, such as bananas, pineapples, oranges, lemons, etc., are plentiful. Extensive forests exist. Porto Eico is the most eastern and the smallest of the Greater Antilles. It is situated 60 miles east of Hayti. Its area is approximately 3670 miles. It has a heavy rainfall and a fertile soil. Its principal agricultural products are sugar, coffee, tobacco, cotton, maize, and rice, together with various tropical fruits. It possesses valuable forests and valuable mineral deposits. Guam, the principal of the Ladrone or Marianne Islands, is situated in the Pacific Ocean north-east of the Philippines, about 900 miles from Manila and 5200 miles from San Francisco. Tutuila, one of the Samoan Islands, is situated in the tropical southern Pacific, 4000 miles from San Francisco, 2200 miles from Hawaii, and 4200 miles from Manila. It contains the magnificent harbor of Pago-Pago. Wake Island is a lonely island in the mid-Pacific, some 1300 miles east of Guam and 2000 miles west of Honolulu. REVIEW QUESTIONS. State the geographical position of the Uuited States, excluding its insular possessions and Alaska. Describe the peculiarities of its coast lines. Name the principal indentations of the eastern coast. Of the western coast. In what respect do the islands which lie north of Cape Cod differ from those which lie south of it ? 24 370 PHYSICAL GEOGRAPHY. What is the origin of the islands off the southern coast of Florida ? Describe the Pacific Highlands. Of what mountains do these consist ? Locate the Great Plains. The Great Basin. Name the principal peaks of the Eocky Mountains. Of the Pacific Mountain chain. Describe the Atlantic Highlands or the Appalachian System. Name the great low plains of the United States. Describe the lake-systems of the United States. What system of inland drainage is found in the United States? In what mathematical zone is the United States situated ? In what physical zones ? Between what isothermal lines does the United States extend ? What difference exists between the climate of the eastern and western coasts ? What are the causes of this difference ? In what wind zone does the United States lie? In what parts of the country does the heaviest annual rainfall occur? The smallest annual rainfall ? For what was the Weather Bureau established ? Describe the following storm or weather signals employed by the U. S. Weather Bureau ; viz., the temperature signal ; the clear or fair weather signal ; the rain or snow signal ; the cold wave signal ; the storm signal ; the information signal. How would the signal flags be displayed to indicate an approaching storm, followed by a cold wave? Under what four characteristic plant regions may the vegetation of the United States be arranged ? Describe the location of each of these regions. Name the principal forest trees of the United States. Name the principal domesticated and wild animals of the United States. Enumerate the principal agricultural productions. Name the principal corn-producing States. The principal wheat-producing States. Name the principal cotton-producing States. The principal rice-producing States. The principal sugar-producing States. Name, in the order of their money value, the principal mineral products of the United States. Name in the same order the most important metals. Where are the principal coal-fields of the United States situated ? The princi- pal coal-oil or petroleum fields? The principal fields of natural gas? The principal beds of salt? What are the limits of the Territory of Alaska ? State its boundaries. What is its area ? Name the principal indentations of the coast of Alaska. What is the extent of its coast line ? Name the principal islands of the western coast. Of the southern coast. Describe the surface structure of Alaska. Describe the river-system of the Yukon. Where is the Kuskokwim Biver ? SYLLABUS. 371 What is the general climate of Alaska ? Describe the vegetation of Alaska. What are the principal trees ? Name the principal food fish of Alaska. Name the principal fur-bearing animals. What other large animals are found in the country? Name some of the different people who inhabit Alaska. Name the principal settlement of Alaska. Name the most important of the insular possessions of the United States. Name the principal islands of the Philippines. What is the number and the combined area of the Philippine Islands? Name and locate the principal city. Name the principal agricultural products of the Philippines. What mineral deposits are said to exist ? What kinds of forests exist ? Describe the climate of the Philippines. When does the rainy season occur? Name the principal of the Hawaiian or Sandwich Islands. The principal city. Describe the surface structure of the Hawaiian Islands. Describe the climate. Name the principal agricultural productions. What is its approximate area of Porto Eico ? Name its principal agricultural productions. Describe its climate. Where is Guam Island ? Tutuila Island ? Wake Island ? State some import- ant fact concerning each. oo^o* MAP QUESTIONS, Describe from the Physical Map of the United States the surface structure of the country, giving the relative position of the High Lands and Low Lands. Describe the Pacific Highlands. Describe the Atlantic Highlands or the Appalachian System. Locate the Black Hills ; the Wahsatch Mountains ; the Sierra Nevada ; the Cascade Mountains; the Coast Mountains; San Luis Park; Pike's Peak ; Long's Peak ; Fremont's Peak ; Mt. Eainier ; Mt. Shasta ; Mt. Whitney ; Mt. McKinley. Describe the drainage of the Great Lakes. Name the principal rivers which empty into the Atlantic. Into the Gulf of Mexico. Into the Pacific. Name the principal tributaries of the Mississippi. Where are the Santa Barbara Islands? The Bahama Islands? Vancouver's Island ? What is the cause of the southward deflection of the isothermal lines in the western part of the United States ? In what portions of the United States is the lowest mean annual temperature found? The highest? Locate the principal indentations of the coast of Alaska. Locate its principal islands. Its principal river systems. Locate the two highest mountain peaks. Where are the Philippine Islands situated? Locate Luzon, Mindanao, Samar, Panay, Palawan, Mindoro, and Negros. Locate the Hawaiian or Sandwich Islands. Locate Porto Eico. 372 PHYSICAL GEOGRAPHY. GENERAL SYLLABUS. Physical Geography treats of the distribution of the land, water, air, plants, animals, and minerals of the earth. The rotundity of the earth is proved : (1) By the appearance of approaching or receding objects; (2) By the circular shape of the horizon; (3) By the shape of the earth's shadow; (4) By actual measurement. Exact geographical position is determined by reference to certain imaginary circles called parallels and meridians. Maps are drawn on different projections : the Equatorial, the Polar, Mercator's, and Gall's, a variety of Mercator's projection, are in the most general use. The rotation of the earth is proved : (1) By Foucault's pendulum ; (2) By the deviation in the direction of falling bodies, winds, and ocean currents. It is rendered probable by analogy. The change of seasons is caused by the revolution of the earth, together with the inclination of the earth's axis at an angle of 66° 33' to the plane of its orbit, and the constant parallelism of the axis with any former position. The proofs of the present highly heated condition of the interior of the earth are as follows : (1) The deeper we penetrate the crust, the higher the existing temperature. (2) The presence of volcanoes, which, in all latitudes, eject melted rock from the inside of the earth. (3) Careful geological observations which prove that the earth's surface is never at rest, but is gradually sinking in some places and rising in others. The gradual cooling of the heated and potentially plastic interior causes : (1) Crater or volcanic eruptions ; (2) Fissure or sheet eruptions ; (3) Gradual changes of level ; (4) Earthquakes. Volcanoes eject from the interior of the earth: (1) Melted rock or lava; (2) Ashes or cinders ; (3) Vapors or gases. Nearly all volcanoes are found near the coasts of the continents or on islands. The principal volcanic regions of the earth are : (1) Along the shores of the Pacific; (2) In the islands of the Pacific; (3) In the neighborhood of the seas that divide the northern and the southern continents ; i. e., the Caribbean Sea, the Mediterranean and Bed Seas, and in the Pacific and Indian Oceans between South-eastern Asia and Australia ; (4) In the northern and central parts of the Atlantic Ocean; (5) In the western and central parts of the Indian Ocean. Fissure or sheet eruptions occur : (1) As extensive vertical sheets filling great fissures in the rocks of all geological formations; (2) As extensive horizontal sheets between parallel strata, or spread out over vast areas of the bed of an ocean or the earth's surface ; (3) In dome-shaped masses called laccoliths or laccolites. Earthquake shocks are more frequent: (1) In winter than in summer; (2) At night than during the day ; (3) During the new and full moon than during any other phase. GENERAL SYLLABUS. 373 Bocks may be divided, according to their origin, into four classes : (1) Igne- ous ; (2) Aqueous ; (3) Metamorphic ; (4) JColian. Eocks may be divided according to their condition into : (1) Stratified ; (2) Unstratified. Unstratified rocks are either igneous or metamorphic. Aqueous rocks are sometimes called sedimentary. During the geological past extensive changes occurred in the land and water surface of the earth, and in the plants and animals inhabiting it. There were six geological eras : (1) The Azoic era ; (2) The Eozoic era ; (3) The Palaeozoic era ; (4) The Mesozoic era ; (5) The Cenozoic era ; (6) The Era of Man. The agencies now causing changes in the earth are : (1) Erosion or denuda- tion; (2) Wind corrasion and transportation; (3) Avalancbes and land-slides; (4) Ocean waves; (5) Man. Erosion is due to three agencies : (1) Weathering ; (2) Corrasion ; (3) Trans- portation. The principal agencies that cause weathering are : (1) Heat and cold ; (2) Alternate freezing and thawing ; (3) Eusting, corrosion, and solution. Of the 197,000,000 square miles of the earth's surface, 144,000,000 square miles are covered by water, and 53,000,000 by land. The proportion between the land and water is very nearly as the square of three is to the square of five. Nearly all the land masses are collected in one hemisphere, and a large part of the water in another. There are two great systems of trends or lines of direction, along which the shores of the continents, the mountain-ranges, the oceanic basins, and the island chains extend. The coast lines of the northern continents are very irregular, the shores being deeply indented with gulfs and bays, while those of the southern continents are comparatively simple. Of the 53,000,000 square miles of the land, 3,000,000, or about one-seventeenth, is composed of islands. Islands are either continental or oceanic. Continental islands are detached portions of the neighboring continents. Oceanic islands are either high or low; the high oceanic islands are generally of volcanic formation ; the low islands are of coral formation. There are four varieties of coral formations : (1) Fringing reefs ; (2) Barrier reefs ; (3) Encircling reefs ; (4) Atolls. The earth's surface is composed of high lands and low lands. The dividing line is 1000 feet above the level of the sea. High lands are either mountains or plateaus. Low lands are either hills or plains. According to their origin plains are: (1) Undulating; (2) Marine; (3) Allu- vial. According to their position they are : (1) Coastal ; (2) Inland ; (3) Worn- down mountain low-lands, old plains, or peneplains. According to the materials that form them they are : (1) Biver-made ; "(2) Dust ; (3) Lava. 374 PHYSICAL GEOGRAPHY. According to their position plateaus are: (1) Marginal; (2) Intermont. According to their age they are : (1) Young or immature; (2) Old or matured. As the earth cooled its interior shrank away from the crust. Enormous lat- eral pressures were produced in the crust as it slowly accommodated itself to the new conditions. These pressures fractured the crust, folded or Dent it, or caused it slowly to rise in some places and to sink in others. Mountains were formed: (1) By flexure; (2) By fracture; (3) By the injection of lava between strata. The following peculiarities are noticeable in the relief forms of the conti- nents : (1) The continents have, in general, high borders and a low interior. (2) The highest border lies nearest the deepest ocean. (3) The greatest prolongation of a continent is always that of its predominant mountain-system. (4) The prevailing trends of the mountain masses are the same as those of the coast lines, and are, in general, either north-east or north-west. Water acquires its maximum density at about the temperature of 39.2° Fahr. Water requires more heat to warm it, and gives out more on cooling than any other common substance. The drainage of the land is of two kinds : (1) Subterranean ; (2) Surface. Surface drainage is either oceanic or inland. Springs are : (1) Hill-side ; (2) Fissure ; (3) Artesian. Constant springs have large reservoirs ; temporary springs, small reservoirs. Hot springs either have deep-seated reservoirs, or their reservoirs are near beds of recently ejected lava. The principal mineral springs are silicious, sulphurous, chalybeate, brines, and acidulous. The quantity of water discharged by a river depends : (1) On the size of its basin ; (2) On the amount of its rainfall. The material eroded by a river is deposited : (1) In the channel of the river ; (2) On the alluvial flats or flood-grounds ; (3) At the mouth ; (4) Along the coast near the mouth. In the upper courses of rivers erosion occurs mainly on the bottom of the channel ; in the lower courses, at the sides. The gradual elevation of river basins may engraft or unite several indepen- dent rivers in a single river system. Its gradual depression dismembers the tributaries of a system, or causes them to become separate rivers. In the Northern Hemisphere the earth's' rotation causes a river to tend to corrode its right bank more than its left bank, and in the Southern Hemisphere, its left bank more than its right bank. The Atlantic and Arctic Oceans receive the waters of nearly all the large river systems of the world. Lakes are : (1) Lakes of new land-areas ; (2) Delta lakes ; (3) Lagoon or sea- shore lakes; (4) Glacial lakes; (5) Playa lakes. Some prehistoric lakes are : (1) Agassiz ; (2) Bonneville ; (3) Lahontan. GENERAL SYLLABUS. 375 Lakes have a comparatively short life because : (1) Their basins are filled by- sediment from their inflowing streams ; (2) Tbeir outlets are cut down by their outflowing streams. The bed of the ocean is less diversified than the surface of the land. The greatest depth of the ocean is, probably, greater than the greatest elevation of the land. The articulation of land and water assumes four distinct forms — inland seas, border seas, gulfs and bays, and fiords. Inland seas characterize the Atlantic ; border seas the Pacific ; gulfs and bays the Indian ; fiords the Atlantic and Pacific. The Pacific occupies about one-half of the entire water area of the earth, the Atlantic about one-quarter, the Indian about one-fifth, the Antarctic about one- seventeenth, and the Arctic about one thirty-fifth. A deposit of fine calcareous mud or ooze, formed of the hard parts of minute animalculse, occurs over extended areas of the floor of the ocean. Tides are caused by the attraction of the sun and moon : spring tides by their combined attractions ; neap tides by their opposite attractions. Constant ocean currents are occasioned by the heat of the sun and the rotation of the earth. The vertical rays of the sun are warmer than the oblique rays : (1) Because they are spread over a smaller area ; (2) Because they pass through a thinner layer of air ; (3) Because they strike the surface more directly, and, therefore, produce more heat. Continual summer characterizes the tropics ; summer and winter of nearly equal duration, the temperate zones ; and short, hot summers, followed by long, intensely cold winters, the frigid zones. The principal modifiers of climate are : (1) The distribution of the land- and . water-areas ; (2) The varying elevation of the land ; (3) The slope of the land ; (4) The position of the mountain-ranges ; (5) The nature of the surface; (6) The distribution of the winds and moisture ; and (7) The ocean currents. Winds are caused by the disturbance of the equilibrium of the atmosphere by heat. The general motion of the surface winds is toward an area of greatest heat ; of the upper currents, toward an area of smaller heat. The general atmospheric circulation is from the equator to the poles, and from the poles to the equator. The principal wind zones are: (1) The zone of equatorial calms; (2) The zones of the trades ; (3) The zones of the calms of Cancer and Capricorn ; (4) The zones of the prevailing westerly winds ; (5) The zones of the polar winds. Cyclones are : (1) Tropical cyclones ; (2) Temperate latitude or extra-tropical cyclones. They are both travelling areas of low barometer. The passage of an extra-tropical cyclone is frequently attended by warm waves in front of the storm and cold waves in its rear. Anti-cyclones are travelling areas of high barometer. 376 PHYSICAL GEOGRAPHY. Moisture may be precipitated from the air in the form of dew, mist, fog, cloud, rain, hail, sleet, or snow. In order that any form of precipitation may occur, the air must he reduced below the temperature of its dew-point. Glaciers are masses of ice and snow, which move with extreme slowness down the higher valleys of mountain-ranges. They resemble rivers in that they receive, through the drainage of their basins, the solid material which flows into them. Glaciers are : (1) Alpine ; (2) Piedmont ; (3) Continental. Moraines are lateral, medial, terminal, frontal, and ground. Till, and other glacial deposits, assume shapes called drumlins, eskers, kames, and valley trains. The snow line is the distance above the level of the sea where the snow remains throughout the year. The height of its lower level above the sea depends: (1) On the amount of the snowfall; (2) On the temperature of the valley; (3) On the inclination of the slope. The unit of electric potential is called a volt ; the unit of electric current, an ampere ; the unit of electric resistance, an ohm. The principal electric phenomena of the atmosphere are thunder and light- ning, St. Elmo's fire, and the Aurorse. The principal optical phenomena are the rainbow, the mirage, halos, and coronae. The earth acts like a huge magnet. Its magnetism is, probably, due to the circulation around it of electrical currents generated by the sun's heat. The true basis for the distribution of vegetation is the distribution of light, heat, and moisture, upon which its existence mainly depends. The variety and luxuriance of vegetation decrease as we pass from the equa- tor to the poles, or from the base of a mountain to the summit. Only those plants that contain green coloring matter can, by photosynthesis, prepare their own food by materials taken directly from the soil. The principal plant groups or societies are : (1) Water plants; (2) Drought plants; (3) Plants occupying a position intermediate between water and drought plants ; (4) Salt plants. Forests are : (1) Evergreen foliage ; (2) Deciduous ; (3) Coniferous ; (4) Leaf- less; (5) Swamp. Deserts are : (1) Tropical ; (2) Cactus ; (3) Salt and alkaline. The principal food-plants of the tropical regions are rice, bananas, plantains, dates, cocoanuts, cassava, bread-fruit, sago, and yams. Coffee, tea, cocoa, pepper, cloves, nutmegs, and vanilla are products of the tropics. The principal food-plants of the temperate zones are barley, rye, wheat, oats, maize or Indian corn, buckwheat, and potatoes. Animals are restricted by conditions of food and climate to certain regions of the earth. They are dependent for their continued existence upon plants. With a few exceptions animals possess but little power of becoming accli- GENERAL SYLLABUS. 377 mated or living in a climate differing greatly from that in which they were created. The principal physiographical harriers opposing the dispersal of animals are : (1) Large bodies of water ; (2) Extensive and elevated mountain-ranges ; (3) Deserts ; (4) Forests. The principal physiological barriers are : (1) Food condi- tions; (2) Climate. The principal animal realms or regions are: (1) The Nearctic : (2) The Noo- tropic; (3) The Pahearctic; (4) The Ethiopian; (5) The Oriental; (6) The Aus- tralian. Marine fauna are : (1) Littoral; (2) Pelagic ; (3) Abyssal. The entire human family has descended from a single pair or species. The primary races of men are the Caucasian, the Mongolian, and the Negro. The secondary races are the Malay, the American, and the Australian. Mineral veins are : (1) Veins of segregation ; (2) Veins of infiltration ; (3) Fissure veins. Exclusive of Alaska and its insular possessions, the United States has an area of 3,026,500 square miles. The coast line of the United States is comparatively simple and unbroken. The predominant mountains are in the west ; the secondary mountains are in the east. The great low plains of the United States are the Atlantic Coast Plain and the Central Plain, or the Plain of the Mississippi Valley. The United States lies in the physical north temperate and the physical torrid zones. The United States, exclusive of Alaska and its insular possessions, lies in the zone of the prevailing westerly winds. The heaviest rainfall is on the coasts, especially on the west and south. There are four distinct plant regions : the Forest, the Prairie, the Steppe, and the Pacific. The Territory of Alaska occupies an area of about 580,000 square miles. It is mainly mountainous. The shore lands of the Arctic are frozen moorlands like the tundras of Asia. The Yukon and Kuskokwim are the principal rivers. Myriads of salmon visit the rivers during the breeding season. Valuable food- fish are found in the waters off the coasts. Numerous fur-bearing animals are found. Named in the order of their money value, the principal mineral products of the United States are pig iron, bituminous coal, copper, anthracite coal, gold, silver, petroleum, building stone, natural gas, lead, cement, zinc, brick clay, salt, and mineral waters. 378 PHYSICAL GEOGRAPHY. GENERAL REVIEW QUESTIONS. Mathematical Geography. Name the planets in their regular order from the sun. What is the position of the solar system in space ? State briefly Laplace's nebular hypothesis. Of what use are latitude and longitude in geography ? Distinguish between Mercator's, equatorial, polar, and conical projections. Upon what does the difference iu the length of the day and night at different seasons of the year depend ? Explain the causes of the change of seasons. The Laud. Enumerate the proofs of the present heated condition of the interior of the earth. What do you understand by the phrase, "A potentially plastic interior"? Distinguish between fissure and crater eruptions. Define laccoliths ; dike ; caldera. Distinguish between primary and secondary volcanic eruptions. Name five principal regions of active volcanoes. What facts have been discovered respecting earthquake shocks ? Name some parts of the earth that are slowly sinking. Name some parts that are slowly rising. Explain the origin of coal. Enumerate some of the changes which are now taking place in the crust of the earth. Define rock ; mineral ; loam ; marl. Into what four classes may rocks be divided according to their origin ? Name the six geological eras. Define rock weathering. Name some of the more important agencies that cause weathering. Define erosion ; corrasion ; transportation ; denudation ; deposition. What are the relative land- and water-areas of the earth ? Describe the land hemisphere. The water hemisphere. What do you understand by lines of trend ? Which of the continents contains, in proportion to its area, the greatest length of coast line ? Which the least? Distinguish between continental and oceanic islands ; between coral and vol- canic islands. State Darwin's hypothesis for the formation of coral islands. Give some other hypotheses for coral formations. Define each of the following varieties of plains: alluvial, fluviatile, marine, coastal, inland, peneplain, dust, lava. GENERAL SYLLABUS. 379 Distinguish between marginal and intermont plateaus. How can the cooling of the earth's interior cause mountains? What peculiarities are noticeable in the general relief forms of the conti- nents ? Which of the continents resemble each other in the general arrangement of their relief forms ? In what respect do they all resemble one another ? The Water. Enumerate some of the properties of water that enable it to play such an important part in the economy of the earth. Distinguish between subterranean and surface drainage. Distinguish between hill-side, fissure, and artesian springs. Between constant and temporary springs. Name some of the principal mineral springs. Define river-system, basin, water-shed, source, channel, and river-mouth. Explain the origin of waterfalls. By what are the inundations of rivers caused ? What is silt ? In what different parts of a river-system may silt be deposited ? Define fluvio-marine formations. What do you understand by a dismembered river? By an engrafted river? By a drowned river? Classify lakes. Name some of the principal ways in which lake basins may be formed. Why are lakes necessarily temporary drainage features? In what two ways may salt lakes be formed ? Name the great fresh-water lake systems of the world. State the composition of ocean water. What is its density? Its boiling point? Tts color? How do the five oceans compare with one another in area? Distinguish between inland seas, border seas, gulfs and bays, and fiords. Explain the origin of the ooze deposits on the ocean's beds. Define cauldron ; furrow ; trough ; bank ; shelf. In what parts of the Pacific, Atlantic, and Indian Oceans are most of the deep depressions situated ? By what are waves caused ? Upon what does their height depend ? How are tides caused? Distinguish between ebb, flood, spring, and neap tides. Where does the parent tidal wave originate ? In what part of the ocean are tides the highest? Why? What are the main causes of constant oceanic currents? In what respects do the currents in the three central oceans resemble one another ? The Atmosphere. Name six constituents of the atmosphere. Explain the operation of the thermograph. Of the barograph. 380 PHYSICAL GEOGRAPHY. By what instrument is the pressure of the atmosphere measured ? What proof have we that the greater weight of the atmosphere lies within a few miles of the earth's surface ? Define climate. Name the principal modifiers of climate. Why are the vertical rays of the sun warmer than the oblique rays ? In what different ways does the atmosphere receive its heat from the sun ? Explain the origin of winds. Define barometric gradient ; isobaric chart. Name the principal regions of tropical cyclones. Distinguish between tropical and extra-tropical cyclones. What influence, on the precipitation of moisture, is believed to be exerted by the dust particles in the atmosphere ? Name the different wind zones of the earth. What is the origin of land and sea breezes ? What resemblance do land and sea breezes bear to monsoons ? What facts have been discovered in regard to the extended storms of the United States? Enumerate the circumstances upon which the rapidity of evaporation depends. State the general law for the occurrence of precipitations. Name the primary forms of clouds. The secondary forms. Explain the peculiarities of the rainfall in each of the wind zones. Define snow line. On what three circumstances does the height of the snow line depend? Describe the formation of a glacier. Name three types of glaciers. Name some of the evidences of the existence of a former glacial epoch in North America. Enumerate the principal electrical and optical phenomena of the atmosphere. What is the probable cause of the earth's magnetism ? Define volt, ohm, ampere. What analogies exist between the flow of water in a pipe and an electric current ? Plant Life and Animal Life. Why should the distribution of light, heat, and moisture form the best basis for the distribution of vegetation ? Define photosynthesis ; proteids ; protoplasm. Define flora. Distinguish between the horizontal and the vertical distribution of vegetation. State the names and limits of the horizontal zones of vegetation. What are the characteristic features of the flora of each of these zones ? State the conditions requisite for the existence of forests ; of prairies ; of steppes; of deserts. Into what different classes may forests be divided ? Deserts? Enumerate the principal cultivated plants of the torrid, temperate, and polar zones. GENERAL SYLLABUS. 381 Define fauna. Upon what is the existence of animal life dependent? Name the principal animal regions or realms. State briefly the limits of each realm. Name a few characteristic animals in each realm. Enumerate the proofs of the probable unity of the human race. Name the portions of the world inhabited by each of the primary and second- ary races. Minerals. Name the principal useful metals. Enumerate the most important natural fuels. In what parts of the world are valuable deposits of coal found ? Name some of the important gold-fields of the earth. Physical Features of the United States. What is the area of the United States exclusive of Alaska and the recently acquired insular possessions? Describe the surface structure of the United States and its drainage-systems. What are the causes of the difference in the temperature of the eastern and western coasts? In what wind zone is the United States situated ? Describe briefly the charac- teristics of the mean annual rainfall. Name the four principal regions of vege- tation. Enumerate the chief agricultural productions of the country. What large animals are found in the United States ? Name the chief mineral productions of the United States. What is the area of Alaska? What are the pi'incipal indentations of its coast? Name its principal islands. Describe the river-system of the Yukon. Name the insular possessions of the United States. GENERAL MAP QUESTIONS. Volcanoes and Earthquakes. Describe the volcanic districts of the Pacific Ocean. In what portions of these districts are volcanoes most numerous ? Describe the volcanic districts of the Indian Ocean. Describe the volcanic districts of the Atlantic. What portions of the world are especially liable to earthquake shocks ? Geological Formations of the Earth. Trace on the map the principal regions of the earth where rocks formed by volcanic and younger eruptive flows occupy the surface. Trace in a similar manner the regions where the archseic rocks and granites occupy the surface. Where the Palaeozoic rocks occupy the surface. The Mesozoic. The Tertiary and Desert sandstones. The Quaternary. What portions of the earth's surface are occupied by sandy and desert wastes ? Over what portions are the exact character of the geological formations unknown? 382 PHYSICAL GEOGRAPHY. Oceanic Basins, Areas, and River-systems. What two oceans receive the drainage of the greatest areas of the continents? State, from a careful inspection of the direction in which the principal river- systems flow, the direction of inclination of the principal slopes of each of the continents. Locate the principal systems of inland drainage in each of the continents. Name the principal lakes and rivers belonging to each of these systems. Name the principal rivers draining into the Atlantic. Into the Pacific. Into the Indian. Into the Arctic. Name the principal rivers of the world which have delta mouths. Name the land and water boundaries of each of the five oceans. Describe briefly some peculiarities of the bed of the Atlantic Ocean. Of the Pacific Ocean. Of the Indian Ocean. Ocean Currents. What is the general direction of the equatorial ocean currents ? Explain the cause of this general direction. What exception can you find to it? What is the general direction of the Arctic currents? Of the Antarctic cur- rents ? What are the causes of these general directions ? Describe the principal currents of the Atlantic ; of the Pacific ; of the Indian Ocean. Name the principal warm ocean currents ; the principal cold ocean currents. Name some cold currents which powerfully affect the climate of parts of the earth. Name some warm currents which powerfully affect the climate. In what respects do the general directions of the currents in each of the cen- tral oceans resemble one another? Name some points of resemblance between the Gulf Stream and the Japan Current. Isothermal Lines and Physical Zones. Point out some of the most striking deviations in the directions of the iso- thermal lines from the parallels of latitude. Explain in each case the main cause of these deviations. In what part of the world do the isothermal lines coincide most nearly with the parallels of latitude? Trace on the map the isothermal line of 70° Fahr. Of 30° Fahr. What is the mean temperature of London for January? For July? What other large cities have nearly the same mean July or January temperature as London ? What is the mean temperature of Bombay for January ? For July ? What other large cities have nearly the same mean July or January temperature as Bombay ? Point out the northern limit of drift ice. The southern limit. GENERAL SYLLABUS. 383 Describe the boundaries of the physical torrid, temperate, and frigid zones. Name the principal countries which lie wholly or in part in each of these zones. Winds and Ocean Routes. State the boundaries of each of the wind zones. What is the general direction of the wind in each of these zones? Name the principal monsoon regions of the world. What is the direction of the rotation of the wind in the cyclonic storms of the northern hemisphere? Of the southern hemisphere? Name the principal tropical cyclone regions of the world. What would be the general route of a vessel in sailing from America to Europe, and back again ? From Europe to San Francisco ? What two sailing routes are there from Europe to Australia or India ? Give the northern and southern limits of drift ice. Vegetation. State the boundaries of each of the plant zones. Name the countries or por- tions of countries which lie in each of these zones. Name some of the useful plants of each of the plant zones. Point out on the map the northern limit of trees. The southern limit. Name some parts of the world where the following are cultivated : rice, oranges, sugar, cocoa, cloves, pepper. Name the different kinds of forests. What are the principal tea- and coffee-growing countries of the world ? Animal Regions. State the boundaries of the principal animal regions or realms. Name some animals that are found in the temperate regions of both the east- ern and the western continent. In the tropical region. In what parts of the world are the whale, seal, grizzly bear, musk-ox, and walrus found ? Ethnographic Chart. Trace on the map the northern and southern limits of permanent habitation. Name all the countries of the world inhabitated by the Caucasian race. In what parts of the world are the Caucasians mixed with other races? Name the different countries of the world inhabited by the Mongolian race. Name some of the different peoples belonging to this race. What parts of the world are peopled by the Ethiopian, or Negro, race? Name some of the different tribes belonging to this race. Name the different countries of the world inhabited by the secondary races of men. Give the names of the principal tribes of each of the secondary races. 384 PHYSICAL GEOGRAPHY. What different races of men inhabit North America? South America? Europe? Asia? Africa? Australia? Physical and Weather Maps of the United States. Describe from the map the forms of relief of the United States. Name the principal mountain-ranges belonging to the predominant and secondary mountain-systems. Describe the drainage-systems of the United States. What large lake-system is situated in the north-eastern part of the United States? Trace on the map the general directions of the principal isothermal lines, showing the hottest and coldest portions of the country. Name the principal islands which lie near the coasts of the United States. Name the fluvio-marine formations of the eastern coast. Name the principal islands that lie near the coasts of Alaska. Name the recently acquired insular possessions of the United States. Locate each group of islands. Name some of the principal products of some of the largest of these groups. Trace the progress of the areas of high and low barometer across the United States on the days indicated on the map. B-O^OO QUESTIONS RELATING TO THE PHYSICAL GEOG- RAPHY OF A STATE. In what physical region is this State principally situated ? Is the general surface of this State more or less than one thousand feet above the sea-level ? Into what body or bodies of water do the principal rivers of this State empty ? Name the principal lakes located within, or that border, on this State, if any. Name the principal rivers or river systems that are partly or wholly situated in this State. What is a navigable river ? Name the navigable rivers of this State, if any. Name the navigable lakes in this State, or that border on the State, if any. Name the principal mountain system of this State, if any. Has this State any coast line ? If so, name its principal inlets, bays, harbors, points, or promontories. Name the islands that lie near the coast, if any. What is the general climate of this State? When is rain most common? Name the cereals of this State. Name the principal fruits of this State? Name the other agricultural products of this State in addition to cereals and fruits. Are any wild animals found in this State ? Name the principal mineral products of this State. INDEX. A. Abyssal Fauna, 305 Abyssinian Plateau, 116 Acclimation, 292 Acidulous Springs, 136 Aconcagua, 104 Active Volcanoes, 49 Actual Barometric Gradient, 203 Humidity, 222 Adirondack^, 337 Aerial Ocean, 186 Movements of, 186 Africa and Australia, Contrasts of, 118, 119 Approximate Dimensions of, 117 Drainage Systems of, 151 Fresb -water Lakes of, 155 Salt Lakes of, 156 Surface Structure of, 115 Agassiz, Lake, 249 Agencies Now Producing Changes in Crust, 72 Agones, 258 Agricultural Productions of Sandwich Islands, 363 Products of Philippine Islands, 360, 361 Air, Elasticity of, 188 Alaska, Inhabitants of, 358 Mineral Deposits of, 358 Albert Nyanza, 117 Algonkian Period, 68 Alleghanies, 337 Alluvial Cones, 144 Pans, 144 Flats or Flood-grounds, 144, 145 Plains, 93 Alpine Glaciers, 239 Shrubs, Mosses, Lichens, and Saxi- frages, Zone of, 276 Alps, Mountain System of, 107 Altai Mountains, 112 Amazon, Selvas of, 106 25 American or Eed Eace, 318 Kace, Groups of, 318, 319 Americas, Contrast of Surface Struct- ^ ure of, 106, 107 Ampere, 251 Anchor Ice, 126 Andes, Cordillera of the, 104 Aneroid Barometer, 189 Animal Barriers, 291 Life, Distribution of, 294 Eealms of, 296 Eealms or Eegions, Enumeration of, 297 Annual Variation of Needle, 258 Anthracite Coal, 70 Anti-cyclones, 216 Appalachian Mountain System, 102, 103, 337 Approximate Dimensions of North America, 103 of South America, 106 Arabia, Mountains of, 112 Plateau of, 113 Aral, Sea of, 114 Archaean Period, 68 Arctic Plateau, 102 Argon, 187 Armenia, Plateau of, 113 Artesian Springs, 130, 131 Wells, 131 Articulation of Land and Water, 166 Aryan Stock, Principal Groups of, 311, 312 Asia, Approximate Dimensions of, 114, 115 Drainage Systems of, 151 Great Low Plain of, 114 Minor, Plateau of, 113 Peculiarities in its Drainage Systems, 151 Salt Lakes of, 156 Surface Structure of, 111 Asphaltum, 326 Asteroids, 14-16 385 386 INDEX. Astronomical Climate, 193 Atacama, Desert of, 106 Atlantic Coast Plain, 337 Currents in, 180 Highlands, 337 Ocean, Tidal Wave in, 175 Atlas Mountains, 116 Atmosphere, Composition of, 187 Form of, 192 Height of, 190 Optical Phenomena of, 259 Atmospheric Circulation, Origin of, 202, 203 Dust, 188 Electricity, 251 Equatorial Currents, 203 Polar Currents, 203 Pressure, 188 Atolls or Coral Islands, 86, 87 Aurora Boreal is, 254 Australasian Island Chain, 85 Australia, Approximate Dimensions of, 118 Drainage Systems of, 151 Great Low Plain of, 118 Salt Lakes of, 156 Surface Structure of, 117 Australian Alps, 118 Race, 317 Region of Animals, 303 Austria-Hungary, Mountains of, 108 Auvergne Mountains, 109 Avalanches, 76 Azoic Era, 67 B. Balkan Alps, 107 Bananas and Plantains, 283 Banks, 165 Barley, 281 Barograph, 190 Barometer, 188 Aneroid, 189 Mercurial, 188 Self-recording, 189, 190 Barometric Gradient, 201 Bavarian Plateau, 109 Bay of Fundy, High Tides in, 176 Bayous, 139 Bays and Gulfs, 167 Beryl, 328 Betel Plant, 286 Big Horn Mountains, 99 Birthplace of Tidal Wave, 173 Bituminous Coal, 70 Black Forest Mountains, 109 Hills of Dakota, 99 or Ethiopian Race, 314 Oak, 287 Blizzards, 215 Block Mountains, 98 Dissected, 98 Blue Color of Sky, 260 Icebergs, 244, 245 Mountains, 337 Bogs, 156, 157 Quaking, 281 Bohemian Plateau, 109 Bolivia, Plateau of, 105 Bonneville, Lake, 154, 249 Border Seas, 167 Bore, 176 Boulders, 248 Boundaries of Oceans, 166 Brazil, Plateau of, 105 Wood, 287 Bread Fruit, 284 Breakers, 169 Breezes, Land and Sea, 207 Broken Plateaus, 95 Brown or Malay Race, 317 Buckwheat, 282 Building Stones, 327 Bunsen's Theory of Geysers, 134 Buttes, 95 c. Cactus Deserts, 280 Calcareous Springs, 135 Caluis of Cancer and Capricorn, Zones of, 205, 206 Cameroons Mountains, 116 Canon of the Colorado, 75, 336 Cantabrian Mountains, 107 Caoutchouc, 287 Carboniferous Period, 69 Carpathian Mountains, 107, 109 Cascade Range, 102 Caspian Sea, 114 Cassava, 283 Cats' Tails or Mares' Tails, 226 Catskill Mountains, 337 Caucasian or White Race, 309 Caucasus Mountains, 110, 113 Cauldrons, 165 Cause of Cyclones, 210, 211 of Deserts, 128,234 Causes Producing Inundations of Rivers, 140 Caverns, 74 Celtic Group of Aryan Stock, 311 INDEX. 387 Cenozoic Era, 71 Cereals, Distribution of, 281 Cevennes Mountains, 107-109 Chalybeate Springs, 136 Change of Day and Night, Cause of, 29, 30 of Seasons, Causes of, 32, 33 Changes in Eiver Courses, 139 Characteristics of Fresh-water Lakes, 157 Charts, Isobaric, 190 China, Plain of, 114 Chlorophyll, 268 Chocolate, 285 Cinchona Trees, 287 Cinnabar or Mercury Sulphide, 324 Cinnamon, 286 Circuit, Electric, 250 Cirro-cumulus Clouds, 227 Cirro-stratus Clouds, 227 Cirrus Clouds, 226 Civilization, Degree of, 308 Classification of Winds, 204, 205 Cray, 326 Climate, 193 and Products of Porto Eico, 365 Astronomical, 193 Continental, 199 Modifiers of, 198 Oceanic, 199 of Philippine Islands, 362 of Sandwich Islands, 363 Physical, 193 Cloud-bursts, 216 Clouds, 224, 225 Primary Forms of, 226 Secondary Forms of, 227 Cloves, 286 Coal, Formation of, 69, 70 Important Deposits of, 325 Oil, 325 Coastal Plains, 93 Cocoa, 285 Cocoanut, 283-287 Coffee, 285 Coke, 325 Cold Springs, 133 Temperate Zone of Vegetation, 274 Waves, 215 Winds, 216 Color of Ocean Water, 162 Colorado, Canon of, 75, 336 Comets, 14 Commercial Geography, Definition of,12 Condition of Earth's Interior, Hypoth- eses Concerning, 42 Cone-bearing Trees, Zone of, 275, 276 Conical Projection, 27 Coniferous Forests, 278 Constant Ocean Currents, 177 General Features of, 179, 180 Origin of, 177, 178 Springs, 132 Continent, Definition of, 78 Continental Climate, 199 Coast Lines, 82 Glaciers, 240 and Insular Areas, Eelation be- tween, 83 Island Chains, American, 83, 84 Asiatic, 84 Islands, Peculiarity of Distribution of, 85 Outlines, Changes in, 81, 82 Beliefs, Peculiarities of, 98, 99 Shelves, 83, 164 Convection, 201, 202 Copper, Extensive Deposits of, 323 Coral Formation, Other Hypotheses for, 90, 91 Variety of, 88 Islands, 84 or Atolls, 86, 87 Darwin's Hypothesis for, 89 Distribution of, 88 Formation of, 87 Cordillera, 96 of the Andes, 104 of the Eocky Mountains, 100 Coronal, 260 Corrasion, 75 Co-tidal Lines, 175 Cotton, 287 Cotton-seed Oil, 287 Counter-currents, 177 Courses or Tracts of Eivers, 137 Cradle of the Tides, 173 Crater or Volcanic Eruptions, 43 Cretaceous Period, 71 Crevasses in Glaciers, 240 Crust, Composition of, 63 Thickness of, 42 Crystal Mountains, 116 Cultivated Plants, 281 Cumberland Mountains, 337 Cumulo-stratus Clouds, 228 Cumulus or Heap Clouds, 226 Currents, Utility of, 181, 182 Cyclones, Cause of, 210, 211 Extra-tropical or Temperate Lati- tude, 213, 214 Peculiarities of, 210, 211 388 INDEX. Cyclones, Progressive Motion of, 209 Eegious of, 210 Spiral Inflowing or Eotary Motion of, 209 Tropical, 209 Cyclonic Rotation of Wind, Cause of, 212 D. Dates, 283 Day and Night, Varying Length of, 36 Dazzling White Icebergs, 244 Dead Sea, 114, 156 Deccan, Plateau of, 113 Deciduous Forests, 278 Trees, Zones of, 274, 275 Declination, Magnetic. 257 Deep-sea Sounding Instruments, 166 Delta, 83 Formation, 145, 146 Mouth, 140 Eivers, 147 Denudation, 72 Deposition of Silt, 142 Desert of Atacama, 106 Belt of Eastern Continent, 235 Deserts, Cause of, 128, 234, 235 Characteristics of, 279 Classification of, 280 of Western Continent, 236 Destruction of Forests, Influence of, on Inundations, 140 Detritus or Silt, 142 Development of River-system, 148 Devonian Period, 68 Dew, 223 Circumstances Influencing Eapidity of Deposition of, 224 Point, 222 Dhawalaghiri, 112 Diamond Fields, 327 Diamonds, 327 Diffusion, 187 Dimensions of the Earth, 21 Dinaric Alps, 107 Dinotherium, 71 Direction of Slope, Effect of, on Tem- perature of Air, 200 Dirt-bedecked Icebergs, 246 Dismembered Eivers, 142 Dispersal of Native Species of Animals, 290 Dissected Block Mountains, 98 Plateaus, 95 Distributaries of Eivers, 146 Distribution of Animal Life, 294 of Cereals, 281 of Human Eace, 306 of Moisture, Effect of, on Tempera- ture of Air, 200 of Vegetation, 271, 272 Diurnal Variation of Needle, 251 Dome-shaped Mountains, 99 Double Continents, 79 Dove's Law of Rotation of Winds, 206 Dragon Mountains, 116 Drainage, 128 Continental, 149 of Ice and Snow, 240 Inland, 129 Oceanic, 129 Subterranean. 128, 129 Surface, 128, i29 Systems, 149 Glacial Disturbances of, 249 of the United States, 338 Drift, Glacial, 242 Drifts or Levels in Mines, 322 Drought Plants, 270, 271 Drowned Eivers, 147 Drumlins. 247 Dunes, 81 Dust, Atmospheric, 188 Particles, Influence of, on Absorption of Heat by Air, 197 Plains, 94 Dykes, 319 E. Eager, 176 Earth, Magnetic Properties of, 256 Earthquake Lake Basins, 153 Motion,, Varieties of, 56 Velocity of, 56 Shocks, Duration of, 56, 57 Sounds Accompanying, 56 Waves, 169 Earthquakes, 55 Distribution of, 58 Facts Concerning, 55, 56 Periodicity of, 58 Principal Cause of, 57 Earth's Magnetism, Possible Origin of, 256, 257 Strata, Order of, 72 Elasticity of Air, 188 Elbruz Mountains, 113 Electric Current, 250 Electricity, Atmospheric, 251 Electrification, 250 INDEX. 389 Electromotive Force, 250 Elevation, Effect of, on Temperature of Air, 199 Embayed Mountains, 98 Engrafted Eivers, 142 Eocene Epoch, 71 Eozoic Era, 68 Equator, Magnetic, 258 Equatorial Calms, Zones of, 205 Currents, Atmospheric, 203 Projection, 27 Winds, Eainy Character of, 230 Era of Mammals, 71 of Man, 71 Erosion, 72 Glacial, 242 of Waterfalls, 138 Erratic Blocks, 248 Eskers, 247 Estuary, 140 Etesian Winds, 216 Ethiopian or Black Race, 314 Eegion of Animals, 301, 302 Ethnography, 306 Europe and Asia. Comparison of Relief Forms of, 115 Approximate Dimensions of, 110 Drainage Systems of, 150 Glaciers of, 242 Great Low Plain of, 107, 110 High, 107 Low, 107 Peculiarities in its Drainage Sys- tems, 150 Relief Forms of, 107 Salt Lakes of, 156 Surface Structure of, 107 Systems of Fresh-water Lakes of, 155 Evaporation, 221 Circumstances Influencing Rapidity of, 221 Evergreen Foliage Forests, 278 Trees, Zones of, 274 Extinct Volcanoes, 49 Extra-tropical or Temperate Latitude Cyclones, 213, 214 "Eye" of Storm, 209 Eyre Lake, 118 F. Falls of Niagara, 138 Fauna, 290 Marine, 304 Ferrel's Law, 204 Applied to River Courses, 145 Fiord Coasts, 249 Fiords, 167, 248 Origin of, 248, 249 Fissure or Sheet Eruptions, Varieties of, 53, 54 Springs, 130 Veins, 320 Peculiarities of, 321 Fixed Stars, 14 Flax, 287 Flaxseed, 287 Flora, 268 Fluviatile Islands, 145 Lakes, 145 Plains, 93 Fluvio-marine Formations, 147, 148 Fogs, 224 Folk Lore, 308 Food-plants of Tropical Regions, 282, 283 Foraminifera, 167 Foraminiferal Land, 167 Forests, 276 Classification of, 278 of Philippine Islands, 361, 362 Formation of Soil, 269 Forms of Relief of the Land, 91, 92 Fossils, 66 France, Mountains of, 108 Fresh Lakes, 152 Water Lakes, Characteristics of, 157 Frontal Moraines, 242 Fruits of the Tropical and Warm Tem- perate Zones, 285 Furrows, 165 G. Gairdner, Lake, 118 Galena, or Lead Sulphide, 324 Gall's Projection, 26 Ganges and Brahmapootra, Delta of, 147 Gangue, 320 Garnet, 328 Gas, Natural, 325 Gems, 327 General Features of Constant Ocean Currents, 179, 180 Geographic Influences, Effect of, on Animal Regions, 298 on Civilization of Caucasian Race, 312 on Civilization of Ethiopian Race, 316 on Civilization of Yellow Race, 313 390 INDEX. Geographical Distribution of Glaciers, 242, 243 Geography, Branches of, 11 Plant, 268 Zoological, 290 Geological Time, Divisions of, 67 Germany, Mountains of, 108 Geyser Regions, 135 Geysers, 133, 134 Bunsen's Theory of, 134 Ghauts, Eastern and Western, 113 Glacial Deposits, 242 Drift, 242-246 Epoch, 246 Groovings or Scratches, 247 Lakes, 153 Sediments, 242 Streams, 241 Glaciers, 239 Geographical Distribution of, 242 Types of, 239 Work of, 241 Globular Lightning, 253 Gobi, Plateau of, 113 Gold, Principal Producers of, 323 Graded Rivers, 141 Gradient, Barometric, 201 Gradual Elevations and Subsidences of Crust, 84, 85 Graphite, 327 Gravitation Theory of Mountain Making, 99 Great Basin, 336 Plateau of, 102 Kinghan Mountains, 112 Lakes of North America, 154, 155 Green Mountains, 337 Groovings, Glacial, 247, 248 Ground Ice, 126 Moraines, 242 Guam, Island of, 365 Guiana, Plateau of, 105 Gulf of Guinea, Monsoons of, 208 Stream, 180 Gulfs and Bays, 167 Gum Arabic, 287 H. Hail, 236 Origin of, 237 Hailstone, Structure of, 236 Halos, 260 Hamitic Stock of Caucasian Race, 310, 311 Harbor of Pago-Pago, 365 Harmattan, 215 Hartz Mountains, 109 Hasheesh, 287 Hawaiian or Sandwich Islands, 363 Haze, 224 Heap or Cumulus Clouds, 226 Heat Equator, 198 Latent, 127 Lightning, 253 Unit, 127 Heated Interior, Effects of, 43 of Earth, Proofs of, 40, 41 Height of Atmosphere, 190 of Land, 102 of Ocean Waves, 168 of Tidal Wave, 176 Hemp, 287 Heredity and Variation, 293 High Europe, 107 Lands, 92 Water, 170 Hill-side Springs, 130 Himalayas, 112 Hindoo- Koosh Mountains, 113 Hindostan, Mountains of, 112 Hoar-frost, 223 Horizontal Distribution of Animal Life, 294 Horse Latitudes, 295, 296 Zones of Vegetation, 272 Hot or Thermal Springs, 133 Human Race, Distribution of, 306 Unitv of, 306 Humidity, Actual, 222 Relative, 222 Hydraulic Mining, 322 Hypsometry, 192 I. Iberian Peninsula, Mountains of, 109 Ice, Anchor, 126 and Snow, Drainage of, 240 Fields, 164 Floes, 164, 246 Foot, 164 Ground, 126 Packs, 164 Sheets, 164 Icebergs, 244 Varieties of, 244 Ice-cold Water, Latent Heat of, 127 Stored Heat Energy of, 127 Ichthyosaurus, 71 Imaginary Circles, 21, 22 Immature River Systems, 141 INDEX. 391 India, Plain of, 114 Rubber, 287 Indian Corn or Maize, 282 Ocean, Currents in, 181 Monsoons of, 208 Tidal Wave in, 175 Indigo, 287 Indo-China, Mountains of, 112 Iuhabitauts of Alaska, 358 Inland Drainage, 129 of the United States, 338 or Mediterranean Seas, 167 Plains, 93 Seas and Lakes, Tides in, 176 Intermont Plateaus, 95 Invisible Load of River, 142 Iran, Plateau of, 113 Iron, Ores of, 322 Irregular Variation of Needle, 258 Islands, Continental, 83 Fluviatile, 145 of Guam and Tutuila, 365 of Japan, 84 of Porto Rico, 361 Isobaric Charts, 190 Isobars, 190 Isoclinal Lines, 258 Isogonal Lines, 258 Isothermal Lines, 197 Italian Peninsula, Mountains of, 109 Japan Current, 181 Japhetic Stock of Caucasian Race, 311 Jura Mountains, 98, 109 Jurassic Period, 71 K. Kafla, Plateau of, 116 Karnes, 247 Kaolin, 326 Karakorum Mountains, 112 Kenia, Volcanic Peak of, 116 Khamsin, 215 Kilimandjaro, Volcanic Peak of, 116 Kiolen Mountains, 109 Kong Mountains, 116 Kosciusko, Mount, 118 Krypton, 187 Kuen-lun Mountains, 112 Kunchinjunga, 112 L. Lacustrine Plains, 93 Lagoon, 153 Lahontan Lake, 154, 249 Lake Basins Due to Changes of Level, 153 Due to Landslides and Lava Streams, 153 Systems of the United States, 338 Lakes Bonneville and Lahontan, 154 in Delta Districts, 152 Fluviatile, 145 Fresh, 152 Glacial, 153 in Lower Course of River, 152 of New Land Areas, 152 Ox-Bow, 153 Playa, 154 Prehistoric, 249 Salt, 152 Sea-shore or Lagoon, 153 Utility of, 157 Land and Sea Breezes, 207 and Water Areas, Distribution of, 198 Articulation of, 166 Forms of Relief of, 91, 92 Horizontal Forms of, 78 Masses, Continental Contrasts of, 80,- 81 Peculiarities in Distribution of, 78, 79 Slides, 76 Vertical Forms of, 78 Lapis-lazuli, 328 Latent Heat of Ice-cold Water, 127 Lateral Moraines, 241 Latitude, Definition of, 23, 24 Value of Degrees of, 24, 25 Laurels and Myrtles, Zones of, 274 Lava, 44 Plains, 94 Law of Precipitations, 223 Layer or Stratus Clouds, 227 Lead and Zinc Ores, Deposits of, 324 Leafless Forests, 278, 279 Levees of the Mississippi, 143 Life History of Volcano, 52 Light, Heat, and Moisture, Geographic Effects of, 77 Northern, 254 Lightning, 252 Varieties of, 252, 253 Light-year, Definition of, 16 Lines, Co-tidal, 175 Isoclinal, 258 Isogonal, 258 Isothermal, 197 of Trend, 80 Littoral Fauna, 305 392 INDEX. Llanos of the Orinoco, 106, 273 Load of River, 142 Lodes, 320 Lofty Mountains, 98 Logwood, 287 Loinbardy, River Plains of, 143, 144 Looming, 261 Longitude, Definition of, 23, 24 Value of Degrees of, 24, 25 Longitudinal Valleys, 99 Loo Choo Islands, 84 Low Europe, 107 Lands, 92 Water, 170 Lower or Plains Course of River, 139 Lupata Mountains, 116 M. Magnetic Attractions and Repulsions, 256 Declination, 257 Equator, 258 Field, 255 Flux, 255 Magnetism, 254 Magnets, 254 Artificial and Natural, 255 Maize or Indian Corn, 282 Malay or Brown Race, 317 Malayic Branch of Brown Race, 317 Mammoth Cave of Kentucky, 129 Man, Distribution of, 306 Mantchuria, Plain of, 114 Map Projections, 25 Marginal Plateaus, 95 Marine Fauna, 304 Plains, 93 Marl, 327 Marsh Lakes, 156 Marshes, 156 Mathematical Geography, Definition of, 11 Zones, 35 Matter, Inorganic, 267 Organic, 267 Matterhorn, 107 Matured River-systems, 141 Maximum Density of Water, 125 Meadows, 279 Mean Annual Isotherms of United States, 339 Rainfall, Chart of, 234 Temperature, 197 Meanders, 145 Medial Moraines, 241 Mercator's Projection, 25, 26 Mercurial Barometer, 188 Meridian, Definition of, 22, 23 Metallic Ores, 321 Meteorology, 186 Mesas, 95 Mesozoic Era, 71 Meteors, 14 Mexican Gulf and Caribbean Sea, Mon- soons of, 208 Middle or Valley Course of River, 139 Mineral Deposits of Alaska, 358 of Philippine Islands, 361, 362 Pitch, 326 Products, Classification of, 322 Value of, 321 Springs, 135 Substances, Varieties of, 321 Veins, 319 Varieties of, 320 Minerals, Definition of, 63 Distribution of, 319 Enumeration of Some Important, 63 Mining, Hydraulic, 322 Miocene Epoch, 71 Mirage, 260, 261 Mississippi, Alluvial Flats of, 144 Delta of, 146 Levees of, 143 Mistral, 216 Mists, 22 Mocambe Mountains, 116 Mock Moons, 260 Suns, 260 Modifiers of Climate, 198 Mongolian or Yellow Race, 312 Monsoon Regions, 208 Monsoons, 207 of Indian Ocean, 208 of the Gulf of Guinea, 208 Mont Blanc, 107 Mont Rosa, 107 Moon, Phases of, 172 Moons or Satellites, 16, 17 Moors, Sphagnum, 280, 281 Swamp, 280 Moraine Deposits, 246 Moraines, Varieties of, 241, 242 Morasses, 156, 157 Mount Everest, 112 Mount McKinley, 102 Mountain Chain, 96 Making, Gravitation Theory of, 99 Passes, 95 Peaks, 95 INDEX. 393 Mountain System, 96 Winds, 209 Mountains, 95 Block, 98 of Denudation, 97 Dome-shaped, 99 Embayed, 98 by Flexure, 98 by Fracture, 98 Origin of, 96 Subtuberant, 99 of Uplift, 97 Mudflats, 83 Multiple or Ribbon Lightning, 253 N. Nanling Mountains, 114 Natural Bridges, 75 Nature of Surface, Effect of, on Tem- perature of Air, 200 Neap Tides, 172, 173 Nearctic Region of Animals, 297, 299 Nebular Hypothesis, Laplace's, 31 Needle, Inclination or Dip of, 258 Variation or Declination of, 257 Negrillo Branch of Ethiopian Race, 314, 315 Negritic Branch of Brown Race, 317 Negro Branch of Ethiopian Race, 315 Negroid Branch of Ethiopian Race, 315 Neotropic Region of Animals, 299, 300 Newfoundland, Fogs of, 225 New Zealand Geyser Region, 135 Niagara, Falls of, 138 Nicaragua Wood, 287 Nieuveldt Mountains, 116 Nile, Delta of, 146, 147 Nimbus or Storm Clouds, 226 Non -periodical Rain Zones, 231 North America, Approximate Dimen- sions of, 103 Culminating Point of, 103 Drainage Systems of, 149 Glaciers of, 243 Great Lakes of, 154, 155 Isolated Water-sheds or Drainage Centres of, 150 Relief Forms of, 100-103 Salt Lakes of, 155, 156 Surface Structure of, 100 Mediterranean Branch of White Race, 311 North-easters of the United States, 214 Northern Light, 254 Northers of Texas, 216 Nutmegs, 286 o. Oats, 281 Oblique Rays of Sun, Heating Power of, 194, 195 Ocean, Aerial, 186 Currents, Constant, 177 Effects of, on Temperature of Air, 200 Drifts, 179 Floor, 165 Ice, 163 Line of Invariable Temperature of, 163 Streams, 179 Water, Color of, 162 Composition of, 162 Density of, 162 Inequalities in Saltness of, 162 Temperature of, 162 Waves, Action of, 77 Oceanic Areas, 166 Climate, 199 Drainage, 129 Island Chains, 86 Islands, Classes of, 85, 86 Movements, 168 Waves, 168 Oceans, Boundaries of, 166 Oil-fields, 326 Old Plains, 94 Plateaus, 95 River-systems, 141 Oligocene Epoch, 71 Olive Oil, 287 Ooze Deposits, 167 Opium, 286 Optical Phenomena of Atmosphere. 259 Ore, Free Milling, 322 Ores, Metallic, 321 Oriental Region of Animals, 303 Origin of Atmospheric Circulation, 202, 203 of Constant Ocean Currents, 177, 178 of Hail, 237 of Salt Lakes, 156 of Saltness of Ocean, 162 of Winds, 201 Orinoco, Llanos of, 106 Orology, 96 Outlets or Distributaries of Rivers, 152 Ox-bow Lakes, 153 394 INDEX. Pacific Highlands, 335 Mountain Chains, 336 Ocean, Currents in, 180, 181 Greatest Depth of, 166 Tidal Wave in, 175 Pago-Pago, Harbor of, 365 Palsearctic Eegion of Animals, 300, 301 Palaeontology, 66 Palaeotherium, 71 Palaeozoic Era, 68 ■ Palm Oil, 287 Pampas of the Eio de la Plata, 106 Pamperos, 216 Paraselena, 260 Parasitic Cones, 47 Parhelia, 260 Pasco, Plateau of, 105 Patagonian Archipelago, Islands of, 84 Pearls, 328 Peat, 326 Bogs, 157 Peculiarities of Continental Eeliefs, 99 of Cyclones, 210, 211 Pelagic Fauna, 305 Peling Mountains, 114 Peneplains, 94 Pepper, 286 Period of Coal Plants, 69 of Mollusks, 68 Periodical Eain Zones, 230 Permian Period, 71 Perpetual Snow, Eegions of, 238 Persia, Plain of, 114 Petrifactions, 66 Petroleum, 325 By-products of, 326 Phases of the Moon, 172 Philippine Islands, 358-363 Agricultural Products of, 360, 361 Area of, 360 Climate of, 362 Map of, 359 Mineral Deposits in, 361, 362 Principal Islands of, 360, 361 Philippines, 84, 85 Phosphorescence, 162 Photosynthesis, 268 Physical Climate, 193 Climatic Zones of the United States, 339 Geography, Definition of, 11 Zones, 197, 198 Physiographical Animal Barriers, 291 Physiological Animal Barriers, 292 Piedmont Glaciers, 237 Placer Deposits of Gold, 322 Plain of the Mississippi Valley, 337 Plains, 93 Alluvial, 93 Coastal, 93 Fluviatile, 93 Inland, 93 Lacustrine, 93 Marine, 93 Plane of Earth's Orbit, Definition of, 32 Planets, 14 Plant Geography, 268 Groups or Societies, 270 Growth, Conditions Bequisite for, 269 Eegions, 277 Plants, Cultivated, 281 Yielding Beverages, 285 Plateau of Arabia, 113 of Bolivia, 105 of Brazil, 105 of Deccan, 113 of Great Basin, 102 of Guiana, 105 oftLabrador, 102 of Pasco, 105 of Quito, 105 Plateaus, Broken, 95 Dissected, 95 Intermont, 95 Marginal, 95 Old, 95 Young, 95 Platinum, Principal Ore Deposits of, 325 Playa Lakes, 154 Plesiosaurus, 71 Pluviometer, or Eain Gauge, 232 Po, Plain of, 110 Polar Circles, 23 Currents, Atmospheric, 203 Projection, 27 Winds, Generally Dry Character of, 230 Bains of, 231 Zone of Vegetation, 276 Zones, 206 Characteristic Climate of, 196 Political Geography, Definition of, 11 Popocatepetl, 102 Porto Eico, Climate and Products of, 365 Island of, 364 Position of Mountain Eegion, Effect of, on Temperature of Air, 200 Potatoes, 282 INDEX. 395 Pound-degree Fahrenheit, 127 Prairies, 279 Precious Stones, 327 Precipitation, Law of, 223 of Saline Substances, 156 Varieties of, 223 Predominant Mountain System, Com- mon. Usage of Term, 100 Prehistoric Lakes, 249 Pressure, Atmospheric, 188 Prevailing Westerly Wind Zones, Rains of, 231 Zones of, 206 Primary Paces of Men, 309 Volcanic Eruptions, 47 Prime Essentials of Vegetation, 270 Prismatic Colors of Sunlight, 259 Proteids, 267 Protoplasm, 267 Pterodactyl, 71 Pyrenees Mountains, 107 Quagmire, 157 Quaking Bogs, 281 Quercitron, 287 Quinine, 287 Quito, Plateau of, 105 R. Paces and Whirlpools, 177 of Men, 309 Eafts, 144 Pain, 228 Geographical Distribution of, 232, 233 How Caused, 229 Quantity of, How Measured, 232 Zones, Periodical, 230 Rainbow, Cause of, 259, 260 Raindrops, Size of, 229 Rainfall, Distribution of, 229 Rain-gauge, or Pluviometer, 232 Rapids and Waterfalls, Formation of, 138 Realms of Animal Life, 296 Red or American Race, 318 Red River, Raft of, 144 Reefs, 81 Regelation, Action of, in Glaciers, 240 Regions of Cyclones, 210 of Perpetual Snow, 238 Rejuvenated Rivers, 141 Relative Humidity, 222 Land and Water Areas of Earth, 77, 78 Relief Forms of Europe, 107 of North America, 100-103 of South America, 104-106 Revived Rivers, 141 Revolution of the Eartb, 30 Ribbon or Multiple Lightning, 255 Rice, 282 Rills, Definition of, 137 Rio de la Plata, Pampas of, 106 River Basin, 137 Channel, 137 Channels, Glacial Disturbances of, 249 Courses, Changes in, 139 FerreFs Law Applied to, 145 or Tracts, 137 Definition of, 137 Load of, 75 Mouth, 137 Mouths, 140 Source, 137 System, 137 Development of, 148 Systems, Immature or Young, 141 Matured or Old, 141 Terraces, 145 Transporting Power of, 142 Valleys, 137 River-made Plains, 94 River's Load, 142 Rivers, Dismembered, 142 Drowned, 147 Engrafted, 142 Graded, 142 Inundations of, 140 Rejuvenated, 142 Revived, 142 Rivulets, Definition of, 137 Rock, Definition of, 63 Rocks, iEolian, 65 Aqueous, 64 Basement, 68 Crystalline, 65 Fossiliferous, 66 Fragmental, 65 Fundamental, 68 Igneous. 64 Me tam orphic, 64 Non-fossiliferous, 66 Plutonic, 64 Primitive, 65 Sedimentary, 64 Stratified, 65 Un stratified, 65 Volcanic, 64 Rocky Mountains, 335 Cordillera of, 100 396 INDEX. Eotation of Earth, Proofs of, 28, 29 Eotundity of the Earth, Proofs of, 19-21 Eye, 281 Sahara, Plateau of, 116, 117 Sailing Eoutes, 216, 217 Saline Substances, Precipitation of, 156 Salt and Alkaline Deserts, 280 Common, 327 Lakes, 152 Origin of, 156 Plants, 271 Springs, 136 Saltness of Ocean, Origin of, 162 Sand-bars, 81 Sand-drifts, 81 Sandwich Islands, Agricultural Produc- tions of, 363 Surface Structure of, 363 or Hawaiian Islands, 363 Sapphire, 328 Sargasso Seas, 181 Scandinavian Peninsula, Mountains of, 109 Scratches, Glacial, 247 Seas, 168 Border, 167 Inland or Mediterranean, 167 Sargasso, 181 Sea-shore Lakes, 153 Secondary Mountain System, Common Usage of Term, 100 Eaces of Men, 309 Volcanic Eruptions, 47 Secular Variation of Needle, 258 Self-recording Barometer, 189, 190- Therinometer, 194 Selvas of the Amazon, 106 Semitic Stock of Caucasian Eace, 311 Sheet or Fissure Eruptions, Varieties of, 53, 54 Lightning, 253 Shelves, 165 Shifting of Wind Zones, 206 Shoals, 165 Shore Lines, 81 Sibiric Branch of Yellow Eace, 313 Sierra Madre Mountains, 101 Sierra Nevada and Cascade Eanges, 102 Eange, 102 Silicious Springs, 136 Silt, Deposition of, 142 or Detritus, 142 Silurian Period, 68 Simitic Branch of Yellow Eace, 313 Simoom or Samiel, 215 Sink-holes, 74 Sirocco, 215 Sky, Blue Color of, 260 Sunset Tints of, 260 Sleet, 229 Snow, 237 Crystals, 237 Line, 238 Circumstances Affecting Height of, 238 Mountains, 116 Societies or Groups of Plants, 270 Soil, Formation of, 269 Soils, Classification of, 269 Solano, 215 Solar System, Position of, in Space, 18 Tides, 171 Solvent Powers of Water, 128 Soudan, 117 Sources, Electric, 250 South America, Approximate Dimen- sions of, 106 Culminating Point of, 104 Drainage Systems of, 150 Fresh-water Lakes of, 155 Belief Forms of, 104-106 Salt Lakes of, 156 Secondary Mountain Systems of, 105 Surface Structure of, 104 Mediterranean Branch of Caucasian Eace, 310 Specific Heat, Definition of, 126 of Water, 126 Sphagnum Moors, 280, 281 or Water Moss, 157 Spheroids, Oblate and Prolate, 19 Spices, Geographic Distribution of, 285 Spring Tides, 172, 173 Springs, Acidulous, 136 Artesian, 130, 131 Calcareous. 135 Chalybeate, 136 Classification of, 132 Cold, 133 Constant, 132 Definition of, 129 Fissure, 130 Hill-side, 130 Hot or Thermal, 133 Mineral, 135 Eeservoirs of, 132 Salt, 136 Silicious, 136 Sulphurous, 336 Temporary, 132 INDEX. 397 Stalactites, 75 Stalactitic Deposits, 136 Stalagmites, 75 Stalagmitic Deposits, 136 Stars, Number of, 14 St. Elmo's Fire, 254 Staubbach, Falls of, 138 Steppes, 279 Stereographic Projection, 26 Stock Languages, 309 Stored Heat Energy of Water Vapor, 127, 128 Storm, " Eye " of, 209 or Nimbus Clouds, 227 Storms, 209 Stratus or Layer Clouds, 227 Streams, Underground, 129 Struggle for Existence, 294 Sub-arctic Zone of Vegetation, 275 Submarine Volcanoes, 49 Subterranean Drainage, 128, 129 Waters, 129 Subtropical Zones of Vegetation, 274 Subtuberant Mountains, 99 Sugar Cane, 284 Sulimau Mountains, 113 Sulpbur, 326, 327 Sulphurous Springs, 136 Sunlight, Prismatic Colors of, 259 Sunset Tints of Sky, 260 Surface and Upper Atmospheric Cur- rents, Interchange of, 204 Drainage, 128, 129 Structure of Sandwich Islands, 363 Survival of the Fittest, Doctrine of, 293 Swamp Forests, 279 Moors, 280 Thicket, 280 Swamps, 156 Swiss Plateau, 109 Table Mountain, 116 Tanganyika Lake, 117 Tapioca, 284 Tchad Lake, 117 Tea, 285 Temperate Latitude or Extra-tropical Cyclones, 213, 214 Zones, Characteristic Climate of, 196 Temperature, 193 of Ocean Water, 162 Temporary Springs, 132 Terminal Moraines, 241 Theory of Tides, 170 Thermograph, 194 Thermometer, 193 Self-recording, 194 Thiau Shan and Altai Mountains, 113 Thibet, Plateau of, 112 Thunder, 252 Thunder-storms and Thunder Squalls, 216 Tidal Wave, Birthplace of, 173 Height of, 176 Waves, 69 Tide, Ebb, 169 Flood, 169 Tides, Cradle of, 173 Definition of, 169 Neap, 172, 173 Solar, 171 Spring, 172, 173 Theory of, 170 Till sheets, 246, 247 Tin, Principal Ore Deposits of, 325 Tobacco, 287 Topaz, 328 Tornadoes, 212 Torrens Lake, 118 Torricelli, 188 Trade Winds, Zones of, 205 Transportation and Deposition, 76 Transverse Valleys, 99 Transylvanian Mountains, 107, 109 Plateau, 109 Travertine, 136 Trend, Lines of, 80 Triassic Period, 71 Tropical Cyclones, 209 Deserts, 280 Eegions, Food Plants of, 282, 283 Zone of Vegetation, 273 Tropics, 23 Characteristic Climate of, 196 Troughs, 165 Tundras, 114 Turko-Grecian Peninsula, Mountains of, 109 Turquoise, 328 Tutuila, Island of, 365 Twin Continents, 79 Types of Glaciers, 239 u. Under-currents, 177 Underground Streams, 129 UDdertow, 169 United States, Area of, 334 Climate of, 339 398 INDEX. United States, Climatic Contrasts of, 339, 340 Coast Line of, 334 Constancy of Climate of, 341 Drainage Systems of, 338 Forms of Belief of, 334, 335 Gulfs and Bays of, 334 Islands of. 334 Mean Annual Isothermal Lines of, 339 Peculiarities of Climate of, 339 Physical Climatic Zones of, 339 Eai'nfall in, 341 Wind Zone of, 341 Unity of the Human Pace, 306 Proofs of, 306, 307 Upper or Mountain Course of Eiver, 137 Ural Mountains, 109, 110 Utility of Currents, 181, 182 of Lakes, 157 V. Valdai Hills, 110 Valley Trains, 247 Valleys, Longitudinal, 99 Origin of, 99 Transverse, 99 Vanilla, 285, 286 Variation or Declination, 257 Varieties of Glacial Drift Deposits, 246 Vegetation, Distribution of, 271 Horizontal Zones of, 272 Prime Essentials of, 270 Vertical Distribution of, 276, 277 Veins, Mineral, 319 of Infiltration, 320 of Segregation, 320 Velocity of Eiver, Causes Determining, 137 Vertical Distribution of Animal Life, 295 of Vegetation, 276 Eays of Sun, Heating Power of, 194, 195 Victoria, Falls of, 138 Nyanza, 117 Vindhya Mountains, 113 Visible Load of Eiver, 142 Volcanic Ashes or Cinders, 45 Dykes, 47 Eruptions, Cause of, 46, 47 Explosive, 48 Non-explosive, 48 Gases or Vapors, 45 Volcanic Lightning, 253 Necks, 47, 48 or Crater Eruptions, 43 Volcano, Life History of, 52 Volcanoes, Active, 49 Extinct, 49 of Philippine Islands, 362, 363 Principal Eegions of, 49-51 Submarine, 49, 51 Volt, 250 Vosges Mountains, 109 w. Wake Island, 365 Warm Temperate Zones of Vegetation, 274 Waves, 214 Water, Circulation of, 124 Composition of, 125 Maximum Density of, 125 Moss, or Sphagnum, 157 Plants, 270 Properties of, 125 Solvent Powers of, 128 Specific Heat of, 126 Vapor, Stored Heat Energy of, 127, 128 Water-bottle, 165 Water-falls, Erosion of, 138 Water-gap, Formation of, 149 Water-shed, Definition of, 137 Water-spouts, 213 Waters, Subterranean, 129 Waves, Earthquake, 169 Force of, 169 Ocean, Height of, 168 Tidal, 169 Weather Bureau, 342 Forecasts, 342 Indications, 342 Weathering, Agencies Producing, 73, 74 Definition of, 72, 73 Wells, 129 Artesian, 131 Western Continent, Deserts of, 235, 236 Wheat, 281 Whirlpools and Eaces, 177- White Caps, 169 Mountains, 337 or Caucasian Eace, 309 Wind Corrasion, 76 Erosion, 76 Gap, Formation of, 149 INDEX. 399 Wind Transportation, 76 Zones, 205 Shifting of, 206 Winds, Classification of, 204, 205 Definition of, 201 Etesian, 216 Mountain, 208 Origin of, 201 Work of Eivers, 142 Woru-down Mountain Low Lands, 94 Yablonoi Mountains, 113 Yams, 284 Yellow or Mongolian Eace, 312 Yellowstone National Park, 335 Geyser Eegion, 135 Yoseinite, Falls of, 138 Young Eiver Systems, 141 z. Zagros Mountains, 113 Zig-zag Lightning, 252 Zinc and Lead Ore, Deposits of, 324 Blende, 324 Zone of Calms, Eains of, 230 of Equatorial Calms, 205 of the Trades, Eains of, 230 Zones of the Calms of Cancer and Capricorn, 205 of Prevailing Westerly Winds, 206 of the Trade Winds, 205 Physical, 197, 198 Zoological Geography, 290 -£, L> o l ■ LIBRARY OF CONGRESS 021 648 977 9