Class QL *lj 5*5 10 Copyright^ COPYRIGHT DEPOSIT Digitized by the Internet Archive in 2011 with funding from The Library of Congress http://www.archive.org/details/lessonsinphysica01drye / LESSONS 6r?> PHYSICAL GEOGRAPHY BY CHARLES R. DRYER, M.A., F.G.S.A. PROFESSOR OF GEOGRAPHY, INDIANA STATE NORMAL SCHOOL NEW YORK •:• CINCINNATI-: -CHICAGO AMERICAN BOOK COMPANY THE LIBRARY OF CONGRESS, Two Copies Received SEP. 6 1901 n COPVRIGHT ENTRY C CLASS 0- XXc N». copy a. Copyright, 1901, by CHARLES R. DRYER. Entered at Stationers' Hall, London. DR. PHYS. GEOG. \V. P. I. PREFACE This book has been written in the belief that physical geography is not only interesting and valuable in its sub- ject matter, but is capable, when properly presented, of developing a scientific habit of mind. No attempt is made to discuss all the physical features of the earth, or those of any special region. The best type forms are selected and treated with sufficient fullness to give a clear and definite picture. From a study of the type general laws are developed, and the student is thus provided with a key for the solution of geographical problems wherever they may arise. Each topic is treated inductively. The essential facts are first given, and the student is then guided to a knowledge of their causes, significance, and results. This plan makes it possible to avoid in some degree the vague generalizations which are characteristic of most text-books. The student is in possession of a sufficient number of facts to enable him to see the basis and appreciate the value of the generalizations which follow. A large number of realistic exercises are introduced which appeal to the actual or possible experience of the student. They are designed not for the purpose of discovery but of realization, and to give some idea of the methods and possibilities of geo- graphic research. These exercises include both field and laboratory work. No pains has been spared to make the book scientifi- cally accurate and representative of the state of geographic 5 6 PREFACE science at the opening of the twentieth century. Due prominence is given to recent developments, but not to the exclusion of any link in the chain which connects the face of the earth with man. Discussions of topics which have a special bearing upon human interests are intro- duced at intervals throughout the book, and the relations of the physical features of the earth to human progress are systematically treated in a final chapter. The book has been written with a view to the needs of the teacher as well as to those of the student. Each topic is treated with sufficient fullness to enable the teacher to see its relation to other topics and to teach it intelligently. An unusually large number of illustrations have been selected with a view only to their teaching value. Appen- dixes give full instructions as to where good material and appliances for teaching may be obtained and how to use them. A bibliography of nearly all the geographical lit- erature available in English is added for the use of stu- dents, teachers, and those wishing to provide a good working library of the subject. In the arrangement of topics the logical order of the science is modified by the pedagogical order of presenta- tion to students. As a rule a topic is introduced where it is most needed in teaching ; but in many cases the order may be modified to suit individual or local conditions with- out inconvenience. Difficulties have not been avoided, but constant effort has been made to find the best way of overcoming them. The best method of learning is also the best method of teaching, and it is the hope of the author that this book may prove to be a substantial help in both. Terre Haute, Indiana. CONTENTS BOOK I. THE PLANET EARTH I. The Earth in Space . II. The Structure of the Earth III. The Face of the Earth 9 26 38 BOOK IE THE LAND IV. Erosion ...... V. The Mississippi River System VI. The Colorado River System VII. The St. Lawrence River System VIII. Underground Waters .... IX. Glaciers ...... X. The Drift Sheet of North America XI. Lakes and Lake Basins XII. The Development of Drainage Systems XIII. Forms of Sedimentation XIV. Mountains XV. Volcanoes . . ' . XVI. Land Sculpture ..... XVIL Coast Forms ..... XVIII. The Physiographic Cycle and the Classification of Land Forms ........ 57 68 81 92 102 108 122 135 152 168 178 194 210 227 239 BOOK III. THE SEA XIX. The Figure of the Sea . . . . . 243 XX. Sea Water . . . . . . ... .250 XXI. Movements of the Sea . . ' . . . . 258 7 CONTENTS BOOK IV. THE ATMOSPHERE XXII. The Air 273 XXIII. Moisture in the Air . . . 280 XXIV. Winds 287 XXV. Insolation and Temperature 293 XXVI. The Distribution of Pressure and Winds 3 QI XXVII. Storms ...... 312 XXVIII. Rainfall ...... 327 XXIX. Weather and Climate .... 335 BOOK V. LIFE XXX. Plant Geography XXXI. Animal Geography ...... XXXII. The Geography of Man Appendix I. The Equipment of a Geographical Laboratory Appendix II. Meteorological Instruments Appendix III. The Construction of a Weather Map . Appendix IV. Reference Books , Index 349 364 333 393 39S 410 412 421 LIST OF IMPORTANT MAPS Heights of the Land and Depths of the Sea. . . 40, 41 Glacial and Champlain Periods, North America . . -125 Glaciated Region of Europe . 133 Distribution of Volcanoes . 208 Mean Annual Surface Temper- ature of the Ocean . . 253 Density of the Surface Water, and Surface Currents 256, 257 Isotherms .... 295 Temperature Zones . . . 298 Annual Range of Average Monthly Temperature . 299 Isobars and Winds . . . 303 Ocean Winds . . . . 305 Polar Whirls . . . 308, 309 Rainfall . . . 32S, 330, 331 Isotherms in United States . 344 Absolute Annual Range of Tem- perature in United States . 345 Rainfall in United States . . 345 Vegetation Regions . . . 358 Animal Realms and Regions . 367 BOOK I. THE PLANET EARTH Alone, unpiloted, unswerving aye, The blind old earth spins on its trackless way. CHAPTER I THE EARTH IN SPACE The Earth appears to be Flat. — One who lives in a moderately level country sees the earth around him as a flat disk which stretches away in every direction to a circu- lar rim or horizon. Above the disk he sees the heavens or sky, like a dome or inverted bowl, with its edge resting all around on the rim of the earth. Every day he sees the sun rise above the edge of the earth on one side, pass over the arch of the sky, and disappear below the edge on the other side. At night the moon appears to follow a similar path, and the sky is studded all over with myriads of bright points. Sometimes clouds float between earth and sky and hide sun, -moon, and stars. In a hilly country it is necessary only to climb up to a high point to observe the same appearances, except that the earth disk seems larger and the sky dome more lofty. From any point of view it is evident also that objects in the distance look smaller than similar ones near by. To one looking along a railroad track, the telegraph poles appear to grow shorter and the rails nearer together, until they meet on the horizon. The same is true of the trees and houses on a long street. A horse or a man a mile away looks less than half his real size. Any object, how- 9 IO THE PLANET EARTH ever large, even a mountain, if far enough away, may ap- pear very small or be indistinguishable. On a bright, clear day distant objects look larger and nearer than on a dark day. A burning brush pile at night can be seen much farther than the unlighted pile by day. So the appearance of distant objects varies with their size and with the quantity of light that comes from them. Realistic Exercises. — Clear and definite ideas of distance can be acquired only by experience and practice. The student should give him- self a thorough training with yardstick and tapeline. Let him measure his book, the desk, schoolroom, building, yard, width of the street, city square, etc., not neglecting to measure heights as well as horizontal distances. He should practice estimating distances by the eye and then correct his estimates by measuring. A very convenient way of meas- uring is by the step or pace. Walk a hundred steps and measure the distance : do this repeatedly until the ability is acquired to step" a known and uniform space. In a short time any one can learn to measure dis- tances quite accurately by counting steps. Let one half of a class arrange themselves in line at regular intervals of one hundred yards, while the other half observe the apparent distance and size of the individuals. The Curvature of the Earth. — More than two thousand years ago scientific observers discovered that the surface of the earth is not flat, as it seems, but curved. When viewed from the water level on the shore of the ocean or of any body of water several miles across, the lower part of a ship or a tree, house, or other object at the same level near the oppo- site shore is hidden behind the curve of the water sur- face. At a distance of one mile the object is hidden to the height of eight inches, at two miles four times eight inches, at three miles nine times eight inches, and so on, according to the square of the number of miles. If the observer ascends a hill or THE EARTH IN SPACE II building, he can see farther over the curve; that is, the circle of the horizon enlarges. This is true also upon land. On a plane surface the horizon would be at an indefinite distance and would not retreat as the observer ascends. At places east of us the sun rises and sets earlier, and at places west of us later, than where we are. At Philadel- phia the sun rises an hour before it does at St. Louis, which is about eight hundred miles farther west. If the surface of the earth were flat, the sun would rise at all places at the same moment and set at the same moment. If the earth's surface were a plane, sun time would be the same at all places. The sun is so far away that it would appear at the same angle from Phila- delphia, St. Louis, and Denver, and if it were noon at one of these places, it would be noon at the others. Vertical lines at the three places would be parallel. But when it is noon at St. Louis it is i p. m. at Philadelphia and 1 1 a.m. at Denver. Vertical lines at the three places converge downward. Therefore the surface of the earth along an east and west line is curved. At a place where the sun is directly overhead at noon an upright post casts no shadow. ■ At places to the north or south an upright post at noon casts a shadow, the length of which becomes greater as the distance from the place of no shadow increases. If the earth's surface were a plane, the sun at noon, on account of its great distance, would appear everywhere at the same angle, and an upright post would cast a shadow of the same length everywhere. ( ps < ps 0S 12 M. PLANE M. EARTH M. DEN VER ST.L OUIS PHILADE LPHIA 12 THE PLANET EARTH To one who travels north or south, stars previously invisible rise in front of him, while stars behind him dis- appear below the horizon. The plane of the horizon tilts as he goes, sinking be- fore and rising behind, which is just what must occur on a curved sur- face. Thus a person at A (Fig. 3) can see the star x, but not y and z, while a person at C can the surface of the earth Fig. 3- see all three of the stars. Hence along a north and south line is curved. The shadow of the earth cast upon the moon during an eclipse always has a circular outline, on whatever side of the earth the moon happens to be. Only a spherical body casts a circular shadow in every position. Lastly, the form and size of the earth have been accu- rately measured by various methods. The mean equatorial diameter has been found to be 7926 miles, the polar diam- eter 7900 miles, and the mean circumference 24,860 miles. Realistic Exercises. — Ascend to the highest point you can reach, the tower or roof of a building, or the top of a tree or hill. How does the horizon change as you go up ? If possible, observe distant objects across a level stretch of country or a large body of water. If you travel several hundred miles east or west, how do you have to change your watch to make it agree with the time of the places you visit ? Raise a lamp or candle slowly from the floor to the level of a table : does its light strike all the objects upon the table at the same moment ? Lower it : does its light disappear from all parts of the surface of the table at the same moment ? If there is a hill in your vicinity, even a small one, stand at the foot of it and note the stars just visible above its top. Ascend the hill and notice how those stars rise higher above it and new stars come in sight. Descend upon the opposite side and notice how the stars disappear THE EARTH IN SPACE I 3 behind the hill. Other objects, as trees and buildings, may be observed instead of stars. The curve of the hill produces in a small space the same effect as the curve of the earth. Consult the almanac for the time when an eclipse of the moon will occur, and observe the circular shadow of the earth. Hold a ball in various positions between a lamp and a wall : if the line from lamp to ball is perpendicular to the wall, what is the shape of the shadow ? Substitute various other objects for the ball, and compare results. The Starry Heavens. — If we observe the position of the conspicuous group of stars in the northern sky called the Great Dipper, 1 and then look for it again a few hours later, we shall find that, while the stars still form the outline of a clipper, the whole group has changed its position. By repeated observations we can map out in the heavens the path along which it is traveling and learn its speed, so that we can predict about where it will be in another hour or two. In this way, from a few evenings' watching, it will be plain that the stars in the northern sky appear to wheel around a central point, in a direction op- posite to the motion of the hands of a clock (counterclockwise), once in about twenty - four hours. The central point is near a not very bright star standing alone in a direct line with the outer Fig 4 - two stars of the Dipper, which are called the Pointers be- cause they always point toward the central star, called 1 The Dipper is near the horizon due north, Sept. 22, at midnight; Oct. 23, at 10 P.M.; Nov. 7, at 9 P.M.; Nov.' 22, at 8 P.M.; Dec. 7, at 7 P.M. In the northeast, Dec. 22, at midnight ; Jan. 20, at 10 P.M. ; Feb. 4, at 9 P.M. ; Feb. 19, at 8 P.M. On the meridian above the Polestar, March 21, at midnight; April 20, at 10 P.M.; May 5, at 9 p.m. ; May 21, at 8 p.m. * 1 * * + +■ * > + m • .* * POLARIS ^ * ' y' \ <, x * '' • *.._ • * t' 1 14 THE PLANET EARTH Polaris or the Polestar. The circumpolar stars inside the circle described by the Dipper never set in our part of the world. If we watch some conspicuous group of stars like Orion, which in the autumn months rises early in the evening 1 % , directly in the east, we shall pieces see fr s l° wr y mount the sky, it * pass a little south of the * zenith, and, about twelve ^ *" bull hours after its rising, set * directly in the west. The other stars outside the circle * of the Dipper will be found i to rise somewhere on the *" eastern horizon, follow a 0RI0N longer or shorter path across Fig 5- the heavens, and set some- where on the western horizon. The whole starry heavens seem to be turning about an axis tvhich passes through the Polestar, just as an open umbrella may be turned about its stick. If bits of paper are gummed to the inside of the um- brella, and it is held close to the head and turned, the papers will perform the same kind of a motion as the northern stars. If we had a large hollow globe with stars fastened all over its inner surface, and could place ourselves at the center of it while it rotates around us, it would imitate the apparent motion of all the stars, and we could see the whole path of each. The great hollow globe which seems to carry the stars, and at the center of which our home on the earth seems to be placed, is called the celestial sphere. 1 Orion is just rising due east, Sept. 22, at midnight ; Oct. 23, at 10 P.M. ; Nov. 7, at 9 P.M. ; Nov. 22, at 8 P.M. ; Dec. 7, at 7 P.M. On the meridian, south of the zenith, Dec. 22, at 11 P.M. ; Jan. 20, at 9 p.m. ; Feb. 4, at 8 P.M.; Feb. 19, at 7 P.M. THE EARTH IN SPACE 15 The Apparent Path of the Sun. — The sun appears to revolve with the celestial sphere, but its path changes a little from day to day. On March 21 * (Vernal Equinox) it rises due east, passes south of the zenith at noon, and sets due west. After March 21 it rises farther and farther north of east, and sets farther and farther north of west until June 2 1, 1 when it shines into our north windows for some time \V4 Fig. 6. (From Todd's New Astronomy.) morning and evening, though it still passes south of our zenith at noon. Then the path of the sun begins to move back southward. On September 23 * it is just where it was on March 21, and on December 22 1 it rises farthest to the south of east, and sets farthest to the south of west. Realistic Exercises. — Choose some convenient place where the view is as little obstructed as possible. An open field is best, but an upper room or tower or roof of a building is good. From this spot deter- mine the four cardinal points on the horizon : the point directly below 1 These dates vary a day or two in different years and centuries. i6 THE PLANET EARTH 57.3 Inches the sun at noon is south, the point directly below the Polestar is north. and the sunrise and sunset points on September 23 and March 21 are east and west respectively. Always standing in the same place, observe once every week or ten days the point on the horizon where the sun rises or sets or both ; also its distance above the south point at noon. Valuable observations may be made of the sunset point from a west room with several windows. Mark upon the floor a spot which com- mands as wide a range of the western horizon as possible. Observe from it the movement of the sunset point north or south from week to week. The extreme points reached on June 21 and December 22, and the middle point on September 23 and March 21 are especially im- portant, and their direction should be marked upon the window sill. The height of the sun at noon may be advantageously observed from a south window. Mark upon the floor from week to week the point reached by the shadow of the window sill at noon, or measure its distance from the wall and record it. with the date. This work may be done more accurately by the use of a de- vice for measuring angles. Find a building or fence the west side of which is in a north- south line. This may be de- termined at night by observing whether it is in line with the Polestar. At a point near the south end, and about six feet from the ground, drive a long nail horizontally so that it will project several inches. With a carpenter's leveling board, or with a square and plumbline, draw a horizontal line at the level of the nail, locate upon it a point 57.3 inches north of the nail, and mark it with a tack (Fig. 7). Fasten to the nail one end of a wire or a string which does not stretch, and with a pencil draw an arc downward from the tack to a point below the nail. This will be a quadrant of a circle three hundred and sixty inches in circumference, and the degrees, each one inch long, should be marked and numbered from the tack downward, and each degree divided into fourths or arcs of 15'. About noon, at the moment when the sun first shines upon the west side of the building, note the point N Fig 7- heliotrope. THE EARTH IN SPACE 17 where the shadow of the nail falls across the quadrant. The angle between the shadow and the horizontal line will be the altitude of the sun, or its angular distance above the south point of the horizon. The angle between the shadow and the vertical line will be the angular dis- tance of the sun from the zenith. The altitude should be read and recorded once a week or ten days for at least six months. Rotation of the Earth. — For thousands of years even the wisest men believed the earth to be the center of the universe, and the sun, moon, and stars actually to revolve around it as they appear to do. ' The sun was supposed to be a small hot ball a few miles distant, and the stars to be only bright points ; therefore the velocity with which they must travel to complete a revolution around the earth every twenty-four hours did not seem so enor- mous to the ancients as it would to us. During the six- teenth and seventeenth centuries, the discoveries of Kepler, Galileo, and Newton 1 proved that the earth-center {geo- centric) theory of the universe is erroneous, and that the heavenly bodies appear to revolve around the earth from east to west because the earth is rotating from west to east upon an axis, the north end of which points toward the Polestar. The earth makes one complete rotation upon its axis in 23 h. 56 m. 4.09 s. Realistic Exercises. — The proof that the apparent motions of the' heavenly bodies are due to the rotation and revolution of the earth be- longs to astronomy rather than to geography, but these motions may be realized in several ways. If we watch the sunset closely, as the face of the sun appears to sink below the horizon, it is easy to see how in real- ity the horizon is rising between us and the sun. Again, as the face of the full moon appears to rise gradually above the horizon it is easy to see that the appearance is due to a sinking horizon. That is, as we watch the sun, moon, and stars rise, march from east to west, and set, we are really seeing the earth rotate from west to east. When riding rapidly upon a railroad train notice how objects in the landscape appear to be rushing by in the opposite direction. When 1 Read about these men in the encyclopedia. DR. PHYS. GEOG. — 2 THE PLANET EARTH sitting in a car, can you always tell whether a car upon the next track is moving one way, or your own car the other way ? Revolution of the Earth. — The earth travels along a nearly circular path through space at an average distance of nearly 93,000,000 miles from the sun. The circumfer- ence of this orbit is 584,600,000 miles. The earth revolves once around the sun (from one Vernal Equinox to the next) in 365 d. 5 h. 48 m. 46 s. About how many miles does it travel in its orbit each second ? The earth's orbit is an ellipse having the sun at one of the foci. • On January 2, the earth is nearly 91,500,000 miles from the sun, and on July 3, nearly 94,500,000 miles. The diameter of the sun is about 880,000 miles, or no times the earth's diameter, and the mean radius of the earth's orbit is 105 times the sun's diameter. Realistic Exercises. — In a room having an unobstructed floor space sixteen feet in diameter drive a small nail into the floor in the middle of the space. On each side of it in a north-south line drive an- other nail at a distance of an inch and a half. These nails should project an inch or two above the floor. Take a string which does not stretch easily, double it, and tie the ends so as to make a loop 94! inches long. Place one end of this loop around the two nails, and with a pencil at the other end draw upon the floor an ellipse having the nails for its foci. Around the north nail draw a circle eight ninths of an inch in diameter. The circle represents the sun, and the ellipse the orbit of the earth in correct proportion, the scale being one inch to a million miles. A proportional earth would be .008 inch in;diarheter. Mark the orbit with a line of ink or paint. Mark upon the ground a similar figure, using a doubled cord 94I feet Fig. 8. — How to draw an ellipse. THE EARTH IN SPACE 19 long (one foot to a million miles) . On this scale the sun will be nearly ten inches in diameter and the earth nearly one tenth of an inch. To realize the motion of the earth among the stars, walk around this path counterclockwise, with your face turned toward the center, (the sun), and notice how the distant objects beyond and on a line with the center continually change. Walk around again with your face turned directly away from the center (the sun), and notice a similar procession of objects in the line of sight passing backwards or clockwise. On account of the brightness of the sunlight we can not see the stars in a line with the sun, but at midnight we can look directly overhead and see the stars which are on the opposite side of the earth from the sun. If we thus observed the zenith at midnight every night for one year, we should see a procession of stars apparently passing westward, and at the end of the year the same stars would be in the zenith at midnight as at the begin- ning of the year. It is not necessary to make these observations every night or at midnight. If we observe the position of any star or group of stars, as Orion, at the same hour, say 9 p.m., once a week or ten days, it will be found in a position farther to the west at each successive observation. Thus in watching the apparent annual westward march of the stars, we are really seeing the eastward movement of the earth around the sun. NOON Fig. 9. The Change of Seasons. — That the sun's rays have greater heating power at noon than at morning or evening is one of the most familiar facts in nature. This is due chiefly to two causes, shown in Fig. 9. At sunrise and sunset, the rays, being horizontal, pass through a greater thickness of air, which absorbs more of their energy, and they are spread over more surface, so that there is less 20 THE PLANET EARTH heat to the square mile. At noon, the rays, being more nearly vertical, pass through less air and cover less space, which makes the heat more intense. 1 If the varying path of the sun in the heavens has been observed through the year (see p. 15), the facts are also familiar that in summer its path is longer and approaches nearer to the zenith than in winter. In June, the sun not only shines more directly in our latitude than in January, but also shines several hours longer each day. In spring and autumn, the angle of the rays and the length of daytime are intermediate. These changes are sufficient to account for the changing seasons. The causes of the variation in the path of the sun remain to be explained. The Attitude of the Earth. — As the earth moves around the sun, its axis always points toward the Polestar, and is always inclined at an angle of about 66^° to the plane passing through the earth's orbit and the center of the sun, or about 23^° from a perpendicular to that plane. Thus the earth presents at different times of the year different faces to the sun, as is shown in Fig. 10. Realistic Exercise. — It is not always easy to get a clear understand- ing of this subject from words and pictures, and resort should always be had to demonstration with some kind of apparatus, perhaps the simpler the better. Darken the room in which the orbit of the earth is marked upon the floor, place a lamp in the position of the sun, and carry a globe (almost any kind of a ball will answer) around the orbit counterclock- wise, holding it in such a position that the axis stands at an angle of 66i° with the floor (the plane of the orbit), and the north end of it points directly north of the zenith. Observe that when the globe is in a position north of the lamp the northern hemisphere leans away from the lamp, the center of the lighted half is south of the equator, and the northern edge of the lighted half falls short of the north pole. When 1 The idea sometimes met with that slanting rays heat less then direct rays be- cause they strike with less force, is erroneous. Rays of heat and light have no force of impact and do not strike at all in the same sense as a ball or an arrow does. THE EARTH IN SPACE 21 Fig. 10. — Position of the northern hemisphere throughout the year. the globe is south of the lamp, the northern hemisphere leans toward the lamp, the center of the lighted half is north of the equator, and its northern edge reaches beyond the north pole. When the globe is east or west of the lamp, the northern hemisphere is still inclined, but neither toward nor away from the lamp, the center of the lighted half is at the equator, and its edge just reaches either pole. Observe that at the center of the lighted half the rays strike the surface of the globe perpendicularly, at other places more and more slantingly as the dis- tance from the center increases, and at the edge of the lighted half the rays just graze the surface horizontally, or are tangent to it. Results of the Earth's Attitude. — If the axis were perpendicular to the plane of the orbit, that plane would always be the same as the plane 22 THE PLANET EARTH of the equator, and the sun would always be vertically above some point on the equator. As it is, the sun is vertical at the equator at two oppo- site points in the earth's orbit, on September 23 and March 21. On June 21 the northern hemisphere is inclined toward the sun so that the vertical rays fall on the Tropic of Cancer, 23I north of the equator, and the tangent rays reach the Arctic Circle 23J- beyond the north pole. On December 22 the northern hemisphere is inclined away from the sun so that the vertical rays fall on the Tropic of Capricorn, 23^° south of the equator, and the tangent rays reach the Arctic Circle 23^° short of the north pole. Where do the tangent rays reach in the southern hemisphere on June 21 and December 22? This change of face presented to the sun is manifested to us in the varying daily path of the sun through the heavens, as it swings back and forth, north and south, once a year. Realistic Exercise. — Admit a beam of direct sunlight through a hole in a curtain or shutter. Hold a book so that the beam strikes its surface perpendicularly ; incline the book and observe the lighted spot grow larger and less bright as the angle increases. From Fig. 9 it is evident that on account of the curvature of the earth's surface the parallel rays of the sun must always strike perpendicularly at some spot and at all others more or less slantingly, and that the area covered by any given bundle of rays increases as their slant increases. Therefore as the daily path of the sun approaches the zenith its heating power in- creases, and as it recedes from the zenith decreases. The Inequality of Day and Night. — Consult the almanac and find the length of the longest day and the shortest day in your latitude. If you can get an English almanac, find the same thing for London. Figure 6 shows how the portion of the sun's path above the horizon varies with the seasons, and Fig. 10 shows why this is so. About December 22 much less than half the northern hemisphere is in the sunlight ; hence the parallel of latitude or path of rotation of any given place, as New York or London, is more than 'half in darkness, and the time required to pass through the night is longer than that THE EARTH IN SPACE 23 required to pass through the day. In the southern hemi- sphere the reverse is true, and on June 21 the condition of each hemisphere is the reverse of that on December 22. At the equator the days and nights are always equal, but the inequality increases toward the poles. Thus the sun's long brush paints the earth with bands of heat which swing back and forth, following the march of the apparent path of the sun and bringing the change of seasons. Equinoxes and Solstices. — The elates on which the sun is vertical over the equator and the days and nights are of equal length, March 21 and September 23, are called the Vernal and the Autumnal Equinox. The dates when the sun is vertical over the tropics, June 21 and De- cember 22, are called the Summer and the Winter Solstice. The earth is nearest the sun on January 2 and farthest from it on July 3. It moves faster in that part of its orbit which is nearest the sun, hence the north- ern summer, from March 21 to September 23, is about six days longer than the northern winter. Location upon a Sphere. — If we take a perfectly plain ball of any kind and mark a point upon it, we shall find it impossible to describe in any way the position of the point. If we spin the ball like a top upon the table or when sus- pended from a cord, and thus establish an axis, two poles, and an equator, the position of any point may then be determined and described by its distance from them. Latitude. — The angular distance north or south from the equator is measured and expressed in degrees of lati- tude from o° to 90 . If the earth were a perfect sphere, every degree of latitude would be g^ of the circumference of the earth ; but the polar diameter being twenty-six miles shorter than the equatorial diameter, the convexity of the earth grows gradually less from the equator to the poles, and a degree of latitude becomes -^^ of the circumfer- ence of a larger and larger circle. Latitude is the angle between the radius of curvature of the earth's surface at 24 THE PLANET EARTH any point and the plane of the equator. The degrees increase in length toward the poles (Fig. n), according to the table on p. 25. NO -„™ LE . Realistic Exercise. — ■ At the equator the Polestar appears just at the horizon, midway between the equator and the pole it is 45 above the horizon, and at the pole it is in the zenith. The altitude or angular distance of the Pole- star above the horizon at any place is equal to the latitude of that place. On March 21 and September 23 the sun is directly over the equator. Observe the noon altitude of the sun on either of these days and subtract it from 90 ; the remainder is the lati- tude of the place. Longitude. — It is evident that all places equally distant from the equator are situated upon a line parallel with the equator and have the same latitude ; and it is necessary to Degrees of lati- tude Fig. 12. — Parallels and meridians, determine the position of each place upon its parallel of latitude. ' This is done by drawing lines called metidians from pole to pole at right angles to the parallels, and meas- uring the angular distance of the meridian passing through STRUCTURE OF THE EARTH 2J The Science of Geography deals especially with the region of contact and interpenetration between the rock sphere, water sphere, and atmosphere, the home of plants, animals, and men. Its business is to describe and explain the distri- bution of all the features found there. While the special field of the geographer lies on what is commonly called the earth's surface, he avails himself of whatever is known con- cerning any part of the earth to assist him in explaining the distribution of surface features. The Centrosphere. — We have no direct knowledge of the interior of the earth. The deepest boring yet made (at Schladebach, Germany) is only about six thousand feet deep, and the deepest natural cut (the Grand Canyon of the Colorado) is about the same depth, which is hardly proportional to a pin scratch through the varnish of a globe. Yet certain inferences may be drawn with great probability concerning the condition of the centrosphere. The pressure within the centrosphere is very great. Every mass of matter in it sustains the weight of the col- umn of rock above it, as the foundation of the Washing- ton monument is under the pressure of the stones placed upon it. It is calculated that at the depth of one hundred and fifty miles the pressure is one million pounds per square inch, and at the earth's center thirty million pounds. Below a depth of eight miles the pressure is sufficient to crush the strongest materials ; therefore, no open space or cavity can exist there. The density of the centrosphere is much greater than that of the crust. The average density of the whole earth has been determined by various methods, and it is thus shown that the earth weighs 5.6 times as much as an equal globe of water. But the average density of the rocks forming the crust is only between 2.5 and 3. Therefore, the material composing the centrosphere must be, on the average, about twice as dense as the crust. The supposition that this material may 28 THE PLANET EARTH be composed largely of iron, gold, and other heavy metals, is not unreasonable. The temperature of the centrosphere is very high. In wells, mines, and tunnels the temperature is always found to increase downward. The rate of increase is variable, but averages i° for every fifty or sixty feet. It is probable that at great depths the temperature does not increase so rap- idly, and that below one hundred and fifty miles the centro- sphere has a nearly uniform temperature of about 7000 F. The centrosphere may be liquid, or solid, or partly liquid and partly solid. If the temperature increases downward at the rate of i° for every fifty or sixty feet, at a depth of fifty miles it is hot enough to melt any substance known upon the surface of the earth. The eruption of melted rock from volcanoes in many parts of the world has encouraged the belief that inside of a thin solid crust the earth is a liquid mass. But people who have held this opinion have failed to take into account the fact that pressure raises the melting point of most substances, and at some depth not very great the pressure may be sufficient to prevent the rock from melting, in spite of its high temperature. The attraction of the sun and moon pulls the sea out of shape and produces a regular rise and fall of the water known as the tides. If the centrosphere were liquid, it would be drawn out of shape in the same way, and the crust would heave up and down as the surface of the sea does. There is no evidence of such a movement ; on the contrary, Professor Darwin and Lord Kelvin have shown that the earth keeps its shape as rigidly as though it were a globe of solid steel. This would not be possible for an earth built on the plan of an egg — a thin shell filled with liquid. The jar of an earthquake in Japan is often felt in England, and the time which it occupies in passing through the earth indicates that it passes through a solid and not a liquid. The crust of the earth is not stretched smoothly over it like the skin of an apple, but is very much wrinkled, folded, and crumpled, especially in mountainous regions. There is some difficulty in conceiving how the crust could become crumpled over a solid globe ; but experiments have shown that under great pressure the most rigid solids, such as steel, act as if plastic and yield slowly to pressure like butter. Consequently the centrosphere, although solid, may. act as if plastic, yielding enough to permit all the movements which have taken place in the crust. STRUCTURE OF THE EARTH 29 Volcanic eruptions may be accounted for by supposing that the lava comes from cisterns of liquid rock of no great extent, or that the rock melts in certain places where the wrinkling of the crust partly relieves it from pressure. Some geologists have been led to suppose that between the solid crust and the solid core there is a shallow layer of liquid matter all around. The condition of the centrosphere is necessarily shrouded in much uncertainty ; but the theory which seems to account best for all the facts supposes the existence of (1) A very large, dense, and hot core, solid because of the pressure upon it. (2) A surrounding shallow layer of liquid or semi- liquid matter, upon which the solid crust or rock sphere floats. So far as the structure of the earth is concerned, it may be roughly compared to a hot iron ball coated with tar and covered with wrinkled leather. Sir William Dawson compares the earth to a plum, peach, or cherry somewhat dried up ; it has a large, hard stone or kernel, a thin pulp, and on the outside a thin skin ; it has shrunk slightly, so as to produce wrinkles in the skin, and in some places the skin has cracked, allowing small quantities of the pulp to ooze out. The Earth-crust ; Mantle Rock. — It is a matter of com- mon observation that the crust of the earth is made up of a variety of materials. This may be seen almost any- where : in a plowed field, in an excavation for a cellar or ditch, in a cut made for a wagon road or railroad, in the banks of a stream, in a gravel pit or quarry. In lowlands and valleys, and on gentle slopes, the surface materials lie in a loose, incoherent mass, which may be removed with spade and pick. This loose and workable material is often called soil ; but it is better to use that term for 30 THE PLANET EARTH only the upper layer of it, and to call the whole mass, however deep, mantle ivck, because it overlies and covers the other rock. The common varieties of mantle rock are clay, sand, gravel, pebbles, and boulders. They are all fragments of older rocks which have been broken up, and wholly or partly pulverized and decomposed. Clay is a soft, almost impalpable powder like flour, sticky and greasy when wet, and easily molded into any desired form. Sand is a mass of loose, hard grains, usually of the mineral quartz. The grains may be coarse or fine, sharp or rounded, but sand is always recognizable by its harsh, gritty feel. Gravel is composed of small stones, larger than sand grains. The stones may be angular or rounded, and of any color or composition, but are usually hard and smooth. Pebbles and Boulders are large fragments of any kind of rock which have been broken off and moved from the parent bed. Mixtures of clay, sand, gravel, and pebbles occur in all proportions. A mixture of sand and clay is commonly called loam ; when it contains also a portion of decayed vegetable matter (humus) it forms a fertile soil. Marl is a whitish mud found at the bottom of some lakes and ponds. It is a mixture of clay and lime derived largely from the decay of the shells of mussels, snails, and other animals. Peat or muck is a black mud formed by the decay of vegetable matter under water. It sometimes accumulates in lakes and marshes to the depth of many feet. Realistic Exercises. — Collect as many kinds of mantle rock as pos- sible and examine them carefully. Note the feel, color, odor, and taste of clay, and its behavior when wet. Under a good microscope the powder of dry clay is seen to consist of minute, flat, translucent scales. Examine different specimens of sand with a hand magnifier, and note the size, shape, and color of the grains. Shake up loam or a mixture of sand and clay in a tall bottle of water and let it settle. The sand will go to the bottom very quickly, while the clay will settle slowly on top of the sand : it makes the water turbid or roily, and it may take twenty-four hours for all the clay to settle and leave the water clear. Examine specimens of clean gravel : it may be necessary to wash STRUCTURE OF THE EARTH 31 out the sand and clay in a stream of water. From a pint or quart of gravel pick out the smooth, rounded stones and count them ; count the rough, angular stones, if any, and determine what per cent they are of the whole number. Try the hardness of each stone with the point of a knife and determine what per cent of them are soft, that is, easily scratched with a knife, and what per cent are hard, that is, scratched with difficulty or not at all. Bear in mind for future investigation the question, Why are most gravel stones hard and smooth ? Fig. 14. — Shale overlain by mantle rock. (Near Terre Haute, lnd.) Examine as many boulders as possible : note their size, shape, and hardness, and whether they are composed of only one kind of mineral or of several kinds. The shape may be rounded, without flat faces ; sub- angular, with flat faces and rounded edges and corners ; or angular, with sharp edges and corners. Examine a specimen of marl and note the fragments of shell. In peat observe the fragments of roots, stems, and leaves. Bed Rock. — If we dig or bore down far enough into the mantle rock, it will be found to be of moderate depth, and 32 THE PLANET EARTH to be everywhere underlain by bed rock. This is a solid, massive, continuous sheet of rock which extends indefinitely in every direction and requires the drill and hammer, or even the use of gunpowder or dynamite, for its removal. On mountains and in regions of steep slope the bed rock is very thinly covered or lies bare and exposed to the weather. A projection of bed rock through the mantle rock often occurs upon a hillside or cliff, and is called an outcrop or exposure. Bed rock is also likely to be exposed in the bed or along the banks of a stream. Over the greater part of the land surface bed rock is found to lie in distinct layers. A set of layers of one kind of rock is called a stratum (plural, strata). Shale, sandstone, conglomerate, and lime- stone are the only common kinds of stratified bed rock. Shale or mud rock (Fig. 14) is nothing but compacted and hardened clay. It is soft and fine-grained, Has the feel, odor, and taste of clay, splits easily into thin, irregular blocks, and is popularly, although wrongly, called " soapstone." It is usually of a gray or brown color, but may contain vegetable matter enough to make it black. The harder and more compact varieties of shale split into thin, regular leaves and are commonly called slate. Sandstone (Fig. 15) is composed of sand grains held together by some kind of cement. It may be fine- or coarse-grained ; tough and Fig. 15. —A sandstone cliff. (Montgomery County, Ind.) STRUCTURE OF THE EARTH 33 Fig 16. —Fragment of conglomerate. compact or loose and friable, so that it can be crumbled with the fingers. The colors vary from nearly white through gray and brown to dark red. The granular structure may be easily recognized by the harsh feel or the appearance under a magnifier. The most common cements in sandstone are clay, lime, silica, and iron, to which the red color is due. Grindstones and whetstones are made of fine and even-grained sandstone in which the cement is silica. The layers may be many feet in thick- ness or as thin as pasteboard. They often contain glistening grains of mica. Conglomerate is cemented gravel and is often found in the lower part of a gravel bed. If the pebbles are rounded, it is called pudding stone ; if angular, breccia. Limestone (Figs. 17, 18), the most abundant of all rocks, is largely composed of the skeletons or shells of animals which lived in water. These may be microscopic, as in chalk, or easily recognizable as shells, corals, crinoid stems ("button molds"), or other forms. Limestone is of all colors and is not too hard to be scratched with a knife. A drop of cold dilute hydro- chloric acid placed upon limestone dissolves it with foaming, due to the escape of gas. Some limestones are formed by the deposit of lime from solution in water, as the stalactites and stalagmites in caves, and the tufa around the mouths of some springs. Many limestones are composed of small rounded grains in ap- pearance like fish eggs. This kind is called concretionary or oolitic limestone. Bituminous coal, or soft coal, is a stratified bed rock formed by the consolidation of peat. Aqueous or Sedimentary Rocks. — All the rocks thus far described, including both mantle and bed rocks, are composed of materials which, in most cases, have been re- moved from their original position and carried some dis- tance by running, water. When the current of a stream slackens its speed or enters the still water of a lake or the DR. PHYS. GEOG. — 3 Shell. Tufa. Fig. 17. —Specimens of limestone. 34 THE PLANET EARTH ocean, the mud, sand, and gravel which the stream has been carrying settle to the bottom. By this process the materials are deposited in nearly level layers or strata and are generally assorted so that each layer is composed chiefly of one kind of sediment. Hence all rocks thus formed are classed together as aqueous, sedimentary, or stratified rocks. Igneous and Metamor- phic Rocks. — If at any place we bore down through the mantle rock and through the layers of stratified bed rock, at a greater or less depth we strike bed rock which is not stratified and which owes its form and structure to the action of heat. In volcanic re- gions, melted rock has escaped in vast quanti- ties through cracks in the crust, spread out over the stratified rocks, and cooled in the form of lava streams and sheets. It is often blown out of volcanoes in the form of lava dust, sand, or gravel. Such rock is called eruptive or volcanic. In many cases the melted rock has not succeeded in reaching the surface, but has forced itself into the cracks and between the lay- ers of stratified rock and solidified there at considerable Fig. 18. — Limestone cliff. (Near Madison, Ind.) STRUCTURE OF THE EARTH 35 depths. Such rock is called intrusive. All rocks which have cooled from a once molten condition are called igneous. Other rocks have not been melted, but have been more or less changed from their original fragmental and strati- fied form by heat and pressure, and hence are called meta- morphic or altered rocks. Igneous and metamorphic rocks are distinguished from aqueous rocks by being often un- stratified and made up, not of fragments, but mainly or wholly of crystals. The crystals may be too small to be seen by the naked eye, but they are often conspicuous from their shape and sparkling luster. Some igneous rocks are not granular or crystalline, but structureless like glass. Granite is a common representative of a large family of igneous rocks, each member of which is composed of a mixture of two or more differ- ent minerals, rather coarsely crystallized and presenting a speckled or mottled appearance. Granite commonly consists of three minerals : ( i ) quartz, in glassy, lustrous grains too hard to be scratched with a knife ; (2) feldspar, in white or reddish crystals which break with flat faces and square angles ; (3) mica, in soft, thin flakes, usually black. The mica is sometimes replaced by hornblende, in greenish or black prismatic or needle-shaped crystals. A mixture of feldspar with horn- blende or mica, but without quartz, is called syenite. If the minerals named above as form- ing granite are not thoroughly mixed, but occur in more or less regular bands, the rock is called gneiss. A mixture containing a large proportion of mica in coarse flakes, and therefore splitting easily, is called mica schist. Basalt or trap rock is a common representative of a group of minutely crystalline or glassy igneous rocks which are usually of a greenish or black color. Gneiss. Granite. Fig. 19. — Igneous rocks. 36 THE PLANET EARTH Realistic Exercise. — Collect as many specimens of rock as possi- ble, both stratified and unstratified. Almost anywhere north of the Ohio and Missouri rivers a gravel pit will furnish a hundred different kinds. Try first to separate the aqueous rocks from the igneous and metamorphic The latter are the more difficult, and to make much progress in studying them a descriptive handbook with a few labeled specimens are necessary. 1 Classification of Common and Typical Rocks Origin Class Texture Bed Rock Consolidated Mantle Rock Unconsoli- dated Aqueous Rocks Deposited by water Mechanical Sediments. Fragmental. Shale. Sandstone. Conglomerate. Clay. Sand. Gravel. or ice. Usually stratified. Chemical or Organic Sediments. Crystalline, Compact. Limestone. Bituminous Coal. Marl. Peat. Igneous Rocks Cooled from a Eruptive or Volcanic. Cooled on the surface. Compact or Crystalline. Glassy. Basalt. Trap (Lava). Obsidian, Pumice. melted state. Unstratified. Intrusive. Cooled below the surface. Crystalline. Granite. Syenite. Stratified. Slaty. Compact. Crystalline. Glassy. Rock Name Original Form Metamorphic Rocks Altered by heat and Slate. Quartzite. Marble. Anthracite Coal. Shale. Sandstone. Limestone. Bituminous Coal. pressure. Unstratified. Banded. Schistose. Gneiss. Mica Schist. Conglomerate or Granite. Shale or Granite. i The Washington School Collection, furnished by E. E. Howell, Washington, D.C., is very good. STRUCTURE OF THE EARTH 37 The Structure of the Earth-crust. — The crust of the earth, so far as, accessible to us, consists of three general layers. (1) On the outside, mantle rock, a layer of loose, uncon- solidated, generally stratified fragments, nowhere more than a few hundred feet thick, and in many places entirely wanting. (2) A layer of stratified and consolidated rock frag- ments, perhaps averaging five or ten miles in thickness. In mountainous regions it has been extensively warped, crumpled, and broken, and in some localities removed by erosion. (3) A fundamental, unstratified layer of unknown thickness, which has cooled and crystallized from a pre- viously molten condition. It has also extensively pene- trated into and through the other layers, and has thus become surface rock in some places. CHAPTER III THE FACE OF THE EARTH " If an observer from the depths of celestial space could observe the surface of our globe as it would present itself to him in the course of a daily rotation, the most striking feature would be the gradual narrowing of the continents toward the south." — Suess. The large and striking features of the face of the earth are due to the fact that the surface of the earth-crust is slightly irregular, and the waters of the sea have accumu- lated in its depressions. Those portions of the crust which project above the water level form the land masses, great and small, continents and islands. The shore line of the sea is the boundary between two strongly contrasted regions : the land with its varied surface and products, and the unbroken expanse of the ocean. Seventy-two per cent of the surface of the solid earth seems to be hidden from observation under a barren blanket of water. But apparatus has been invented by which we may feel down through the water and gain con- siderable knowledge of the solid crust beneath ; so that we can now represent to our imagination the main features of the earth as they would appear without water in the sea. On the relief map of the world, pp. 40, 41, the most promi- nent feature is the crookedness of the lines which mark the different levels and the consequent irregularity of the areas which they inclose. The largest tracts of approximately uniform level occur on the sea bottom between six thou- sand and eighteen thousand feet below the surface (medium shades of blue). Figure 20 shows the proportions of the 38 THE FACE OF THE EARTH 39 earth-crust which lie at the various levels. More than half (57 per cent) of its surface lies under water six thousand feet or more in depth. This is called the area of depres- sion or deep sea floor. The dry land (red and white) rests upon a block or foundation (lightest blue) a little larger than itself, which rise^-rather steeply from the sea floor. This continental block, comprising the land and the narrow belt of sea bottom around it, over which the water is less than six thousand feet deep, is called the area of elevation. Per Cent of Ar.ea of Earth-Crust Surface (10$ =19,700,000 Sq. M.) Fig. 20. — Generalized profile of the earth-crust. (Hypsographic curve — after Wagner.) Realistic Exercise. — Upon a hollow rubber ball five or six inches in diameter mark the poles and equator. Draw meridians and parallels 30 apart : using these as guides, mark the outline of the continental block, including within it the Gulf of Mexico, Caribbean Sea, Mediterranean Sea, Red Sea, the seas between Australia and Asia, and the Arctic Ocean. Cut the ball along the outline marked : one portion thus made will have the form of the area of elevation (the Arctic Ocean being at the center), and the other portion the form of the area of depression. The Plan of the Earth. — The continental block sur- rounds the Arctic depression at a distance of about 20 160 180 160 140 120 100 4 THE PLANET EARTH from the north pole, and extends thence south- ward in three great arms. The longest arm is occupied by North and South America, a land mass which extends to 56 south latitude. The second arm is occupied by Europe-Africa, which ends at 35 south, and the third arm by Asia- Australia, which ends at about 40 south. The south polar regions are probably occupied by land, which projects in a few places a little beyond the Antarctic Circle, but north of this land the sea forms a belt around the globe, from about io° to 30 in width, and sends northward three great arms which interlock with the arms of the land. The longest arm, the Atlantic Ocean, lies between the Americas and Europe-Africa, and is wide open at both extremities, forming a channel of com- munication between the two polar regions. The broadest arm, the Pacific Ocean, lies between the Americas and Asia-Australia. It is nearly closed at latitude 65 north. The third arm, the Indian Ocean, lies between Africa and Australia, and ex- tends only to about 25 north latitude. In this plan there are several striking peculiarities. (1) There is a large excess of water in the southern hemi- sphere. (2) Each of the three continental arms is more or less broken by deep inlets of the sea which serve to separate them naturally into grand divisions of the land. The Ameri- can arm is thus broken at io° north, the Europe-African at 35 north, and the Asia-Australian near the equator. (3) Most of the resulting land masses are triangular, with bases to the north, and tapering points toward the south. (4) The great land masses and ocean basins are set over against each other on opposite sides of the globe. Europe- Africa is opposite the Pacific, Asia-Australia opposite the Atlantic, and North America opposite the Indian, while South THE FACE OF THE EARTH 43 America forms an exception in being opposite to the island region of the western Pacific. This antipodal arrangement of land and water has led some geographers to think that the earth is a spheroid slightly flattened on four sides, which determine the positions of the Arctic. Atlantic, Pacific, and Indian depressions, while the land masses, including the Antarctic continent, occupy the projecting edges and corners. The Region of Depression. — More than three fourths of the sea floor (78 per cent) lies between 6000 and 18,000 feet below sea level. Its generally smooth surface is broken by a few narrow ridges which support groups or chains of islands, and by valleys or holes (colored darkest blue on the map), called deeps, a few of which exceed 24,000 feet in depth. The average depth of the sea is about 11,500 feet, or nearly 2\ miles. The greatest depth yet found is about 31,600 feet, near the Ladrone Islands in the western Pacific. The Region of Elevation. — From the deep sea floor the continental block rises in most places rather steeply, so that its sides, comprising the region between 600 and 6000 feet below the sea level, form only 10 per cent of the area under water. Its upper surface is but slightly elevated, nine tenths of the land being less than 6000 feet above sea level. Much ingenuity has been expended in the effort to discover some unity of plan in the relief of the several grand divisions, but without much success. Continents do not take the form of raised domes plun- ging steeply on all sides toward the sea ; neither is there a central back- bone of high land, a feature very prominent in large islands ; nor are nnrginal highlands with a central depression between them the com- mon rule. South America is bordered on the western side by the narrow wall of the Andes Mountains. In North America the two par- allel systems of the Rocky Mountains and the Sierra Nevada-Cascade form a double barrier. The southeastern and southern parts of Asia are occupied by an irregular patchwork of lofty plateaus and mountains, which are prolonged through southern Europe to the Atlantic. In Africa and Australia the principal highlands lie along their eastern 44 THE PLANET EARTH sides. The only general plan of continental relief consists in the occur- rence of an elevated margin next to the Pacific and Indian oceans, and of extensive lowlands next to the Atlantic and Arctic oceans. The Coast Shelf. — The gentle slope of the lowlands often extends out to sea, forming a wide submerged shelf between the shore and the boundary of the steep slope of the continental block. Along a great part of the Atlantic coast the thin edge of the sea thus transgresses upon the land. On the other hand, the steep slope of the highlands near the Pacific coast usually continues under water, and the sides of the conti- nental block plunge abruptly downward to the deep sea floor. The un- broken descent from the summit of the Andes to the sea floor amounts in some places to a fall of 42,000 feet in 80 miles. The slopes of the Gulf of Guinea are in some places as much as 2000 feet per mile. 50 Miles 100 Fig. 22. — Profiles of coast shelves. Irregularity of Land Surface. — The surface of the land is characterized by general roughness and irregularity, in contrast with the comparative smoothness of the sea floor. A profile of the land almost anywhere, drawn upon a scale large enough to show the smaller features, appears some- what jagged, and in elevated regions it may resemble the teeth of a saw. An examination of continental relief in detail reveals the presence of numerous inclosed basins nearly or wholly surrounded by mountains, especially in the central regions of Asia-Europe and Africa, and in western North Amer- ica. As much as one fifth of the whole land surface is so inclosed as to have no outlet for drainage to the sea. THE FACE OF THE EARTH 45 A few of these depressions in the land lie below sea level ; of these the basin of the Caspian Sea is the largest and that of the Dead Sea the lowest. More than one fourth (26.7 per cent) of the land surface of the globe lies between sea level and 600 feet, and nearly three fourths (73.7 per cent) below 3000 feet. The greatest height yet measured is Mount Everest in the Himalayas, 29,000 feet. The average elevation of the land is approximately 2300 feet, or a little less than half a mile. Characteristics of the Grand Divisions. — In Europe more than half the surface (54 per cent) is less than 600 feet above the sea and only one tenth more than 6000 feet. It is the least elevated of all the grand divisions and is characterized by extensive low plains. Of all' the grand divisions Africa has the smallest part (12.5 per cent) of its surface below 600 feet. It is characterized by extensive plateaus. Australia resembles Africa, but its elevation as well as its area is much less. Asia is distinguished by the height and massiveness of its mountain chains, which give it the greatest absolute height, 29,000 feet, and the greatest average height, 2884 feet. Thirty-eight per cent of it lies above 3000 feet. The Americas exhibit all the great relief forms, low plains, high plateaus, and mountain chains, without marked predominance of any. Their long north and south mountain systems are not continuous, but are separated in Central America by a system now partly submerged, but reappearing eastward through the West Indies. Islands. — All the large islands except Madagascar and New Zealand, and many small ones, stand upon the conti- nental block and seem to be the tops of peaks or ridges rising from its partly submerged surface. Oceanic islands rise from the region of depression and are of volcanic origin. 46 THE PLANET EARTH Comparative Smoothness of the Crust Surface. — In comparison with the size of the earth the irregularities of the crust are trifling. The lowest point known is a little over 31,000 feet below sea level and the highest point is 29,000 feet above sea level, so that the range of relief or vertical distance between them is only about 60,000 feet, 11^ miles, or 7 ^g of the earth's diameter. Upon a globe 7 feet in diameter the range of relief proportional to that of the earth would be one eighth inch, which is considerably less in proportion than that of the roughness of the skin of an orange. If the elevations of the crust were used to fill the depressions and the whole surface were graded to one level, that surface would be 1 .44 miles below the present sea level and would be covered with water 1.56 miles deep. Sea and Land. — The sea itself, with an average depth of about 2\ miles, is only a thin skin upon the globe which, like a shallow pool upon a sidewalk after a rain, serves to mark the outlines of a depression which would be otherwise scarcely noticeable ; yet its volume is nearly thirteen times as great as that of the land above water. The posi- tion of the water surface or sea level determines the most important boundaries of the world. From it all heights and depths are measured, and by it all coast lines are fixed. A slight rise of sea level would submerge large areas of land and change entirely the outlines of continents ; while a lowering of six thousand feet in its level would not materially change the outline of the continents, but would unite them into a single mass. Causes of Relief. — The causes of irregularity in the sur- face of the earth-crust are not fully understood. This prob- lem has been the subject of much study and speculation, and many hypotheses have been proposed to account for the depression of the deep sea floor and the elevation of the continental block. Diastrophism. — The upper layers of the earth-crust on land are composed largely of sedimentary rocks, such as are now forming on the sea bottom near shore, and they contain the fossil remains of animals THE FACE OF THE EARTH 47 which live only in shallow sea water. Hence we feel sure that nearly all the present land surface was once covered by the sea and has been raised from that position to its present elevation. The sedimentary strata have not only been raised bodily hundreds or thousands of feet, but they have, also been broken, tilted, folded, crumpled, and crushed together in a manner which shows that they have been subjected to enormous horizontal pressure (see pp. 178, 192). The movements of ele- vation, depression, fracture, and dislocation are still in progress. Marks placed upon the rocks along the coast of Sweden many years ago show that the land is slowly rising, in some places three or four feet in a cen- tury. Buildings erected three or four hundred years ago on the west coast of Greenland are now under the sea. Buried stumps of trees on the coast plain of New Jersey furnish evidence that a slow sinking is in progress there. A Spanish magazine built near the mouth of the Mis- sissippi about two hundred and thirty years ago is now more than ten feet under water. During an earthquake in 1822 the coast of Chile was suddenly raised two or three feet, and again an equal distance in 1835. Fig. 23. — Cape Maysi, Cuba. The coast of Cuba presents a series of raised benches which were cut' by the waves when the land stood at lower levels than now. Any solu- tion of the problem of the causes of diastrophism, or movement in the earth-crust, must account for the elevation of great continental areas and for the crumpling and breaking of the strata. Isostasy. — It is thought by many- geologists that the earth-crust under the ocean is denser and heavier than it is under the land, and that in consequence the sea floor sinks more deeply into the underlying centrosphere while the continental block is floated higher above it. Realistic Exercises. — Float two blocks of wood of the same size and shape, one of oak and the other of pine, in a vessel of water, and note the different heights of their upper surfaces above the surface of the water. Fill a U-shaped glass tube nearly half full of water. Pour oil into one arm and note the different levels at which the liquids stand. The shorter column of relatively heavy water balances the longer column of relatively light oil and holds its surface up to a higher level. 48 THE PLANET EARTH Several lines of evidence seem to indicate that the material compos- ing the great plateaus and mountain ranges of the world is actually lighter than the average of the earth crust. These regions are probably not held up at their high level by the rigid support of the surrounding parts of the crust, as the roof of a building is supported by the walls, but they are pushed up by the heavier masses of the sea floor, the pressure of which is transmitted in every direction, as if through a liquid or plastic layer beneath the crust. This balancing of sea floor and con- tinental block has been called by Dutton isostasy. Contraction. — The wrinkling and folding of the earth-crust has long been accounted for in another way. It seems certain that the earth was once much hotter than it is now, and that it is constantly radiating heat into the cooler space around it as a hot stone or iron radiates heat in a cold day. If the globe has been cooling, it must also have been contracting or growing smaller. The heat of the sun keeps the crust at a nearly constant temperature, but the centrosphere has kept on cooling and contracting until the crust has become too big for it and is compelled to fold and wrinkle by its own weight. The wrinkled skin of a withered peach or a cold baked apple is an example of a simi- lar change. Realistic Exercise. — Inflate a rubber toy balloon with air, cover it with a thin layer of flour paste, and rotate it in flour until a smooth dry coat- ing is formed an eighth of an inch thick. Attach to it by a glass con- nector a rubber tube provided with a pinchcock. Immerse the end of the tube in water, and let the air escape from the balloon a few bubbles at a time. As the balloon contracts, the coating of paste will become folded and wrinkled in a manner quite similar to the folding of the earth-crust. The hypothesis of isostasy, or balancing weights, seems best to account for the great regional elevations and depressions (continents and ocean basins), and the hypothesis of cooling and contraction best to account for such smaller features as mountain ranges. The Representation of Relief. — The facts of geography are most conveniently expressed by the use of maps. The, fundamental idea of a map is a drawing which shows npon a horizontal plane the location, direction, distance, and area of the features of the earth's surface as they are distributed. A plan of a house showing the arrangement, shape, and THE FACE OF THE EARTH 49 size of the rooms, doors, windows, etc., and perhaps the location of pieces of furniture, is an example of a simple map. More complex maps may be made not only to show arrangement "on the flat," but also to indicate the "ele- vation " or relief of the surface. The map on pp. 40, 41, makes use of a common device for showing relief. The areas of different elevations are printed in different colors, various shades of blue being used for the sea floor, and shades of red for the land. Each boundary line of a color or shade is level or everywhere at the same distance above or below the sea level, measured vertically. These lines of equal elevation upon a map are called contour Hues or simply contours. The lightest shade of red shows all the land between sea level and three thousand feet above, but does not show where the land is just one hundred or twenty-nine hundred feet. The uncolored area shows all the land above twelve thousand feet, but does not show just how much above that level any point is. The boundary lines between the different colors or shades indicate exactly the elevation of the places through which they pass. The line between the red and the blue is the coast line and is everywhere at sea level, the line between the two lightest shades of blue is everywhere exactly six thousand feet below sea level, and the line around the outside margin of the darkest red is everywhere exactly six thousand feet above sea level. By drawing contour lines at sufficiently small intervals relief may be indicated with any desired degree of precision. When contour lines are drawn at small intervals the spaces between them are frequently left uncolored. Figure 24 shows a sketch or picture of a landscape and Fig. 25 a contoured map of the same region. In the foreground is a portion of the sea, the shore line of which forms the basal or zero contour. Con- tours are drawn upon the map at intervals of fifty feet measured verti- cally from the sea level, and they mark the lines where the seashore would be if the sea should rise fifty, one hundred, etc., feet. Where the slope is steep, one would have to travel only a short distance to rise fifty feet ; hence the contours are close together. Where the slope is gentle, one would have to travel far to rise fifty feet ; hence the contours 5o THE PLANET EARTH are farther apart. By shortening the contour interval to five or ten feet, as may be done upon a large-scale map, the elevation of every point may be shown very precisely. For showing exact elevation no device is equal to the contoured map ; but it has the disadvantage of not being graphic, that is, of not being understood by everybody at a glance. Fig. .24. Fig. 25. One must learn to interpret such a map before he is able to form a mental picture of the region shown. Realistic Exercise. — Upon a table or floor make a clay model of a simple hill with both steep and gentle slopes. Lay a block of wood one inch thick beside it : with a pointed stick make a mark upon the side of the clay hill all around at the height of the upper surface of the block, THE FACE OF THE EARTH 51 which should be moved around to guide the stick. Lay another block an inch thick upon the first and make another mark around the hill two inches above the table. Continue to add blocks until the top of the hill is reached. Each mark upon the hill will be one inch above or below the next, and will indicate exactly the height, above the table or floor, of the points through which it passes. Measure the horizontal distance between the marks upon the gentle and the steep slope. Now look down upon the hill from some distance above, and draw upon paper the lines as they appear from that point of view. The drawing will be a contoured map of the hill in which the contour interval is one inch. Fig. 26. Fig. 27. A very common device for showing relief upon a map is the use of hachures, or fine lines running up and down the slopes, and so drawn as to show the steepness of the slope by the depth of shading. Ha- chured maps may be made very graphic and almost equal to a picture. Figures 26 and 27 show the relation between hachured and contoured maps of the same area. A combination of the two is the best possible method of showing relief upon a map. Models are miniature reproductions of portions of the earth's surface in sand, clay, paper, plaster, or other mate- rial (see p. 393). They are often called relief maps. The vertical heights are usually exaggerated in order to show small details. This exaggeration is sometimes excessive, the elevations being made forty to one hundred times as 52 THE PLANET EARTH f^g 6" 'S high as they ought to be in proportion to the widths shown. Thus all slopes become so steep and the forms of moun- tains and other features so unnatural as to ren- der the model worse than useless because it teaches more error than truth. A good model, in which the heights are not exaggerated more than ten times, is gen- 2 erally the best represen- 2 tation of relief. Pictures 1 of models are very good substitutes for the mod- E els themselves, but are subject to the same re- strictions in regard to exaggeration of heights. K profile shows the ele- vations and depressions of surface along any given line, which may be straight or crooked. It has two scales, the horizontal and the ver- tical. The latter is gen- erally exaggerated, and often greatly so. The student should THE FACE OF THE EARTH 53 notice the amount of exaggeration in a model or a profile, and guard against the erroneous impression which it would otherwise give him. The stereogram, or block picture, is a combination of model and section, and may be used very effectively to show the relation of relief and structure. See Fig. 1 54. The Earth as the Home of Plants, Animals, and Men. — Life forms as we know them — plants, animals, and men ■ — are able to live and flourish upon the earth because they have become adapted to a multitude of conditions which probably do not exist in the same combination upon any other planet. The most important conditions which make the earth habitable are dependent upon its position, form, attitude, motions, size, structure, and plan. The position of the earth — its distance from the sun — determines the amount of heat which it receives. This is sufficient to maintain at all places upon the face of the earth a temperature which never falls lower than about 120 below the freezing point of water ( —88° F.), and never rises higher than about 120 above the freezing point (152 F.). This makes it possible for large quantities of water to exist in each of three forms, — solid ice, liquid water, and gaseous vapor. The form of the earth determines the angle at which the nearly parallel rays of the sun strike its face at different latitudes, and consequently the amount of heat received per square mile. This gives a variety of temperatures ranging from the torrid to the frigid. The attitude of the earth, or the inclination of its axis, in combination with its daily and yearly motions, determines a change of seasons, or variation of temperature, at all latitudes, and prevents both the uniformity which would exist if the earth's axis were perpendicular to the plane DR. PHYS. GEOG . — 4 54 THE PLANET EARTH of its orbit, and the excessive variation which would result if the axis were nearly parallel to that plane. The revolution of the earth around the sun at a nearly- uniform speed in an orbit which is nearly circular brings about the regular succession of seasons and years, each of which is of moderate length. The succession of warm and cool, or wet and dry, seasons gives to plants and ani- mals alternating periods of comparative rest and activity. The rotation of the earth upon its axis exposes the greater part of its face to alternations of heat and cold, light and darkness, at short intervals, and imposes upon living beings correspondingly short and frequent periods of rest and activity. It also enables man to look out at night into space, see the moon and stars, and learn something of the universe of which the planet earth forms an insignificant part. The size and density of the earth determine its mass, or weight, and consequently the force of gravity. The attraction of the solid earth is sufficient to prevent the atmosphere from escaping into space and to give it such composition and density as to support plant and animal life. The attraction of the earth also determines the weight of every object upon its face, and the strength or rigidity of plants and the muscular power of animals are nicely adapted to support or to move their own and other weights. The structure of the earth gives a firm crust for the sup- port of all creatures which live upon the land, while the outer layer of pulverized mantle rock furnishes a permea- ble bed for the roots of plants and a storehouse of availa- ble food. Even the centrosphere contributes to the food supply ; for the masses of igneous rock which have escaped from it gradually crumble into mantle rock and are converted into fertile soil. The presence of large fluid THE FACE OF THE EARTH 55 masses of water and of air makes it possible for extensive systems of currents to circulate in the atmosphere, in the sea, and on the surface of the land. Thus the materials of the earth are kept in motion and its face is made to un- dergo perpetual change. It is this which keeps the earth a living planet as distinguished from a dead one like the moon, and contributes to that variety and beauty of sky and landscape which make it a pleasant home for man. The sea is the home of millions of living forms, and it furnishes water for all those which live upon the land. The atmosphere not only rests upon the land, but pene- trates to the bottom of the sea and supplies all creatures with the breath of life. From it plants obtain the carbon which forms the greater part of their own substance, the food of animals, and the material of fuel. It absorbs and retains the heat of the sun, tempering its intensity by day and preventing its too rapid escape by night. Currents of air carry the water-vapor from the sea and distribute it as rain and snow over the land. The air and the water which falls from it attack the solid crust of the earth and break it up into mantle rock. The plan of the earth presents a vast expanse of water broken at intervals by large and small masses of land. While the land masses predominate in the northern hemi- sphere, their longer axes extend north and south through so many degrees of latitude as to traverse all the zones of climate. This variety is made still greater by diversities of elevation, relief, and distance from the sea. The num- ber and variety of living forms probably decrease from near sea level downward to the deep sea floor and upward to the mountain tops, but the great expanse of sea surface and the low average elevation of the land make a very large proportion of the face of the earth available for a 56 THE PLANET EARTH dense population of some kind. The arrangement and relief of the land masses are such that the moisture evapo- rated from the sea is very unevenly distributed over them. Some portions receive an excess of rainfall, while exten- sive areas in every continent are so dry as to be very un- favorable to the existence of life. Land plants and animals are generally unable to cross oceans, deserts, or mountains, and the presence of these natural barriers has largely controlled the migration and distribution of man himself. If, from the whole face of the earth, those portions are deducted which are either too high, too low, too hot, too cold, too wet, or too dry, there remains not more than one tenth part which is suitable for the home of a dense population of civilized men. The infinite variety of situation, relief, soil, and climate has brought about a corresponding variety of living forms, each adapted to the peculiar set of conditions under which it lives. Probably no large part of the sea or land is entirely devoid of life ; but the sphere of life is strictly confined to the thin shell of the earth where land, water, and air inter- mingle. Not far below it lies the fervent heat of the cen- trosphere, and not far above it the intense cold of stellar space. Reference Books. — For a list of reference books on subjects included in Book I, see Appendix IV, especially pp. 413, 414. BOOK II. THE LAND The hills are shadows, and they flow From form to form, and nothing stands : They melt like mist, the solid lands, Like clouds they shape themselves and go. — lENNYSON'S In Memoriam. CHAPTER IV EROSION Weathering. — The contrast between the roughness of the land surface and the smoothness of the sea floor is due to the fact that the land is exposed to the action of the atmosphere, while the sea floor is protected from it. The direct action of the air and the weather upon the earth- crust is a complex process called weathering, accomplished and assisted by various agents. Its general result is to break up and crumble the surface of the land. (i) Oxygen is an active chemical agent which causes rocks to decay and crumble by a process like the rusting of iron. Carbon dioxide attacks igneous rocks and converts them into materials from which stratified rocks are formed. (2) Rainwater dissolves the cement present in many rocks, which in consequence fall to pieces. It also me- chanically removes and washes away loose particles. (3) Frost is one of the most efficient agents concerned in breaking up rock masses. When water freezes in the pores and crevices of rocks, it expands, and thus enlarges the cracks or makes new ones. When the morning sun, after a frosty night, strikes against a cliff, there is often a 57 58 THE LAND continuous shower of rock fragments which have been loosened by the freezing and thawing. At high altitudes, where changes of temperature are frequent and severe, mountain peaks are rapidly broken down by this process. (4) Changes of temperature which do not include freezing and thawing act in a similar manner. When rock at any temperature is warmed it expands, and when cooled it con- Fig. 29. — Weathered granite. (Near St. Cloud, Minn.) tracts. Repeated expansion and contraction tends to break it up, especially to scale off thin sheets from the surface. This process is often used to break in pieces large boulders. They are first heated by a fire, and then suddenly cooled by throwing water upon them. (5) Gravity is continually pulling every mass of rock downward, and if the rock mass is insufficiently supported EROSION 59 it breaks off by its own weight. Gravity is not a part of the atmosphere, but in all processes of weathering it is a silent partner which never forgets or lets go for a moment. (6) Wind, by its own force, but more efficiently by blow- ing sand against rock, wears it away. Even the hardest materials, as steel and glass, are rapidly carved and eroded by wind-blown sand. (7) Plants and animals, although not a part of the atmosphere or factors of weather, may be included among the agents of rock disintegration. The roots of plants penetrate crevices in the rock and by their growth force the sides farther apart. Decaying vegetation fur- nishes an acid which in- creases the solvent power of water. Various burrowing (Near Prescott, Ariz.) and boring animals accomplish some less important work in rock destruction. By the action of all these agents large masses of rock are disintegrated and reduced to smaller and smaller fragments. Some rocks are also chemically decomposed and changed into other minerals. Weathering is the process by which massive bed rocks are converted into mantle rocks, and its products are clay, sand, gravel, pebbles, and boulders. Of these, only clay is a product of chemical decomposition and bears little resemblance to the original igneous or metamorphic rock from Fig. 3°. Cliffs eroded by wind and sand. 60 THE LAND which it was produced. The other kinds of mantle rock are clearly recognizable as fragments of larger masses. Weathering is not con- fined to the surface of the earth-crust, but extends as far down as air and water penetrate. It is most active in the zone between the surface and the level of permanent ground-water. The bed rock is sometimes found to be broken up or " rotten " to the depth of one hundred feet or more. In some places the mantle rock lies undisturbed in the position where it was formed, and the transition from soil to bed rock is so gradual that it is impossible to tell where one ends and the other begins. (See Fig. 14.) More frequently the loose fragments have been re- moved some distance and deposited in another place. Mantle rock in place is called residual soil, while that which has been transported is often called drift, — wind, glacial, or stream drift, according to the agent of transportation. Weathering is most rapid and extensive (1) in regions of heavy rainfall, (2) at high altitudes, where changes of temperature, especially freezing and thawing, are frequent, (3) on steep slopes, where gravity acts most efficiently and the mantle rock promptly falls, slides, or is washed away, and (4) in regions of fragile or soluble rock. Realistic Exercises. — Examine pebbles and boulders and observe the difference between the weathered surface and the surface of a fresh frac- ture. One may be lighter or darker, rougher or smoother, than the other, according to the kind of rock. The depth to which weathering has pen- etrated is often plainly visible, and some specimens may be found in a crumbling condition throughout. If a cliff of bed rock is accessible, observe the talus or pile of fallen fragments which lies at its foot. Ex- amine the place of contact between the bed rock and the mantle rock above and observe whether the change is gradual or abrupt. If a section of bed rock is freshly exposed, as in a quarry, the depth to which air has penetrated is often shown by the staining of the rock along the cracks. Valleys and Streams. — It is hardly possible to travel anywhere upon the land surface without coming across a valley. Of all land forms it is the most common, — so common as to attract no attention unless it is unusually deep or otherwise troublesome to cross. Valleys exist in great variety and in all dimensions, from a barely visible furrow to a canyon a mile deep ; but almost without excep- EROSION 6l tion they are alike in having a stream at the bottom. This universal association of a stream with a valley does not excite surprise, because we expect water to flow along the lowest level. If valleys exist, it is but natural, as we say, that streams should find and follow them. But let us turn this proposition around and consider its reverse side. If streams exist, is it not natural that their courses should be marked by valleys ? Run-off. — If the course of a stream is followed up, it will be found to be joined at intervals on either side by tributaries, each of which flows in a valley usually propor- tioned to the size of the stream. The main stream and its valley grow smaller above the mouth of each tributary until they are reduced to a tiny rivulet flowing in a furrow, and finally come to an end at a spring, pond, or swamp, or upon the smooth slope of a hillside. If any tributary is followed up, it also is found to divide like the trunk of a tree into smaller branches and rivulets. The sur- face of the land on either side slopes toward the stream or one of its tributaries, and at the same time there is a continuous slope downstream from the head or tip of every branch. If the slope is ascended from the stream, at a greater or less distance a point is reached where the surface begins to slope away from that stream toward some other stream. A more or less definite line may be found which marks the junction of the two slopes and separates the water flowing into one stream from that flowing into the other. If this divide or water' parting is followed, it is found to pass around the heads of all the tributaries and to inclose the basin or area from which water drains into the stream system. Some part of the rain falling upon any basin sinks into the ground, but a large part runs down the slopes. At first this water forms a thin and scarcely 62 THE LAND perceptible sheet; but it soon gathers into little rills which join one another and grow larger until they flow into one of the permanent branches of the stream system. The smallest branches flow only while it rains, and their grooves or gullies are dry most of the time. The permanent branches are supplied from ponds, swamps, glaciers, or springs. Near the sources of the stream the slopes are apt to be steep, the current swift, the channel narrow and deep and perhaps interrupted by rapids and falls. The bed is strewn with boulders, pebbles, or coarse gravel. Farther down, as the slope becomes more gentle, the bed is smoother, rapids are less frequent, and are separated by long reaches of quiet water, and the channel becomes wider, shallower, and more crooked. The loose material is less coarse and consists chiefly of fine gravel and sand. Here the water course is likely to become double and to consist of a wide outer valley which the stream covers only at high water and through which the narrower channel winds irreg- ularly from side to side. Still farther down, the valley may be- come very much wider and con- sist of an extensive flood filaiti bounded by bluffs. Here the ordi- nary channel follows a meander- ing course, full of zigzag bends and horseshoe curves. The slope is gentle, the current sluggish, and the bed obstructed by sand bars and mud banks. The stream finally empties into a larger stream, or into a lake or the sea. These are the usual conditions of run-off or the escape of rainwater from a hydrograpJiic basin. ask ^^fedHWHcOfy "Gfeft^ KpT*^ '-'""' '* iij Mr ■ ' A pdpsr? ■ wm^''''' : ' :[ ' i mm • ! '"- : - Jijfv' r jSk Fig 31— A mountain stream. (Rainbow Falls, Ute Pass, Colo.) EROSION 63 Fig. 32. — Small stream meandering in a flood plain ; sand bar and small terraces. Transportation. — But this is only half the story. It does not require a very close study of a stream to discover that it is not only a stream of water but also a stream of mantle rock, that the crust of the earth itself is flowing away through the same channel. Some streams are clear and some are muddy, but all carry a portion of mantle rock. The work of the stream is most rapid and most impressive at high water and in the upper parts Fig. 33. -Boulders in bed of stream. of itg CQms ^ where the current is swift it rolls and pushes pebbles along the bottom, and at times of flood is able to move even large boulders. 6 4 THE LAND In the middle course, where the current is moderate, it may be seen to carry sand or to roll it along the bottom, and the inside of every bend is marked by a deposit of sand left as the water went down after the last flood. In the lower course, where the current is very gentle, only clay or the finest sand is carried, all the coarser material having been dropped farther upstream. ^0" Fig. 34. — Sand bar and bluff at bend of river. (Mississippi R., below St. Cloud, Minn.) Rock fragments of all sizes are buoyed up by the water so as to lose one third or more of their weight, and are in consequence more easily moved than when out of water. The current of a stream does not flow smoothly onward, but is disturbed by the irregularities of its bed so as to be thrown into ripples and eddies. This irregular motion helps to keep the fragments from settling. In a smooth, gently flowing river the mud may often be seen boiling up from the bottom in hundreds of places where an upward current comes to the surface. Even over a smooth bed the current has a wavy up and down movement which throws the sand at the bottom into cross ridges or ripples, with the longer slope upstream. The finer the particles of rock, the more slowly do they settle in water and the more easily are they kept in suspension ; therefore, sand is carried along by a slower and smoother stream than gravel, and clay by a slower stream than sand. The size and weight of the particles which a stream can carry in suspension increase rapidly with the velocity of its current. A current of one third of a mile an hour will carry clay ; of two thirds of a mile, fine sand ; of two miles, pebbles as large as cherries ; of four miles, stones as large as an egg. Material in suspension usually manifests itself by making the water turbid or muddy, but streams also carry invisible rock material in solu- tion. The most common materials transported in solution are salt and EROSION 65 lime. It is dissolved lime which makes water hard, and leaves a crust on the bottom of a kettle in which hard water is boiled. The ability of a stream to carry material in solution is not affected by the velocity of the current, for the material is not deposited except by evaporation or some chemical change. Corrasion. — Wherever a stream runs over bed rock it wears the rock away slowly by solution, but a stream which carries sediment in suspension may wear away such rock rapidly. A clear stream acts upon rock like a piece of paper rubbed upon wood, a muddy stream like a piece of sand paper. If a stream is not overloaded with sediment and can carry sand or coarser particles rapidly, it cuts or files its way down into the crust of the earth through the hardest rocks. The grains of sand and gravel not only wear away the stream bed, but wear upon one another. Boulders and pebbles which are rolled and tumbled about in the current have their corners and edges worn off, and become smaller and more rounded as they travel on. Only the hard ones can. endure such harsh treatment without being reduced to powder. This explains why gravel stones are seldom angular or soft. Corrasion is most rapid where (1) the slope is steep, (2) the volume of water large, (3) the quantity of sedi- ment sufficient, but not too great, and (4) the bed rock soft or friable. Erosion. — All over the surface of a stream basin the crust of the earth is being crumbled to pieces by the agents of weathering. Gravity and the wash of the rain drag and push the mantle rock thus formed down the slopes into the stream. The current of the stream trans- ports the material to lower levels, and in doing so cuts its own channel deeper. Thus the land is everywhere being torn down and carried away toward the sea. The 66 THE LAND lowering of the land surface by weathering, transportation, and corrasion is called erosion or degradation, and its most efficient agents are gravity and running water. In regions of small rainfall and steep slope, corrasion is more effect- ive than weathering, and erosion goes on much more rapidly near the streams, which cut their channels and valleys deeply into the face of the country. In regions of large rainfall and gentle slope, weathering and rainwash are more efficient than corrasion, and erosion is more uni- formly distributed over the basin, though still most rapid near the streams. Summary. — ■ From these facts it appears not only that a stream of running water is competent to make a valley, but that it must necessarily do so, and that a small stream is able to make a large valley if it is given time enough. The surface of the land is cut by innumerable valleys ; running water is the only agent everywhere present which is known to be capable of doing such work : therefore a stream valley is regarded as the depression or trench which the stream itself has cut. 1 Its bottom is or has been at some time covered by the stream, and it is bounded by relatively steep banks or bluffs. The channel sometimes occupies the whole width of the valley and sometimes only a small portion of it. Realistic Exercises. — Let the student visit any convenient stream and see for himself as many of the above described features and processes as can be found. Let him watch the stream in action, during or just after a rain, and see what it does with sediment under varying conditions of fineness and current. At some favorable point supply it with fine and coarse material and observe the result. Shake up clay, sand, and gravel in a tall bottle of water and observe the manner in which they settle. A stream a foot wide is doing the same kind of work in the same way 1 Care should be taken to distinguish between the valley and the basin, which is popularly called valley. EROSION 6/ as the largest river, and one may be known by studying the other. It is of the greatest importance that stream action be actually studied in the field. "Every river appears to consist of a main trunk fed from a variety of branches, each running in a valley proportioned to its size, and all of them together forming a system of val- leys, communicating with one another, and having such a nice adjustment of their slopes that none of them join the principal valley either on too high or too low a level : a cir- cumstance which would be infinitely improbable if each of these valleys were not the work of the streams which filozv through them.''' — John Playfair, 1802. CHAPTER V THE MISSISSIPPI RIVER SYSTEM The basin drained by the Mississippi River and its tribu- taries has an area of about one and a quarter million square miles and is one of the largest in the world. On the west DRAINAG AND PRINCIFAL STREAMS Fig- 35 it is separated from the basins of streams flowing into the Pacific by the crest of the Rocky Mountains. On the north the divide between it and the basins of the Nelson and the St. Lawrence is low and flat. On the east it is 68 THE MISSISSIPPI RIVER SYSTEM 6 9 3jnns5]; S[i[d A!0 bounded by the Appalachian highland, and on the south a slight elevation forms a part- ing between its waters and those of the minor streams flowing into the Gulf of Mex- ico. The main stream flows from the north- west corner of the basin along a nearly central line more than 4000 miles to the Gulf. It is divisible into three sections, which differ widely in volume of water and other characteristics. (1) The Missouri ex- » tends from its source to its junction with the I upper Mississippi near St. Louis, more than 5* 2800 miles. (2) The middle Mississippi ex- tends from the mouth of the Missouri to ; the mouth of the Ohio, 200 miles. (3) The I lower Mississippi extends from the mouth I of the Ohio at Cairo to the Gulf, 1075 miles. ! The principal tributaries from the western I highland are the Yellowstone, Platte, Ar- ' kansas, and Red. The volume of the main \ stream is almost doubled by the upper Mis- j 1 sissippi, which joins it from the north, and it receives a still larger accession of water from the Ohio, which drains a northeastern arm or lobe of the basin. The distance in a straight line between the sources of the Missouri and the Ohio is nearly 1800 miles, and from the extreme northwest corner of the basin to the mouth of the river is about 2000 miles. The Missouri River. — The Missouri rises at the crest of the Rocky Mountains in southwestern Montana by three forks, the DR. PHYS. GEOG. — 5 70 THE LAND longest of which, the Jefferson, is fed by the melting snow which fills an old volcanic crater surrounded by peaks 9000 to 11,000 feet in elevation. It flows through deep gorges and glacial lake basins, a mountain torrent de- scending 4200 feet in 400 miles, to the point of junction with the Madison and Gallatin forks. Thence the Mis- souri breaks through the Big Belt Mountains by the Fig- 37- — Great Falls of the Missouri. "Gate of the Mountains," a canyon 1200 feet deep, passes a series of rapids and falls, one of which, the Great Falls, has a perpendicular drop of 87 feet, and near Fort Benton, 600 miles from its source, enters the plateau known as the "Great Plains." The Yellowstone escapes from the volcanic region of the National Park over falls 350 feet high and through a canyon 800 feet deep, and joins the THE MISSISSIPPI RIVER SYSTEM 71 Missouri 400 miles out on the plains. From its source to the Great Falls the Missouri has an average fall of ten feet per mile ; from the Falls to the mouth of the Yellow- stone, two feet four inches per mile ; from the Yellow- stone to the junction with the Mississippi, less than one foot per mile. The rainfall of the Missouri basin is less than twenty inches a year, and the source of water supply is largely from the melting SnOWS Fig- 38. —Gorge of the Yellowstone. upon the mountains. The volume of water varies greatly, being at high water in June about thirty times as great as at low water in November. The loss by evaporation is so great that the river in summer actually grows smaller as it advances across the plains, and it succeeds in dis- charging at its mouth only twelve per cent of the total yearly rainfall in its basin. As a result of this the river is overloaded with sediment, and justifies its name, " Big Muddy." The Missouri Valley. — Through a great part of its length the valley consists of three trenches, one within another. The widest trench, or valley proper, five or six miles across, consists of a flat or gently sloping plain, covered with the deposits of the river and its tributaries, and bordered by bluffs or terraces. Winding through the valley is a second trench or bed of the river occupied at high water, and on the bottom of this the low water channel swings from side to side. The Platte, Arkansas, and Red Rivers. — The Platte and the Arkan- sas stretch from the mountains across the dry plains, and are subject 72 THE LAND to conditions similar to those of the Missouri. Both grow smaller by evaporation, and the Platte is at times a mile wide and only six inches deep. The Red is far enough south to catch the rains from the Gulf, and is less variable in volume. The Upper Mississippi rises among the lakes and forests of northern Minnesota, at an elevation of about 1 500 feet above the sea. It flows by a tortuous course from lake to lake, with intervening rapids, about 600 miles to St. Paul ; its average fall in this course is about fifteen inches per Fig- 39- -Flood plain on upper Mississippi River. (Near St. Cloud, Minn.) mile. From St. Paul to its junction with the Missouri, 660 miles, its average fall is about five inches per mile. From St. Paul to the sharp bend at Muscatine, the valley is gen- erally two or three miles wide ; below Muscatine its width is seldom less than five miles. The upper Mississippi has a basin whose area is only one third that of the Missouri, but the rainfall is thirty-five inches, which enables it to flow as a strong stream more than half a mile wide at its mouth, and to contribute nearly as much water as does the Missouri. It is subject to less fluctuation in volume than either the Missouri or the Ohio, but has THE MISSISSIPPI RIVER SYSTEM 73 a period of high water from February to July. It is lowest in Decem- ber, but is always navigable without difficulty as far as St. Paul. Its banks are marked by many high and picturesque bluffs. The Middle Mississippi resembles the upper Mississippi in the character of its valley, which is five to seven miles wide, and bordered by limestone bluffs. Its channel is much divided by islands formed by deposits from the muddy water of the Missouri. Its fall is seven and one half inches per mile. The Ohio. — The basin of the Ohio is less than one half as large as that of the Missouri, but the rainfall is forty- three inches, and the river discharges about three fourths as much water as the Missouri and upper Mississippi com- bined. It rises by two nearly equal branches, — the Monon- gahela from West Virginia, and the Allegheny from north- western Pennsylvania. After a course of about 300 miles each from the summit of the Alleghany Mountains, and a fall of 1000 feet, they unite at Pittsburg, 700 feet above the sea, and nearly 1000 miles from the Mississippi. Be- tween Pittsburg and Cairo the river presents a series of shoals and rapids separated by reaches or pools in which the water is deeper and the fall very slight. At the Louis- ville rapids there is a fall of 23 feet in 2.25 miles. The valley of the Ohio is deeply cut into the Alleghany plateau, which slopes westward from the mountains to Indiana. The valley is usually not more than one or two miles wide, and bounded by steep bluffs from 100 to 400 feet high (see Frontispiece). The river is from a half mile to one mile in width, and is bordered by a continuous flood plain. It is subject to excessive fluctuations of volume, and at the highest stage discharges thirty-four times as much water as at the lowest. The spring rains and melting snows of February and March sometimes raise its level at Cincinnati seventy feet above low water mark, and the droughts of summer sometimes reduce its depth to two or three feet. In the last 200 miles of its course the bluffs become lower and the valley wider, with a corresponding expansion of the flood plain to ten or more miles, 74 THE LAND SCALE OF MILES GULF OF MEXICO Fig. 40. —Lower Mississippi flood plain. through which the river winds in a manner similar to that of the lower Missis- sippi. The Ohio is gen- erally navigable for small boats as far as Pittsburg. The Lower Missis- sippi. — The alluvial valley of the lower Mississippi from the mouth of the Ohio to the Gulf is 600 miles long, but the distance as the river flows is nearly twice as great (1075 miles). The width of the flood plain va- ries from 25 to 80 miles. It is bounded on the east by clay bluffs 100 to 300 feet high. There are also bluffs on the west side as far down as the Red River, but they are not so prominent as those on the east. The course of the main channel of the Mississippi is near the east bluff as far as Memphis, then it crosses to the west side of the val- ley at Helena, but soon crosses again to the east bluff, which it strikes at THE MISSISSIPPI RIVER SYSTEM 75 Vicksburg and follows as far as Baton Rouge. The surface of the valley is mostly below the level of the river banks, and is traversed by an intricate network of side channels and sluggish streams, some of which receive water from the Mississippi as well as discharge into it. The valley contains four large basins. The St. Francis basin lies on the west side of the river, and extends from a point above Cairo nearly to the mouth of the Arkansas. The St. Francis River flows through it parallel with the Mississippi, receives all the small tributaries from the west, and empties into the main stream. The Yazoo basin lies on the east side of the valley and is drained by the Yazoo River, which begins in a branch leading out of the Mississippi above the mouth of the St. Francis, flows near the east bluff, receives all the eastern tribu- taries, and joins the main stream near Vicksburg. The Tensas basin lies on the west side below the Arkansas, and is drained by the Tensas bayou through the Black River into the Red River. Below the mouth of the Red lies the basin of the Atchafalaya, a river which re- ceives water from both the Red and the Mississippi, and pursues an in- dependent course to the Gulf, which it enters 100 miles west of the mouths of the larger stream. Thus the alluvial valley is traversed throughout its length by a secondary channel on the opposite side from the main stream and parallel with it, which receives water both from the main stream and from the tributaries, to be discharged at some point farther down. The main channel is extremely tortuous, and frequently divides into two or more channels, which inclose islands or bars. Figure 41 shows its course near Greenville, Miss., where the river appears to be wriggling from side to side in a series of S curves. On the inside of each bend, and on the downstream side of each tongue of land, there is a sand bar, while on the opposite side the water is deep and swift. The swifter current on the outside of each bend, and on the upstream side of each tongue, cuts away the bank at those places, while the slower current on the oppo- site side, 'by depositing sediment, builds a new bank. Thus the tendency of such bends is to grow more crooked and to travel downstream. There is a tendency also for the nar- row neck of land, which separates one bend from the next, 7 6 THE LAND Fig. 4i- to grow narrower. Finally, at some time of high water, the river cuts through or runs over the neck. The slope and fall previously distributed throughout the long bend are now con- centrated in the new and short cut, and the current there has such a velocity as to widen and deepen the channel very rapidly, form- ing a permanent cut-off. Lake Chicot and Lake Lee (Fig. 41) are old bends of. the river, which have been cut off and partly filled with sediment. Thus while the river is growing more crooked in one place, it straightens itself in another, and the average sinuosity remains about the same. The valley abounds in horseshoe- or cres- cent-shaped lakes, all of which were once portions of the chan- nel, and show how the river, in the course of ages, has shifted its bed from one side of the valley to the other, and has occupied at some period every portion of it. Floods. — As may be in- ferred from what - has been said of the Missouri and Ohio, the lower Mississippi is subject to great floods. THE MISSISSIPPI RIVER SYSTEM 77 which give it more than ten times the volume it has at low water. The water rises 53 feet at Cairo, 36 at Memphis, 48 at Helena, 53 at Vicksburg, and 15 at New Orleans. If not artificially re- strained, the river at high water spreads out and covers nearly the whole valley from bluff to bluff. As the water overflows its banks, the current is checked rather suddenly by its shallow- ness and by the willows Fig 42. -Levee, Mississippi River. and other vegetation. It consequently drops the larger and coarser part of its load of sediment within a mile or two of the channel, and builds up its banks higher than the general level of the valley floor, forming natural levees. The flood water deposits a thin layer of fine mud over the whole submerged country, and returns through the nu- merous bayous, or side channels, to the main stream farther down the valley. Thus the spaces inclosed by the network of channels become platter-shaped de- pressions,, many of which are occupied by cypress swamps. The natural levees upon both sides of the river Fig. 43. -Crevasse, Mississippi River. haye been raised by arti . ficial embankments of earth designed to prevent- the high water from flooding the valley ; but frequently the river breaks through the levee, forming a crevasse, which rapidly widens and transmits a raging and destructive torrent of water. Below the mouth of the Red the rise of y8 THE LAND the river is much less because the surplus water escapes through the Atchafalaya, which sometimes carries all the water of the Red and one third that of the Mississippi. The Delta. — At a point 300 miles above the mouth of the Mississippi, the Atchafalaya, the first distributary, or branch which does not rejoin, leaves the river (Fig. 40), and thence carries part of its water to the Gulf. This point is the present head of the Mississippi delta, an area of 10,000 square miles of lowland and marsh, much of which is scarcely above the level of the sea. The Missis- sippi River carries to the Gulf a load of fine mud, sufficient to cover a square mile about 270 feet deep every year. Thus the delta is being gradually extended into the Gulf. About fifteen miles from the sea the river divides into three arms called "passes," which subdivide into smaller arms, each of which has built for itself natural levees which appear above the waters of the Gulf as narrow tongues of land, the whole forming a tract called "the Goosefoot." The mouth of the South Pass is kept open for large vessels by jetties or embankments built upon each side in such a manner as to confine and quicken the current and compel it to deepen the channel across the bar. Other Features. — The alluvial valley of the lower Mississippi is a very broad and shallow trench cut through loose sedimentary material, chiefly sand and clay. The depth of this material varies from 100 feet at Cairo to more than 1000 feet at New Orleans. The width of the river channel varies from a half mile to two and a half miles ; the depth of the river at low water, from five feet to 150 feet. The fall from Cairo to the head of the delta is less than six inches per mile ; through the delta at low water it is but a small fraction of an inch per mile. The average vol- ume of water discharged is about 600,000 cubic feet per second, and the velocity of the current varies from one mile to four miles per hour. The lower Mississippi is one of the muddiest rivers in the world, the greater part of its sediment being furnished by the Missouri. The increase of volume and slope at high water quickens the current and enables it to deepen and straighten the channel. At the same time the overflow deposits sediment and builds up the general level of the flood plain. THE MISSISSIPPI RIVER SYSTEM 79 At low stages of water the current is feeble and easily deflected by any obstruction. Consequently it staggers from side to side under a load which it drops at one place and picks up again at another. Thus it wriggles about all over the surface of the valley and maintains it, on the whole, at about the same width. There is some evidence which indicates that it is slowly raising its bed and banks. The final result of its work is the extension of its delta out into the Gulf and the build- ing up there of a pile of sedimentary strata more than 1000 feet thick. Work of the Mississippi System. — The vast system of the Mississippi furnishes examples of almost every variety of stream and stream work. The head waters of the Missouri rush over rapids and cataracts, and are able to corrade mountain gorges and canyons. The upper Mississippi is busy in draining and slowly destroying a multitude of gla- cial lakes. The Ohio has cut a deep trench in the Alle- ghany plateau, and is now engaged chiefly in widening it. The lower Mississippi has reached its base-level, or the lowest level to which its current and load will permit it to reduce its bed, and is now sawing sidewise at places into the restraining bluffs. The whole system is tearing down the land, and carrying it into the Gulf, at a rate which lowers the average level of its basin one foot in about 3500 years. The lower Mississippi is said to be in its old age because its destruc- tive work is nearly accomplished, and it can now do little more than transmit the waste material supplied by its tributaries. The Ohio and its tributaries have hardly yet reached maturity because they have the greater part of their possible task of land degradation still before them. The branches of the upper Missouri are infant streams, which have just begun the work of tearing down and carrying away the great mass of the Rocky Mountains. If nothing interferes, the condition of old age already approaching in the lower Ohio will creep up that stream and up the Mississippi and the Missouri and their tributaries, until the plateaus and the mountains disappear and the Mississippi basin is reduced to a low and nearly featureless plain. Summary. — The Mississippi system may be taken as a typical example of most of the great river systems of 80 THE LAND the world. Its head waters descend from lofty mountains, and its middle course is through a region of moderate elevation and of large average rainfall. The slope of its bed is steep near its source, and decreases more and more slowly to its mouth ; that is, its longitudinal profile is con- cave to the sky (see Fig. 36). The ramification of its main stream into numerous branches, like the limbs, branches, and twigs of a spreading tree, the occurrence of rapids, cataracts, gorges, and lakes among its head waters, and their absence elsewhere, the wide and gently flowing cur- rents of its middle course, and the large proportions of its alluvial valley and delta are common characteristics of great rivers. Exercise. — Examine physical maps of the different continents and compare the following rivers with the Mississippi, as to area and form of basin, length, fall, tributaries, origin of head waters, course, alluvial valley, and delta : Amazon, Orinoco, Plata, Hoang, Yangtze, Ganges, Amur, Lena, Yenisei, Obi, Volga, Danube, Mackenzie, Yukon. Kongo, Niger. CHAPTER VI THE COLORADO RIVER SYSTEM The Colorado River system drains a series of lofty pla- teaus surrounded by still loftier mountains. The greater part of its basin lies between 5000 and 10,000 feet above the sea. Its farthest sources are in the mountains of western Wyoming, at an elevation of 12,000 feet, whence the Green River flows southward about 400 miles. In southeast- ern Utah the Green is joined by the Grand, which rises among the highest peaks of the Park Range in Colorado. The junction of these rivers forms the Colorado, which flows southwest- ward 200 miles, then turns to the west for 150 miles, then flows south about 300 miles to the Gulf Of California. The Fi « 44- -Basin of the Colorado River. whole length of the Green-Colorado, by the windings of the river, is not far short of 2000 miles. Its principal tribu- taries, — - the Grand, San Juan, Little Colorado, and Gila, — are all upon its eastern side, its basin is widest near the 81 82 THE LAND FGET 10 000 8 000 ^ 6 000 ^V i v ( r i 000 I - a ^o ^o '<>• 2 ooo B k / O S. "SO .-R.i ve r x ..MI a .-.'-is o n~ Jil v e 7 Fig. 45. —Profiles of the Green-Colorado and Ohio-Mississippi rivers. mouth of the river, and in contrast with the wide-spreading, symmetrical, elmlike branchings of the Mississippi, the map of the Colorado system resembles a bent and broken trunk with a few straggling and one-sided limbs. Over scale onun.ES 6' ' ' '5 io 15 20 Fig. 46. — Part of Green River. THE COLORADO RIVER SYSTEM 83 Fig. 47. —Canyon of Lodore, by which Green River leaves Browns Valley. most of the basin the rainfall is less than ten inches per year, and the principal sources of water supply are the rains and melting snows upon the mountains which stand along 8 4 THE LAND its border, the Rocky Mountains on the east and north, and the Wasatch on the west. In northern Utah the Uinta Mountains project eastward from the Wasatch half- way across the basin. The Upper Green River. — North of the Uinta Mountains the Green River traverses a plateau into which it has cut a broad valley about iooo feet deep. On reaching the northern foot of the mountains the river enters a canyon which penetrates directly into the range to a point within five miles of the crest, then turns abruptly to the east and runs along the axis, gradually crossing toward the south. The easterly course of the river for forty miles is through a broad valley (Browns Valley) which continues to the end of the range. The river, however, does not follow this valley around the end of the mountain ridge, as it ap- parently might, but turns sharply to the southwest and crosses the range through the Canyon of Lodore (Fig. 47). On the south side of the moun- tains it cuts off the end of a projecting plateau at a point where a course a few miles longer would have carried it around that elevation. The Terrace Canyons. — Be- tween the valley of White River (Fig. 53) and the mouth of the Grand, a distance of about 150 miles, the surface of the country is like a staircase which has been tipped back- ward so that each tread or step Fig. 48. - The Terrace Canyons. slopeg up ^^ thg riser next below (see Figs. 48 and 53). A person traveling southward parallel with Green River ascends a moderate slope for 60 miles and reaches an elevation of 3000 feet above the river. At this point the upward slope ends in a line of very steep cliffs, which drop down in a few miles nearly to the river level. The next sloping step is 25 miles wide and the cliffs THE COLORADO RIVER SYSTEM 85 at its edge are 2000 feet high. The third step is 50 miles wide, and its border cliffs are 1200 feet high. Green River cuts across the terraces from north to south directly against the slope of the steps, and in doing so forms three canyons, each of which is shallow at the upper end and gradually increases in depth to the lower end. Cataract Canyon. — South of these terraces the river makes a slight easterly turn and by doing so runs into an elevated ridge, but before reaching its central axis turns again westerly and runs out of the ridge. In the canyon thus formed (Cataract Can- yon) the walls increase in height to about 2700 feet near the middle, then decrease to the lower end. In the midst of it the Green and Grand rivers unite to form the Colo- rado at a depth of 1200 feet below the general level of the country. Glen and Marble Canyons. — From the mouth of Dirty Devil River to the mouth of Fig ' 49 - Marble Canyon ' the Paria the Colorado flows through Glen Canyon, which has nearly perpendicular walls from 200 to 1600 feet high, carved into a great variety of glens, alcoves, and amphitheaters. Below Glen Canyon is Marble Canyon, which increases in depth from 200 feet at its head to its foot, where the walls are 3500 feet high. Its width is about twice its depth, and it has been cut through a bed of marble 1000 feet in thickness, which stands in smooth, precipitous cliffs on either side. Grand Canyon. — At the foot of Marble Canyon, the Little Colorado comes in from the east, the main stream changes its general direction from southwest to west, and DR. PHYS. GEOG. — 6 THE LAND : Ml m i the Grand Canyon begins. This so far surpasses all other canyons in magnitude as to render them comparatively insig- nificant. The river passes through a series of plateaus, the surface of which lies from 6000 to 8000 feet above the sea, by a channel which varies from three- quarters of a mile to more than one mile in depth. The slope of the river is steep and broken by many rapids. The sur- face of the plateau is highest near the upper end of the canyon, where it stands 6000 feet above the river; but owing to the rapid descent of the river bed, the canyon is seldom less than one mile deep all the way to its mouth at the Grand Wash, where the plateau terminates in a line of cliffs facing westward. The pla- teaus are about 130 miles in width, but the course of the river is so crooked that the Grand Canyon is 218 miles long. In its upper or eastern portion the walls are very irregular and cut by side canyons into recesses, alcoves, and amphitheaters. It is here a valley from eight to twelve miles wide, out of which rise a multitude of ridges, spurs, gables, towers, and pin- nacles, mountainous in size and endless in variety of detail, forms which have been carved out of the massive strata of the plateau. Through the midst of them, but sunk far below, winds the slender thread of the river from one fourth to one THE COLORADO RIVER SYSTEM 87 half mile wide. In the lower or western half, the walls are more regular and the canyon is distinctly double, consisting of an upper and outer canyon five or six miles wide, and 2000 feet deep, through which winds an \S Fig. 51. — Section of double canyon. inner gorge one or two miles wide and 3000 feet deep. Fig. 52.— Grand Canyon at foot of Toroweap, showing double canyon. Lower Colorado. — A few miles below the mouth of the Grand Can- yon, the Colorado turns abruptly to the south and flows 300 miles (by the meanders of the river 500 miles) through a nearly rainless country to the Gulf of California. As it approaches the gulf, its flood plain is ten or more miles in width, and it resembles the lower Mississippi. The Gulf of California once extended more than 100 miles farther to the northwest than it now does. The mouth of the Colorado River was then on the eastern side of the gulf, but the river extended its delta until it formed a barrier which cut off the head of the gulf from the main body. The water evaporated from the basin thus formed, and now known as the Salton Desert. This is 266 feet below sea level, and is sometimes temporarily flooded by the waters of the Colorado. Summary. — The characteristics of the Colorado River distinguish it above all other rivers in the world. From the north side of the Uinta Mountains to the foot of the 88 THE LAND COLORADO PLATEAUS GRAND CANYON Fig. 53. — Section along the canyons of the Gre Grand Canyon, a distance as the river flows of nearly 1000 miles, the stream follows a course entirely regardless and apparently in defiance of the surface and slope of the country. Mountain ranges and massive plateaus stand across its path, but they seldom turn it aside. In several instances it seems to go out of its way to cut through them or to run into a ridge and out again. Not only in particu- lar cases, as through the Terrace Canyons, but in the greater part of its course, it flows directly opposite to the general slope of the country. The elevation of the Colorado pla- teaus above the sea is several thousand feet higher than that of Browns Valley. As a result of this, the river flows for a thousand miles, with trifling exceptions, at the bottom of a steep-walled trench, sunk thousands of feet below the general level of the country. Origin of Canyons. — The first impression made upon one who sees these canyons, or pictures of them, is likely to be that some force acting from the interior of the earth has broken the crust apart and made a great crack, which the river had only to follow. But this theory will not bear investigation. Cracks in the earth-crust do occur, but none have ever been found so crooked as this series of canyons. Cracks are almost always accompanied by fault- ing, or a displacement of the rock on one side up or down ; but in the canyons the strata on one side correspond to those on the other, as if they had once extended across the chasm. Several faults occur in the Colorado plateau region (see F, Fig. 53), but they are not parallel with THE COLORADO RIVER SYSTEM 89 . „„^bmJth PANYON GRAY C. DESOLATION CANYON CATARACT CANYON ^TERRACE CANYONS ,200 100, MILES Jolorado River, shortened by omitting bends. the river, which runs across as regardless of them as it is of mountains. Each tributary river has a canyon of its own, and so, too, has each smaller branch. Each canyon is adjusted to the size of the stream which flows through it, and the level of its bottom is usually adjusted to that of the larger canyon into which it empties. Thus, the whole tract of plateaus and mountains is divided by a labyrinth of ramifying canyons into irregular blocks. Every rod of this network, from source to mouth and from top to bottom, shows evidence of being the work of running water. The conclusion is unavoidable, that the streams have carved their own canyons. A stream can not begin to cut a valley until it has begun to flow along the course of the valley. It begins at the top and works downward. It is evident that the Green- Colorado River never could have flowed over mountains, up slopes, and across plateaus on such a surface as now exists along its course. The only solution of the problem is found in the supposition that the river established its channel when the surface sloped continuously, or nearly so, in the direction of its flow, and that it has maintained nearly the same general course and level through all the subsequent movements of the earth-crust in its basin. It has acted very much like the saw in a sawmill, which cuts a groove into anything presented to it. The earth-crust has been pushed up, arched, and broken, and the blocks have been tilted at various angles, while the river system has been sawing its canyons. 90 THE LAND The river has been able to corrade thus deeply because (i) its slope is steep, through the canyons six to ten feet to the mile, and its current swift, (2) it is supplied with a full stream of water from the mountains, and (3) it carries a sufficient load of sediment for active corrasion without being overloaded and choked. The canyons are nar- row, because at elevations below 8000 feet the region is nearly rainless, weathering goes on very slowly, and the "tributaries which would cut down the walls and widen the valley are few, short, and inconstant. The double gorge of the Grand Canyon furnishes striking proof of the fact that since the river began to flow the country has stood at a much lower level than now. When the wide outer gorge (see Fig. 51) was completed, its bottom was' not far above sea level, the river had a feeble current and wound from side to side, eating back the cliffs until its valley became in some places fifteen miles wide. Then came a slow upheaval of the land which gave the river a rapid descent, quick- ened its activity, and caused it to cut into the floor of the old valley a narrower and deeper canyon. Work of the Colorado System. — The Colorado River furnishes on the largest scale and in the greatest variety examples of the work of a river which drains an elevated and arid region. Empowered by its rapid descent, it has engraved upon the face of the earth, in plain characters, the story of what running water, together with running sediment, may accomplish. Its short, steep tributaries, flat divides, and narrow canyons are evidences of scant rainfall and relatively small water supply. If the rainfall in the Colorado basin had been forty inches instead of less than ten inches per year, weathering would have been rapid and the river would have been much larger, with longer and more numerous tributaries. In the time which it has taken to carve its narrow canyons, it would have worn down the mountains and plateaus, and perhaps filled the Gulf of California with their debris, as the Mississippi has filled much of the Gulf of Mexico. The Colorado has been called a "precocious infant" because although it may be called young it has accomplished a great work. Yet it has THE COLORADO RIVER SYSTEM 91 done very little in comparison with what remains for it to do. There are other rivers which present characteristics similar to those of the Colo- rado, but on a smaller scale. The Rio Grande drains an area of arid plateaus lying southeast of the Colorado basin. In passing through the ranges of mountains between Presidio and the mouth of the Pecos it traverses a series of narrow canyons 1000 to 5000 feet deep and 350 miles long. The Snake River, rising from the same mountains as the Missouri and the Green, flows westward through the lava beds of the Columbia plateau by a series of canyons, one of which is fifteen miles wide and 4000 feet deep. The rivers which drain the lofty plateaus of central Asia to the east and south — the Hoang, Yangtze, Mekong, and Brahmaputra — descend to the lowlands through stupendous gorges, some of which have never been fully explored. Exercise. — Write a comparison of the Mississippi and the Colorado in regard to basin, tributaries, divides, fall, current, rapids, cataracts, valley, and amount of sediment carried. CHAPTER VII THE ST. LAWRENCE RIVER SYSTEM The St. Lawrence basin may be regarded as a shallow de- pression in the long eastward slope of North America and as a broad gap in the 'eastern highlands, connecting the Fig- 54- — Basin of the St. Lawrence River. central plains with the Atlantic coast. Few places in the divide which surrounds it are more than 1500 feet above the sea, and on the south the divide is in many places less than half as high. About one sixth of the basin is covered by the five Great Lakes which lie close to its southwestern border. The tributary streams, except the Ottawa and Saguenay from the north, are short. The level of Lake Superior is 602 feet above the sea, and the descent from it, through the St. Marys River, to the level of the twin 92 THE ST. LAWRENCE RIVER SYSTEM 93 lakes, Michigan and Huron (581 feet above sea level), is twenty-one feet. Lake Huron is connected by St. Clair River and Lake and Detroit River with Lake Erie, which lies eight feet lower (573 feet). From Lake Erie to Lake Ontario, through the Niagara River, there is MILES 12 15 18 21 24 27 MILES 500 1000 1500 2000 Fig. 55. — Profiles of the St. Lawrence and Niagara rivers. a drop of 326 feet in thirty miles. The St. Lawrence River leaves Lake Ontario at an elevation of 247 feet, and with an average fall of one foot per mile reaches sea level at Three Rivers, 500 miles from its mouth. Between Lake Ontario and the mouth of the Ottawa at Montreal, the river is wide, straight, swift, clear, bank-full, without floods or flood plain, and with numerous rocky islands and rapids. Some of these peculiarities may be accounted for by the influence of the lakes. Sediment carried by streams into lakes settles there, and the water flows out clear. In the absence of sediment, the river lacks the tools necessary for corrasion, and makes very little impression upon the bed over which it flows : hence the stream channel is but slightly depressed below the top of its banks. Any tem- porary excess of water supply is spread out on the broad surface of the lakes, and has no appreciable effect in rais- ing their level ; consequently floods can not occur in the river below. From Montreal to the mouth the current is 94 THE LAND SCALE OF MILES ^ r^~T^^ Fig 56 — Laurentian channel. affected by the ocean tides. Below Three Rivers the river is 1000 feet or more in depth, widening into a great estuary, the Gulf of St. Lawrence. A channel about 2000 feet deep extends along the bot- tom of the gulf, and 300 miles out to sea. The course of the St. Lawrence, includ- ing the Great Lakes, presents striking pecul- iarities. In its upper part it resembles a stream which has been obstructed by a series of dams, above each of which the water is held back in a great pond or reservoir. In its lower part it is not now a river, but an arm of the sea which extends up the valley 500 miles. When the river made this valley and the chan- nel upon the bottom of the gulf, it must have had in this part of its course a current of con- siderable velocity and it must have carried sedi- ment. These conditions point back to a time when the St. Lawrence system did not contain many lakes and when its basin stood at an elevation of 2000 feet or 10 20 30 40 50 Fig- 57. — Hudson channel. THE ST. LAWRENCE RIVER SYSTEM 95 more above its present level. Since that time the course of the old Laurentian river has been obstructed by a num- ber of dams above which the waters of the Great Lakes are held up to their present levels, and the land has sub- sided so as to let the sea into the lower valley, converting it into the present broad gulf. Drowned Valleys. — The lower portions of stream valleys which have sunk below sea level are called drowned valleys. _ The lower St. Lawrence is per- haps the greatest example of a drowned valley in the world, but many other rivers are in the same condition. The old channel of the Hudson River may be traced upon the sea bot- tom about 125 miles be- yond its present mouth (Fig. 57), and its valley is drowned as far up as Troy, 150 miles. The sea ex- tends up the Delaware River to Trenton, and Chesapeake Bay with its many arms is the drowned valleys of the Susquehanna and its former tributaries (Fig. 58). Many of the most famous harbors in the world, as San Francisco Bay, Puget Sound, the estuaries of the Thames and the Mersey, and the Scottish firths, are drowned valleys. The Niagara River and Falls. — The strip of country be- tween Lake Erie and Lake Ontario, twenty-five miles wide, .30,. 40 50 Fig 58- — Delaware and Susquehanna channels. 9 6 THE LAND consists of two plains lying at different levels. The upper plain extends from Lake Erie northward eighteen miles to the edge of an escarp- ment or cliff, where the surface drops steeply down 200 feet to the level of the lower plain, which borders Lake On- tario. Both plains are underlain by strata of sandstone, limestone, and shale, which are not quite horizontal, but slope southward about thirty- five feet to the mile. The arrangement is such that the strata outcrop on the surface in the following order from the shore of Lake Erie: (1) hard limestone (Corniferous), (2) soft shales (Salina), Fig. 59. - Niagara plains (3) hard, thick -bedded limestone (Niagara), extending to the edge of the escarp- ment, (4) soft shales and thin-bedded limestones (Clinton) forming the lower part of the cliff and the surface of the N I A G 4 fi| A L ! M H S T O N ALES L t M E S T 0/ Buffalo LAKE ERIE Fig. 60. — Section of Niagara plains. lower plain (Figs. 59, 60). The Niagara River flows across these two plains from one lake to the other. Its course is quite direct, so that it makes the whole descent THE ST. LAWRENCE RIVER SYSTEM 97 of 326 feet in about thirty miles. In the first thirteen miles the river is five to twenty feet deep and about one mile wide except where it is divided by Grand Island. The banks are low, and the current moderate. The stream resembles the St. Lawrence below Lake Ontario, and has corraded its channel to a very slight depth. Where it reaches the Niagara limestone, rapids begin, and after rushing down a slope of fifty-three feet in half a mile, the river drops perpendicularly 160 feet into a narrow gorge, which extends seven miles to the escarpment, and there opens out upon the lower plain. The width of the gorge varies from 600 to 1200 feet, and its nearly perpen- dicular walls rise 200 feet above the water. The slope is very steep and the water rushes through the narrow chan- ABOVE THE FALLS Fig. 61. — Cross sections of the Niagara. nel in a succession of boiling rapids (Fig. ||| 62). Midway in the length of the gorge is the Whirlpool, where an expansion and a bend cause the current to circle around in a complete loop. After passing under itself, it escapes at right angles to the course of the incoming stream. The peculiar features of the river which demand explanation are the falls, the gorge, and the sud- den change in the character of the valley from wide and shallow to narrow and deep. In the walls of the gorge, strata of varying composition and hardness are exposed. At the top the Niagara limestone forms a bold, perpen- dicular face about fifty feet high. Below, a series of soft Clinton shales are weathered into a steep slope. About midway of the height harder strata of Clinton limestone form a low cliff, below which soft shales and sandstones again form a steep slope to the water's edge. The strata on one side of the gorge correspond exactly in character and position with those on the other side, so that there is no suggestion of a fault or displacement. At the falls the same strata occur in the same order. 9 8 THE LAND Fig. 62. ^Niagara Gorge. Fig. 63. — Niagara Falls. THE ST. LAWRENCE RIVER SYSTEM 99 The upper (southern) part of the river joins the gorge at a right angle (Fig. 59), so that the portion of the stream which forms the American Fall drops over the side of the gorge, while the larger Canadian or Horseshoe Fall plunges in at the end, the two streams being separated by Goat Island (Fig. 63). At the brink of both falls the Niagara limestone overhangs like the cornice of a house, so that a considerable space intervenes between the falling water and H - the face of the precipice (Fig. 64). This space behind the Fi S- 64. -Section of Niagara Falls. American Fall is called the "Cave of the Winds," and is large enough to admit visitors. At the foot of the American Fall lie many great blocks of limestone which have fal- len from above (Fig. 65). From the brink of the Horseshoe Fall similar blocks many yards in area have been observed to fall from time to time. A com- parison of the positions of the brink of the Horseshoe Fall as determined by sur- veys made in 1842 and in 1 890 (Fig. 66) shows that the fall has moved upstream on an average five feet a year. Method of Recession. — The water passing Fig. 65. - The American Fall. over the brink of the falls strikes the bottom with great force, and boils upward again, while a portion of it constantly splashes back against LoFC. 100 THE LAND the face of the precipice behind. In winter blocks of ice are hurled back against the wall, and add to the destruc- tive effect of the splashing water. The soft shales are worn away, leaving the limestone above unsupported, which sooner or later fails by its own weight. In the Horseshoe Fall the force of the water is sufficient to toss the fallen blocks about, and to use them as tools to undermine the limestone still farther. The American Fall is too feeble to break up and carry away the blocks. Consequently they have accumulated and Fig. 66. —Map of Horseshoe Fall. ,, . . now protect the precipice somewhat from further attack. From all these facts it seems evident that at some time in the past the Niagara River began to fall over the escarpment (Fig. 59), and that, by the processes just described, the falls have traveled upstream to their present position. The Niagara gorge, therefore, has not been made by downward corrasion, for which the stream, on account of the absence of sediment, is poorly fitted ; but it is the result of excavation and under- mining by the falls. This work has been made possible by the position of the hard Niagara limestone on top, and the softer strata beneath, and by the fact that the water carries little sediment. If the rock in the bed of the river above the falls were softer, or if the river carried sediment, it would corrade downward and reduce the height of the fall by beveling off its edge. When the falls have receded to a point where the soft Salina shales are on top, and the Niagara limestone at the bottom, they will change from a THE ST. LAWRENCE RIVER SYSTEM ioi perpendicular cataract to a succession of rapids. A harder layer on top and a softer layer beneath are necessary con- ditions for the maintenance of a perpendicular fall. Such conditions occur frequently, and on almost any stream may be found falls which reproduce on a small scale the over- hanging ledge, the deep pool, and the gorge which exist at Niagara in such magnificent proportions. The St. Lawrence basin has had a long and eventful history. The river was once mature and had a bed which sloped continuously in a curve concave to the sky, like the Ohio and Mississippi. By processes which will be de- scribed in Chapters X and XI, it has been rejuvenated, or compelled to begin again the work of eroding its basin and grading its channel. It is now an example of a ponded stream, which, by reason of the lakes in its course, is almost deprived of sediment and hence of the ordinary means of stream corrasion. It is cutting down its bed very slowly, except in the Niagara section, where peculiar conditions may enable it, in time, to extend its gorge back to Lake Erie. That lake will then be drained and its bed will be traversed by a river which, by deepening its channel, will drain in turn the three upper lakes. Exercise. — Using any available source of information, learn the characteristics of the Nile River and compare it with the type rivers described in Chaps. V, VI, and VII. In what respects does it resemble the Mississippi? the Colorado? the St. Lawrence? In the same way study the Rhine, the Zambezi, the Indus, and the Euphrates. CHAPTER VIII UNDERGROUND WATERS Ground-water. — Some portion of the rain which falls upon the surface of the land sinks into the ground. The quantity varies with the steepness of the slope, the cover- ing of vegetation, and the porosity of the rock. Fine clay and compact limestone absorb water, but permit very little to pass through. Sand and coarse sandstone absorb rather less water than clay, but transmit it quite freely. Any rock which is traversed by joints and cracks, as is usually the case in nature, allows the rainfall to penetrate the crust of the earth. Some regions of limestones and lavas are so broken up by fissures that there are no surface streams, the entire drainage being through underground channels. Ground-water is continually rising by capillary attraction through the soil and keeps growing plants alive in dry weather. It is also the source of supply for wells and springs. If a well is bored to a depth below the level at which the ground is saturated with water it fills up to that level. If the water level outcrops on the surface, a spring occurs at that point. In Fig. 67 the rain falling on the surface HT penetrates through the sand until it lg 67 ' reaches the surface of the clay beneath, and moves slowly toward its lowest point S. But it stands higher in the sand than the level of the top of the clay, because a certain pressure is necessary to overcome friction and force the water through 102 UNDERGROUND WATERS 103 the sand. The lowest level of ground- water is at a height where the resistance due to friction just counterbalances the pressure due to the accumulated water. Since the friction increases with the distance which the water has to flow through the sand to its point of escape, it will hold the water up to a higher level below T than below H. There will be a spring at S, and a well sunk at W down to g will strike water. Both spring and well will be unfailing if the rainfall is sufficient to supply the outflow from them. If a permeable stratum, as gravel, lies below an impermeable stratum, as clay, and receives rain upon its outcropping surface, as at O, it may become filled with water up to the level of O. Then if a well starting at a lower level, as at A or B, is sunk until it taps the water-bearing gravel, the water will rise above the mouth of the opening, and a flowing or artesian well will be ob- tained. In a boring at B the pressure may be sufficient to raise the water to the top of a house or to make a fountain. The ground-water everywhere tends to flow or creep slowly toward the valleys, where it accu- mulates or feeds the surface streams. In regions of small rainfall and deep, permeable soil the greater part of the drainage may take place through the mantle rock in- stead of on its surface. By digging in the bottom of a dry stream bed, a good sup- ply of water may often be found, and a dam sunk to the proper depth may force the hidden stream to rise to the surface. Streams sometimes increase in vol- ume more rapidly than can be accounted for by their visible tributaries. In such cases they receive additions from the percolating ground-water. Fig. 68. — Stream flowing from a cave. (Donaldson's Cave, Lawrence County, Ind.) Underground Streams. — In some limestone regions the drainage is wholly subterranean and the earth-crust is ic>4 THE LAND honeycombed with tortuous passages and tunnels which frequently widen into large and lofty chambers or caves. The surface of such a region is pitted with funnel-shaped Fig. 69. — Section of caves. depressions or sinkholes which have no outlet except at the bottom. In some cases a stream enters an opening in the side of a cliff or hill, and after flowing some distance underground reappears upon the surface. Many surface streams in limestone re- gions flow from caves (Fig. 68). Caverns. — The rain- water percolates through the soil, enters the small crevices and joints of the limestone, and by reason of the carbon di- oxide which it contains, is able to dissolve the rock and gradually to enlarge the passage. It often follows some plane of Stratification, hollow- Fig-. 70. —Natural Bridge, Virginia, jng put large, irregular rooms along that level, and, finding its way to a lower level, repeats the process there. The result is a cave in two or more stories, connected by numerous passages. UNDERGROUND WATERS 105 In places the intervening floor breaks down and a lofty hall is opened from top to bottom. The place where the roof of a cave has fallen in is marked upon the surface by a sinkhole or inclosed valley without visible outlet. Where the roof of a large tunnel has fallen in, a portion may remain standing and form a natural bridge which spans the now open valley. The Natural Bridge, in Vir- ginia, was formed in this manner. Where water carrying lime in solution drips from the roof of a cave, it may evaporate, or lose some of its carbon dioxide, or both, and thus becoming incapable of holding the lime, deposit it in a long, pendant stalactite, like an icicle. At the point where the dripping water strikes the floor, more lime is deposited and a slender, co- lumnar stalagmite is built up to meet the stalactite. Thus columns, statues, " curtains," "altars," " organs," and Qther strange and beautiful forms are added to the characteristic scenery of caves. Mammoth Cave in Kentucky, Wyandotte and Marengo caves in Indiana, and the Luray Cavern in Virginia, are among the most famous and extensive in the world. Wyandotte Cave has a measured length of more than four miles, and contains one room 210 feet long, 90 feet wide, and 65 feet high. Mineral Springs. — Water which percolates a consider- able distance through the earth-crust meets with a variety of minerals which it dissolves and transports to the surface, where it emerges as a mineral spring. The nature and quantity of the mineral matter held in solution vary with the character of the rocks traversed and the temperature of the water. Fig. 71. - Stalactites and stalagmites. (Marengo Cave, Ind.) DR. PHYS. GEOG. io6 THE LAND Many springs of moderate depth and temperature form deposits of lime, iron, or other minerals about their mouths, but springs of hot water in volcanic regions bring to the surface vast quantities of silica which contribute to the formation of extensive masses of rock. Hot springs sometimes take the form of geysers, from which, at regular intervals,, the water spouts to a great height. Old Faithful, in the Yel- lowstone Park, throws a stream of hot water 150 feet high about once Fig. 72. — Hot spring terraces, Yellowstone Park. an hour (Fig. 73). These periodic outbursts are due to the gradual accumulation and final explosion of steam at great depths. The Work of Ground-water contributes to the same end as that of surface water. It dissolves and eats away the substance of the earth-crust and transports it, in some cases to higher levels, but finally, by one route or an- other, to the sea. Its channels take the form of covered tunnels or caves, but these are often changed into valleys by the caving in of the roof. It extends the processes of UNDERGROUND WATERS 107 weathering and erosion to indefinite depths, prepares the rock for more rapid at- tack by surface agents, and plays an important part in the tearing down and re- moval of the land. Realistic Exercises. — Men en- gaged in sinking wells can fur- nish much information concerning the ground-water of any locality. The student should investigate the depth of wells in his neighbor- hood, the materials through which they pass, and in which water is found ; the source, quantity, and permanence of the supply ; the quality and temperature of the water ; the levels at which springs occur ; the deposits, if any, at their mouths ; and the nature and ex- tent of caves, if any exist. Mines, quarries, and other excavations often show the penetration of ground - water and the streams which traverse the joints and fis- sures of the rock. Fig 73 — Old Faithful. CHAPTER IX GLACIERS Upon the tops of mountains and in the polar regions upon lower land, most of the moisture which falls from the clouds is in the form of snow. If the quantity which falls is greater than the quantity which melts and evapo- rates, the difference remains from year to year, and the ground is always cov- ered with a mantle of snow. The line above which snow is always present is called the snow line. Its height above the sea is great- est near the equator and in regions of dry climate, and least near the poles and in re- gions of moist climate, varying from 18,000 feet to near sea level. On mountain tops snow is blown off the peaks and slides down the slopes until it accumulates in the valleys to the depth of hundreds of feet. In the summer part of the snow is melted by day and frozen again at night, rain occasionally falls upon it, and it changes from dry, loose snow to a coherent mass, half snow and half ice, called neve. The pressure of the upper layers upon those below consolidates them and finally changes the neve into clear, solid ice. It is the same proc- ess of thawing, wetting, freezing, and pressure by which 108 • . ' , . .. frife^l • \\L '"H k\h iiiiMTriy^wV *fr E^S^' ~~nir m-&> .., 1 , Fig 74. — Snow-capped mountains. (Mont Blanc, Switzerland.) GLACIERS 109 boys make hard, icy snowballs. When the pile of ice, neve, and snow becomes deep enough, it begins to spread out at the bottom under the pressure of its own weight. A basin filled with such material overflows by a stream of ice, somewhat as a basin filled with water overflows by a river. Ice thus formed from snow instead of water is called gla- cial ice ; and any large mass of it is called a glacier. Alpine Glaciers. — Glaciers were first studied in the Alps, and those mountains still offer to the tourist and student one of the richest and most accessible fields for glacial observation. The snow line on the Alps lies at a height of about 8500 feet, and the longest glaciers descend in the course of ten or fifteen miles to the 4000-foot level. They are essentially rivers of ice, each of which conforms to the windings and irregularities of its own valley. The rate of motion is seldom more than two feet a day, or more than 250 to 500 feet a year, and therefore quite imperceptible to ordinary observation. If, however, a row of stakes is set across the ice in a straight line with stakes on the banks, the line will gradually become more and more convex downstream and the rate of movement of each stake may be measured. By this method it has been discovered that the motion is more rapid in the middle than at the sides, in summer than in winter, by day than by night, on steep than on gentle slopes, and in the narrow than in the wider parts of the val- — ley. The ice does not fill every > ! / / / nook and recess of the valley or set back into side ravines as water 4 1 i j £"•■-< ^^--* -Vr V ^-«-- j y 1 would. If Stakes are driven into Movement at side of Movement on surface the side of the glacier in a vertical a glacier. of a glacier row, after some weeks the line will lg ' 75 be found to incline downstream (Fig. 75), showing that the upper layers move faster than the lower. Realistic Exercise. — In a long box or trough place a mass of some plastic substance like pitch, tar, shoemaker's wax, or asphalt. Stick a row of upright pins in a straight line across it and set the box in an inclined position. If the material is kept moderately warm, it will flow slowly downward, and after a few days the row of pins will be found to be convex and inclined downstream. Why ? HO THE LAND Crevasses. — The surface of a glacier is traversed in various directions by cracks called crevasses. One set extends from each side toward the center diagonally up- stream. These are due to the unequal rates of motion. The ice in the center moves faster, while that at the sides drags against the valley walls, and the ice is pulled in two, or breaks at right angles to the direction of the strain (Fig. j6). In passing around a bend the ice upon the outer side is put upon the stretch, and crevasses appear which often close up again after the bend is passed. At points where the valley bottom is convex or the angle of slope increases abruptly, Diagonal crevasses. Longitudinal section of an ice fall. Cross section of a glacier. Fig. 76. the ice becomes deeply crevassed (Fig. j6). Such places correspond to ripples in a river, which remain stationary while the water moves on. Wherever the ice is suddenly subjected to a pulling or stretching strain, it breaks readily like a brittle body, and in many cases it becomes so broken by a maze of cracks that it resembles a heap of sharp, angular blocks. Where the stream moves on more smoothly, many of the cre- vasses close up again, and their sides unite so completely that all trace of the break disappears. If a crevasse remains open long, the warm air gains access to its surfaces, which melt unevenly, and when the sides come together again, they do not fit and the break remains unhealed. Ice which has been extensively crevassed never fully recovers its former solidity and smoothness. Causes of Glacial Motion. — To discover how a rigid, brittle body like ice can be squeezed out from under the weisrht of its own mass and can then move clown a wind- GLACIERS III Fig- 77- — Davidson Glacier, Alaska. ing valley in conformity with its varying direction, width, and slope is a problem of great difficulty. Plasticity. — While ice is very brittle under sudden strain, it is slightly plastic, and will stretch or bend or flow like very stiff molasses candy without breaking, if it is only given sufficient time. Breaking, Pressure Melting, and Regelation. — The readiness with which ice breaks under small strains and its cracks are again healed has been already described. When water freezes, it expands, as broken pitchers and burst pipes testify every winter. Conversely, when ice is compressed it melts. If two blocks of ice with dry surfaces are pressed together, slight melting occurs ; when the pressure ceases, the water thus formed freezes and the blocks are cemented together. (Try the experiment.) This process is called regelation (freezing again). The consolidation of snow into ice and the movements of the ice may be accounted for by the fact that breaking, pressure melting, and regelation are constantly going on. Realistic Exercises. — Suspend a lump of ice weighing about twenty pounds in a loop of wire. The wire will slowly cut into the ice and pass completely through it, but the cut will heal as fast as made, and only a layer of air bubbles will remain to show where it was. The ice is melted above the wire by pressure and freezes again below it. Fill a strong iron box or cylinder with damp snow or small pieces of ice and subject them to great pressure under a screw or lever press ; they will be consolidated into one mass having the form of the box. I 12 THE LAND Melting and Expansion by Freezing. — A glacier moves more rapidly in summer than in winter, by day than by night, and in the warmer region near its lower end than in the colder region near its source. The fact that wherever and whenever there is the most water in the ice, it moves fastest, indicates that its motion is due partly to melting. Also whenever the water formed by melting freezes again, it expands and tends to push the whole mass down the slope. Fig. 78. — Aletsch Glacier, Switzerland. The causes and methods of glacial motion seem to be complex. The principal forces at work are gravity, heat, and expansion by regelation. It is impossible to deter- mine just how much each contributes to the result. The whole truth can not be expressed by such a simple state- ment as that a glacier slides or flows or creeps down its valley. It probably moves by sliding, flowing, and creeping. GLACIERS 113 Ablation. — Throughout the length of a glacier the ice is disappearing more or less rapidly by evaporation and melting, but, of course, most rapidly in the warmer region toward its lower end. On a clear summer day the sur- face of the ice is traversed by streams of water which unite into drainage systems similar to those upon the land. After a longer or shorter course they usually drop into some crevasse and disappear in the depths below. Around these cascades the ice melts more rapidly, and a cylindrical well (inoulin) is formed, extending downward out of sight. The melting and evaporation are sometimes suffi- ciently rapid to lower the general surface of the ice as much as a foot in one day. The glacier finally reaches a point in its course where the ablation or destruction of ice equals the supply brought down, and the glacier comes to an end. At this point a stream of yellowish or milky water issues from the mouth of a cave or tunnel in the ice and carries away the whole drain- age of the valley above. It is as if a long block of ice were pushed toward a hot stove at such a rate that it melts as fast as it comes. The ice as a whole is moving forward, but the end remains at nearly the same point. The end of a glacier is not strictly stationary, but retreats or advances with changes of season and climate. Transportation. — An Alpine glacier carries upon its surface and in its substance a large quantity of rock debris which it has gathered from the sides and bottom of its val- ley. On the steep slopes of the mountains weathering goes on rapidly, and great avalanches, or slides of snow, rock, and earth, descend upon the surface of the ice stream. This rock and earth are piled near the margin in a long ridge called a marginal moraine. When a tributary joins the main glacier the united marginal moraines continue down the central part of the combined glacier as a medial moraine. These piles of rock and dirt protect the ice beneath them from melting, so that by the ablation of the ii4 THE LAND bare ice they may come to lie upon a ridge a hundred or more feet high. The morainic material rolls down the sides of this ridge and spreads out over a wider band, so The long undulating arrow follows the line of most rapid motion of "Mer de Glace" in the Alps. The amount of movement of the surface of the glacier - in inches, per 24 hours in summer- is also indicated. Fig. 79- that at the lower end of the glacier the whole surface of the ice is often buried under a mass of gravel and boulders. A large amount of rock debris, known as ground mo mine, accumulates at the bottom of the glacier and is pushed and dragged along with it. It is largely composed of clay, sand, Fig. 8o. — Terminal moraine. (Middle Blase Dale Glacier, Disco I., Greenland.) and gravel. The whole mass of rock and earth carried by a glacier upon its surface, in its substance, and at its bottom is called glacial drift, and by the final destruction of the ice GLACIERS 115 it is dumped at the lower end in a confused heap known as a terminal moraine. Abrasion. — Pure ice moving over a rock surface would probably do little more than sweep away loose material ; but a mass of ice a thousand feet thick, having sand, gravel, and boulders frozen into its bottom, acts like a flexible rasp which fits the irregularities of its bed and abrades or wears it away in a peculiar and striking manner. All sharp angles and corners are rubbed off. The softer portions of the bed rock are scooped out into hollows and the harder portions are left projecting ; but all the slopes and outlines are smoothed and rounded. If the bed rock is hard and fine-grained, it may be polished as finely as any marble or granite monument. More than this, the glacier leaves its signature upon the rock in the form of parallel strics, or scratches, from the finest hair lines to grooves a foot or two deep (Fig. 96). Pebbles and grains of sand, grinding along over the rock floor under the weight of the ice above, wear away the surface and leave scratches all running in the same direction. A rock surface which has been thus planed, polished, and striated is said to be gla- ciated. The pebbles and boulders them- selves are subjected to the same process, and every terminal moraine contains thousands of them which present one or more glaciated faces. The general effect of a glacier upon its valley is to deepen it, to change its cross section from a V-shape to a U-shape, and to leave it with a gently undulating surface more or less covered with glacial drift. Glacial Drift is distinguished from all other deposits by well-marked characteristics. (1) Where it has not been redistributed by the water flowing from the melting ice, it is unassorted and unstratificd. All kinds and sizes of Fig. 81. — Glaciated boulder. n6 THE LAND sediment are mixed up together higgledy-piggledy. (2) The ground moraine is largely a tough clay as full of gravel stones as a pudding is of plums, and containing Fig. 82. — Map of Muir Glacier. glaciated boulders of all sizes. It is called till or boul- der clay. (3) The terminal moraine is likely to contain more sand and gravel than clay, and any number of large boulders of every variety of rock existing along the course GLACIERS 117 of the glacier which brought them. The stones are gener- ally angular or subangular, and may be glaciated on one or more sides, but are not smoothly rounded, like water-worn pebbles. (4) Glacial drift is largely composed of foreign material, that is, of rock fragments which have come from a distance and are unlike the bed rock upon which they lie. Exercise. — Write a comparison of an Alpine glacier and a river in regard to origin, course, movement, transportation, corrasion, deposits, and work accomplished. "The track of a glacier is as unmistakable as the track of a man or a bear." If a glacier should entirely disappear by ablation, what evidences of its former existence would remain ? Fig. 83. — Muir Glacier, showing ice wall. Alaskan Glaciers. — Some of the glaciers which descend the mountainous coast of Alaska are different in form from any known elsewhere. The Muir Glacier (see Figs. 82 and 83) is fed by twenty or more ice streams, which descend n8 THE LAND into and fill an amphitheater thirty to forty miles in diam- eter. The medial moraines, marking the lines of flow, converge toward a single outlet about a mile wide, through which the surplus left from ablation, the drainage of 800 square miles of snow field, escapes into an arm of the sea. The ice stream ends in a jagged wall not far from 1000 "=&■ =.-!*«.-- Fig. 84. —Map of Malaspina Glacier. feet high and standing 200 feet above the water. Large masses break off from this cliff and fall into the water with a loud roar. Other large masses are loosened from the foot of the cliff beneath the water and rise to the surface with violent splashing. Thus a continuous proces- sion of icebergs float away and melt in the sea. GLACIERS 119 The Malaspina Glacier, at the foot of the Mt. St. Elias range, is in form the reverse of the Muir (see Figs. 84 and 82). Many separate streams from the mountain valleys unite into a plateau or lake of ice which spreads out to a width of fifty miles. The ice front extends along or near the shore of the ocean for a distance of seventy miles. The sur- face of the glacier is un- dulating, like the western prairies, and in the cen- tral portion is mostly free from moraines and dirt, but broken by thousands of crevasses. The outer edge of this ice sheet seems to have been for a long time stagnant and has become covered by a thick coating of sand and gravel derived from the moraines. Upon this soil a dense growth of trees and shrubs has sprung up, forming a for- est under which the ice is, in places, 1000 feet thick. The Greenland Ice Cap. — Greenland is a plateau about 1500 miles long and 800 miles wide in its widest part. Two thirds of its surface is buried beneath a sheet of per- petual snow and ice. The general elevation of the ice plateau is 7000 to 8000 feet in the central area, gradually 100 200 300 400 Fig. 85. —Map of the Greenland ice cap. 120 THE LAND decreasing to 2000 or 3000 feet toward the coast, giving to the island a surface form like that of a loaf of bread, gently rounded in the middle and steeply sloping at the edges. Beyond 50 or 75 miles from the coast no mountain peak, rocky islet, or other sign of land rises above the sea of neve. The white, featureless expanse is unbroken by crevasses or water courses and unstained by dirt or dust. The whole mass seems to be moving outward in all directions, and, as it approaches the coast, becomes broken by projecting peaks of rock and extensively crevassed. Its edge is divided into numerous long tongues which es- cape down the nar- row valleys to the sea. The largest of these yet described forms the Humboldt Glacier, which ad- vances into the sea Fig. se. -iceberg. with a wall 60 miles long and 200 to 300 feet above the water. From the va- rious projecting tongues of ice innumerable bergs break away and crowd the adjacent waters. The rate of motion in the Greenland glaciers sometimes reaches 50 or 100 feet per day. The Antarctic Ice Cap. — The region around the south pole as far as latitude yo° seems to be covered with an ice cap similar to that of Greenland, but of vastly greater extent. Explorers sailing in that direction are stopped by an unbroken wall of ice, 200 to 300 feet high, from which flat-topped bergs (Fig. 87), often half a mile in breadth, break off and float away. The area included within this GLACIERS 121 ice wall is about 4,000,000 square miles, or larger than the whole of Europe. The neve fields, if not continuous over the whole region, must be very extensive and moving out- ■ SB m --»»—. -' J , ' ' .'; "'■ Fig. 87. — Antarctic iceberg. ward in all directions to supply the quantity of ice which is discharged. Continental Glaciers. — -Glaciers which are not confined to valleys but spread over wide tracts of country, like the ice caps of Greenland and the Antarctic region, are called continental, and are the only surviving representatives of vast ice sheets which once covered a large part of North America and Europe. DR. PHYS. GEOG. CHAPTER X THE DRIFT SHEET OF NORTH AMERICA Fig. 88. — Boulders from a terminal moraine. (St. Joseph County, Ind.) The greater part of the northern half of North America is covered with a sheet of mantle rock similar in essential character to the ground moraine now forming under the glaciers of the Alps, Alaska, and Greenland. In the United States this sheet of mantle rock extends as far south as the Ohio and Missouri rivers. Its thickness varies from a few feet to several hundred feet, its average depth being not less than ioo feet. The greater part of its mass consists of a stony clay containing pebbles of all sizes, many of which are glaciated. There are also exten- sive deposits of sand and gravel, often well assorted, but also mixed with each other and with clay in all proportions. More conspicuous than these, but constituting only a small THE DRIFT SHEET OF NORTH AMERICA 123 percentage of the whole, are thousands of boulders of all sizes up to that of a small house. The pebbles and boulders represent a great variety of material. An hour's search is often sufficient to collect fifty or one hundred species of rock nearly all foreign to the region where they lie, and the major- ity of them foreign to the United States. Most of these " errat- ics " or " lost rocks " are recognizable as frag- ments of the igneous and metamorphic rocks of the old Laurentian high- land of Canada, and in some instances they can be traced back to a defi- nite locality from which they must have come originally. Large masses of metallic copper from the shores of Lake Supe- rior have been found buried in the soil of In- diana, and some of them are glaciated. Boulders of a peculiar conglomer- ate, consisting of pebbles of red jasper dissemi- Fig. 89. — A boulder. (Near Camden, Maine.) nated through a ground mass of white quartz, are scattered over Ohio, Indiana, and Illinois, and on account of their striking colors attract popular attention. They must all have come from one parent ledge of similar rock on the north shore of Lake Huron. The surface of this sheet of mantle rock is traversed by a complex system of ridges which have the form and com- position peculiar to terminal moraines. In hundreds of places where the bed rock has been exposed by natural or 124 THE LAND artificial means it is found to be glaciated, the grooves and scratches having a general north-south direction. These features admit of but one explanation. This sheet is a vast deposit of glacial drift. The evidence has now accumulated in such mass, variety, and accordance as to make it impossible to doubt that at a comparatively recent period the northern part of North America was covered with an ice sheet like that of Greenland, extend- ing as far south as the glacial boundary shown on p. 125. If the lines of glacial scratches are traced back northward, they point to the region around Hudson Bay as the location of the central snow field. From this region the ice moved southeastward over New Eng- land, southward over the Middle states, south westward over the West- ern states, westward nearly to the Rocky Mountains, and northward toward Alaska and the Arctic Ocean. The farthest point from the center reached by the ice was in Kansas, a distance of 1500 miles. The area covered was about four million square miles, but it is not probable that it was all covered at any one time. The Older Drift. — Close examination reveals the fact that the drift is not simple and uniform over the whole area, but is made up of several distinct sheets which overlap one another, like the shingles on a roof. The lowest and outermost sheets together form what is known as the older drift, which lies on the surface in parts of Ohio, Indiana, Illinois, Iowa, Missouri, Kansas, and Nebraska. The margin of the older drift is not usually marked by a ridge or terminal moraine, but thins out to a vanishing edge along the glacial boundary from central Ohio westward. The older drift is extended and partly covered by deposits of fine silt (loess), probably the allu- vial sediment deft by glacial floods. It is also characterized by the occurrence within its mass of buried timber and vegetable debris, so common that the well diggers call such an accumulation "Noah's brush- heap " or "Noah's barnyard." It is probable that the ice sheets which deposited the older drift advanced to their southernmost limit and at once retreated without pausing anywhere in the United States long enough to form a well-marked terminal moraine. !25 126 THE LAND The Newer Drift. — Partly overlapping the older drift lies a much thicker and more complex sheet of more recent drift, the southern margin of which is marked by a series of. terminal moraines, almost continuous from Cape Cod to Alberta. As will be seen from the map. p. 125, the terminal moraine in the east coincides with the glacial boundary, but in central Ohio the two part company. In the Mississippi valley they are 500 miles apart, but run close together again through the Dakotas. In the interval between them in Wisconsin there is a large area entirely free from drift. The drift sheet is quite thin in New England, but increases in mass west- ward until -in Ohio and Indiana it attains a depth of from 100 to 500 feet. The Moraines. — The principal irregularities of the sur- face of the newer drift are due to the long lines of terminal moraines which traverse it. The margin of the ice sheet which deposited the newer drift not only occupied the line of its farthest advance long enough to deposit a massive moraine, but during its retreat it halted at frequent inter- vals or temporarily readvanced. The line held at each period of halting is marked by a moraine roughly parallel with the previous one. The method of retreat was a step backward and then a long pause, as an army retreating from an enemy's country marches by day and at night halts and throws up intrenchments. Between the Ohio River and Lake Superior the lines of moraines indicate sixteen successive halting places. A very noticeable feature is the looped or festooned form of the moraine groups, indicating that the ice sheet was divided into several lobes or tongues which advanced independently of one another. This lobation was due to the broad, open valleys of the region. In the valleys the ice was thicker than on the bordering highlands, and consequently advanced farther and melted back more slowly. The basins of the Laurentian lakes seem to have exerted a controlling influence upon the lobing of the ice margin. There was an Erie lobe in Ohio and Indiana, a Saginaw lobe from Lake Huron in Michigan and Indiana, a Lake Michigan lobe in Michigan, Indiana,' and Illinois, a Green Bay lobe in Wisconsin, and THE DRIFT SHEET OF NORTH AMERICA 127 Fig. 90. — Map of Erie moraines. a Superior lobe in Minnesota. In each lobe the ice spread out from the center toward the margin, and in the reentrant angles between the lobes piled up iuterlobate moraines of huge proportions. The Surface of the Moraines. — At the edge of the ice the newer drift material was dumped pell-mell in long heaps, while a portion of the ground moraine was pushed forward and a portion gathered under the edge where the ice current was too feeble to carry it farther. In many places the morainic material was deposited in shallow lakes which stood along the ice front, or was carried by outflow- ing streams far down their valleys. The moraines formed under these conditions have a varied aspect. The simplest are long ridges or swells, rising above the level surface of the drift plain like dead ocean waves. The most massive consist of a belt of hills from two to twenty miles wide, where the drift is piled in a confused assemblage of 128 THE LAND domes, knobs, peaks, and irregular ridges, with corre- sponding hollows between, all in the utmost disorder. Fig. 91.— A hilly moraine. (St. Joseph County, Ind ) The predominating materials are gravel and sand. The feebler moraines present the same features on a smaller scale, forming the " mound and sag " type of surface ; or the sags may be absent and the moraine consist of a belt of Fig. 92. - A kettle hole. (Near Morristown, N.J.) low, broad mounds rising from a plain. A moraine line is sometimes marked only by a broad belt or strip of sur- THE DRIFT SHEET OF NORTH AMERICA 129 face thickly strewn with large boulders. The relief of a morainic surface forms a unique type of topography, which once seen and understood can be readily recognized. Kettle Holes form one of the most characteristic features of terminal They are bowl-shaped or funnel-shaped basins of all sizes moraines. Fig- 93- — A kame. (Tippecanoe County, Ind.) and depths, having no outlet, and often occupied by small lakes. Each marks the place where a large block of ice detached from the main mass and partly buried in drift has melted and left a depression, as ice melt- ing under sawdust often does. Kames are heaps of'sand and gravel which have been deposited along or near the edge of the ice by outflowing streams of water. They take the form of mounds and wind- ing ridges with a hummocky and rapidly undulating outline. The material is more or less perfectly stratified. They oc- cur in connection with mo- raines and are often difficult to distinguish from them. Eskers, or " serpent kames, 11 are long, winding ridges of gravel which ex«tend often for many miles across hills and valleys in the direction of ice movement. They are accumulations formed in the tunnels of sub- glacial streams or in ice-walled canyons open to the sky. Drumlins are peculiar rounded and elongated lenticular hills of boul- der clay, which were formed under the ice some distance back from the Fig. 94. — An esker. (In Auburndale, near Boston, Mass.) 130 THE LAND Fig- 95- — A drumlin. (Near Amherst, Mass.) margin, and perhaps correspond to the sand bars in a river. They do not usually occur singly, but in groups which occupy the whole face of the country. General Results of the Ice Invasion. — The foreign boulders arid glacial scratches upon the White, Green, and Adirondack mountains indicate that the ice overrode their summits and ::: was not less than a mile thick over northern New Eng- land. Its thickness over the Laurentian highlands may have been two miles. On account of the ab- sence of land projecting above it, the surface of 'the ice sheet was clean, and lateral and medial moraines were wanting. The drift was gathered up from the bottom, and, except a portion which in some manner became incorporated in Fig. 96. — Glaciated rock. (Summit of Mt. Monadnock, N.H.) THE DRIFT SHEET OF NORTH AMERICA 131 the body of the glacier, was dragged along as a ground moraine. The ice sheet may be pictured as combining the features of the Greenland ice cap with those of the Malaspina Glacier. The great central expanse was smooth and clean, but for many miles back from its margin it was probably covered with gravel and boulders laid bare by the ablation of the upper layers. It may even have resembled the Malaspina in sup- porting a growing forest. The action of the ice sheet was vigorous and prolonged and its effects correspondingly great. The present sur- face features of the region which it covered are largely the result of its work. The regions of ice accumulation in Canada and New England were regions of greatest Fig. 97. — Drift plain. (Tippecanoe County, Ind.) abrasion. Not only were they swept nearly bare of mantle rock, but hills and mountains were worn down, and the surface of the bed rock was pitted with thousands of shallow depressions now occupied by lakes. The rock debris thus formed was carried southward and spread over southern Canada and northern United States. In the region of glacial deposition, previously existing hills and ridges were rubbed down, valleys were filled up, and the surface of the country plastered over with a coat of drift, as a mason plasters a rough stone wall with mortar. 132 THE LAND Except in the mountain regions the old surface features were obliterated and a new and much smoother surface was created. The contrast between the broken surface of the country south of the glacial boundary (Fig. 98) and the monotonous smoothness of the drift plain north of it (Fig. 97) is very striking, and in some places the change from one to the other is abrupt. Fig. 98. — Dnglaciated region. (Near New Albany, Ind.) The Drift Plain is relieved only by shallow valleys which the streams have cut a little way into it and by the belts of morainic hills which rise here and there from fifty to three hundred feet above its surface. The moraine belts are studded with thousands of ponds and small lakes, and the plain itself abounds in swamps and marshes. On account of the gentle slopes and the short time during which the streams have been at work, the whole region is poorly drained. The drift, however, is the " grist of the glacial mill,'" and consists of an intimate mixture of rock flour and fragments ground from a great variety of minerals. It contains all the elements of plant food and forms one of the most productive and enduring soils in the world. The drift regions are preeminent in their agricultural resources. The Preglacial Drainage of the glaciated region underwent profound modification. The St. Lawrence River system was completely changed THE DRIFT SHEET OF NORTH AMERICA 133 in character, a subject which will be more fully discussed in the next chapter. The old outlet of the Allegheny and Monongahela rivers, which formed a single northward-flowing stream, was dammed with drift, and their waters were permanently diverted to the Ohio. The northern outlet of the Winnipeg basin in Canada was dammed and the basin occupied by a fresh-water sea (see Lake Agassiz on map, p. 125), which emptied through the Minnesota River into the Mississippi. The course of the Missouri through the Dakotas was displaced one hundred miles to the westward. From the Maine coast the ice extended far out to sea, the lower portions of the stream valleys in Maine were deepened, and by subsequent drowning they have been converted into fiords, which give the coast its present extremely ragged outline. Fig. 99. — Glaciated regions of Europe. Other Glaciated Regions. — During the glacial period northern Europe passed through a series of ice invasions similar to those of North America, and it now presents similar characteristic features. The general movement of 134 THE LAND the ice, the glacial boundary, and the principal terminal moraine are shown upon the map, Fig. 99. The Scandina- vian mountains formed the chief gathering grounds, with secondary centers of dispersal in the Scotch highlands and the Alps. The Baltic and North seas were filled with solid ice, as Hudson Bay was. The glacial period closed at least 10,000 years ago, yet it was so recent as compared with other great changes, and its effects in Europe and North America were so pro- found and far reaching, that it may well be regarded as one of the most important events in the recent physical history of the world. Realistic Exercises. — Any student who lives north of the glacial boundary should make himself acquainted with the glacial features in his vicinity. Boulder clay may be readily found and distinguished from other clay ; foreign pebbles and boulders may be picked up by the thousand. Glaciated pebbles and glacial scratches on the bed rock are likely to be found anywhere. Deposits of loess, the thickness of the drift, the occurrence of buried timber, drift-filled valleys, changes in stream courses, moraines, kettle holes, lakes, kames, eskers, and^ drumlins should be looked for and investigated. No region offers better facilities or more interesting subjects for elementary field work than the areas covered by glacial drift. CHAPTER XL LAKES AND LAKE BASINS In an ideal drainage basin the slope is continuous from the divide to the mouth of the stream. But in nature slopes are interrupted by depressions which are completely sur- rounded by a rim of higher land and act as reservoirs which detain and store up a portion of the rainfall. Such a de- pression, if the rainfall is sufficient, fills with water up to the level of the lowest point in the rim and becomes the bed of a lake or pOnd. Lakes may be regarded as ex- pansions of the streams with which they are con- nected. They vary in form and size from quiet pools or reaches, where the current of a stream is imperceptible, to veri- table inland seas, like the Great Lakes. Diastrophic Basins The Great Basin.— The largest basins are due to Fi e I0 °- the warping or irregular elevation and depression of the earth-crust by internal forces. The Great Basin of west- i35 136 THE LAND ern United States, lying between the Sierra Nevada and Wasatch Mountains, has an area of about 210,000 square miles. Its surface is divided by parallel mountain ranges into numerous valleys and subordinate basins. The rain- fall is scanty and almost confined to the mountain tops. Great Salt Lake in Utah is the shrunken remnant of a body of water (Lake Bonneville) which was nearly ten times as large as the present lake, stood about 1000 feet higher, and had an outlet by way of the Snake River to the Columbia. During the period of overflow its waters were fresh, but a decrease in rainfall caused its surface to fall below the level of the outlet, and it has become increas- ingly salt. At various levels around its inclosing rim, its former shore lines, with their wave-cut cliffs, bars, spits, terraces, and deltas, record the work of the waves and in- flowing streams of the ancient lake (see Fig. 132). Lakes of Nevada. — At the time of the greatest extension of Lake Bonneville a large body of water (Lake Lahontan) occupied a very irregular area of 1500 square miles in the western side of the Great Basin, receiving drainage from the Sierra Nevada. This lake, at its highest stage, had a depth of nearly 900 feet, but never had an outlet. Pyramid, Winnemucca, Walker, Humboldt, and Carson lakes now occupy the lower portions of the old lake bed. They are subject to great variation in volume from year to year. At many points in the Great Basin, wet weather lakes gather in times of rainfall and soon dry away, leaving ftlayas, or beds of mud, which bake to a hard and cracked crust. Asiatic Basins. — The great plateaus of Asia include ex- tensive basins and inland drainage systems similar to those of the Great Basin of the United States. The country lying between the Kuenlun and Altai Mountains is of this char- acter. The desert of Gobi is the bed of a dried-up sea, containing in its lowest parts salt lakes and marshes which rise and fall with the uncertain water supply. Snow-fed LAKES AND LAKE BASINS 137 streams creeping out from the mountain valleys sometimes reach these lakes, but their waters are generally lost or evaporated. Southwestern Asia, including large portions of Persia and Arabia, contains basins either entirely dry or holding in their lowest parts shrunken salt or bitter lakes. The Caspian Basin. — To the north and west of the pla- teau country of Asia, and including a large area of south- eastern Europe, lies a great low plain which has no outlet to the sea. It contains the Caspian Sea, which has an area of 1 70,000 square miles and a maximum depth of 3000 feet. The surface of the Caspian Sea is about 90 feet below the level of the sea. The Caspian seems to have been originally part of a gulf which extended southward from the Arctic Ocean, from which it was cut off by the rising of the intervening land. The Aral Sea and Lake Balkash are salt lakes of the same origin as the Caspian. Rift Basins. — In east Africa there are two extensive chains of lakes and dry basins which are long and narrow and lie, like fiords, between precipitous cliffs thousands of feet in height. One chain extends from Lake Nyassa on the south, through Tan- ganyika and Albert, to Rudolf, where it is joined by another chain from the south. Thence the line continues northward as a long strip of low land, dotted with lakes and old lake basins, some of which are below sea level, to the southern end of the Red Sea. At the north end of the Red Sea a similar line of depressions extends from the Gulf of Akabah to the Dead Sea and the valley of the Jordan River in Palestine. This deep, narrow valley, nearly 4000 miles long, containing the Red Sea and DR. PHYS. GEOG. — 9 Fig. 101. — Map of east Afri- can lake chains. 138 THE LAND more than thirty lakes, has been produced by a series of parallel faults, or cracks in the earth-crust. The block between the faults has subsided, forming the " Great Rift Valley," bounded by high, precipitous walls on either side. The bottom is quite irregular and has been obstructed by many outflows of lava. Lake Nyassa is 350 miles long, 50 miles wide, and 300 to 600 feet deep, and empties southward by the Shire and Zambezi rivers to the Indian Ocean. The largest lake, Tangan- yika, is 400 miles long, 20 to 40 miles wide, and 500 to 2000 feet deep, and overflows westward into the Kongo River and Atlantic Ocean. Lake Albert is one of the sources of the Nile. Most of the lakes have no outlet. The Dead Sea is remarkable for the extreme saltness of its waters and for the fact that its surface lies nearly 1300 feet below the level of the sea. In the northwestern part of the Great Basin there are numerous rift valleys, some of which are occupied by small lakes. Of these, Alvord and Warner valleys in Oregon, Surprise Valley in California, and Long Valley in Nevada are the most notable. Long and Warner valleys STEIN MTS. Fig. 102. — Section of Alvord Valley, Oregon. are continuous, and form a narrow basin 100 miles in length, walled in by sheer precipices in some places 2000 feet high. The Stein Moun- tains rise 4000 to 5000 feet above Alvord Lake. (See Figs. 150 and 151.) A series of rift valleys extends from central New Mexico, through western Texas, into Mexico. Glaciated Basins Lakes are more numerous in glaciated regions than in any other parts of the world. (See maps of northern Europe and North America.) Basins in glaciated regions are of two classes: (1) bed-rock basins, most numerous in regions where the ice was thickest and abrasion most active ; and (2) drift basins, most numerous in regions of glacial melting and deposition of drift. The great moraine sys- tems which stretch across the United States from Cape LAKES AND LAKE BASINS 139 Cod to the Dakotas, and across Europe from the Valdai Hills to Denmark, are belts of small lakes. Morainic basins due to the irregular deposition of drift are ex- tremely variable in form, but may be classified as kettle, channel, and irregular basins. MORAINIC LAKES NORTHEASTERN INDIANA. Moraines 5 Beaches Fig. 103. Kettle basins or kettle holes (Fig. 92) are roundish, caldron-, or funnel- shaped depressions which owe their existence to the melting of detached masses of ice left, during the glacial retreat, partly buried in drift. Those which have a clay bottom are filled with water, but those with a gravel bottom are generally dry. Channel basins are long and narrow, and were made by streams which temporarily drained the melting ice front. Irregular basins are combinations of kettle holes, channels, and other depressions which fill and overflow into one another, forming connected bodies of water at the same level. 140 THE LAND Fig 104. The Finger Lakes. — Glaciated regions abound in long, narrow rock basins occupied by lakes, among which those LAKES AND LAKE BASINS 141 in central New York called, on account of their form and rela- tive positions, the Finger Lakes, are of peculiar interest. The northern slope of the Alleghany plateau is here trenched by many long, narrow valleys from 1000 to 2500 feet deep, some of which contain lakes, while many do not. Of larger and smaller lakes there are more than a ^"Vgc dozen. Seneca and Cayuga —e each Fig. 105. — Profile of northward slope of Finger Lake plateau. about forty miles long and one to three miles wide. Seneca is 441 feet above sea level and 618 feet deep. Cayuga has an elevation of 378 feet and a depth of 435 feet. On the plateau between the lakes tributary streams flow in broad, shallow valleys until within a short distance of the lake, where the val- leys end in the air, as if cut off (Fig. 106), and the streams drop into deep, nar- row gorges whjch continue almost to the lake shore. Small deltas and alluvial terraces occur at various elevations on the hillsides. The Finger Lakes oc- cupy basins which were in preglacial times the valleys of streams flowing into the Ontario basin. When the ice sheet moved from the north over this region it was comparatively thin on the ridges, but much thicker in the valleys. By glacial abrasion the valleys were widened and deep- ened, and the slopes on either side made more steep. During the Fig. 106.— Taughannock Falls, near Cayuga Lake. 142 THE LAND retreat of the ice the ridges were probably uncovered first, while con- siderable masses of ice still occupied the valleys. As the ice gradually melted and the lake surfaces fell from one level to another, the tributary streams on the plateau entered the lakes at different levels, forming deltas and terraces which they afterward cut through or abandoned, building others at lower levels. The lake basins had been so much deepened by glacial erosion that the old tributary valleys were left far above trit present lake levels, and the streams, compelled to cascade down the steep slopes, began to cut back their present gorges or glens. SCALE OF FEET 1000 2000 3000 Fig. 107. — Cross section of Cayuga Lake valley. (Vertical and horizontal scales the same.) The Cayuga basin was probably made 350 to 450 feet deeper by ice abrasion ; but the valley is still only a broad, shallow groove. Mountain Valley Basins. — The Alps and other moun- tain regions which have been recently glaciated contain many basins similar in essential characteristics to those of the Finger Lakes. The valleys head in vast cirques or am- phitheaters upon the flanks of the mountains, the sites of former neve fields, whence they descend by irregular steps through successive basins to the lowest, which lies near the end of the old glacier and sometimes extends out into the surrounding plain. The lower ends of these basins are usually bordered by morainic dams. The Italian lakes, Como, Lugano, Garda, and Maggiore, and the Swiss lakes, Geneva, Constance, Zurich, and Lucerne, occupy basins of this kind. Landslips, moraines, and deposits of sediment by lateral streams have built the natural dams which hold back the waters of many mountain lakes, but their basins are due largely to glacial erosion. Such mountain lakes are numerous in Scotland, Scandinavia, New Zealand, and the Rocky Mountains. LAKES AND LAKE BASINS 143 The Scotch highlands have been subjected to very extensive glacial erosion, and contain numerous lakes, of which Loch Katrine is one of the most beautiful. Its length is eight miles, its width one mile,and its depth 495 feet. It fills a single symmetrical basin which is closed at its lower end by a belt of very hard and dura- ble rocks. During the later stages of the glacial period the direction of ice move- ment was down the valley, which was scooped out to a Fig Io8 ' depth of 130 feet below sea level; but the more resistant rocks were left as a barrier which holds the water up to its present level. The Laurentian Lakes. — Of special importance, on account of their great size and interesting history, are the Great Lakes of the St. Lawrence system. Lake Superior is the largest body of fresh water on the globe, and it con- tains 280 of the 570 cubic miles of water stored in this chain of reservoirs. Their areas, levels, and depths are given in the following table : — Loch Katrine. Area Elevation, ft. Maximum Average in sq. mi. above sea level. depth in feet. depth in feet. Superior 31,200 602 1,008 475 Huron 23,800 581 73° 250 Michigan .... 22,450 581 870 325 Erie 9,960 573 2IO 70 Ontario 7,240 247 738 300 They occupy a series of comparatively elongated basins, separated by small areas of land, and joined near their ends by short streams or straits. The shape of the lakes, their nearness to one another, their end connections, 144 THE LAND and their trend suggest an overgrown stream line with a succession of immense reaches, a repetition on a grand scale of the characteristics of almost any meadow brook. They occupy basins which seem to be the broken and obstructed sections of a great stream valley. This impres- sion is strengthened by the course of the line of greatest depth, as shown by the heavy line in Fig. 109. The bot- toms of all the lakes except Erie are below sea level. MAP OF THE PREQLACIAL LAURENTIAN RIVER S — Superior Valley P—Pewamo Valley T- Trent Valley D—Dundas Valley C— Cuyahoga Valley Fig 109 Evidences of Glaciation. — The whole St. Lawrence basin was deeply buried under the ice sheet, evidences of which are abundant in the grooved and scored rock surfaces on the islands and shores of the lakes, in the lobed and irregular shape of their southern shores, and in the arrangement of the terminal moraines of the glacial lobes around and between them (see map, p. 125). Evidences of Tilting. — The lake basins are surrounded by numerous old beaches or shore lines, which mark the height and limits which their waters have at some time reached. But these old shore lines are no longer level. They gradually rise toward the north and east. One of them, known as the Algonquin beach, is 25 feet above the southern LAKES AND LAKE BASINS 145 end of Lake Huron, and 635 feet above its northern end. The depth of the lakes below sea level, and the extensive drowning of the St. Lawrence valley (see p. 95), show that the whole basin onee stood at a considerably higher level than at present ; while the occurrence of bones of the whale and other marine animals along the shores of Lake Ontario and Lake Champlain shows that these lakes and the lower St. Lawrence valley once formed a great arm of the sea, an extension of the Gulf of St. Lawrence. All these facts point to the conclusion that the basin of the St. Lawrence has been subjected in the past to extensive depression and upheaval, which was in the nature of a tilting along a northeast and southwest line. Many of the peculiarities of the basins of the Great Lakes may be attributed to the agency of the Laurentide ice sheet, which, creeping forward from the Canadian high- lands, flowed into, filled, and crossed these basins. The pre- glacial valley of the Lauren tian river was probably widened and deepened in some parts by the removal of material, and obstructed in other places by its deposition. Borings have revealed many deep valleys which lead into or con- nect the lakes, but are now filled with drift. Among these are the Pewamo or Grand River valley across Michigan, the Trent valley between Georgian Bay and Lake Ontario, the Dundas valley between Erie and Ontario, and the valley of the Cuyahoga at Cleveland (see Fig. 109). The slight depth of Lake Erie indicates that its basin was not a part of the main valley, but a tributary to it. With the exception of Lake Superior, which is an old diastrophic basin, the Great Lakes are old river valleys, first cut wide and deep by weathering and stream erosion, then depressed, uplifted, and tilted by movements of the earth-crust, and finally widened, deepened, and cleaned out here and choked up and obstructed there, by the North American ice sheet. Ice-dammed Lakes. — When the ice sheet began to melt away, and the southern divide of the Laurentian basin was uncovered, the water 146 THE LAND {'J- LAURE N-T ID p , , — MI !- ES , J 6 .*,? . <,. .. ,-;<• .. V u E /,q ^ So Too collected at several points along the ice front and formed a number of temporary lakes of varied and changing size and form. They were bounded and held in on the north by the wall of the retreating ice front, but their outlines and outlets can still be traced by the beaches formed where their waves beat against the land. Figure no shows three out of the many successive stages in the long and compli- cated history of the Laurentian lakes. Lake Agassiz. — The largest of the ice-dammed lakes of the period of glacial recession occupied the basin of Red River in Minnesota and North Dakota and extended far north- Fig, no. LAKES AND LAKE BASINS 147 ward into Canada (see map, p. 125). Its outlet was through the Minne- sota River into the Mississippi, but the opening of an outlet through the Nelson River into Hudson Bay drained its waters until only Lakes Winnipeg and Winnipegosis remain. Its sediments now form the soil of the great wheat fields of the Red River region. Barrier Basins . Many examples have already been cited of basins which are partly due to the formation of natural dams or bar- Fig, in. —A barrier basin. (Lake McDonald, in the Rocky Mountains, Mont.) riers across a valley. There are few lakes which do not owe their existence, more or less, to this cause. The dam may be of hard rock, as in Loch Katrine ; of glacial drift, as in the Great Lakes ; or a terminal moraine, as in the case of mountain lakes. A lava stream from 148 THE LAND Fig. 112. — Intermorainic lakes, Idaho. a volcano sometimes obstructs a valley and forms a coulee lake. Landslides often form temporary dams, be- hind which water accumulates for a time and then breaks through with destructive violence. The deposit of a tribu- tary stream may set back the waters of the main trunk into which it flows. The growth of coral reefs and the forma- tion of sand bars re- sult in the cutting off of portions of a sea or lake from the main body of water. Thus shore lagoons of great variety and extent are pro- duced. Probably glacial moraines act more often as barriers to drainage than any other species of dam. Wherever they occur in series, the valleys between usually contain many intermorainic lakes. Other Basins Volcanic Basins. — Crater Lake, in southern Oregon, is circular in form, with a diameter of five miles and a depth of 2000 feet. Its surface is 6239 feet above sea level, and it is bordered all around by precipitous cliffs from 500 to 2200 feet high. From the crest of the encircling rim the country slopes away on all sides. The roughly bedded layers of rock also slope outward and downward from the lake shores. The lake, as its name suggests, occupies the crater of an extinct volcano. The angle of its slopes indicates that its summit may have been a mile above the present lake surface. The material which once LAKES AND LAKE BASINS 149 ,--"""' Crni er Lake ~"~~fe^rr | Sea Level Fig. 113. —Crater Lake, Oregon. formed the cone and filled the crater was probably not blown out by an explosion, but has disappeared by sinking into the depths from ,.- x which it came. Basins of this kind are not nu- merous, but Lakes Alba- no and Averno in Italy, the Laacher See in Ger- Fi «- »4.- crater Lake, Oregon, many, and Lake Taupo in New Zealand belong to this class. Alluvial Basins. — The oxbow or horseshoe lakes, very common in flood plain regions, result from the cutting off of a river bend and the silting up of its ends. ' They have been fully discussed in connection with the Mississippi River. Basins by Solution. — In regions of limestone rocks, where subterra- nean drainage channels exist, the falling in of the roof of a cavern often forms a sinkhole basin which partly fills with water. Deep pits or wells are sometimes formed by the escape of water through beds of salt, gypsum, or other soluble rock. This is the origin of many of the small lakes in Florida. 150 THE LAND The Relation of Lakes to Rainfall and Drainage. — The existence of a lake in any basin depends upon the amount of rainfall, which must exceed the amount of water re- moved by percolation and evaporation. In arid regions the rainfall is generally insufficient to fill the basins to overflowing, the minerals brought in by tributary streams accumulate, and the water becomes salt or alkaline. Such lakes may dry up or fill with mineral deposits, leaving thick beds of salt, soda, borax, gypsum, or tufa ; but on account of the absence of outlet streams which would cut down the rim, such basins are relatively permanent. In regions of abundant rainfall, lakes regulate the flow of out- let streams, preventing floods. They also act as settling basins for sediment, so that a stream flowing out of a lake is usually clear. Of all features of the landscape, lakes are the most ephemeral. A combination of agencies is at work toward their speedy destruction. The inflowing streams are fill- ing them with sediment and minerals deposited from solution, while the outflowing streams are cutting deeper channels through the retaining ' barriers. In the case of small, shallow lakes the growth of vegetation is one of the most efficient agents of destruction. Aquatic plants find anchorage and rich soil in the lake bottom, while they absorb the greater bulk of their food from the atmosphere. They grow and decay year after year, and the lake becomes filled with vegetable matter. Thus it is gradually converted into a peat bog or a muck meadow. A lake may be buried by the accumulation of vegetable matter which floats upon its surface. A large majority of lakes occur in glaciated plains, or in rugged mountain regions ; that is, upon land surfaces which have not been long ex- posed to atmospheric agencies. They are characteristic of LAKES AND LAKE BASINS 151 the youthful stages in the development of relief. The lakes of arid regions are no exception to this rule, because erosion and deposition go on there with extreme slowness. Realistic Exercises. — The student should make a list of all the agents and processes concerned in the formation of lake basins and classify the basins according to their origin. The topics to be considered in summary and review are: the most efficient agents in basin forma- tion, the relation of lakes to climate, to rivers, and to the development of relief, and the general absence of lakes from old mountain regions like the southern Appalachians, and from extensive plains like southern Russia and southeastern United States. The formation of basins and some of the characteristics of lakes may be studied in any locality, at least on a small scale. Any lake, pond, or pool exhibits some of the phenomena and processes peculiar to the life history of a lake, just as a small brook has many of the characteristics of a large river. Let the student investigate the origin of the basin, whether it be by excavation or damming or both. Inflowing streams are filling the basin with sediment and, perhaps, building deltas at their mouths. The outlet stream is usually clear, but may be cutting its channel deeper and lowering the water level. This lowering may be hastened by an artificial ditch. Low, level, and marshy land about the borders of the lake shows the former extent of the basin and the amount of Fi § "5- ~ Shore lines on S ravel bank filling which has taken place. Old beaches or shore lines may be found at some distance from the water's edge. The growth of vegetation may be noted, and the formation of peat around the shores. A tem- porary pool or puddle of water formed during wet weather and disap- pearing in a few days often furnishes an opportunity for the study of shore lines at successively lower levels, either cut into the face of a little cliff or built up where the shore is shelving. When it dries up, the fine mud brought in by feeding streams is left as a coating on the bot- tom and hardens into a genuine ftlaya. CHAPTER XII THE DEVELOPMENT OF DRAINAGE SYSTEMS The Life History of a River. — By the action of running water the face of the land is being carved into ever-chan- ging patterns of relief. While this work is going on, the streams themselves undergo a parallel series of changes. Every stream system has its life history, during which it develops from a stage of youthfulness, when it has just begun the task before it, through maturity toward old age, when its possible work upon the land has been accom- plished. From a study of existing streams we may picture to ourselves an ideal river which passes through this series of changes without accident or interruption. For the simplest case, suppose a considerable area of the earth-crust to be slowly elevated above the sea. Let it be composed of rock strata originally in a nearly hori- zontal position ; but suppose the strata, while in the process of elevation, to be somewhat crumpled and folded, forming a long ridge from which the surface slopes toward the sea in opposite directions. The result of this movement is a coastal plain, rising gradually into a plateau and then more steeply to a dividing ridge. The surface is slightly irregular, diversified by broad, shallow basins or elongated depressions, with broad, flat divides. This newborn land is exposed to a temperate climate and a moderate rainfall. Weather and running water be- gin their work at once. The run-off is at first in sheets rather than in streams. Each basin is filled until the water runs over into the next J 52 THE DEVELOPMENT OF DRAINAGE SYSTEMS 153 lower one, and each long depression transmits a shallow flood until continuous lines of waterway are established from the high land to the sea. A waterway whose course is determined by the original irregularities of the surface of its basin, is called a consequent stream. The waters charged with sediment begin to corrade the surface over which they flow, and soon engrave it with delicate channel lines. The valleys contain numerous lakes, but extend in the general direction of the steepest slopes to the sea. Each stream has its steepest slope near the divide, but its volume there is small because it drains a small area. Near the sea it has a large volume but a gentle slope. There- fore the middle portion, having the requisite volume and swiftness, intrenches itself most rapidly. At first each drainage line is an almost limbless trunk, but as it sinks its channel deeper into the earth-crust, the lakes are drained and lateral branches appear which traverse the intervals between the main streams. As the main stream deepens its channel its branches are given a steeper slope, their currents are quickened, and they develop other branches as the main stream has done. Thus the drainage system grows by extending its branches upward and outward like a tree, until their tips reach the crest of the main ridge and interlock with the tips of the branches of the next system on either side. Water falling upon any portion of this land finds a system of continuous channels by which it returns to the sea, and the drainage is complete. In the middle portion of a river, where corrasion is most rapid, a larger number of strata are cut through, some of which prove to be harder and some softer : consequently rapids and cataracts appear which retreat upstream, leav- ing gorges below them. On account of the disturbed and DR. PHYS. GEOG. — IO 154 THE LAND crumpled condition of the rock strata in the. highest ridge, alternations of hard and soft layers are frequent, and the head waters present a series of cascades which persist a long time because the streams are too small to wear the strata away. In the lower reaches of a river the valley is soon cut down to base level, where the slope is gentle and the cur- rent too slow to carry the full load of sediment it receives. Deposition occurs and downward corrasion ceases. The stream begins to swing from side to side, to undermine its bluffs, and thus to widen its valley. Floods are frequent, and spread over the wide valley floor their successive layers of sand and mud. Thus a wide flood plain is built up and becomes characteristic of this part of the river's course. Meanwhile the river may be depositing a delta at its mouth and pushing it out to sea, and thus building a clam of sediment. The base- leveled and flood-plain condition gradually extends itself up the main stream and thence up the larger tributaries in succession, until at length corrasion and valley deepening continue only in the torrential head waters and in the middle portion, where it is less vigorous than at first. The head-water streams are small in volume, and the load they carry is the coarsest, but they are able to move it because of their steep slopes. In the middle course the load of sediment gathered by the tributary streams is large, but in its descent from the upper valleys it has been ground finer. The volume of water is larger and its flow is sufficiently swift to enable it to carry its greater load. In the lower course the load is still larger and finer, but the volume of water is pro- portionately great, and through all its writhings and shiftings the river staggers on, dropping sediment here and now, and picking it up again there and hereafter, but in the long run getting it delivered finally to the sea. A river which has acquired a perfect adjustment be- tween its volume, slope, and load is said to be a mature, graded stream. It is a condition toward which all streams are tending, but which few ever reach. By their degree THE DEVELOPMENT OF DRAINAGE SYSTEMS 155 of approach to it, not by years, is their age reckoned. A man at twenty years is young, but a horse of twenty is old, not in time, but in stage of development. If a stream system should ever reach base level through- out the extent of its trunk and principal branches, and should reduce its basin to a plain faintly sloping from low, indefinite divides to wide flood plains, it would have reached a condition of old age. The Development of Valleys. — By downward corrasion a stream cuts a steep-sided trench of its own width, but weathering, gravitation, and the wash of the rain widen it and make the sides sloping. The form of the valley in cross section varies with the stage of development, and with the material into which it is cut. A very young val- ley is a simple V-shaped d o__jb a b c a groove, as may be seen in any hillside gully. As time goes on, it becomes deeper and more flaring, as shown in b and c, Fig. 116. When base level is reached, and the valley passes into a flood-plain condition, the form is radically changed, as in d. If the walls of the valley contain strata of BASE LEVEL Fig. 116. c b a b c unequal hardness, the form is modified by the projection of hard layers and the retreat of the softer ones, as 117. If the strata are Fig. 117. shown in Fig. not horizontal, various unsymmetrical forms are produced, as in Fig. 118. The Development of Divides and Profiles. — The divides and ridges between stream valleys pass through a corre- i 5 6 THE LAND sponding series of changes. They are at first broad and flat or gently rounded, as a— a, Fig. 119. As the valleys widen, the interstream h c ch , 7 d c b a r-~7fi -\-;t— - « b c d_ ridges grow narrower and sharper and are finally lowered. A slope made ba s £ level irregular by weathering is Flg - " 9 - steeper in hard material and more gentle in soft, and the tendency of water running over it is to wear away the pro- cb a jecting corners, where the flow is swiftest, more rapidly than the reentrant angles, where the flow is slowest. The tendency also is to leave only the coarse sediment near the top of the slope and to spread the finer far Fl «- I2 °- out from its foot (Fig. 120). It follows from these conditions that slopes produced by weathering tend to be irregular, that a perfectly graded slope grows steeper toward the top, and that its pro- file is a curve concave upward, with the greatest curvature at the upper end. This is called the curve of corrasion or stream Fig. 121. Curves of corrasion in a stream and its eWSWtl (Fig. I2l). tributaries. A graded slope is usually flattened at the top, where the rivulets run only while it rains and are too feeble to corrade. The curve of ram-wash is convex upward. The relative ex- tent of these two curves depends mainly upon elevation. On high plateaus and mountains the curves are concave to the very top and the ridges are sharp and angular (ABC, Fig. 122). On hills and plains the curves are convex, and the elevations are broad and rounded (BD, Fig. 122). Combinations of the two curves exist in all proportions. The progress of erosioii tends toward the final flattening of all slopes. Fig. 122. THE DEVELOPMENT OF DRAINAGE SYSTEMS 157 The Migration of Divides. — During the development of drainage systems a struggle goes on between adjacent trunk streams for possession of the territory. Some rivers, by reason of larger volume, steeper slope, or softer mate- rials to work in, are able to extend their branches and head waters more rapidly than others, and thus to push back the divides and invade the basins of their neighbors. This may occur gradually by a general widening of the valley of the stronger stream, as shown in Fig. 119, or it may occur rather suddenly by capture or piracy. Fig. 123. The Chattooga River, at the western corner of South Carolina, was formerly the upper part of the Chattahoochee ; but the Savannah had a shorter course to the sea and a more rapid fall. One of its tributaries was able to extend itself until it tapped the Chattahoochee and robbed it of its head waters LM (Fig. 123). The divide was thus shifted from the line AB to the line AC. The Oconee will probably repeat this process in the near future. An elbow or right angle in the general course of a river is frequently an indication that it has thus beheaded one of its neighbors, as at E. i 5 8 THE LAND The St. Joseph River in southern Michigan was originally the upper part of the Kankakee in Indiana. While the edge of the Michigan ice lobe stood along the termi- nal moraine, a large stream now represented by the Dowa- giac drained the ice front and emptied into the Kankakee at the site of South Bend. When the ice withdrew, the Dowa- giac turned aside through a gap in the moraine to the basin of Lake Michigan. A portion of its channel was thus aban- doned by the main stream and left to transmit in a re- versed direction a small tribu- tary. This tributary had a fall of more than three feet to the mile, while the Kankakee had Fig. 124. only one third as much ; and it did not take very long for the more rapid stream to eat back the low divide at its head, and to divert the St. Joseph from the Kankakee and Mississippi to Lake Michigan and the St. Lawrence. The Development of Meanders. — A straight stream is an impossibility in nature. The current through an arti- ficial ditch soon shows a tendency to become crooked. A slight inequality in the firmness of the bank, or an acci- dental obstruction, is sufficient to turn the current to one side, from which it is deflected toward the other, and incipient meanders are established. The course of a stream consequent upon the irregulari- ties of a surface newly raised above the sea or renewed by glacial action is crooked in an irregular manner. As a stream continues its work it develops meanders accord- ing to its conditions. The steeper the slope and the swifter the current, the more direct is the course. As the stream THE DEVELOPMENT OF DRAINAGE SYSTEMS 159 approaches base level the available energy is so reduced that a slight obstruction is sufficient to turn it aside, and it develops in its flood plain the wide curves so character- istic of large rivers (see maps of the Mississippi). These are roughly symmetrical because the material of the flood plain is nearly homogeneous. Subsequent Streams. — As consequent streams deepen their channels they are liable to find differences in the hardness of the rock over which they flow, and are A. Consequent drainage. B. Subsequent drainage, with water and wind gaps. Fig. 125. obliged to adjust themselves to these conditions. Suppose the young consequent streams shown in Fig. 125 A to flow down a moderate slope across which two strata of hard rock extend at right angles to the streams. The strongest stream (b) is able to cut gaps through the hard strata more rapidly than the weaker ones. It extends its branches to the right and left in the softer strata, and finally not only beheads its neighbors, but dismembers them, and adds their fragments to its own system, as 160 THE LAND shown in Fig. 125 B. A stream which has thus adjusted its system to the structure of its basin is called subsequent. The map of such a system looks like a grapevine trained upon a trellis, and it is hence called trellised drainage (Fig. 125 B). Where the main stream crosses the softer strata, weathering and lateral corra- sion are most rapid, and the valley is wide and open ; but its level can not be sunk lower than that of the stream where it cuts through the hard stratum below. Each hard stratum acts as a dam which deter- mines the base level of the stream above. In this dam the river slowly cuts a narrow notch, which, in the progress of erosion, becomes a sluice or gateway through a ridge standing out above the level of the more easily eroded country on either side. Such a gateway is called a water gap (see Fig. 159). The shallower notches made by the other streams before they were dismembered, and now abandoned, remain as wind gaps. Disturbances of Stream Development. — It is very sel- dom, if ever, that a stream is permitted to pass through all the stages of development from youth to old age in a regular and normal manner. The most important inter- ruptions arise from elevation or subsidence of the stream basin and from glaciation. The general effect of elevation is to put new life and vigor into a stream by giving it a steeper slope. It begins over again the work of deepen- ing its valley and extending its head waters. Thus a new set of narrower, deeper, and straighter waterways are cut down into the floors of the old ones, and rocky shelves or terraces are left to mark the old valley floors (see Figs. 51, 52). Subsidence has an effect the reverse of that due to elevation and makes a stream prema- turely old. Its slopes are diminished, its current slackens, the lower portion of its valley is drowned, the middle portion fills up with sedi- ment, and only the head waters are able to continue actively the work of corrasion. Changes of climate which increase or diminish the rain- fall have a corresponding effect upon the volume and force of streams. Glaciation. — Some of the effects of glaciation upon streams have already been noticed. During the existence of an ice sheet nearly all the streams in the glaciated THE DEVELOPMENT OF DRAINAGE SYSTEMS 161 region are temporarily obliterated, and after its disap- pearance their courses are found to be permanently di- verted, or their valleys dammed and choked with drift. Fig. 127. — Cross section of a filled valley. (T, terrace; R, river; F, flood plain.) Fig. 126. — Alluvial terraces. (Mississippi valley, near St. Cloud, Minn.) Many of the valleys which drained the ice front were flooded with water and half filled with sand and gravel. Since the disappearance of the ice the diminished streams have been at work cleaning out their old valleys, with imperfect success. In most cases, the «o- stream has cut a new and smaller channel through the drift filling, leaving massive alluvial terraces on either side. Summary. — A young stream is one which has accom- plished but little of the work of erosion and degradation of the land which is possible for it to do. It is charac- terized by irregular profile, steep slope, swift current, narrow valley, and numerous rapids, cataracts, and some- times lakes. The Colorado River is an extraordinary ex- ample of a young river. The St. Lawrence, once a mature stream, has been rejuvenated, or restored to infancy, by glaciation. 162 THE LAND A mature stream is one which is well advanced in the work before it. It has graded or nearly graded its valley, and its profile is nearly the ideal curve of corrasion (see Fig. 121 ). Its valley is broad and its lower portion is in the flood-plain condition. The rapids, cataracts, and lakes have disappeared or linger only along the head waters. The Mississippi river system has nearly reached the stage of x maturity. An old stream is one which has reduced its basin to the lowest possible level. It would resemble the lower Mis- sissippi ; but there is probably not a river in the world which has reached that stage throughout. Rivers are sel- dom permitted to reach old age, but are either drowned by the sea or restored to youth by the accidents of upheaval or glaciation. A consequent stream is one whose course is determined by the relief of the surface of its basin. All streams are at first consequent. A subsequent or adjusted stream is one whose course has been modified by the structure of its basin. The relief of its basin is largely the result of its own work. An antecedent stream is one which has maintained its original consequent course in spite of upheavals of the land and the growth of plateaus and mountain ranges in its basin. Some portions at least of the Green-Colorado system are antecedent. A superimposed stream is one whose course has been determined by the relief of some previously existing sur- face, which has been removed or greatly modified by ero- sion. Its course, like that of an antecedent stream, has little or no relation to the present relief. When the con- sequent streams of the glacial drift have swept it away and laid bare the surface of the bed rock beneath, they THE DEVELOPMENT OF DRAINAGE SYSTEMS 163 will become superimposed. Some portions of the Green- Colorado may have been superimposed from a surface which has since been removed by degradation. The Susquehanna, Delaware, and Potomac are superimposed streams (see pp. 185, 186). Streams as Factors in Human Life. — Drainage. — The natural function of streams is drainage. They carry away the surplus water supplied by rainfall, in excess of that which is evaporated. Most of the ground-water finally reaches the streams. Drainage is most rapid and complete where the slopes are steep, the surface compact, and vege- tation scanty. In mountainous regions generally these conditions prevail in a high degree, the rainfall runs off quickly, and the streams are subject to great variations in volume, being flooded during a storm, and nearly dry in clear weather. The removal of forests and the cultivation of the ground render drainage more rapid and complete. More rapid run-off carries away greater loads of mantle rock, and hastens the process of stream erosion. In some cases, the destruction of forests has resulted in washing away the soil and leaving the surface barren and worth- less. On the other hand, slow and imperfect drainage may be as objectionable as too rapid drainage. A region which is imperfectly drained abounds in marshes, ponds, and lakes, which support a vegetable and animal life pe- culiar to themselves. Such regions are comparatively unfavorable for human occupation until their drainage is artificially improved. The cutting of ditches and the laying of underdrains may remove the stagnant water, and render the rich accumulations of humus, or decayed vegetable matter, available for the growth of agricultural plants. Some of the best areas for farming and gardening have been obtained in this way. 1 64 THE LAND Flood plains. — The most productive lands in the world are flood plains. At every period of high water, a stream brings down mantle rock from the higher grounds, and deposits it as a layer of fine sediment over its flood plain. A soil thus frequently enriched and renewed is literally inexhaustible. In a rough, hilly, or mountainous country, the finest farms and the densest population are found on the "bottom lands" along the streams. The flood plain most famous in history is that of the river Nile in Egypt. For a distance of 1500 miles above its mouth this river flows through a rainless desert, and has no tributary. The heavy spring rains which fall upon the highlands about its sources produce in summer a rise of the water, which overflows the valley on either side. Thus the lower Nile valley became one of the earliest centers of civilization, and has supported a dense population for 7000 years. The conditions in Mesopotamia, along the Tigris and Euphrates rivers, are similar to those along the lower Nile, and in ancient times this region was the seat of a civilization perhaps older than that of Egypt. The flood plains of the Ganges in India, and the Hoang in China, are the most extensive in the world, and in modern times the most populous. The alluvial valley of the Mis- sissippi is extremely productive of corn, cotton, and sugar cane. Irrigation. — In all ages the extent and value of flood plains have been increased by artificial means. Dikes or levees are built to regulate the spread and flow of the water and to protect the land from destructive floods. Dams and reservoirs are constructed for the storage of water, which is led by a system of canals and ditches to irrigate large tracts of land which would be otherwise worthless. By means of irrigation, the farmer has control STREAMS AS FACTORS IN HUMAN LIFE 165 of his water supply and is able to get larger returns than are possible where he depends upon the irregular and uncertain rainfall. It is estimated that in the arid regions of western United States there are 150,000 square miles of land which may be made available for agriculture by irrigation. Perhaps in the future the valley of the lower Colorado may become as productive as that of the Nile. Routes of travel and commerce. — Streams are the easiest routes of travel and commerce. In uncivilized countries they are almost the only ones. Explorers take advan- tage of them to penetrate the interior from the sea, and the first settlers follow the same routes. A river usually furnishes from its mouth well up toward its source a smooth, graded highway, upon which a cargo may be transported with much less effort than overland. If obstructions occur in the form of rapids or falls, boat and cargo are carried around them. It is often easy to pass by a short portage or " carry " from one stream system across the divide to another. In a new country, homesteads, towns, and cities spring up first along the streams, and the density of population and the value of land decrease back toward the divides. As the country becomes more thickly settled, wagon roads and railroads are built, and the waterways become less important. But in regions which are not very level the easiest grades in every direction are found along the streams, and the main routes of land travel follow the stream valleys. In traversing a mountainous region, a railroad follows the windings of some river up to the crest of the divide, which it crosses through a pass, or often by a tunnel, and descends the valley of some stream on the other side. If a stream is too shallow or too much obstructed for easy navigation, a canal may be built either around 166 THE LAND the obstructions or through the whole length of the valley. In the most highly developed countries the large rivers still form highways of commerce. Their mouths furnish good harbors for sea-going vessels, and lighter craft can penetrate far inland. The Mississippi is navigable to St. Paul, iooo miles from the sea, and the St. Lawrence with the Great Lakes carries more tons of freight than any other inland route in the world. Water power, etc. — Streams with rapid fall furnish water power available for running machinery. Regions where such streams are numerous and accessible, as in New England, furnish unusual facilities for manufacture, and the country becomes densely populated. A fall or rapid upon .any stream is likely to become the site of a manu- facturing and commercial center. The water power at the Falls of Niagara, now partly utilized, is sufficient to run all the machinery in several of the largest cities on the continent. Thus, by furnishing easy routes for transportation, and cheap power for manufacturing, rivers have determined the principal lines of human travel and migration, and the principal centers of human settlement. Nearly all the large cities of the world, and thousands of smaller ones, owe their location and growth to the advantages furnished by some stream. Streams furnish a supply of fish and other animals val- uable for food or clothing. Uncivilized peoples often depend upon them mainly for support, and in cases like that of the salmon of the Columbia and other streams of the Pacific slope, fish become an important article of com- merce. Streams have trenched their valleys into the crust of the earth and exposed the bed rock, which is thus made accessible to the quarryman. STREAMS AS FACTORS IN HUMAN LIFE 167 Sources of knowledge. — The long and often deep sec- tions cut by streams enable us to observe the structure of the earth-crust, and to read its history. In this respect no other river has done so much for us as the Colorado. In its canyons the layers of stratified rock are exposed down to the granite core of the earth, and we have been able to interpret there the story of the past, and to learn the processes by which the various forms and features of the land have been produced. Beauty of scenery. — Man is largely indebted to streams for the variety and beauty of scenery. Running water itself is attractive to young and old. A landscape without water lacks its chief charm. A child instinctively finds its way to the brook, and the man seeks beside the river the pleasure and recreation which no other place affords. Streams have carved the surface of the land into an end- less variety of beautiful forms, and a land where stream valleys are few or shallow is monotonous and tiresome. The most common as well as the most celebrated beauty of scenery in the world, from the tiny meanders of a meadow brook to the unequaled grandeur of the Colorado canyons, is largely due to the presence and action of streams. CHAPTER XIII FORMS OF SEDIMENTATION Run-off of Mantle Rock. — There is not only a constant run-off of water from the face of the land, but also a run- off of the land itself, and the two are closely connected. A snow field accumulates by additions at the top until an ice sheet is formed which escapes down a valley or spreads over a large part of a continent. A sheet of mantle rock grows by progressively deeper weathering of the bed rock at the bot- tom. If it rests upon a nearly level surface it may remain where it was formed and consti- tute a residual soil ; but if formed upon a slope, it creeps downward. On very steep slopes enormous masses of earth sometimes slip suddenly, form- ing a landslide analogous to an avalanche of snow. Gravity is the chief agent, but it is assisted by the forces of freezing and thawing. The movement is most rapid along the face of a vertical cliff, from which fragments fall by their own weight and form a talus with a slope as steep as the character of the material will permit. Wind Deposits. — The wind is an important factor in the movement of mantle rock, transferring it up grade as well as down. Along the shores of the sea and of large lakes, the winds from the water blow the sand up Fig. 128. —A landslide. (Sawtooth Mountains, Ida.) FORMS OF SEDIMENTATION 169 Fig. 129. — Talus slopes. (Devils Lake, Rocky Mountains.) in wavelike heaps which resemble the sand ripples in bottom of a stream, sweeping it up the long slope the crest and dropping it over the other side. Thus the dime ( Fig. 131), as it is called, travels slowly inland and buries forests and farms which may lie in its way. The most extensive dune systems are found in desert regions, where the sand is con- tinually passing through a se- ries of shifting forms like the waves of the sea. The loess the to Fig. 130. —Sand ripples. (Algerian Sahara.) which occurs so plentifully in connection with the glacial drift p. 124) is thought by some to be, at least in part, a wind deposit. DR. PHYS. GEOG. — 1 1 (see 170 THE LAND Glacial Drift. — The agency of ice in the transportation and removal of mantle rock has already been discussed in detail (see Chapters IX and X). Drift sheets and plains, moraines and drumlins, are forms assumed by mantle rock on its way to the sea when deposited by ice. Fig. 131. —Sand dune on the shore of Lake Michigan. Kames and eskers are similar forms which are produced by the com- bined action of ice and water. When a glacier reaches the sea a portion of its load is floated away by the icebergs and distributed over the sea bottom as they melt. Ten pounds of ice is sufficient to float one pound of rock. Alluvial Deposits. — The movement of mantle rock would be very sluggish indeed if it were not for the fact that with the help of running water the rock assumes a con- dition in which it is virtually liquid. Suspended in water the sediment is transported according to the laws explained in Chapter IV. A slight decrease in the velocity of a stream greatly diminishes its power to carry sediment. The coarsest part of its load is dropped first, and afterwards the finer parts. Running water thus has a remarkable power of assorting materials. If its velocity is gradually and con- tinuously checked there is a continuous deposit of sedi- FORMS OF SEDLMENTATION 171 ment, varying downstream from coarser to finer. If its velocity is suddenly checked, coarse and fine sediments are deposited together without much assorting. As the volume and velocity of a stream vary at any given place, it may deposit there at different times coarse or fine sedi- ment or none at all. Thus alluvial deposits are always characterized by stratification, or division into more or less distinct layers. Each layer is made up of similar frag- ments, and represents a period of continuous deposition. Each division plane represents a pause in deposition, or an abrupt change in the character of the material. Alluvial Cones and Fans. — A mountain stream with a very rapid fall brings down a mass of coarse debris, and at, the foot of the slope, where ^"/f^a^ 'm# ■e Fig. 132.— Alluvial cone. (Near Salt Lake City, Utah.) the current is suddenly checked, deposits it in the form of a steep alluvial cone. Where the sediment is finer it is spread out into a low, broad alluvial fan. Alluvial Plains. — The alluvial plains formed in the lower courses of great rivers have already been described 172 THE LAND (see pp. 74-78). In some cases a diastrophic valley may be- come filled with sediment to a great depth and be converted into an alluvial plain. One of the most extensive alluvial plains in the world is the valley of California, between the Sierra Nevada and the Coast Ranges. It is 400 miles long by 80 miles in width, and has been filled with sediment, brought mostly by the streams from the Sierra, to the depth of two thousand feet. Lake basins which have been filled with sediment form lacustrine plains. Deltas. — A delta differs from an alluvial fan in being a deposit in still water instead of on land. It is generally larger, broader, and flatter than a fan, but the two are not always easily distinguished. Any stream, large or small, flowing into a pond, lake, or sea, may build a delta if the conditions are favorable. Strong waves, tides, and currents in the receiving body are unfavorable conditions, but large rivers are able to accomplish the work in spite of them. One of the largest deltas in the world, that of the Ganges-Brahma- putra, has been built in the Bay of Bengal where the tides rise and fall sixteen feet. Large deltas are but extensions of flood plains, and they grow more rapidly where the water off the mouth of the river is shallow. Their extension is often retarded by the fact that the crust of the earth slowly sinks under the load of sediment, as seems to be the case at the mouth of the Mississippi, where the deposit has acquired a vertical thickness of a thousand feet or more. Coast Shelves. — Through whatever forms mantle rock may pass on its downward way, its final destination is the bottom of the sea. The streams of ice and water dis- charge it into the shallows along the shore, and the waves, tides, and currents help to distribute it over a. wider belt. The coarser material is not carried far beyond the exist- ing shore line, but the very finest may be lodged several FORMS OF SEDIMENTATION 173 hundred miles out. Thus along the coast of every land mass the coast shelf is being built up at a rate which varies with the quantity of sediment and the depth of the water. Deposits from Solution. — Another portion of the material carried from the land remains to be noticed. It is not mantle rock and it plays no part in the construction of the forms just described. Those mineral constituents of the earth-crust which are dissolved by rain water become actually liquid and their run-off is free and rapid. In the Mississippi River the quantity of mineral matter carried in solution forms more than one fourth of the whole amount discharged into the Gulf. It consists chiefly of carbonate and sulphate of lime and common salt. Fig 133. — Alkali plains, Arizona Ground-water is nearly everywhere charged with lime, salt, and other minerals in varying quantities, and Wherever it evaporates from the surface of the earth a saline crust is slowly formed. Thus in dry regions the soil gradually becomes charged with " alkali,"' forming "alkali plains." In lakes which have no outlet streams vast quantities of salts accu- mulate. Beds of rock salt hundreds of feet thick are of frequent occur- 174 THE LAND rence in the earth-crust, and mark the sites of ancient lakes or seas which have dried up. Every -stream delivers to the sea its quota of salts in solution, and sea water would grow indefinitely more salty if it were not for the agencies which bring about a partial deposit of its mineral matter. Foremost among these agencies is animal life. Most of the animals and some plants which live in the sea have a shell or bony skeleton composed of lime which the animal or plant extracts from the water. When the organism dies the skeleton sinks to the bottom, and con- tributes to the formation of a lime deposit. Animals and plants thus convert the dissolved lime into an organic sediment which is subject to the same laws as any other sedi- ment. One third of the sea bottom is covered with a soft gray ooze or mud made up entirely of the shells of minute animals which live in the surface waters. This deposit only needs consolidation to produce rocks resembling the chalk that is abun- dant in England and other parts of the world (see p. 33). Coral Reefs. — The most peculiar and interesting accu- Fig 134. -ooze, magnified. mulations of limestone in the sea are the coral reefs. The rock of coral reefs is chiefly made up of the skeletons of various species of the coral polyp. The individual polyp varies in size from a pinhead to a foot or more in diameter, but most of them are small. In the reef-building species the individuals are not free and separate, but thousands of them are connected to- gether in one head or mass which is attached to the bot- tom and grows by branching out somewhat like a bush. The young polyp is produced by budding from the side of the parent, and remains attached to the parent stem. The combined group of individuals secretes a lime skeleton FORMS OF SEDIMENTATION 175 which forms a foundation upon which new generations build, a head of coral being alive only at the tips of the branches. These animals flourish in warm, clear water where a strong cur- rent brings them plenty of food. They can not live in muddy water, or if exposed to the air, or at depths greater than about 300 feet. The bottom of the sea is traversed by numerous ridges from which vol- , . , Fig. 135. — Branching coral, canic peaks rise nearly to the surface or project above it, forming small islands. The submerged peaks present conditions favorable to the growth and multiplication of myriads of sea animals, whose remains accumu- late until a shoal or bank covered with shallow water is formed. Upon this foundation patches of coral begin to grow upward and to spread out as far as the shallow water extends. As the top of the patch approaches the surface of the sea (Fig. 136, a), the polyps near the center die from „ _ , Sea Level crowding, want of food, occasional exposure to the air, and the deposit of mud and sand, while those near the edges, having plenty of room, food, and clear water, con- tinue to grow and extend outward (Fig. 136, b). The waves break off pieces of the living coral rock and pile them up on the reef until its edge is raised in some places five to fifteen feet above the sea. At the same time the water dissolves out the dead coral rock from the middle (Fig. 136, c). The result is an irregular, broken ring of land surround- Fig. 136. ■■■■■^■■iii Fig. 137. —Section of actual coral reef. 176 THE LAND ing a shallow lagoon or lake of water. These ring-shaped islands, or atolls, are of all sizes, from one to a hundred miles in length, but the Fig. 138. — An atoll. area above water is very small, often consisting of a few islets rising here and there from the submerged reef. A volcanic island often has a fringing reef of coral along its shore. In other cases it is sur- rounded by a barrier reef some distance out from the shore, where the food supply and the absence of mud favor a vigorous growth of coral. The waves are continually breaking up the reef and grinding it into calcareous sand and mud, which fill the spaces between the coral branches. The whole mass rap- idly becomes cemented into solid limestone rock which shows little or no indication of its origin. Thus the tops or slopes of the volcanic peaks become crusted over with limestone known in one case to be at least 1000 feet thick. Fig- !39- — Map of an atoll. Fig. 140. — Part of a coral reef. (Great Barrier Reef, Australia.) (From a photograph loaned by the American Museum of Natural History, New York.) FORMS OF SEDIMENTATION 177 Coral reefs and islands are most numerous in the west- ern Pacific Ocean. They also occur in the northern part of the Indian Ocean and in the western Atlantic. All the extensive coral formations lie in the track of the great equatorial currents (see p. 265), which bring to the growing polyps abundance of warm water and food. They are ac- cumulations of lime which has been extracted from solution in sea water, solidified, and deposited by living plants and animals. Their place in the economy of nature corre- sponds to that of the salt beds and alkali plains on land. Results of Sedimentation. — By these various processes of sedimentation the material washed away from the land is finally deposited on the bottom of the sea, being assorted and laid down in layers which are level or only slightly inclined. Each layer of mud or sand is somewhat irregu- lar in thickness and limited in extent, gradually thinning out to an edge where it overlaps and is overlapped by other layers. By pressure and the cementing action of sea water, probably assisted in the deeply buried layers by the internal heat of the earth, the beds of mud and sand near the shore are gradually consolidated into strata of shale and sandstone, while farther from shore, beyond. the reach of the coarser sediment from the land, limestones are formed by the rain of shells upon the bottom. Thus mantle rock is reconverted into bed rock, the forms destroyed upon land are reconstructed in the sea, and over thousands of square miles the thickness of the earth-crust is increased by the addition of sedimentary rocks in horizontal strata. Realistic Exercise. — The student should return to the study of the streams in his vicinity in the light of Chapters XII and XIII. Nearly all forms of slopes, valleys, divides, cones, fans, alluvial plains, terraces, and deltas may be found ; or they may be made at will by the use of a mound of earth and a sprinkling pot. CHAPTER XIV MOUNTAINS w®jm~-\~ -^szWm^s&k fig. 141. — Folded strata, Maryland. Faulted and Folded Strata. — As shown in the last chapter, sedimentary rocks are always laid down in nearly horizontal strata. The greater part of the land surface is found to be made up of (or underlain by) similar strata which must have been formed originally under similar conditions, and subsequently upheaved to their present elevation (see Figs. 14, 15, 18). Over large areas of plains and plateaus the strata have been lifted bodily upward with little or no displacement of parts or disturbance of their original smooth horizontality. In other regions they have been tilted, folded, and broken into every degree of confusion. A fault is a fracture accompanied by displacement of the strata. It may be accompanied by a bending up or down of the strata. The 178 MOUNTAINS 179 amount of throw or vertical ' displacement is sometimes as much as 20,000 feet. A syncline is a downfold- ing of the strata in the form of a trough, as at a, Fig. 143. An anticline is an up- folding of the strata in the b Fig. 142. — A fault. form of an arch, as at b, Fig. 143. An anticline is sometimes over- thrust, as in Figs. 144 and 145. Compressed folds are a series ofsharply bent synclines and anti- clines in which the connecting: Fig. i44- Fig. 145. limbs are parallel and nearly vertical, as shown in Fig. 146. A fan fold is an anticline which has been pinched at the bottom until it is narrower there than at the top, as shown in Fig. 147. In nature these forms are seldom ,,- .., found complete, but more or less ' / \ extensively eroded. i8o THE LAND Mountains. — Any relatively great elevations of land having steep slopes and a sharp or narrow top are popu- larly regarded as mountains. The term includes features which vary widely in form, dimensions, structure, and origin. A single mountain rarely exists by itself except in the case of volcanic cones (see Chapter XV). A mountain range is a long ridge having two principal slopes and a crest, which may be jagged or level like the ridge- pole of a barn. It may be straight and simple, or, as is more commonly the case, it may be curved or irregular and send out many branching spurs. The important part of the range is not the peaks, which may attract attention, but the broad continuous mass which supports them (Fig. 149). Usually but a small portion of a range can be seen at once or iP^r ^ L4 Fig. 148. An irregular mountain range. (Park Range, Colo.) Fig. 149. — Teton Range, Wyoming. MOUNTAINS ISI shown in a picture ; hence erroneous ideas are apt to be acquired con- cerning their steepness, ruggedness, and confusion. Ranges are often found combined in chains, and chains in systems. Mountains are usually portions of the earth- crust which have been not only uplifted but also deformed and dislocated. The special character of the mountains is primarily determined by the nature of the deformation. Block Mountains. — Probably the simplest mountains in existence are those of southern Oregon and northern Cali- fornia and Nevada. This region is traversed by numerous ridges which extend north and south and are separated by barren valleys and playa lakes. They are from 10 to Fig. 150. — Section of block mountains. 100 miles long, 3 to 20 miles wide, and 1000 to 5000 feet high. They have a steep slope on one side, often rising in a sheer precipice to a height of 2000 feet or more, and may be easily recognized as broken blocks of the earth- crust which have been tilted, some one way and some an- other. They resemble the roofs of old-fash- ioned houses with un- equal slopes. The long back slopes B, Fig. 151, are formed by the surfaces of the strata, while the steep slopes E are formed by their broken edges. T is a trough block, forming a " rift valley " by its subsidence. Many of the mountain ranges of the Great Basin are of similar structure ; but most of them are much more complicated. The younger Fig- 151- 182 THE LAND Fig. 152. —Half-buried mountains, Utah. ranges still retain their straight crests and even slopes. The older ranges have been carved by pro- longed erosion into ex- tremely rugged forms. The crests are jagged, the slopes are ridged by spurs and valleys, and the original form has been changed until it is hardly recognizable. The val- leys between are broad plains of sand and gravel washed from the moun- tains. On account of the scanty rainfall there are few permanent streams to carry away the debris, which has accumulated until the ranges are half buried in their own waste. Simply Folded Mountains. — The Uinta Mountains in northeastern Utah extend east and west about 120 miles. The width of the range is forty miles. The crest follows an irregular line north of the center, is cut by a few shallow notches or passes, and rises at some points to a. mile and a quarter above the surrounding plateau. The slopes are broken by parallel spurs, which branch out from the crest and are separated by transverse valleys. The range has been formed out of a single broad arch or flat anticlinal fold, which has been greatly eroded. The structure is slightly complicated by the occurrence of a fault along the north side. From the dip of the strata upon each side, the original height of the arch above the present crest has been calculated, and it appears that a thickness of three and one half miles of rock has been removed. This does not imply that the mountains were ever MOUNTAINS 183 three and one half miles higher than at present : for erosion has gone on at the same time with upheaval. Figure 153 shows in the foreground In the background the Uinta /old is supposed to have re- mained tmeroded, the foreground shows the Uinta Monn ains as they exist Fig 153 the mountains as they are, and in the background the mountains as they would be if the eroded material were restored. The Jura Mountains in France and Switzerland consist of a series of parallel ridges and valleys in which each ridge is an anticline and each valley is a syncline. They are so young that only the topmost lay- ers have been eroded from the arches, and the floors of the troughs have been thinly covered with mantle rock. Fi £- x 54- — Sterogram of Jura Mountains. Complexly Folded Mountains. — Few mountain systems are as simple as those just described. In most cases they owe their existence primarily to extensive foldings and THE LAND faulting which combine to give them a very com- plex structure. Figure 1 5 5 shows the variety of structure which exists in the Appalachian high- land, and the extent to which erosion has modified the original forms in that region. The forms produced by the process of upheaval have been almost entirely destroyed, and the present moun- tains are quite different from the original Appa- lachians. Strata having a thickness of at least five miles have been removed from the highland, and scarcely more than the stumps of the old mountains now remain. At 1, Fig. 155, a series of compressed folds has been worn down to a plain, the surface of which is formed by the edges of the nearly vertical strata. At 2 an anticline has been reduced to a valley, and at 3 a syncline is left standing as a ridge made up of concave strata like a pile of platters. The ridges at 4 are projections of hard strata above the more easily eroded ones on either side. Most of the present ridges are of this character, and their parallel zigzags are shown in Fig. 156. With level crests rising to a nearly uniform height, they stretch across the country like gigantic walls. Adjacent ridges frequently approach each other and unite at a sharp angle, inclosing a valley, the structure of which is shown in Figs. 157, 158. In Fig. 157 the bed of hard sandstone which forms the ridges is continuous under the valley, and its shape re- sembles the prow and bottom of a canoe. The region occupied by the Appalachian valleys and ridges is about seventy-five miles wide, and extends through Pennsylvania, Maryland, and Virginia into Tennessee. It is bounded on- the east by a much older mountain range, the Blue Ridge (B, Fig. 155), and on the west by the eastern escarpment of the Alleghany plateau (A, known as the Alleghany Moun- tains), in which the strata have been but slightly disturbed. The drainage of the Appalachian highland pre- sents many interesting peculiarities. The principal MOUNTAINS I8 5 Fig. 156. — Part of the Appalachian Mountains, in Pennsylvania rivers, the Delaware, Susquehanna, and Potomac, rise in the western plateau and flow southeastward directly across the trend of the ridges, through which they pass by means of water gaps. The tributary streams follow the valleys between the ridges, and with their branches present systems of trel- lised drainage (see p. 160) in great perfec- tion. The main streams are independent of the relief, and flow across the trend of the ridges rather than parallel with it ^S x 57- — Eroded syncline ; canoe valley. Such relations between relief and drainage must be the result of a long period of adjustment. The level sandstone-topped ridges of uni- form height and the softer strata of shale and limestone in the valleys DR. PHYS. GEOG. — 12 THE LAND upon were between suggest the expla- nation. The original Appa- lachian folds were in past ages worn down to a nearly level plain sloping gently to the southeast, across which the large rivers followed the same courses as at present. The plain was then uplifted to the height of the present ridge crests, and as the re- vived streams cut their val- Fig. 158. —Eroded anticline. 1 • . ■. .1 1 6 ° leys into it, they came down the alternations of hard and soft rocks. While the main streams slowly sawing their gaps into the sandstone, the tributaries were Fig- 159. —The Delaware water gap. able to erode wide valleys out of the limestones and shales (see Figs. 157, 158, and 160). By a series of adjustments as explained on page 1 59, all the smaller streams in each valley combined into one system tributary to a master stream which was able to cut its gap down more rapidly than the rest. Thus the trunk streams became superimposed MOUNTAINS 187 upon the new surface and their tributaries maturely adjusted to its structure. Every ridge and valley in the present Appalachian Moun- tains is the product of erosion, but the arrange- ment of hard and soft strata is due to the orig- inal upheaval and fold- ing. The internal forces of the earth furnished a block of peculiar struc- ture, from which air, rain, frost, and running water have carved out the pres- ent peculiar and elaborate pattern of relief. I n many lofty moun- tain ranges the strata have been doubled up, crushed, contort- ed, overturned, and shoved upon one an- other in wild con- fusion. Fig. 160. — Water gaps of the Susquehanna. Figure 161 shows a portion of the typical structure of the Alps. The central core of such ranges usually consists of granite or some allied igneous rock, from which the layers of sedimentary rock which once covered it have been removed. The unstratified granite and the edges of the highly inclined strata have been split by frost into a thou- sand sharp and jagged ridges, peaks, needles, and "horns." Although the peaks, ridges, passes, and valleys are conspicuous and occupy the whole landscape, they are super- ficial features, and form but a small part of the great mass of the range which supports them (compare Fig. 162 with Fig. 149). Fig 161. THE LAND Alpine scenery. Relict Mountains. — In many cases mountains of com- plex structure have been worn down to their roots, and their surface forms bear apparently little relation to the arrangement of the material in their mass. But the peaks and ridges mark the place of harder and more resisting rocks which have been left prominent by the removal of less resisting rocks around them. The mountains of Fig. 163. — Mount Monadnock, in southern New Hampshire. MOUNTAINS 189 southern New England (see Fig. 163) and the Scotch Highlands are examples of this class, which may be called relict mountains. Their rounded smoothness of outline is due to glacial abrasion. Plateau Mountains. — Some massive elevations which have the appearance of mountains and are popularly so called hardly deserve the name. They are really dissected Fig 164 —Dissected plateau. (Scott County, Tenn.) plateaus. The strata have been but slightly if at all dis- located by faulting or folding, but are deeply cut by stream valleys. The interstream ridges are mountainous in size, but the strata of which they are composed remain nearly horizontal, and may be traced from one ridge to another across the valleys. The Catskill Mountains in New York, and the Alleghany plateaus of Pennsylvania and West Virginia, are plateau mountains. 190 THE LAND Summary. — The general term mountains includes a variety of land forms which differ in their structure and origin. They have the common characteristics of large mass, elongated outline, and great elevation. They are the products of two sets of forces, one of which acts within or below the earth-crust to produce elevation, and the other acts on the surface of the crust to produce degradation. In mountains like the Oregon blocks and the Jura the internal forces are supreme and almost wholly responsible for the form. In mountains like the Appalachians internal forces have folded and dislocated the strata, but the present forms are almost wholly due to erosion. Every degree of gradation between these extremes may be found. In every range internal forces have raised the massive block out of which external forces have carved the details of ridge, spur, peak, pass, and valley. It follows that lofty mountains, like the Himalayas, Alps, Rocky, and Sierra Nevada, are lofty because they are young ; and that low, subdued mountains of complex structure, like the Appalachians and the Scotch Highlands, are low and subdued in outline because they are old. Earthquakes. — The elevation, depression, folding, and faulting of the earth-crust show that it is subject to a variety of stresses and strains. When it finally yields to an increasing stress and a displacement suddenly occurs, a violent jar results, which is propagated through the crust, like that which is- painfully felt when a stick bent in the hands suddenly breaks. The speed with which the shock travels is about three miles per second, and it often extends completely through or around the earth. The focus or place where the break or slip occurs may be at a depth of several miles, but the jar travels upward and out- ward in ever-widening circles, diminishing in violence MOUNTAINS 191 as it proceeds. The surface movements thus produced constitute an earthquake, which is most violent at a point directly above the focus. The actual distance through which any given point of the earth's surface is moved sel- dom exceeds a small fraction of an inch, but the velocity of the motion may be so great as to make the jar exceedingly destructive. Great cracks open in the earth, and from them mud and hot water are sometimes expelled. Landslides are started, and streams are dammed or turned out of their courses. Buildings are cracked or thrown down, and cities destroyed with great loss of human life. Some of the most de- structive effects are produced by earth- quakes under the sea, which disturb the water so violently that great waves rise over the shores and sweep every- thing before them. Pi &- 165. — Earthquake crack, Arizona. Earthquakes occur chiefly in regions which are still under- going movements of elevation and folding, and hence are intimately associated with young mountains and volcanoes. They are especially frequent along the borders of the Pacific Ocean, where the slopes of the continental plateau are steepest. In Japan noticeable shocks occur almost daily, and delicate instruments show that the earth-crust is in a continual tremor. Causes of Folding and Faulting. — The phenomena of folding and faulting evidently depend upon deep-seated conditions and forces in the interior of the earth. The thick beds of sedimentary rock which compose the mass 192 THE LAND of great mountain systems or rise high upon their flanks were certainly laid down in a nearly horizontal position upon the bottom of the sea. They have been not only elevated to their present position, but in the process of elevation have been extensively fractured, contorted, folded, and crushed in a manner which indicates that they have been subjected to enormous and prolonged lateral or horizontal pressure. The folded strata which underlie a certain area in Penn- sylvania sixteen miles wide would, if smoothed out to a horizontal sheet, extend ninety- six miles. The folding which has occurred in the Appalachian region has re- duced its original breadth about eighty-eight miles. Folded structure like that of Fig. 154 can be most easily imitated by laying a pile of sheets of rather stiff ....,....-.-.•.•.,•. --...... ••.•..-..-. ■,--■- ■:•:■ paper upon a table and com- pressing them lengthwise. Fault- ing and other important details may be imitated by compressing layers of clay, plaster, or wax by means of a screw. Most of the typical displace- ments and deformations of strata may be easily and naturally accounted for by supposing that the earth-crust has in some way become too large for the centrosphere. Readjust- ment wider compression seems to be the key to most of the problems of mountain structure. Either the crust has grown larger than it was originally or the centrosphere has grown smaller. The theory that the earth has been for ages a cooling and therefore a contracting globe, and that while the crust on account of exposure to the heat of the sun has Fig. 166. —Clay compressed lengthwise. MOUNTAINS 193 ceased to cool and contract the centrosphere continues to do so, is not free from objections and difficulties ; but in the present state of our knowledge it seems to be the most satisfactory explanation yet proposed. Regions like the Great Basin, which are traversed by faults, form exceptions to the rule that the earth-crust is under compression. As shown in Fig. 151, each fault block is wedge-shaped and those with a broad base have gone up while those with a narrow base have gone down. This is equivalent to the insertion of a series of wedges into the crust so as to enlarge it. The tilted blocks of the Great Basin act as if they were floating upon a liquid layer below, like blocks of ice upon water, which may be the actual case. It seems evident that in this region the earth-crust has been subjected to stretching instead of compression ; otherwise the blocks would not have found room to rise or sink between their neighbors. This may be accounted for by supposing that this portion of the crust forms the crown of a broad arch, and in crust fold- ing the crowns of the arches or anticlines are always put upon the stretch. CHAPTER XV VOLCANOES Fig. 167. — Stromboli. Stromboli. — The island of Stromboli rises from the Mediterranean Sea north of Sicily. It is a conical pile of material resembling cinders or the slag of an iron furnace. It is 4 or 5 miles in di- ameter and 3000 feet high. At a point on the steep slope about 1000 feet below the summit there is a cir- cular hole from which a cloud of steam con- tinually escapes as if Fig 168. - crater of stromboli. from a chimney. It 194 VOLCANOES 195 one climbs to a point which commands a view of the hole from above, the steam is seen to issue from cracks in a black crust which forms the bottom of a bowl-shaped hollow or crater. From some of the cracks steam is blown out in puffs with a loud snorting noise like that made by a loco- motive engine. In other cracks a thick semiliquid sub- stance heaves up and down, until finally a great bubble bursts with a rush of steam which carries fragments of the liquid several hundred feet into the air. At night the liquid is seen to be white-hot and the crust glows with a dull red color. Whenever the crust is broken by the burst- ing of a bubble, an incandescent surface is exposed from which the light flashes up on the steam cloud above, as it does when a locomotive fireman opens his furnace door. Stromboli is a volcanic cone and in its crater may be seen in a mild and simple form the essential features of a volcanic eruption. It will be observed that it is not a "burning mountain " ; in fact, there is next to nothing combustible in it. The appearance of flame above the crater is due to the illumination of the steam cloud by the white-hot lava, or melted rock, below. The lava in the crater acts very much like a kettle of mush, porridge, or molasses set over a fire. Steam is formed at the bottom, but can not escape readily on account of the thick, viscid character of the substance. The liquid boils and bubbles, gradually rising until it overflows the edge of the kettle or until a sudden and violent outburst of steam throws it in every direction. The island of Stromboli is entirely made up of material which has thus been expelled from the crater and piled up ,.,„,. , , Fig. 169. —Section of Stromboli; around it. i he base 01 the pile rests upon the sea bottom, where the crater must have at first opened, more than 3000 feet below the surface. The eruptions of Stromboli are sometimes more violent than those just described, but they are always due to the formation of steam in a mass of melted rock and the explosion of the bubbles. 196 THE LAND Fig 170. Monte Somma; Vesuvius in the back- ground. Vesuvius is a conical mountain 4000 feet high, rising from a plain on the shore of the Bay of Naples. The upper part of the cone is half surrounded by a semicircular ridge of nearly equal height called Monte Somma. At the beginning of the Christian era this ridge formed part of. the wall of a complete crater about three miles in diameter, the bottom of which was occupied by a forest. In the year 79 this volcano suddenly burst into violent activity. The explosions blew away the south half of the crater rim, and scattered the material over the country, burying the cities of Pompeii and Herculaneum with mud and ashes. Since that time the present cone has been built up in- side the rim of the old crater. Vesuvius has long periods of rest, or of mild activity in which it resembles Stromboli. Globular masses of steam escape in rapid puffs and form a spreading cloud above the mountain. At the same time hot stones are hurled into the air, and fall back with a rattling sound like that of coal thrown into a cellar. The escaping gases, like the steam from the nozzle of a boiling tea- Fig. 171. Vesuvius and Monte Somma, as seen from Pompeii. VOLCANOES 197 kettle, are at first transparent, but change as they rise into bluish white fleecy clouds, while a peculiar " wash-day " odor is very noticeable. Small streams of liquid lava may be seen flowing down the side of the cone like liquid iron from a furnace. The surface of the lava soon cools and hardens into a stiff scum which becomes wrinkled like the cream on milk which is being poured from the pan. At irregular intervals the eruptions become much more violent. The explosions occur so rapidly as to make a continuous roar, the whole of the neighboring region is shaken, vast volumes of steam mixed with dust rise three or four times as high as the mountain, the cone is split by fissures from which lava streams flow, and the summit seems to sweat fire. Volcanic bombs, or whirling masses of lava, are thrown thousands of feet into the air ; the steam condenses into rain which is made dirty by the dust in Fig - I?2 - ~~ Vesuvius in eruption, April, 1872. the air and, mingling with the sand and stones, produces torrents of mud which overwhelm fields and villages. The eruption may continue for several days, but it gradually subsides, leaving the cone and crater changed in form and dimensions. These examples illustrate the essential feature of every volcanic eruption. It is the escape or expulsion of solid, liquid, and gaseous material from the interior of the earth. The hole or fissure from which the material escapes is the pipe or cliimney, the hollow around its top the crater, and the heap of materials piled around it the volcanic cone, or mountain. Steam forms more than ninety-nine per cent 198 THE LAND of all the gases given off. The lava or liquid portion is simply melted rock, while the dust, often called "ashes," sand (lapilli), and larger stones or " cinders " are portions of lava blown into spray or clots by the explosion of steam. Pumice is a glassy lava which has been puffed up by steam bubbles into a light, spongy substance. Masses of lava having a coarsely cellular structure like bread are called scoria. Most of the volcanoes of the world resemble Vesu- vius in general character, but some are more violent, and others less so. Krakatoa. — The most violent and destructive volcanic eruption of modern times occurred in 1883 from the island of Krakatoa in the Strait of Sunda. During a series of explosions a mass of rock esti- mated at one cubic mile was blown into the air in the course of a few hours. A column of steam and dust rose to a height of seventeen miles and spreading out covered the sky with a black cloud which carried the darkness of midnight for scores of miles around. A rain of dust, sand, and fragments of pumice covered the sea and land. The noise of the explosions resembled that of heavy cannonading, and was heard at many places more than 2000 miles distant. The finest dust blown into the upper air was distributed by air currents all around the earth and did not completely subside for two or three years. Air waves were started which traveled three and a half times around the earth. The sea waves rose on the neighboring shores to a height of fifty feet, destroyed the lives of 35,000 persons besides a vast amount of property, and were felt on the shores of America 12,000 miles distant. About one half the island of Krakatoa was blown away, and in the place where a peak half a mile high had stood, the water is now 1000 feet deep. Hawaiian Volcanoes. — The island of Hawaii is the largest volcanic pile in the world. It is a mass of lava from 70 to 90 miles in diameter, rising from water 15,000 feet deep to a height of 14,000 feet above sea level. There are four principal craters, of which two are now active. The summit of Mauna Loa, one of the highest points of the island, is a flat plain in the midst of which is a pit 3 miles long and nearly 2 miles wide and 1000 feet VOLCANOES 199 deep. This often contains a lake of lava thirty or forty acres in extent. From the surface of the lake columns of lava shoot up like a fountain to the height of several hun- dred feet. The lava seldom overflows the rim of the crater, Fig 173. —Map of Hawaii. but bursts through the side of the mountain at lower levels, spouting high into the air and forming a river of molten rock. After this the level of the lake in the crater subsides. On the eastern slope of Mauna Loa and nearly 10,000 200 THE LAND feet below its summit is another crater called Kilauea. This pit is from two and a half to three and a half miles in diameter. Near its center is a pool or boiling spring of lava. Part of the time this pool is covered with a black crust showing a rim of fiery liquid around its edge. Jets and fountains shoot up here and there, play for a few minutes, and subside. At intervals the whole crust be- comes broken by a network of cracks, each separate piece turns edge down- ward, and sinks, and the pool is left an un- broken expanse of glowing lava. Then the surface of this lake of rock freezes again. The pool is undoubtedly the top of a column of lava which extendsdown- Fig. 174.— Kilauea; lava lake. i ^i i r ward thousands 01 feet. By repeated overflows of such lava pools the vast crater gradually fills nearly to the brim ; then, as the lava is drawn off through some subterranean outlet, its floor subsides until the pit is 1000 feet deep. - Volcanic vents like Mauna Loa and Kilauea may be thought of as springs or wells of liquid rock, the level of which varies with the supply and the facilities for escape. They are called calderas, or caldrons, and are distinguished by their great size, the extreme fluidity of their contents, the absence of violent explosions, and their habit of draining off at lower levels instead of overflowing. The lava streams which flow down the slopes of Mauna Loa are some- times half a mile to three miles wide and attain a length of forty-five VOLCANOES 20I miles. When, as occasionally happens, a stream flows into the sea, the water is made to boil, the lava is shivered into fragments by the sudden cooling, and the air is filled with a fine glassy dust. The flow is at first very rapid and broken by cascades like those of a river, but as the gentler slopes are reached at lower levels, the stream spreads out, cools, stiff- ens, and is checked by its own want of fluidity. In flowing through for- ests it surrounds the trees, and may kill with- out destroying them. Fig 175 — Lava flowing into sea (Lava flow of iSSr, Hawaii. ) Islands of living forest are sometimes left in mid-stream. In highly liquid lavas like those of Hawaii a solid crust often forms over the sur- face while the liquid'm the interior drains away, leaving a long, winding Fig. 176. —Ropy lava, Hawaii. DR. FHYS. GEOG. — I 2 202 THE LAND tunnel or cavern. In other cases the crust breaks up into huge blocks which are carried along like cakes of ice in a river, and when the stream ? 3S> Mt. Shasta, California. Mt. Hood, Oregon. Mauna Loa, Hawaii. . Fujiyama, Japan. Fig 177— Profiles of volcanoes, showing slopes. finally solidifies are left in confused heaps which are difficult and dan- gerous to travel over, if not impassable. The surface resembles that of an ice gorge in a river or of the pack ice piled up by the currents in the polar oceans. If the lava stream is thin, the surface does not break up into cakes, but becomes wrinkled, ropy, and diversified with rounded projections, resembling a tangle of cables (Fig. 176). The slope of a volcanic cone depends upon the nature , ,, „ of the material. If the cone is made V"" i ^ — ■■■> all, as far as it goes. Most ...,\ of the forms that are due to atmospheric forces are char- acterized by relief, elevation, verticality ; most of the forms due to marine forces are characterized by horizontality. Deltas differ from all other coast forms in being the work of land streams, which build them in spite of the opposition of waves, tides, and currents. The work of a river is to convey its load of sediment to the sea and to push forward a delta lobe for each distributary. The work of the sea is to destroy this irregularity in the coast line, to cut off the lobes, and to carry the material to a position of rest. Fig 224. -Island and bar. Fig. 226. — Blocked rivers. COAST FORMS 235 In most cases the sea is stronger than the river, and well-developed deltas are exceptions rather than the rule. A stream which empties into a drowned valley builds a delta at the head of the bay and may in time fill it, thus assisting the sea in. its efforts toward a smooth coast. The Mississippi delta is a typical form in which the river forces are entirely superior to those of the sea (Fig. 40). Many varieties, more or less lobed, rounded, pointed, or straight, may be found be- tween this extreme and the opposite where the river is entirely overcome by the sea. In such cases the river is turned aside and compelled to flow a long distance parallel with the beach before finding an outlet, or its mouth is completely blocked by a straight bar, and the water escapes by percolation through the sand. Effect of Tides. — The general direction of tidal forces is at right angles to the coast line, and therefore at right angles to the alongshore currents which build beaches and bars. The daily flow and ebb tends to scour out bays and inlets, to break through the beach ridges, and to establish runways or channels leading across the foreland, marsh, or coastal plain. The Eastern Coast of the United States. — The coast of North Amer- ica from Yucatan to Nova Scotia exhibits, in great variety of detail, the results of the complex interaction of all the forces which shape the forms of the coast. The old Appalachian highlands have been worn down, and their debris has been spread over a belt 200 to 400 miles wide. Slow elevations and depressions have caused the shore line to swing back and forth across this belt repeatedly. Whatever position the shore line has occupied, the streams have always deposited the bulk of their sediment in the shallow water just off shore. Thus the zone of greatest deposition has swung back and forth with the shore line, and the sediment has been widely distributed. The result is a plain which slopes gently and evenly from the Piedmont plateau to the edge of the coast shelf. At the present time about half of this plain is above water and forms the Atlantic and Gulf coastal plain, which begins at 236 THE LAND New York and widens southward to Texas. The other half is under water and forms the surface of the continental or coast shelf. The present shore line south of Cape Cod is for the greater part of its length double. The inner or mainland coast is indented by numerous bays, those in the north being deeper than those in the south, the result of a moderate depression which has drowned the lower portions of all the river valleys. From one to ten or more miles off the points of the main- land, the outer shore line stretches in the long, smooth, swinging curves of a barrier beach. Be- tween the two lies a belt of lagoons, tidal marshes, and sounds. Along the Texas coast, tidal action is very feeble, and the beach is unbroken in one stretch of 102 miles. Off the great Mississippi delta the beaches are wanting or fragmentary. The form of the Florida coast is modified by the growth of coral reefs (Fig. 230). Along Georgia and South Carolina tidal action is strong, and the beach expands into the " sea islands," broken by many inlets (Fig. 229). The North Car lina coast is bordered by an almost continuous bar which extends in long curves from one cusp to another, fam Fig 227 ^/ |cs t„.i %± Cape Florida '.lief- ^ Fig. 228 Fig. 230. 2 3 8 THE LAND Cape Cod CAPE NANTUCKET SOUND Fig. 231. the most prominent being Capes Fear, Lookout, and Hatteras. At the north end of New Jersey the beach skirts the foot of a sea cliff and projects far into the lower bay of New York, as the spit of Sandy Hook (Fig. 228). The southern shores of Long Island, Rhode Island, Marthas Vineyard, and Nantucket exhibit an almost con- tinuous line of lagoons and beaches. Cape Cod is an eroded headland (Fig. 231) which projects far into the sea, and from which long bars extend like wings to right and left. North of it, the characteristics of a glaciated coast become more prominent as the beaches disap- pear and are succeeded by the strongly contrasted coast of Maine, with its rocky islands, cliffs, and fiords (Figs. 227, 214). Realistic Exercise. — Students who live far inland, and have no opportunity to observe forms on the seacoast, may find most of the coast forms well developed along the shores of the Great Lakes. Even a small pond or temporary pool often presents in miniature the character- istic features of wave and current action. Perhaps the g^f^^ most favorable opportunity for the study of coast forms is furnished by the Fig. 232. Bonneville shore lines. shore lines of extinct lakes, like Bonneville (see p. 136), from which the water has retreated, leaving the bottom exposed to view. A dried-up pool beside the road will often repay careful study. CHAPTER XVIII THE PHYSIOGRAPHIC CYCLE AND THE CLASSIFICATION OF LAND FORMS The present form of the face of the earth is due to the action of two sets of forces. One set, derived from the internal heat of the earth itself, produces movements of de- formation in the earth-crust, or brings about a transference of matter from the interior to the exterior. The other set, derived from the heat of the sun, sets in play the various activities of the atmosphere, which. are chiefly outside the earth-crust. Gravitation is the constant ally and silent part- ner of all. Internal forces determine the larger features, such as ocean basins, continental blocks, mountain ridges, and volcanic domes. External forces modify these by pro- ducing out of them lesser features. One set rough-hews great blocks, which the other set shapes into forms of infinite detail. To one is due the apparently boundless expanse of oceans and continental plains which give an impression of vast sameness and monotony. To the other is due an equally vast variety. One produces an island like Hawaii (see Fig. 173), which the other transforms into an island like Santa Rosa (Fig. 233). The completed work of the one consists of profound deeps and lofty heights, areas of strongly contrasted elevation ; the completed work of the other would be the removal of this contrast, the lowering of elevations, the filling up of depressions, and the reduction of the face of the earth to a graded plain near sea level. Earth heat and sun heat work together, but at cross purposes : one to build up, the other to tear down. 239 240 THE LAND Fig. 233. Yet each of these processes supplements the other. The waste of the mountains and plateaus goes to form beds of sediment which fill basins and valleys, build deltas, and bury coast shelves. Igneous granite and lava are con- verted into clay, sand, gravel, and dissolved salts ; these are consolidated into shale, sandstone, conglomerate, and limestone, and thus the total quantity of fragmental, aque- ous, and stratified rock is ever increasing. These beds ac- cumulate until they attain sufficient thickness, and are in turn upheaved to form again the massive strata of plateaus and mountains. Under the persistent stress of all these forces the materials of the earth-crust pass through a re- current cycle of forms of which the plateau or mountainous elevation and the graded plain near sea level are the ex- treme members. The regular succession of changes is often interrupted and the orderly procession of forms interfered with ; but every square mile of land surface is in some stage of development on the way from a plateau THE PHYSIOGRAPHIC CYCLE 241 •SpU13|SI DIUBOOQ •saSuBJ ucE^unoj/\j ■sXa||BA 01L|doJlSBIQ ■s>po|q peiijj. 's;u9LudjB0S3 ■s;|n^j ■sppj. ;snjL|;jaAo •sp|oj. passgjdujo^ •sppj ubj •sauipuA's 'S8uipi;uy , s9U!pouo/\| ■■e;bj;s pauipu •s? i_j ; ^r 10 : : ; ; ! : 35" 35 9600 10800 12000 ! „ y* : | 33'.S 35.6° ! 34 33 ! 35 '4 "35^, ! ! Fig. 239— Section of the Atlantic Ocean, showing temperatures. floor is overlain by water having a temperature below 40 , while only three per cent of the floor has a temperature always above 6o°. Eighty per cent of all the water in the sea is below 40 , and the average temperature is between 38 and 39 . The sun has been shining upon the sea for many millions of years, and, although its rays do not penetrate deeply, there has been time enough for the heat to reach the bottom by the slow process of conduc- tion. It might also be expected that the internal heat of the earth would do something toward keeping the lower waters warm. The fact that while the temperature of the earth-crust increases down- ward, the temperature of the sea decreases in the same direction, consti- tutes one of the most interesting problems of oceanic geography. The Mediterranean Sea is a large body of deep water (13,000 feet) shut off from the ocean by a barrier at the Strait of Gibraltar which rises to a level of 1200 feet below the surface. The temperature of the Mediter- ranean water falls from 75 at the surface to 55° at a depth of 750 feet, where it ceases to fall and remains constant at 55 all the way to the bottom. The temperature of the Atlantic water outside falls continu- ously from 75 at the surface to 37 at the bottom. The Red Sea has a temperature of 70 from 1200 feet to the bottom at 3600 feet. Here nature has contrived a suggestive experiment for us on a large scale by 254 THE SEA confining a body of deep water and showing that it can be kept warm to the bottom. It is also true of the Gulf of Mexico and other inclosed bodies of water, that their bottom temperature is about the same as that of the bottom of the deepest inlet from the ocean. The lesson plainly is that the low bottom temperatures of the open sea are due to a circu- lation which carries the cold polar waters along the bottom toward the equator, where they rise, become warmed, and evaporate or return toward the poles on the surface. The shallowness of the warm water at the equator is explained by the rise of cold water from the bottom. The escape of any considerable quantity of cold water from the Arctic basin is prevented by the shallowness of the openings, but the basins of the Atlantic, Pacific, and Indian may be regarded as great gulfs opening from the Southern Ocean, which acts as a refrigerator and controls their bottom temperatures, making them lower in the southern than in the northern part of each. The movement is not in the nature of a current, but a creep of the whole lower mass of water so slow that it does not stir the finest sedi- ment, and is discoverable only by its effects upon temperature. It is probably supplied by a sinking of the water at about 50 south latitude, where the cold Antarctic and salt tropi- cal waters meet. Figure 240 shows the effect upon tempera- Fig. 240. . . _ . , ture of a ridge in the ocean bottom. The northward movement of the water is so slow that it is stopped by the barrier, and the bottom temperature on the north side of it is no lower than the temperature at the level of its top. Average Temperature of the Sea at Various Depths 40° 39° 38° -«= 37° 36° Uniformly 36° J \ 35° 34° 33° 32" Depth Temperature 6oo feet 60.7 1,200 " 50.O 3,000 " 40. 1 ° 6,ooo " 36-5 13,200 " 35-2° SEA WATER , 255 Pressure and Density. — The pressure of a liquid is equal in all directions and proportional to its depth. The pressure of sea water one foot deep is 0.445 pound upon every square inch of surface. At the depth of a mile the pressure is 2350 pounds, or the pressure of sea water is more than one ton per square inch per mile of depth. The pressure at a depth of five or six miles would crush a hollow vessel of almost any material if it were not sus- tained by pressure within. Deep-sea thermometers have to be protected from pressure. A sealed glass tube containing air, lowered to a depth of 12,000 feet, was crushed to a fine powder. If water were easily compressible, its density at great depth would be greatly increased by the pressure of the water above. Water is but slightly compressible, so that 140 cubic feet of surface water lowered to a depth of one mile would be compressed to 139 cubic feet, and at a depth of five miles its density would be increased 3! percent. The density of sea water varies also with the temperature, but depends chiefly upon the quantity of salts held in solution. It is greatest in the tropical regions of rapid evapora- tion, and least in the equatorial region of heavy rainfall and the polar regions of melting ice (see map, pp. 256, 257). Density of Water under Various Conditions Pure water at 39. 6° F. . . . . . . .1.00 Surface sea water at 6o° F. . . . . . 1.024 to 1.03 Sea water at a depth of five miles ..... 1.06 Pure water at 212 F. . . . . . ; . .95 Pure ice . . . . . . . . . .92 Sea ice (with included air) .... about -9175 On account of the low temperature and high pressure in the depths of the sea, the density of the water there must be considerably higher than that due to its saltness alone. To ascertain the actual density of the water, the density determined after the water has been raised to the surface must be corrected according to the temperature and pressure which exist at the depth from which it was taken. When this is done, it is found that the water lies in quite regular horizontal layers which increase in density downward. RELATIVE DENSITY OF THE SURFACE WATER,_ REFERENCE ~| Specific Gravity less than 1 .025 " 1.025 to 1.026 » 1.026 to 1.027 " 1.027 to 1.028 " " more than 1.028 'Warm Currents -^-> Cold Currents 120 140 160 180 160 140 ANTA *CTIC CIRCLE 100 80 256 CHAPTER XXI MOVEMENTS OF THE SEA " No drop of the ocean, even at its greatest depth, is ever for one moment at rest." — Wharton. Waves may be produced by any disturbance of the sea surface, but they are usually the result of the friction of the wind. Waves are a series of parallel ridges and hol- lows which follow one another across the surface of the water. The parts of a wave are crest and trough; and the dimensions are length, of the distance from one crest to another, and Jieight, or vertical distance from the bottom of the trough to the top of the crest. Each wave appears to consist of a mass of water moving forward in the direc- tion of the wind, but, except in shallow places, the water moves forward and back and up and down. The wave moves forward by taking in continually new water in front and dropping out water behind. The motion may be imitated by shaking a sheet or strip of cloth up and down ; waves pass through it from end to end, but no portion of the cloth travels lengthwise. A flag blown out from the staff and waving in the wind performs similar vibrations. The movement of the water in a wave is not quite so simple as that of the cloth and is shown in Fig. 241. The line AB represents the surface of a water wave whose length is at, moving in the direction of the long arrow. Suppose the wave length to be divided into eight equal parts, ab, be, etc. If the water were still, the particles 1, 2, 3, etc., would lie directly above the points a, b, c, etc, but each particle is moving in a circular path in the direc- tion shown by the curved arrows. Particle 9 is at the lowest point, 8 258 MOVEMENTS OF THE SEA 259 has moved one eighth of a circle farther than 9, 7 one eighth farther than 8, etc., 5 being half a revolution farther along than 9 and at the highest point. When the wave has advanced one half its length, each particle will have moved through one half a circle, 1 and 9 will be at the crest, and 5 at the bottom of the trough. Thus while the wave advances its whole length each particle of water makes a complete revolution. As shown by the small arrows, the column of water under the trough is moving backward, the column under the middle of the front slope up- - ^ v r- —^ f- I : 1 \ \ 4- a b c d e f g It i Fig. 241. ward, the column under the crest forward, and the column under the middle of the back slope downward ; but the distance moved diminishes with the depth as shown by the small circles. If the water were at rest, the dotted lines ai, &2, etc., would all be vertical and the columns of water inclosed by them would be of uniform width; but in the wave motion the lines sway back and forth, like the stalks of grain in a field when the wind blows, and the columns under the troughs become wider at the top, and the columns under the crests narrower. By the rise of the water the crest is continuously transferred to a point in front of its position at any given moment, and the trough is transferred in the same direction by the fall of the water. The path of a particle is not always circular, but varies with the form of the wave. It is often an ellipse with its long axis inclined or horizontal. The surface of the sea is never still. When the air is calm the influence of previous winds or of distant storms keeps up a long, low undulation called the ground swell. Storm waves sometimes reach a height of 50 feet and a length of 1 500 feet, and travel 60 miles an hour. As waves approach the shore and reach shallow water, there is not water enough to build up the front half to its full 260 THE SEA dimensions, the front slope becomes steeper than the back slope, then perpendicular, and finally overhanging until the crest falls forward, forming a breaker. On shelv- ing shores the waves sometimes rise to a height of ioo feet or more, and the crests, containing many tons of water in rapid forward motion, strike blows which hardly anything can resist. Cliffs are pounded to pieces, light- houses destroyed, and breakwaters built of the heaviest masonry washed away. The pressure sometimes reaches 2000 pounds per square foot. Fig. 242. — A breaker. Tides. — The level of the sea is subject to a regular, periodic rise and fall which is called the tide. It varies in amount at different places. On the deep, open ocean it is probably less than one foot. On the coasts of oceanic islands it is not more than six or seven feet, while at the heads of funnel-shaped inlets, like the Bay of Fundy, it amounts to as much as fifty feet. If we should watch the tide from any point along the coast at low water, we should see the rocks, bars, and portions of the beach and sea bottom laid bare ; then the water would slowly flow or creep up for several hours and cover them. High water would be MOVEMENTS OF THE SEA 26l followed by an ebb or fall, lasting six hours or more. The interval between two periods of high water or low water is twelve hours and twenty-six minutes, but it is not always equally divided be- tween ebb and flow, Fig. 243. — Low tide. the rise being gen- erally more rapid than the fall. The difference of level between high and low g ' 244 ' ~~ lg * e ' water varies not only at different places, but at different times at the same place. These phenomena must have been observed by all peoples who have lived along the shore of the sea, and it must have been noticed at a very early period that the times of high and low water have some relation to the position and phases of the moon. The connection between the moon and the tides was not understood, however, until Newton's discovery of the law of gravitation. If the earth were a globe of water, it is easy to under- stand how the attraction of the moon would draw it out .of shape and produce a slight elongation or bulging in the direction of the moon. The effect upon the spheroidal shell of sea water is the same as though it were a complete sphere. Realistic Exercise. — Fill a toy balloon with water, but not too full, and fasten a cord to its neck. A gentle pull upon the cord will cause the sphere to become elongated. Swing the balloon around in a hori- zontal plane and the long axis will become horizontal. To produce this effect there must be two forces acting in opposite directions. We commonly think of the moon as revolving around the 262 THE SEA earth, but the exact truth is that the earth and moon revolve together around their common center of gravity. If the earth and moon were connected by a rigid bar without weight, but strong enough to support both, the point in the bar where they would balance would be the center of gravity. Although the bar would be 240,000 miles long, the earth is so much heavier than the moon that the center of gravity falls within the mass of the earth about 3000 miles from the center. The earth and moon revolve around this point once in about twenty-eight days, if it were not for the centrifugal force generated by this revolution, the mutual attraction of the two would cause them to fall together, and their distance from each other is determined by the exact balancing of these two forces. At the center of the earth the balance is perfect, but on the side of the earth which is nearest to the moon attraction is a little stronger than at the center, and causes the water to bulge slightly toward the moon. On the side of the earth which is farthest away from the moon the attraction is a little weaker than at the center, and the unbalanced centrifugal force causes the water to bulge slightly away from the moon. In Fig. 245 G is the common center around which the earth and moon revolve. The arrows indicate by their length the L L Fig. 245. relative values of the two forces at the places in question. Thus there are at any given moment two places of high water, H, on opposite sides of the earth and two places of low water, Z, between them. If the moon were always above the same point on the earth, there would always be high water at that point, the moon would cause no change in the level of the sea any- where, and, consequently, there would be no lunar tides ; MOVEMENTS OF THE SEA 263 but, as the earth rotates on its axis from west to east, the point directly under the moon and the other points of high and low water travel around the earth from east to west at the same rate as the apparent motion of the moon. Thus every part of the sea has two stages of high water and two of low water within the time between two transits of the moon over any given place (24 hours and 52 minutes). The period is more than twenty-four hours, because the moon is actually moving in its orbit eastward in the same direction as the rotation of the earth, and after one rotation of the earth on its axis, it takes fifty-two minutes for any given point on the earth to overtake the moon. ^FIRSTI QUARTER Fig. 246. The sun also produces tides in the sea in the same man- ner as the moon, but on account of its greater distance the solar tides are much smaller than the lunar. At new moon and full moon the sun, earth, and moon are all in the same straight line, as shown in Fig. 246, and the lunar and solar tides combine to produce a greater rise and fall than usual, called spring tide. At intermediate periods the sun and moon act at right angles to each other and produce a smaller rise and fall than usual, called neap tide. The variations in the moon's path and distance, the form and depth of the ocean basins, the outline of the continents, and the configuration of inlets produce endless variations in the height and period of the tides. The rise and fall are greatest at the heads of open-mouthed bays, where the crest of the tidal wave finds less and less room as it progresses 264 THE SEA toward the head, and least in landlocked seas like the Mediterranean and Gulf of Mexico. The tidal movement in the north Atlantic seems to be a slopping of the water back and forth as in a tilted basin, high water occurring on the west side at the same time as low water on the east. In straits con- necting two bodies of water which receive the tidal wave at independent mouths, as in East River between New York Bay and Long Island Sound, high water does not occur at both ends at the same time, and powerful and sometimes dangerous currents, called races, flow alternately in Fig. 247. — Bore, on the Seine River. opposite directions. Where the tide enters the mouth of a river, the water sometimes piles up into a wave with perpendicular front which travels upstream at high speed, washing away the banks and upsetting or filling boats. Such waves are called bores, and are especially nota- ble in the Amazon, Seine, and Severn rivers. Currents. — The surface waters of the sea are not only- subject to the to-and-fro movement of waves and tides, but also take part in a vast system of circulating currents by which the water is transferred to distant regions, tempera- ture and saltness are partly equalized, and the climate of MOVEMENTS OF THE SEA 265 the land masses is greatly modified. The map on pp. 256, 257 shows the location, direction, and extent of the princi- pal surface currents. The largest members of the system or trunk streams of each ocean basin extend parallel with the equator on each side of it. In the Pacific the North Equatorial current starts from the west coast of Mexico and flows westward about 8000 miles. Its width is 600 miles, its depth about 600 feet, and its velocity averages one mile per hour. The volume of water in motion is several hundred times greater than that carried by the largest river on land. It reaches the continental barrier at the Philippine Islands, where it divides, and the larger branch turns northward and eastward past the Japan Islands, where it is called the Kurosiwo or Japan current. In middle latitudes this current returns eastward across the north Pacific to the coast of North America. A large branch forms a reversed eddy along the coast of Alaska, but the greater part flows south, and, completing the circuit, rejoins the Equa- torial current. The South Equatorial current leaves the west coast of South America and flows westward 4000 miles without interruption. In mid-ocean it encounters the submarine ridges of the southwestern Pacific and sends numerous branches southward. A part of its water finally reaches Aus- tralia and flows south along the eastern coast. All these branches finally join the Antarctic drift, which moves eastward around the globe between 40 and 6o° south latitude, and some portion of the water, fol- lowing the coast of South America northward, completes the circuit. Between the two westward-flowing equatorial currents a small Counter current, made up of branches from either side, flows back eastward and completes an independent circuit. In the Atlantic Ocean the North Equatorial current leaves the coast of Africa and moves westward to the West Indies, where it is joined by a large branch from the South Equatorial current. A portion of the combined streams passes through the Caribbean Sea and Yucatan chan- nel, rounds the western end of Cuba, and emerges from Florida Strait as the Gulf Stream. A larger portion flows between and outside of the West India Islands and joins the Gulf Stream east of Florida and Geor- gia. The combined currents are here seventy-five miles wide with an average velocity of four miles per hour, and sweep the bottom at a depth of 2500 feet. The Gulf Stream follows the coast of the United 266 THE SEA States as far as Cape Hatteras, where it leaves the land and crosses the ocean to the vicinity of the Azores Islands. Here it divides, a part returning southward and completing the circuit, while a greater part, spreading out over the north Atlantic, drifts slowly past the British Isles and Norway, far into the Arctic Ocean. This inflow of water into the Arctic is compensated by a strong outflow along the east coast of Greenland. This current rounds Cape Farewell, sends a large eddy northward to occupy Baffin Bay, and finally, under the name of the Labrador current, fills the space between the coast of North America and the Gulf Stream as far south as Chesapeake Bay, where it disap- pears by subsidence and mixture with the warmer water. In the south Atlantic the Equatorial current flows from the Gulf of Guinea westward to South America, where the sharp angle at Cape St. Roque splits it in two. The northern branch crosses the equator to join the northern circuit, but the southern branch follows the coast of South America nearly to its southern extremity. The water gradually turns eastward and, reenforced by the Antarctic drift, returns to the African coast and passes northward to the Equatorial, completing the circuit. The Equatorial Counter current in the Atlantic is pinched for want of room, but appears in the eastern part as the Guinea current. In the Indian Ocean south of the equator the circuit is similar to that of the other oceans, from Australia to Africa, southward to the Antarc- tic drift and back to Australia. The basin of the Indian Ocean north of the equator is small and obstructed by the peninsula of India. A circuit, however, exists, but with the peculiarity that in summer (May to October) the movement is in the regular direction, westward in mid-ocean and eastward along the Asiatic coast, and in winter (October to May) these directions are reversed. Generalization. — As a general statement, it may be said that the equatorial waters flow westward until they strike the edge of the continental block, where they turn pole- ward, recross the oceans in middle latitudes, and, returning toward the equator, complete the circuit. The principal movement of the water forms a great eddy on each side of the equator, turning in the northern hemisphere in the same direction as the hands of a clock (clockwise), in the MOVEMENTS OF THE SEA 267 southern hemisphere in the opposite direction (counter- clockwise). In the north Pacific the principal eddy throws off a smaller reverse eddy along the Alaskan coast. In the north Atlantic the Gulf Stream drift into the Arctic and the return by the Greenland-Labrador current form a reverse eddy as large and important as the primary one. In the Pacific and Atlantic Counter currents form small reverse eddies between the Equatorials. In the southern hemisphere the open water permits an Antarctic drift, 600 to 1000 miles wide, to encircle the globe, into which the southern extremity of each continent projects and inter- cepts a portion of the stream. CURRENTS OFTHE GREAT LAKES i PENNSYLVANIA Cleveland Fig. 248. Cause of Currents. — It is well known that the wind blow- ing steadily for a considerable time is able to start surface currents in the same direction. This is demonstrated in the currents of the Great Lakes, where there are no differ- 268 THE SEA ences of temperature or saltness. It is also demonstrated by the currents of the Indian Ocean north of the equator, where the currents reverse their direction twice a year about one month after a similar change in the monsoon winds. It has been calculated that the force of the trade winds is sufficient in 100,000 years to set water in motion to the depth of 12,000 feet. If the map of the ocean cur- rents, pp. 256, 257, is compared with the map of the pre- vailing winds, p. 305, the correspondence is at once evident. The equatorial currents have the same position and direc- ,V\\ \ f ^ C/vV Fig. 249. Fig. 250. Currents in pans of water, set up by blasts of air. tion as the trade winds, the eastward-flowing currents are in the same latitudes as the prevailing westerly winds, and the centers of the great oceanic eddies are coincident with the tropic calms, while the counter currents flow eastward in the belt of equatorial calms. The Gulf Stream - Green- land eddy follows the same course as the southwesterly and northeasterly winds which circulate around a center south of Greenland. The correspondence is so nearly complete as to warrant the conclusion that the principal cause of the surface currents is the prevailing winds. Realistic Exercise. — Sprinkle sawdust upon the surface of a pan or tank of water, and with a bellows, foot blower, or the mouth, blow MOVEMENTS OF THE SEA 269 through a tube a current of air parallel with the surface of the water. At the place where the air current strikes the water there will be a slight depression of the surface, the water will flow in toward it from both sides, and a current will be set up which will move to the farther side of the pan, parallel with the blast of air, and returning will form a com- plete circuit on each side. An Englishman, Mr. Clayden, has made and exhibited a model of the Atlantic Ocean, over the water of which blasts of air are blown in the place and direction of the prevailing winds. The result is a repro- duction of the actual surface circulation, not only in its main features, but even in details like the turn of the Greenland current around Cape Farewell and the Baffin Bay eddy. It has been maintained that differences of temperature and saltness might be causes of ocean currents, and it is probable that they do exercise some influence upon the movement, but it is impossible to determine just what that influence is. They play a subordinate part, and their ef- fects are masked and overcome by other forces. As accu- rate knowledge of the sea increases, it becomes more and more evident that the happy guess made by Benjamin Franklin more than a century ago is the true explanation, and that nearly every detail of the circulation of surface waters in the sea may be satisfactorily accounted for by the force of the winds and the effect of land barriers. Effects of Ocean Currents. — Surface currents carry their own temperature into the regions which they penetrate, and, by imparting it to the air above, modify the climate of neighboring land masses over which this air moves. Be- tween the tropics the western part of each ocean is flooded with water which has made a long journey under a nearly vertical sun and has become heated to a high temperature ; hence the belt of water above yo° F. is widest and deepest along the eastern coasts of the continents. The east sides of the oceans in the same latitudes are supplied with water 270 THE SEA by currents coming from higher latitudes and also by water which rises from below in place of the water blown away by the trade winds ; hence the west coasts of the con- tinents are washed by water of comparatively low temper- ature. This contrast is plainly seen on the opposite sides of Africa and South America (see map, p. 253). In middle and high latitudes of the northern hemisphere, west coasts are washed by warm currents and east coasts by cold currents. The most extreme case of this kind is presented by the north Atlan- tic, where on the east side the isotherms are carried far northward by the Gulf Stream, and on the west side far southward by the Labrador cur- rent (see map, p. 253). Thus the ocean currents cooperate with the prevailing winds to make northwestern Europe habitable and to keep its harbors free from ice almost to the Arctic Circle, while Greenland and Labrador in the same latitude are ice-bound, snow-covered, and desolate. The same contrast exists in a less degree on opposite sides of the north Pacific. Sea Ice. — The freezing point of sea water varies with its saltness from 32 to 26 . In the Arctic Ocean the ice forms every winter to a thickness of ten or fifteen feet. Thzfloe ox pack thus formed is broken up by tides and storms, and driven about by winds and currents. The pieces are forced against one another and the shore, and piled up in extreme confusion. The surface becomes so rough that it is almost impossible to travel over it. Lieutenant Markham's party north of Greenland was able to accomplish only seventy miles in forty days. Nansen's ship, the Fram, was able to make way through the ice by blasting it with dynamite. The largest masses are called flocbergs. The shores of Greenland are crowded with both floebergs and icebergs (Fig. 86) derived from the glacial tongues of the ice cap (see p. 120). Both the sea and the land ice are carried far southward by the Labra- THE SEA AND MAN 271 dor current, and the largest bergs finally melt in the Gulf Stream. In the winter of 1 872-1 873 a part of the crew of the ship Polaris drifted on an ice floe from the north of Baffin Bay to the coast of southern Labrador, nearly 2000 miles, in six and a half months. A still larger quantity of ice is supplied by the Antarctic ice cap, from which bergs 200 to 500 feet above water and sometimes several miles in length are continually be- ing discharged. The Antarctic bergs are flat-topped and steep-sided (Fig. 8y), and exhibit a stratified structure of alternate blue and white layers, which probably represent the snow-fall of successive summers and winters. As only i 1 1 -i An ,- ^ I B# Fig. 251. about one tenth of the mass of ice stands above water, the total thickness of these bergs must be from 2000 to 5000 feet. As the form of an iceberg changes by irregular melting, it may turn over several times, always assuming a position in which its center of gravity is as high as possible. A berg having the form A (Fig. 251), unless weighted at the bottom, would take the position B, and C would turn upside down like D. Icebergs often transport considerable quan- tities of stones and dirt which have fallen from shore cliffs or have been dragged from the land. As the ice melts, these are distributed over the sea bottom far from their place of origin. The Sea and Man. — The sea is the original source of the moisture in the air. The vapor, borne by the winds DR. PHYS. GEOG. — 1 7 272 THE SEA over the land, falls as rain, supplying water for the use of plants and animals, and the flow of streams. The most productive lands are those which are near the sea or are accessible by winds from the sea. The sea teems with living forms, many species of which, like the codfish, mackerel, herring, and oyster, form staple articles of food. The whale furnishes oil and the fur seal furnishes fur, both of such value that these animals have been nearly exterminated. From the sea are obtained sponges, corals, and pearls of great commercial value. Large numbers of people seek the sea for health and pleasure. A sea voyage is a favorite method of travel, and the most popular resorts are those which afford sea air and sea bathing. The sea was once regarded with dread and terror as being dangerous and destructive. It is often thought of as a barren, unproductive "waste of waters." To many peo- ples it has been an impassable barrier to migration. At first men crept timidly in small boats along the shore ; but gradually they gained courage to venture out of sight of land and, guided by the stars, to find their way across the waters to distant countries. The most progressive peoples now use the sea as a means of communication and trade. Large vessels traverse it in all directions, carrying the products of every land to every other land. Civilized man has changed the sea from a barrier to a broad, easy high- way of commerce. Many of the great cities of the world are great because they are seaports. The most prosperous and enlightened countries have a long seacoast. Russia loses no opportunity to secure ports upon the Pacific and Atlantic, and Great Britain has gained her high place among nations through her control of the sea. BOOK IV. THE ATMOSPHERE CHAPTER XXII THE AIR Composition. — The atmosphere, or gaseous portion of the earth, forms a complete spheroidal shell which sur- rounds the solid and liquid globe, and not only rests upon the surface of land and sea, but also penetrates them to a great depth. Its thickness, which is not definitely known, is certainly several hundred miles and may be many thou- sand. Its bulk is almost entirely made up of five gases, which are present in the proportions given in the following table : — Composition of the Air Per cent of Volume Density Nitrogen Oxygen Water vapor (average) . Argon Carbon dioxide (average) Air 76.95 20.61 1.40 1. 00 0.03 99.99 .971 1. 105 .624 1.380 1.529 1. 000 These gases are not united or combined in any way, but are almost entirely independent of one another. They act like five separate and distinct atmospheres occupying the same space at the same time. The space which each gas occupies is determined by the balance between its own 273 274 THE ATMOSPHERE expansive force, tending to make it expand indefinitely, and gravitation, which holds it down to the earth. Car- bon dioxide, being the heaviest of all these gases, does not extend so far upward as the others. Oxygen is a little heavier than nitrogen, and its relative proportion decreases slightly in the upper air. Water vapor is the lightest of all, but its existence as vapor is so far depend- ent upon a warm temperature that it is almost absent at great heights. Properties and Functions. — Oxygen combines freely with nearly all the elements, and in its numerous com- pounds forms about one half of the whole weight of the globe. By the process of respiration it supports the life of all plants and animals, and it is the universal agent of combustion. By respiration, combustion, decay, and other processes of oxidation the quantity of oxygen in the air is being continually diminished. This loss is partly compen- sated by the oxygen set free from plants in the process of food manufacture. Nitrogen is extremely inert and enters into combination with other elements with difficulty. To it is due nearly three fourths of the pressure and density of the air. Without it birds could not fly, clouds and smoke would settle to the ground, and the force of the wind would be proportionately diminished. Argon resembles nitrogen, with which it was confounded until near the end of the nineteenth century. Carbon dioxide (C0 2 ), or carbonic acid gas, is a com- pound of carbon and oxygen formed in the active growing parts of plants and in the tissues of all animals and given off by them in the process of respiration. It is also pro- duced by the combustion of all the ordinary forms of fuel, and sometimes escapes in large quantities from active vol- THE AIR 275 canoes, old volcanic regions, and from many mineral springs. It forms the chief food supply of plants. The green parts of plants in the sunlight absorb carbon dioxide, separate it into its elements, retain the carbon, and give off the oxygen. Carbon dioxide plays an active part in rock formation, entering into combination with lime and other bases to form limestones. It also enters largely into the composition of the bones and shells of animals. While the absolute quantity of carbon dioxide is the least of all the principal constituents of the air, the part it plays in the economy of nature is second to none. Water vapor is supplied by evaporation from all damp surfaces, but chiefly from the sea. When cooled it condenses again into water and falls as rain and snow. The quan- tity present in the air at different times and places is very variable, amounting sometimes to three per cent. Visibility of Air. — The air is sometimes visible. When thrown into agitation by heat it may be seen rising from a stove or from the heated ground. Under proper conditions of illumination and background the wind may be seen as plainly as a current of water. Weight and Pressure. — At sea level a cubic foot of air weighs about one ounce and a quarter, and the weight of all the air above sea level produces an average pressure of 14.74 pounds upon every square inch of sur- face. This pressure is equal in all directions, — downwards, upwards, or sidewise at any angle. The pressure of the air is measured by the barometer. If a glass tube about 32 inches long is filled with mercury, inverted, and the open end inserted into a cup y Fig. 252. — Simple forms of barom- eters. 276 THE ATMOSPHERE of mercury, the mercury in the tube will fall until only enough remains to balance the weight of a column of air of the same size extending to the top of the atmosphere. Such an arrangement is essentially a mer- curial barometer. We can not weigh the column of air directly, but we can weigh the column of mercury which balances it. The height of such a column at sea level averages about 30 inches, and, if one square inch in area of cross section, weighs 14.74 pounds. If the barometer is carried to higher elevations, there will be less air above it, and the mer- cury will fall. If the pressure of the air increases, it will drive more mercury into the tube. The pressure of the air is measured and ex- pressed in terms of the height of the column of mercury which it sup- ports. When the air pressure is said to be 29.50 inches, it means that the air pressure is sufficient to support a column of mercury 29.50 inches high. A description of the standard barometer and instructions for its use are given in the Appendix, pp. 400, 401. Density. — The air being easily compressed, its density is proportional to the pressure to which it is subjected, and consequently diminishes as the height above the sea increases. Density and pressure are also influenced by other conditions, of which temperature and humidity, or quantity of water vapor it contains, are the most impor- tant. When air is heated it expands and becomes less dense. The same effect is produced by the addition of water vapor. On warm, damp days the pressure and density are less, and the barometer stands lower than on cold, dry days. Temperature. — The temperature of the air is deter- mined by the amount of heat received and absorbed from the sun and earth. As the sun heat passes through the air on its way to the earth, about one third of it is absorbed by the air and goes to raise its temperature, while the re- maining two thirds reaches the surface of the land and water. A part of this is reflected back without warming the earth and another part, being absorbed, goes to raise or maintain the temperature of the land and water. The THE AIR 277 Barometer in Inches. Density of air. Height in F A ■■ " - )■ ' HIMALAYA MOUNTAINS Fig. 253. — Decrease of density and amount of air with increase of altitude. warm earth in turn warms the air next to it slightly by conduction and still more by radiating its heat upward. Of the heat reflected and radiated from the earth about 60 per cent is absorbed by the air and goes to raise its temperature still further. The heated air radiates some of its heat back to the earth, and so a continual exchange of heat is going on between the air and the earth ; but on the whole and in the long run as much heat escapes from the earth as it receives. The air takes toll as the heat passes through it both ways, coming and going, and temporarily retains about 70 per cent of the whole amount supplied from the sun'. T/ie lower air absorbs much more heat than the upper air, and consequently is maintained at a higher tempera- ture. This is due largely to the presence of cloud, fog, dust, and smoke, which may be regarded as atmospheric 278 THE ATMOSPHERE sediment held in suspension somewhat as fine mud is suspended in water. The larger proportions of carbon dioxide and water vapor in the lower air also increase its absorptive power for heat. If the air were perfectly clear, dry, and free from carbon dioxide, the heat of the sun would reach the earth with slight obstruction, and in the daytime the land would become excessively heated. In the night the heat would escape with equal rapidity, and the land would become excessively cooled. Upon lofty mountains which reach up through the zone of dust, cloud, and water vapor this is the actual condition. The clouds act as a blanket to protect the earth from the fierce heat of the sun by day and to prevent the escape of heat at night, thus maintaining a more equable temperature. Fig. 254. In Fig. 254 the horizontal line at the bottom represents the surface of the land or water, and the dotted line indicates an elevation of ten miles. The total heat received from the sun is represented by ten arrows, of which three are stopped by the air, and seven reach the earth. The seven arrows pointing upward represent the heat given off from the earth, of which four are stopped by the air and three pass directly through into space. The temperature of the air diminishes on an average one degree for every 300 feet of elevation. The average temperature of the air also diminishes from the equator to the poles at the rate of about one degree for every degree of latitude ; this is due chiefly to the spheroidal form of the earth, which causes the sun's rays to strike more obliquely and to be distributed over more space toward the poles (see p. 22). The dis- tribution of temperature in the atmosphere is subject to these two gen- eral laws, but is made quite irregular by various influences, which will be discussed later. THE AIR 279 The Measurement of Temperature. — Temperature is measured by the thermometer, several varieties of which are described in the Appendix (pp. 398, 399). Weather Observations. — Every student should provide himself with the best thermometer he can afford. Its error may be determined by comparison with a standard thermometer in the laboratory. Compari- sons should be made at two or more temperatures, one at or below freezing, and one near ioo°. The thermometer should be placed in a suitable position at home. The north side of a post at some distance from any building, and four feet from the ground, will answer the purpose. At two periods every day, morning and evening, as between 7 and 8 a.m., and between 7 and 8 p.m., let the student read and record the temperature, observing and recording at the same time the direction of the wind as shown by a vane placed above trees and buildings ; the state of the sky as to clearness or cloudiness ; the fall of rain or snow, and any other notable phenomenon of the weather, as fog, hail, frost, etc. These observations should be continued for at least three months, and, if possible, for a year. The record may be kept in the following form : — Wind Date Hour Temp. _ Remarks and Sky The direction of the wind may be indicated by an arrow flying with the wind as on a weather map, the state of the sky by an open or shaded circle, attached to the arrow ; O means a northwest wind with clear sky ; < Q an east wind with sky half cloudy ; >f a south wind with sky overcast. * CHAPTER XXIII MOISTURE IN THE AIR Evaporation. — Under suitable conditions, evaporation takes place from all damp surfaces and the water vapor mingles with the surrounding air. The heat in the water makes the molecules vibrate and some of those at the sur- face fly off into space. Ice evaporates as well as water, but the higher the temperature, the more rapid is the evapora- tion, and at boiling point molecules escape from all parts of the water, forming bubbles of' steam. At the moment of evaporation, water expands to about 1 700 times its liquid volume and is transformed into an invisible gas or vapor. The quantity of vapor which can exist in any given space de- pends upon the temperature of the vapor. When the space contains all it can hold, the vapor is said to be saturated. Grains of Saturated Water Vapor in a Cubic Foot at Various Temperatures IO° •776 34° 2.279 58 5-37o 82 I I.626 12° .856 36° 2 457 6o° 5-745 84 I2.356 14° .941 38 2 646 62 6.142 86° 13.127 1 6° 1.032 40° 2 849 64° 6.563 88° 13-937 1 8° 1. 128 42° 3 064 66° 7.009 90° 14.790 20° 1-235 44° 3 294 68° 7.480 92° 1*5.689 22° J-355 46° 3 539 70 7.980 94° 16.634 24° 1.483 48 3 800 72° 8.508 96 17.626 26° 1.623 50° 4 076 74° 9.066 98" 18.671 28° 1-773 52° 4 372 76° 9-655 IOO° 19.766 3°° i-935 54° 4 685 78 10.277 I02° 20.917 32° 2. 113 56° 5 016 8o° 10.934 104° 22.125 280 MOISTURE IN THE AIR 281 The quantities given in the table on p. 280 are the same whether the space is a vacuum or is already filled with air or other gases. The air has nothing to do with evaporation except to retard it. Water evapo- rates more rapidly into an absolutely empty space than into dry air, but at a given temperature the same quantity will evaporate into each. When water vapor is added to air, the expansive power of the mixture is in- creased, the surrounding drier air is pushed away, and the whole mass of moist air expands until its density becomes less than that of the drier air. If a cubic foot of dry air at a temperature of 8o° and weighing 516 grains rests upon a body of water and evaporation takes place until 1 1 grains of water vapor is added, the whole mass of the mixture will weigh 527 grains, but a cubic foot of it will weigh only 510 grains, and will be less dense than the original dry air. Humidity. — The quantity of water vapor actually present in space or air is called its absolute humidity. The quan- tity which the space might contain if full or saturated is called its capacity. The ratio of the absolute humidity to the capacity is called the relative humidity. If an eight-ounce bottle contains two ounces of water, its absolute humidity may be said to be two ounces^ its capacity eight ounces, and its relative humidity two eighths or 25 per cent. Absolute humidity answers the question, how much is there in it? capacity answers the question, how much will it hold ? relative humidity answers the question, how full is it? If the relative humidity of air is above 80 per cent, it may be said to be damp air ; if below 50 per cent, dry air. Whether air is dry or damp depends not only upon the quan- tity of moisture which it contains, but also upon its capacity as deter- mined by temperature. Air at 32 containing two grains of water vapor to the cubic foot is very damp because nearly saturated. If heated to 70 , ij; would still contain two grains, but would be very dry because only one fourth saturated. On the other hand, dry air may become damp by cooling without the addition of any moisture. Realistic Exercise. — Fill a bright tin cup half full of water at the temperature of the room, add a few lumps of ice, and stir the mixture with a thermometer. Watch carefully the outer surface of the cup and at the moment it becomes dulled by the formation of dew, read the thermometer. The vapor in contact with the cup has been cooled to saturation and has begun to condense. The temperature at which this 282 THE ATMOSPHERE occurs is called the dew-point. Since vapor at the dew-point is satu- rated, absolute humidity at dew-point equals capacity. By reference to the table on p. 280 the absolute humidity may be found. Suppose the dew-point to be 40 , then the absolute humidity or actual quantity of vapor present is 2.849 grains per cubic foot. If the temperature of the room is jo°, its capacity according to the table is 7.980 grains per cubic foot, and its relative humidity is 2.849 "*" 7-9%°i or 35-7 P er cent. Hygrometer. — A more convenient method of measuring relative humidity is by means of the hygrometer (see Appendix, pp. 401, 402). Condensation. — When water vapor is cooled below the point of saturation, condensation takes place and the vapor changes to fog, cloud, rain, snow, hail, dew, or frost. Cooling in the atmosphere is brought about by several processes. (1) Expansion. — Wherever air rises and reaches successive levels of less pressure, it expands and some of its heat energy is expended in pushing away the surrounding air. Thus without any transfer of heat to other bodies, it is cooled simply by its own expansion one de- gree for every 183 feet of ascent. This is called mechanical cooling and is one of the most efficient causes of condensation. Air at yo° F. and of 50 per cent relative humidity would become saturated by a rise of 4000 feet. As soon as condensation begins, the latent heat of the water vapor is liberated and the cooling by expansion is retarded. Descending air is warmed by compression one degree for every 183 feet of descent. (2) Radiation. — Air is cooled by radiating its heat to cooler objects in the vicinity, as the ground, the sea, a mass of ice or snow, or a body of cooler air. This cause is most efficient in currents of air moving in any direction from warmer to cooler regions. (3) Conduction. — When air comes in actual contact with a cooler body, it loses some of its heat by conduction. This process is com- paratively unimportant because air is a poor conductor of heat, and only a thin layer of it next to the cooler body is affected. (4) Mixture. — Air is often cooled by mixture with cooler air. This is not a different and distinct process, but furnishes favorable conditions for rapid radiation and conduction. Clouds. — The condensation of water vapor in the air near the earth produces fog ; at higher altitudes, cloud. MOISTURE IN THE AIR 283 Clouds are composed of minute particles of liquid water or of ice, a sort of water dust. Unless borne up by a rising current, they settle slowly through the air, but on reaching a stratum of unsaturated air again evaporate. Generally condensation continues above and thus the cloud persists, although continually destroyed and renewed. When vapor is carried horizontally forward by an air current, it may condense at one place and evaporate farther on : thus the cloud appears to be moving forward, but does not ex- tend beyond a certain point. This process is strikingly shown by the " banner cloud " which sometimes hangs for hours from _. a mountain peak, like a nag at- tached to a staff. A current of saturated air chilled by the mountain condenses on the leeward side and evaporates at some distance beyond. This cloud is a temporary form assumed by the vapor as it passes through a certain space. The ever changing forms of clouds are largely _, ._ . due to evaporation and renewal. Cloud Forms. — All the numer- ous cloud forms may be classed under four principal types : — (1) Cumulus clouds are ■i: :% rounded masses like heaps of wool, generally formed at the top of an ascending column of air. Their horizontal base marks the level where saturation is reached, and above this condensation may continue until the cloud is piled up to the height of five miles or more. Cumulus clouds are char- Fig. 256. — Cumulus. acteristic of the equatorial re- gions and of warm summer afternoons elsewhere when the columns of air started upward by the heat of the sun have reached a considerable height. They often result in showers and thunderstorms. 284 THE ATMOSPHERE Fig. 257. — Cirrus (3) Stratus clouds extend in long, horizontal bands or layers and vary in height from 1000 feet to three miles. (4) Nimbus clouds are those from which snow or rain is fall- ing. They may be formed from stratus or cumulus clouds. Many combinations and in- termediate forms occur, of which cirro-stratus, cirro-cumulus, and (2) Cirrus clouds are light and feathery, resembling ostrich plumes, dabs of thin white paint, loose wisps of straw, a cat's tail, and various fan- tastic forms. They are formed at heights of five or more miles and consist of minute ice crystals or snowflakes. Fig- 259. —Nimbus. strato-cumulus are the most common. Precipita- tion. When water vapor condenses into parti- cles so large that the air MOISTURE IN THE AIR 285 can no longer support or evaporate them, a falling or pre- cipitation occurs in the form of rain, snow, or hail. As the particles fall through saturated air, condensation con- tinues, and the drops grow larger up to a certain limit of size, when they break into smaller drops. If the condensa- tion occurs at a temperature below freezing, the vapor crystallizes into snowflakes. Of these there are numerous forms, but all agree in having angles of 6o° between their branches and in being six-pointed or six-sided. Snow or rain may evaporate before reaching the earth. Fig. 260. -Snow crystals, magnified. Hailstones are masses of ice, or of ice and snow, which are some- times as large as hens' eggs or even larger. Their structure is often complicated by alternate layers of snow and ice, showing that they have passed through a variety of atmospheric conditions. The exact method of their formation is not well understood. Measurement of Precipitation. — Rainfall is caught in a metal cylinder called a rain gauge, and its depth is meas- ured in inches (see Appendix, p. 402). Snowfall is de- termined by melting the snow in the gauge and measuring the depth of water produced. On an average ten inches of snow makes one inch of water, but the proportion is very variable. Dew and Frost. — When the temperature of any surface falls below the dew-point of the surrounding air, water 286 THE ATMOSPHERE vapor begins to condense upon it in the form of dew. If the temperature of the surface is below freezing, the vapor crystallizes directly into hoarfrost. The dew does not fall, but is formed at the place where it appears. Frost is not frozen dew any more than snow is frozen rain. Much of the vapor which goes to form dew escapes from the ground or is given off from the surface of growing plants. Dew is heavier on a clear night because the heat is then radiated from the earth more rapidly than on a cloudy night. A tree, board, piece of paper, or cover of any kind acts like cloud and may keep the air beneath from cooling to dew-point. The under side of a board or stone next to the ground may be covered with a heavy dew while the upper side remains dry. This is due to the rise of vapor from the ground. Dew is heavier upon grass than upon bare ground, because of the excess of vapor given off from the grass, and because grass is a better radiator than earth. Dew is heavier in a valley than upon a hill top, because there is more moisture in the ground there to evaporate, and because the cooler and heavier air settles down into the valleys and lifts the warmer air out. A breeze prevents the formation of dew by keeping the air near the ground stirred up and mixed with the drier and warmer air above. The con- ditions favorable or unfavorable for the formation of dew and frost are often very delicately adjusted, and a slight difference in elevation, ex- posure, or condition of the surface will determine whether dew or frost will occur or not. CHAPTER XXIV WINDS Atmospheric Convection. — When air is heated or made more damp by addition of water vapor, it expands and becomes less dense than the surrounding air, which crowds in from all sides and buoys the lighter air upward. The draught in a stove or lamp is a familiar example of the rise of light air, and the smoke from a fire burning in the open air shows that there is an upward current in this, case also. Careful observation will discover the movement of the cool air toward the fire. If there is no wind stirring, the rising column of smoke may be seen to spread out horizontally when it reaches a stratum of air of its own density. A downward movement at some distance from the fire takes place, but is too slow to be easily detected. Every wind that blows is a part of some convection circuit, which may be hundreds or thousands of miles in extent. The air is set in motion and the movement is kept up by a difference in the atmospheric pressures over different parts of the earth's surface. The upward and downward currents of the circuit are usually beyond the reach of ordinary observation, and in the regions where they leave or reach the surface of the earth the air is apparently calm. The lower horizontal currents consti- tute the commonly observed winds, but somewhere in the upper air there is always a current in a nearly opposite direction, which is sometimes made perceptible by the movement of clouds. DR. PHYS. GEOG. — l8 287 THE ATMOSPHERE Pressure and Winds. — The direction and force of the winds are always dependent upon differences of atmos- pheric pressure and can be explained as the result of the distribution of pressure. The location of regions of high and low pressure are shown on a map by the use of isobars or lines drawn through places of equal pressure. Figure 261 shows the isobars of the eastern part of the United States for the morning of March 26, 1898. The figures attached to each line show the height of the barometer along that line. The highest pres- sure was 30.8 inches in New England and the low- est 29.9 inches in Wiscon- sin, the difference being 0.9 inches. If a barome- ter could have been carried very rapidly westward along the line AB, it would have fallen at the rate of one tenth of an inch for every 100 miles. The rate of change of pressure along any line crossing the iso- bars is called the baro- metric gradient or pressure slope. It is evident that the rate of change of pressure is greatest, or, in other words, the slope is steepest, along a line which crosses the isobars at right angles, and that where the isobars are close together the slope is steeper than where they Fig. 261. Isobars. Isotherms.) are far apart. Gravitation tends to make the air move by its own weight down the steepest slope from high to low pressure. The arrows on the map fly with the wind and show that the wind was moving down the slope along lines parallel to CB, and not in the direction of the steepest slope AB. The cause of this slant in the direction of air movement WINDS 289 will be explained on pp. 290-292. In Fig. 262 the directions of slope AB and ofthewind^/ Care opposite to those in Fig. 261. The isobars are farther apart, showing that the slope is less steep (one tenth of an inch to 150 miles), and the wind has a smaller velocity. These examples il- lustrate the first law of winds : By the force of gravitation winds always blow from a region of high pres- sure to a region of low pressure with a veloc- ity which varies with the steepness of the pressure slope. Fig. 262. Velocity of the Wind. — The average wind velocities in the United States vary at different localities between four and fourteen miles per hour. The velocity is greater over the sea than over the land and increases very rapidly with altitude for a few hundred feet and then more slowly. Velocities of 200 miles per hour have been observed at high altitudes and in tornadoes on the surface of the land. (See Appendix, p. 404.) Effect of Rotation of the Earth. — If a person is walking upon the deck of a moving steamer toward some fixed object upon the shore ahead of the ship, and the pilot turns the steamer to the left while the walker continues in the same direction as before, his course will be deflected toward the right-hand Fig. 263. 290 THE ATMOSPHERE side of the steamer and he will describe upon the deck a curved path leading to the right-hand side. The same effect is noticed in walking through a car while it is running around a curve. The walker tends to move straight on and is thrown against the seats on one side of the car. The rotation of the earth tends to produce a similar effect upon all moving bodies. If a globe is viewed from a point directly above the north pole while it is rotated from west to east, the northern hemisphere will be seen turning counterclockwise about the pole as a center. Every north-south line is constantly changing its direction in space. The same is true of any portion of an east-west line. These facts are shown upon the map, Fig. 264. Each meridian, as A, is carried by the rotation of the earth to new positions B, C, D, etc. If an arrow starting northward on A continues in the same di- rection, when carried around to C it will be moving northeast- ward. An arrow starting west- ward on E, when carried to H will be moving northwestward. An arrow starting southward on /, when carried to K will be moving southwestward. An arrow starting eastward on Mi when carried to P will be mov- ing southeastward. At all points on a rotating earth, except at the equator, directions are con- tinually changing so that if any moving body could maintain absolutely its original direction it would move toward all points of the compass in the course of one rotation. On account of friction no moving body can maintain absolutely its original direction, yet it is deflected by rotation more or less rapidly with a force which increases from the equator to the poles. The deflec- tion is to the right in the northern hemisphere and to the left in the southern. This is known as FerreVs Law. Fig. 264. WINDS 291 M Fig. 265. N Figure 265 shows the path of a body starting northward from any point O in the northern hemisphere and moving without friction. As it reaches higher latitudes, the deflection is more rapid and it turns to the east and south. As it returns toward the equator the deflection is less rapid toward the west, and it would thus describe a series of loops around the earth toward the west, between the parallels M and N. The form and limits of the loops would vary with the latitude and the speed. Realistic Exercise. — The subject of deflection by the rotation of the earth is a difficult one to explain and to understand, and has been erro- neously stated in many text-books. The deflection is not a lagging behind or running ahead due to increasing or lessening speed of rota- tion. Perhaps the simplest illus- tration of the deflective effect of rotation may be made as follows : On a sheet of pasteboard draw two straight lines crossing at the center at right angles, and mark the ends of £ the lines north, south, east, and west. Lay the sheet on the table in such a position that the south-to-north line extends from the observer toward some fixed object beyond. Start a pencil along the line toward the north, and while the sheet is rotated counterclockwise, keep the pencil moving toward the fixed object. The line made by the moving pencil will curve away from the straight line to the right or to the east of north on the sheet. If the pencil is started east along the west-to-east line, the result will be a curve to the south of east. The pencil does not change its direction in space, but as the sheet rotates under it, its direction on the sheet continually changes, and. always to the right of its course at any given moment, as in the northern hemisphere. If the sheet is rotated clockwise, the lines will curve to the left, as in the southern hemisphere. The more rapid the motion of the pencil, the less sharp will be the curve ; the more rapid the rotation, the more sharp the curve. A similar illustration may be made with chalk on W— S Fig. 266. 292 THE ATMOSPHERE a black globe. An open umbrella mounted so as to turn upon its stick as an axis answers the purpose of a black globe. The winds are probably deflected more than any other moving body by the earth's rotation, and from this arises the second law of the winds : On account of the earttis rota- tion the path of the winds down a pressure slope in the northern hemisphere is to the right of a line perpendicular to the isobars, and in the southern hemisphere to the left. (See Figs. 261, 262.) The angle at which the wind crosses the isobars increases with the latitude. CHAPTER XXV INSOLATION AND TEMPERATURE The Distribution of Insolation. — The distribution of tem- perature, of pressure, of winds, and of rainfall over the face of the earth are so closely related that they can not be understood separately. They all depend primarily upon the distribution of the rays of the sun, or insolation, and this is determined chiefly by the form, attitude, and motions of the earth, as explained in Chapter I. On account of the spheroidal form of the earth there is but one ray of the sun that strikes its surface vertically, and the amount of insolation received decreases in every direc- tion from the point which receives the vertical ray. At the equinoxes, March 21 and September 23, the vertical ray strikes the equator, but at the winter solstice, December 22, the vertical ray strikes the Tropic of Capricorn, and at the summer solstice, June 21, it strikes the Tropic of Cancer. Thus the tropics bound a zone of maxi- mum insolation which receives the vertical ray of the sun during some portion of the year. At the equinoxes the tangent rays reach to either pole, but at the solstices they strike the polar circles, 231 beyond one pole and 23J short of the other. Thus the polar circles bound areas of minimum insolation which receive the tangent rays of the sun at noon during some portion of the year. Between the tropics and polar circles are zones of medium insolation. The belts of equal insolation on any given day are bounded by parallels of latitude, but they swing back and forth, north and south, through the year, following the appar- ent daily path of the sun through the heavens. During the year the sun shines an equal number of hours upon all parts of the earth's surface, but in the polar regions the insolation is nearly all received in the summer, while near the equator it is almost equally distributed through the year. The general result is shown in Fig. 270 (at the left), which gives the 293 294 THE ATMOSPHERE total insolation received in a year at different latitudes, expressed in percentages of that received at the equator. All places within the tropics receive more than 90 per cent as much insolation as the equator, while all places within the polar circles receive less than 50 per cent, the amount at the poles being about 42 per cent. The Distribution of Temperature is shown upon a map by means of isotherms or lines of equal temperature. Figure 267 shows the mean annual temperature, and Figs. 268, 269, the mean temperatures of January and July, each being the coldest month in one hemisphere and the warm- est in the other. The isotherms are based upon millions of temperature observations made in all parts of the world. For the annual isotherms the average of the temperatures for all the days of the year in each area of one or two square degrees is calculated ; for the monthly isotherms the average of all the temperatures recorded for the month. In these maps the effect of elevation is eliminated and all temperatures are reduced to what they would be at sea level, by adding a definite amount to the ob- served temperature. The observed mean annual temperature of Salt Lake City is 5 1 .3°, its elevation above the sea is 4300 feet, 8.7 is added and the annual isotherm of 6o° is drawn through it. Isotherms show- ing the actual temperature as affected by elevation would be too crooked and irregular to be shown upon a map of this scale. On the ordinary weather and temperature maps of a limited area, like the United States, the isotherms show the actual surface temperatures. It is evident from the maps that the distribution of tem- perature is quite different from that of insolation. While the isotherms extend in a general east-west direction (why ?) they are irregular in course and spacing. The irregularity is greater in the northern hemisphere than in the southern, and in January than in July. Effect of Land and Water. — If the surface of the earth were all water or all land of uniform elevation, the iso- therms would be parallels of latitude and the temperature would decrease regularly and equally along each meridian from the equator to the poles. In the winter the isotherms 296 THE ATMOSPHERE bend toward the equator over the land, and away from the equator over the water, showing that the land is colder than the water. In the summer these conditions are reversed. At least five causes combine to produce this result. (1) Difference in Capacity for Heat. — It requires about twice as much heat to raise the temperature of a cubic foot of water one degree as it does to raise the temperature' of an equal bulk of land. Hence from this cause alone a land surface receiving the same amount of insola- tion as an equal water surface is warmed twice as rapidly. (2) Difference in Penetrability for Heat. — The sun's rays can not penetrate the land deeply, and as the land is a poor conductor of heat only a thin layer is warmed, while the rays penetrate the water to the depth of 600 feet, and the heat is distributed through a much larger volume of water than of land. (3) Difference in Mobility. — The land is fixed while the water is movable. The water which is warmed often flows away and its place is taken by cooler water. Thus the heat received by the land is concen- trated and that received by the water is diffused. (4) Difference in Evaporation. — About one half the heat received by the water is expended in producing evaporation and does not raise the temperature of the water. (5) Difference in Cloudiness. — Cloud and fog are more prevalent over the water than over the land, and these retard the heat on its way to and from the water. In spring and summer the land is heated more rapidly than the water, and in autumn and winter, being a better radiator and having less heat to lose, it cools more rapidly. The rise and fall of temperature in the water is less than on the land and is retarded in time, reaching a maximum in the northern hemisphere in August, and a minimum in February. The temperature of a land surface is subject to great variation, that of a water surface to small variation. Effect of Winds and Currents. — The irregularity of the iso- therms is partly due to ocean currents and prevailing winds which carry their own temperature into regions which other- wise would be warmer or colder. The northward bend of INSOLATION AND TEMPERATURE 297 the isotherms on the west coasts of Africa and South Amer- ica is due to the cold ocean currents from the Antarctic drift, and the southward bend on the east coasts is due to the warm equatorial currents. The great bend of the iso- therms northward on the European side of the north Atlan- tic is due to the Gulf Stream and the southwest winds which accompany it, and the southward bend on the American side is due to the Labrador current. Regions of Maximum and Minimum Temperature. — In January the regions of lowest temperature are in northeastern Asia and Greenland (why ?), and the regions of highest temperature in Australia and South Africa (why ?). In July the areas of low temperature are in the vicinity of the poles, and the areas of high temperature in North Africa, south- western Asia, and southwestern North America (why ?). The lowest temperature ever recorded is — 96° in northeastern America, the high- est shade temperature 1 54 in the Sahara. A line drawn through the points of highest temperature on each meridian is called the thermal equator. It swings north and south with the sun, but is much farther from the geographical equator in July than in January (why ?). Zones of Temperature. — The tropics and polar circles do not divide the face of the earth into zones of temperature, but of insolation. The true temperature zones are bounded by isotherms. Any division of zones must be somewhat arbitrary, but the isotherms which mark the monthly aver- ages of 30 and yo° are convenient boundaries. The tor- rid zone lies between the isotherms of 70 on each side of the equator, the temperate zones between those of Jo° and 30 , and each frigid zone is inclosed by that of 30 . If the position of each of these zones in January and July is observed on the maps, it will be seen that they all swing north and south with the sun. The change of position is greater in the northern hemisphere than in the southern. The torrid zone shifts about fifteen degrees in latitude and is widest in July, especially over Asia. In January the north temperate zone is narrow and irregular, but in July it widens so far as to crowd the north frigid zone out of existence. 298 INSOLA 437. THE ATMOSPHERE Fig. 270. — Temperature zones. (Insolation in percentages at left.) By drawing the isotherms of Jo° and 30 for January and July upon one map, as in Fig. 270, we obtain a set of zones which are not shifting but fixed, and reveal in a striking manner the temperature conditions of the globe. Upon this map hot means an average temperature in the hottest month above jo°, cold an average temperature in the coldest month below 30 , and temperate an average monthly temperature between 70 and 30 . The space between the tropics is mostly occu- pied by a belt which is always hot, a truly torrid zone. This is bordered upon either side by a belt in which the summers are hot and the winters temperate. In the southern hemisphere there is a truly temperate zone in which both summer and winter are temperate. In the northern hemisphere this temperate zone is confined to the oceans. Over the land masses it is replaced by regions of exactly opposite conditions, a truly intemperate zone, in which the summers are hot and the winters cold. Beyond the temperate zones are belts of cold winters and tem- perate summers, and in the southern hemisphere only there is, around the pole, a truly frigid zone which is always cold. Range of Temperature. — The difference between the lowest and the highest temperature at any given place is called the range. It may be reckoned between the high- 300 THE ATMOSPHERE est and lowest temperatures observed in twenty-four hours, which gives the daily range ; between the average tempera- tures of July and January, which gives the annual range ; between the absolutely highest temperature observed dur- ing the year and the absolutely lowest, which gives the absolute annual range; and in other ways. The map, p. 299, shows that the average annual range increases with the latitude (why ?), and with distance from the sea (why ?), and is greater in the northern hemisphere than in the southern (why ?). The centers of maximum range nearly coincide with those of minimum tem- perature. The greatest absolute range is 182° at Verkhoyansk, Siberia. Figures 271 and 272 show the influence of latitude and of land and water upon range of temperature. J. F. M. A. M. J. J. A. s. 0. N. D. 86° J. F. M. A. M. J. J. A. s. 0. N. D. "53" _B_ B B A N, M 50° 32° 14 -4° M jvr A Bd V V . _F_ FT, v^_ ,8'P V s P/ \n FC -40° FC NFL JEN CE )F ATI ruD ~~ INF .UE ICE OF LAN D A JD SEA Fig. 271. Fig. 272. Annual variation of temperature. B, Batavia latitude 6° 8'S M, Madeira latitude 32°38' N A, Algiers " 3 6° 47 ' N Bd, Bagdad " 33°2o' N P, Paris " 48°5o* N V, Valentia " Si°SS' N SP, St. Petersburg " 59° 5 6' N N, Nerchinsk " 5i°58' N FC, Fort Conger " 8i°44' N CHAPTER XXVI THE DISTRIBUTION OF PRESSURE AND WINDS The Distribution of Pressure. — Isobaric maps may show the distribution of the average pressure for the year or month, or the actual pressure existing at a given day and hour. As on isothermal maps, the effect of altitude is eliminated, and all pressures are reduced to sea level by adding to the observed pressure the pressure of a column of air extending from sea level up to the height of the place of observation. The quantity to be added to the reading of the barometer varies with the temperature and density of the air at the time and place of observation, and furnishes a problem of unusual difficulty. The aver- age addition is about one tenth of an inch for every 100 feet of elevation. (See p. 407.) Figure 274 shows that the regions of high average annual pressure (above 30 inches) form two nearly continuous belts around the globe, situated near 30 south latitude and 40 north latitude. The northern belt is the more irregular and is widest over the land. In the equatorial and polar regions the pressure is low, or below 30 inches, the lowest so far as known being at about 6o° south latitude. Figure 275 shows that in January the northern belt of high pressure is expanded so that it covers the greater part of the land surface, but is interrupted by a large area of low pressure over the north Atlantic and Arctic oceans and Greenland and a smaller one over the north Pacific. The southern belt of high pressure is broken up into three centers which lie over the oceans. The highest pressure, 30.50 inches, is found in central Asia; the lowest in the northern hemisphere, 29.50 inches, near Iceland ; and the lowest of all, 29 inches, in the Antarctic regions. Figure 276 shows that in July the northern belt of high pressure shrinks 301 302 THE ATMOSPHERE to two centers situated over the oceans, and that central Asia is occu- pied by a large area of low pressure, tailing at the center to 29.40 inches. The southern belt of High pressure is nearly continuous along the tropic, with centers of higher pressure over the oceans and Australia. In general, the pressure rises from about 29.90 inches at the equator to a maximum of from 30.10 to 30.20 inches at 30 south latitude and 40 north latitude, then falls toward either pole. Toward the south pole the fall is regular and very rapid ; toward the north pole it is intef- N 70 60 50 40 30 10 20 30 40 .*>.,. i2_ W: t-Ot JO ly\. ^ v~ C. JAN. \ A /& i c - V w c 'c % Fig. 273. —Variation of atmospheric pressure along prime meridian in January, and 40 W. Longitude in July. rupted by local depressions. Figure 273 shows the varia- tion of pressure along the meridian of o° in January and of 40 west longitude in July. Relations of Temperature and Pressure. — A comparison of the isobaric and isothermal maps reveals the funda- mental relations between pressure and temperature. The persistently high temperature in the equatorial belt is ac- companied by persistently low pressure. In the seasonal changes the low temperature over the land in winter is accompanied by high pressure, the high temperature in summer by low pressure. These changes and contrasts in both temperature and pressure are more marked in the northern hemisphere than in the southern, and are extreme over Asia, the largest land mass. In January the low tern- 304 THE ATMOSPHERE peratures which prevail over the northern hemisphere are accompanied by prevailing high pressure, and the cen- ters of low pressure occur over the warmer oceans. In July the universal high temperature in the northern hemi- sphere is accompanied by almost equally widespread low pressure. The centers of high pressure occur over the cooler oceans. These correspondences are in accordance with the well-known law that the density of air varies in- versely with the temperature. The fall of pressure with the fall of temperature from middle latitudes toward the poles, and the extremely low pressures in high southern latitudes, are apparent contradictions to this law and must be due to some other cause which overcomes the effect of low temperature. This subject will be considered later in connection with the winds (p. 311). The Relations of Pressure and Winds. — The direction and force of the prevailing winds have been determined by millions of observations in all parts of the world, but are best known over the oceans from the reports of sailors and naval officers. They are shown upon the wind maps, Figs. 277, 278, and the isobaric maps, pp. 303, 308, 309, by arrows which fly with the wind. The intimate relation which exists between wind movement and the distribution of pressure is clearly evident upon the isobaric map for January. The strong centers of high pressure in the southern oceans are each surrounded by a mass of air which is moving spirally outward, counterclockwise. The strong centers of low pressure in the northern oceans are each surrounded by a mass of air which is moving spirally in- ward, counterclockwise. In July the strong centers of high pressure in the northern oceans are each surrounded by a mass of air which is moving spirally outward, clock- wise. All centers of high and low pressure are accom- panied by similar movements, more or less regular and ex- ocean winds. January and February Less_ than 13 Miles an hour —^ Varjab[e mds j. Steady ii Fig 277. OCEAN WINDS. JULY AND AUCUST Less than 18 Miles an hour \ VarlabJe Wlnds Over Fig. 278. 305 ^ St&ac/y n 306 THE ATMOSPHERE tensive. Figure 279 shows that these movements are in ac- cordance with the laws of the winds given on pp. 289, 292 and are the results of gravitation and the rotation of the earth. NORTHERN SOUTHERN NORTHERN SOUTHERN CYCLONE ANTICYCLONE Fig. 279. Gravitation tends to make air move out from a center of high pres- sure down the steepest pressure slope, that is, along radial lines. The earth's rotation deflects the moving air to the right of the radial line in the northern hemisphere and to the left in the southern. Gravitation tends to make air move in toward a center of low pressure along radial lines. The earth's rotation deflects the moving air to the right of the radial line in the northern hemisphere and to the left in the southern, but can never make it move up the pressure slope against gravity. The result, in the northern hemisphere, is a curve in the form of the figure 6. As the wind approaches the center, its path becomes more nearly paral- lel with the isobars, and a whirl or eddy is set up. A movement of air spirally inward toward a center of low pressure is called a cyclone. A movement of air spirally outward from a center of high pressure is called an anticyclone. Near the center of a cyclone the air moves spirally up- ward ; near the center of an anticyclone there is a downward movement. Wind Belts. — The arrangement of the centers of high pressure in belts on each side of the equator causes the prevailing winds also to be arranged in more or less definite belts around the earth. The air moves from the belts of high pressure toward the equator on each side and is deflected westward by the rotation of the earth. This constitutes the trade winds, from the northeast in the northern hemisphere, and from the southeast in the south- ern. They blow with great steadiness throughout the year and are called constant winds. The air also moves THE DISTRIBUTION OF PRESSURE AND WINDS 307 from the belts of high pressure toward each pole and is deflected eastward. This constitutes the antitrade winds or prevailing westerlies, from the southwest in the north- ern hemisphere and from the northwest in the southern. This movement is most regular and forcible in the southern hemisphere, where the pressure slope is steep and constant. Between these belts of prevailing winds are the belt of equatorial calms, where the air is rising, and the belts of tropical calms, where the air is descending. The complete ideal scheme is shown in Fig. 280. Monsoons. — The belts of pressure and prevailing winds are not absolutely fixed, but swing north and south with the sun. The equatorial calm belt coin- cides not with the geographical equator, but with the thermal equator, and is most variable in position. The belts of the southern hemisphere are nearly con- stant. The widest departure from the ideal system is brought about by the large and elevated land mass of Asia. In summer this region ceases to be a part of the northern belt of high pressure, and becomes virtually a part of the equatorial belt of low pressure. Consequently the regular northeast trade winds are suspended. The southeast trades cross the equator and continue far northward as south and southeast winds over the western Pacific Ocean and eastern Asia, and southwest winds over the Indian Ocean and southern Asia. In winter the regular northeast trades prevail over these regions. These southerly summer winds and northerly winter winds are called monsoons. The Polar Whirls. — The air moving from the belts of high pressure toward the poles is deflected eastward, and acquires a high velocity. It thus forms, especially in the southern hemisphere, a great cyclonic whirl from west to east around the polar regions, rising gradually as it nears the center (see maps, pp. 308, 309). Fig. 280. 3°9 3io THE ATMOSPHERE There is some evidence to show that at the very center of the south polar whirl there is a small anticyclone, from which southerly winds blow outward with great violence. In the northern hemisphere the polar whirl is much interrupted by the land masses, and in summer it almost disappears. In winter it is divided into two portions which circulate around the centers of low pressure over the north Atlantic and north Pacific oceans. Probably a portion of the air moves in a circuit which incloses both centers. The General Circulation of the Atmosphere. — Thus far only the movements in the lower layers of air next to the surface of the land and water have been considered. Our knowledge of the upper air by direct observation is much less extensive and accurate than of the lower air. It has been gained by means of observations made upon mountains, by occasional balloon ascensions, from the drift of high clouds and vol- canic dust, and by means of kites and unmanned balloons carrying self-re- cording instruments, such as the thermograph and barograph (see Ap- pendix, pp. 402, 403), to great heights. The movement of the upper air is everywhere toward the poles, with an eastward deflection. In other words, the polar whirls in the upper air cover the whole of each hemi- Fig. 281. — Direction of primary air currents. (After Ferrel.) sphere and have a common circumference at the equator. As the currents approach the poles, they descend and re- turn at intermediate heights toward the equator. Below THE DISTRIBUTION OF PRESSURE AND WINDS 311 this system of circulation, and fed by it, are the surface currents with which we are most familiar. The whole sys- tem of atmospheric circulation is shown by map and sec- tion in Fig. 281. On the map or shaded part of the figure, the complete arrows show the direction of surface currents, and the broken arrows that of upper currents. The mass of warm air rising from the equatorial regions, at and above a height of two miles, turns to the northeast and south- east. By a very circuitous, spiral course, pass- ing round the earth many times on the way, but moving with increasing velocity, it ap- proaches the poles and gradually descending returns toward the equator. At the tropical belts of high pressure, the return currents drop down to the surface of the earth and continue as the trade winds to the equator. A part of them turn back at the tropics and form the prevailing surface westerly winds or anti- trades, which rise as they approach the poles and rejoin the intermediate return currents. Figure 282 shows a diagram of the upper currents and a simplified section of the whole system. Effect of the Polar Whirls. — The effect of the polar whirls may be seen in the rapid rotation of water in a pan or bowl. The centrifugal force throws the water away from the center, where the surface becomes depressed, and piles it up around the sides, where the surface becomes elevated, as in Fig. 283. The water being deeper' at A and B than at C, its pressure upon the bottom is proportionately greater. A similar effect is produced by the whirl of the air around the polar regions. It is thrown away from the polar regions and piled up around the circumference of the whirl. There is less air above the polar regions than above latitude 3o°-40°, and the atmos- pheric pressure is correspondingly low at one place and high at the other. Thus the centrifugal force of the polar whirl makes the pressure low in spite of the low temperature. The position of the tropical belts of high pressure is a resultant of the high temperature of the equatorial regions on one side and the polar whirls on the other. Fig. 283. DR. PHYS. GEOG. — 19 CHAPTER XXVII STORMS Cyclones. — The regularity of the general system of prevailing winds is subject to local and temporary dis- turbances called storms. A storm is usually characterized by an increase of wind velocity, accompanied by precipita- tion. A large majority of storms are cyclonic whirls in which the air moves spirally toward a center of low pres- sure. The isobars are seldom circular, but extend in more or less elliptical curves around the low center. The general course of the winds is across them, but at a smaller angle as the center is approached, as shown in Fig. 279. The motion becomes more rapid and more nearly circular as the air rises around a central calm. The cyclonic or vortex movement may be regarded as. the normal air move- ment on a rotating earth. Each of the great polar whirls in the upper air covers half the earth. Within these are smaller cyclones of the second, third, and even fourth order, down to little dust whirlwinds a few feet in diame- ter. The temperate or mid-latitude regions of the northern hemisphere are much frequented by cyclonic storms which are of great extent, but not violent. They often attain a diameter of more than a. thousand miles and bring their characteristic weather conditions to a correspondingly large area. While the movement of the air at every point within their circumference forms a part of a cyclonic system, the center of rotation moves forward in a general easterly direction at the average rate of about thirty miles an hour. Thus the whirl travels through the atmosphere STORMS 313 Fig. 284. — Upward movement of air at center of a cyclone. as an eddy moves through still water, constantly taking in new air in front and dropping out air behind. The air rises as it approaches the center, and at the height of a mile or more spreads out in a reverse direction toward the circumference. The maps, Figs. 286-288, show the progress of a cyclonic storm across the United States and the weather conditions which accompany it. As" in this example, the isobars encircling the center of low pressure are usually more or less elongated in a north-south direction. The result is that in the front or eastern half of the storm the prevailing winds are from the southeast and south, and in the rear or western half from the northwest and north. Over a small area on the north side east winds occur, and on the south side west winds. The southerly winds coming from the Gulf of Mexico and Atlantic are warm and damp, and- as they advance northward are cooled by radiation and con- duction. Consequently they bring cloudy weather with rain or snow, which on account of the rising and mixture of air around the center often extends over a large area on all sides. On the west side the northerly winds coming from the interior of the continent are cool and dry, and as they advance southward are warmed. Consequently they evaporate the clouds and bring clear weather. As the storm advances, these two strongly contrasted types of weather prevail in succession at lg ' 2 5 ' every point in its path. Fig. 285 shows the curves of pressure in a cyclone ; notice that the changes of pressure are greater and more rapid along BA than along ED. See also Fig. 301. STORMS 315 Anticyclones. — The areas of relatively high pressure between the cyclones sometimes take the form of irregular "ridges," but more often they appear as definite centers of high pressure, or anticyclones, as in Figs. 286-288. The conditions in an anticyclone are the exact reverse of those in a cyclone, as shown on p. 306. At the center the air is descending, and when it reaches the surface of land or water it spreads down the pressure slope in all directions. The rotation of the earth gives it a spiral motion, clockwise in the northern hemisphere. The anticyclonic centers move eastward along paths similar to those of cyclones. On the eastern side the winds are chiefly from the north and northwest, and bring cold, clear weather. On the western side the winds are chiefly from the south and southeast, and bring higher temperature to the regions over which they blow ; but owing to the fact that these currents are supplied with dry air which descends from above at the center and is warmed by compression, they do not usually bring cloudy or rainy weather, as do the winds in front of. a cyclone. Warm and Cold Waves. — The southerly winds in front of a cyclone carry the isotherms northward, as shown in the eastern part of the map, Fig. 287. As the cyclone advances eastward, it carries a wave of rising temperature in front of it. This effect, however, is not usu- ally so pronounced as the cold wave which precedes an anticyclone. The northerly winds in front of an anticyclone cause the isotherms to curve away from the center of high pressure, as shown in Figs. 289- 291. When a cyclone passes across the southeastern part of the United States, followed by an anticyclone in the northwest, a wave of falling temperature spreads over the greater part of the country. In winter freezing temperatures may be carried nearly to the tropic and zero weather to the Gulf states. In the northwest the air is sometimes filled with extremely fine ice crystals driven by a high wind. Such a storm is locally known as a blizzard. In the north and east, the storm usually brings a heavy fall of snow. .5? o 316 STORxMS 317 The Procession of Cyclones and Anticyclones. — Through the greater part of the year, but especially in the winter months, the eastern part of North America is traversed by a more or less irregular but continu- ous procession of cyclones and anticy- clones which succeed one another at inter- vals of a few days. g ' 292 The result is a rapid succession of weather changes, which are often sudden and decided in character. If the cyclones and anticyclones all pursued the same path at regular intervals, the result would be as shown in Fig. 292. An approach to this condition appears upon the maps, Figs. 286-288, but it usually happens that the paths of the different centers are not uniform and the spacing between them is unequal. Thus the order of their occur- rence is irregular. Cyclones and anticyclones also vary greatly in development. Some are small and feeble, some large and strong, and the degree of control which they exercise over the weather conditions varies accordingly. Figures 286-288 show the progressive development of a temperate cyclone. On 286 the low center in Montana, with a. pressure of 29.7 inches, is surrounded by only three isobars, which are far apart. The pressure slope is gentle, and the winds are light and irregular. On 287, the center has moved to Illinois, with a pressure of 29.6 inches, and is surrounded by five isobars. The slope is steeper, the winds are stronger and show very little variation from a regular spiral whirl. On 288, only the rear half of the cyclone is shown, but the pressure at the center, now off the New England coast, has fallen to 28.8 inches and it is surrounded by fourteen isobars, which are closely crowded. The slope is very steep, and the velocity of the wind is high, amounting near the center to a gale dangerous to shipping. The crossed line shows the path pursued by this cyclone across the United States and 3 i8 THE ATMOSPHERE the position of its center from day to day. The maps, Figs. 289-291, show a cyclone which developed very rapidly, having on the second day nineteen isobars, and a difference of pressure between center and cir- cumference of 1.80 inches. Two low centers may combine into one, or a single one may break up into two. Cyclones usually pass off into the Atlantic Ocean, where they gradually die out, or they may con- tinue across Europe as far as central Asia. Storm Paths. — On Fig. 293 the paths of many individual cyclones are shown. The heavy line marks the path of the greatest number. Fig. 304 shows favorite paths across the United States. Weather Maps. — The daily weather maps issued by the United States Weather Bureau should be consulted for exam- ples of cyclones and anticy- clones. The maps for January, February, and March furnish the best and most numerous specimens. The local weather conditions as observed by the student should be compared each day with those shown by the weather map. Observation and map study carried on to- gether for two or three months will make clear the laws which govern the apparently capricious changes of the weather, and will enable the student to predict those changes with a fair degree of accuracy. Weather maps may be obtained by addressing the local officer in charge of the nearest observing station (see Appendix, p. 411). Tropical Cyclones. — In the region between the tropics, cyclones occur which are much smaller than those of temperate regions, but are of proportionately greater vio- lence. They are developed over the western parts of the equator Fig 293 STORMS 319 TO 80 90 100 HO 120 130 140 Fig. 294. — Paths of typhoons. oceans, those in the north Atlantic being called hurricanes, and those of the Pacific and Indian, typhoons. They orig- inate in the belt of equatorial calms when farthest from the equa- tor. From a small be- ginning they increase to a diameter of 300 to 500 miles. At the same time the velocity of the wind increases to a de- gree which becomes destructive to shipping upon the seas and to buildings, forests, and crops on land. After a career which lasts many days or even weeks, they gradually die out. Their paths are more uniform and regular than those of temperate cyclones. They move westward and poleward at right angles to the trade winds, until they reach latitude 25 to 30 , where they turn rather abruptly into the path of the antitrades and move poleward and eastward. West India Hurricanes. — In the months of August, Septem- ber, and October the West India Islands are subject to cyclones which arrive from the east and southeast, and depart toward the northeast along or near the coast of the United States. The winds acquire a spiral move- ment which becomes nearly circular around a central region of calm Fig 295. —Paths of hurricanes. 320 THE ATMOSPHERE which is from ten to twenty miles in diameter. The circumference of the storm is marked by a slight rise of the barometer and the appear- ance of fine cirrus clouds which form in the air, blowing out from the top of the approaching whirl. The barometer begins to fall, the wind freshens, and the cloud mass becomes more dense. As the center comes nearer, the wind rises to a gale, and the clouds gather into a black mass of nimbus from which heavy rain falls. Within about fifty miles of the center, the barometer sinks rapidly, the wind attains full hurricane strength, the clouds are so dense as to change daylight into the darkness of midnight, and the rain pours down in torrents, accom- panied by frequent flashes of lightning. At the center of the whirl these conditions change very abruptly. The wind falls to a calm, the rain ceases, and the clouds break away, showing a clear sky. The barometer now reaches its lowest point, which may be less than 27 inches. This calm, clear, central space is called the eye of the storm, and it may occupy an hour or two in passing. Then the hurricane begins again with sudden and extreme violence, but the winds are re- versed in direction. All the phenomena observed in the first half of the storm are repeated in reverse order but in somewhat more rapid succession. The barometer rises, the violence of the wind gradually abates, the rainfall becomes more gentle, the dark nimbus clouds lighten and at last disappear, the lofty cirrus clouds recede, and the storm has passed away. Destructiveness. — Tropical cyclones are much dreaded by ship cap- tains. When the warning signs of their approach are observed, the ship is put upon a course which usually takes it out of the path of the central portion. If caught in the most violent part of the storm, it is liable to be wrecked by the force of the winds and waves, and can hardly escape without serious injury. In passing over the land the hurricane causes great destruction to life and property. Hardly any- thing of value is left in its path. The smaller islands are sometimes' literally swept clean of trees, crops, buildings, and almost of popula- tion. Perhaps the most complete ruin is accomplished along the coast, which suffers from the combined action of wind and wave ; for the low atmospheric pressure at the storm center and the inblowing winds cooperate to produce a heaping up of the water to a height of many feet above the usual sea level. Causes of Tropical Cyclones. — The evidence seems to point clearly to the conclusion that a tropical cyclone is STORMS 321 a part of a system of convection currents set up and main- tained by a difference of temperature and subject to the influence of the earth's rotation. Under the direct rays of the tropical sun the lower air becomes excessively heated and in contact with the sea excessively humid. It is thus made less dense than the air which overlies it — a condition which is as unstable as would be a layer of oil under a layer of water. Sooner or later the lighter air below breaks through the heavier air above and drains away upward like a draught in a chimney. The sur- rounding air crowds in from all sides toward the bottom of the updraught and soon acquires the usual spiral motion. As the air currents approach the center the rapidity of rotation be- comes so great that centrifugal force overcomes gravitation and prevents the incoming winds from reaching the center, which is left as a calm and comparatively emptied of air, like the core in the center of a water eddy. The air escapes by a spiral movement upward, and since there is no more efficient cause of cooling and condensation than the expan- sion of rising currents (see p. 282) the great mass of nimbus cloud and the downpour of rain necessarily follow. In the eye of the storm the air is not rising and there is probably even a slight downward draught, which tends to produce a clear sky. Duration and Force. — The amount of energy required to maintain the high velocity of a hurricane in a mass of air 300 miles in diameter is enormous and can not be derived from the original heat energy which started the updraught. One typhoon has been known to continue for thirty-five days and to travel the whole length of the heavy line on Fig. 293 from the Philippine Islands to central Europe, a distance of more than 14,000 miles. A very large supply of energy is derived from the liberation of latent heat which accompanies the rapid condensation of water vapor in the rising column. Thus the cyclone maintains at its own center a virtual furnace which keeps up the temperature as long as water vapor is supplied for condensation. When it passes over the land and is fed with dry air, it rapidly loses force and is finally over- come by friction. Course. — The westward and poleward course of a cyclone within the tropics is probably a resultant of two forces. Its lower portion is in 322 THE ATMOSPHERE the current of the trade winds, which tend to carry it westward, while its upper portion rises into the current of the antitrades, which tend to carry it poleward. The result is movement in a direction between the two. Beyond the tropics it follows the northeastward or southeast- ward drift of the antitrades. Origin of Temperate Cyclones. — The conditions under which temperate cyclones originate are so different from those which prevail at the birthplace of tropical cyclones that it seems impossible to attribute them to similar causes. Temperate cyclones are more frequent and violent in win- ter than in summer. Many of them are developed over land where the air is dry and cold, and the conditions are unfavorable to convection. The theory that temperate cyclones are eddies set up around the margin of the polar whirl is a plausible one. The map on p. 309 shows that the north polar whirl in winter is divided into two whirls around the centers of low pressure in the north Atlantic and north Pacific oceans. The margin or circumference of the whirl is along the axis of the belt of high pressure which surrounds these areas of low pressure, and Fig. 293 shows that the most frequent path of cyclones nearly coincides with this axis. The Atlantic whirl seems to be more prolific of cyclones than the Pacific. The oblique flow of the upper and lower winds into the ever narrowing space around the pole, the return of the air at intermediate levels, and the friction of continents may well give rise to local crowding and disturbance, and the temperate cyclones may be eddies driven by the general winds, like the eddies produced in a river where its banks and bottom are irregular. Form of Temperate and Tropical Cyclones. — These whirling masses of air are not tall and slender columns as we are apt to imagine, but rel- atively thin, flattened disks, not more than five miles in thickness and from 300 to 1500 miles in diameter. A circular disk one inch in diam- STORMS 323 eter cut from a leaf of this book would not be too thin to represent their average proportions. Tornadoes. — A tornado is a small and violent cyclone which appears as a funnel-shaped cloud with the small end down. Its formation or approach is preceded by the rapid movement of cloud masses toward some central point. The clouds may look as if lighted up by a great fire or like dense volumes of smoke, or may have a peculiar greenish hue. The funnel- shaped cloud de- scends as a pendant from the larger cloud mass, like an elephant's trunk, and dangles above or upon the ground, writhing and twist- ing about, touching here and there, and often skipping over a portion of its regu- lar path. The tor- nado travels toward the northeast, in the northern hemi- sphere, at the rate of about forty miles an hour, and seldom continues more than two hours. Along a path which varies in width from a few rods to half a mile, it is extremely destructive. The velocity of the wind in the whirl often reaches 200 miles per hour and occasionally twice as much. Nothing except the solid earth itself' can Fig. 296. — How a tornado looks. (Woods County, Okla., May 18, 1898.) 324 THE ATMOSPHERE withstand its force. It creates a deafening roar like the rumble of a railroad train over a bridge, greatly intensified. In front there is a gentle southerly breeze or a dead calm with oppressive heat. The tornado passes in a minute or two and is followed by a sudden fall of temperature. Through a forest a tornado cuts a swath like that of a mower through a meadow, the trees being twisted off or uprooted. From plowed fields it removes the loose soil, and sucks up the water from small ponds, leav- ing them dry. Even large boulders and masses of iron are taken up and transported hundreds of feet. Buildings of all kinds which stand in its way are demolished, and their fragments scattered over the sur- rounding country. Animals and human beings are lifted and whirled about and sometimes transported a half mile or more. They are often killed by flying debris and sometimes seem to be literally torn in pieces. Heavy structures are removed from their foundations and locomotive engines lifted from the rails. The smaller work of a tornado is equally impressive, such as the stripping of feathers from fowls and the cloth- ing from persons. Wire hairpins have been driven through fence boards and straws driven into oak wood. Almost any story of a tor- nado's energy may be true, because the truth is beyond the power of human imagination to invent. A part of a house may be reduced to fragments while the rest is left un- disturbed. People have been carried long distances and deposited unhurt. Heavy objects are removed and light and fragile ones left in place. Fragments of furniture from the same room or from the same piece are often widely scattered in opposite directions. These and other mysterious freaks are probably due to irregular and confused cur- rents in the general whirl. The walls of buildings are often thrown outward as if by an explosion from within. Tornadoes always occur some hundreds of miles to the southeast of the center of a temperate cyclone, where a current of warm, moist air is underrunning a layer of colder air. They occur chiefly in the summer months and in the afternoon of hot days. These conditions are very favorable for the starting and maintenance of strong convection currents. Fig. 297. — Pressure during the passage of a tornado. STORMS 325 Tornadoes seldom occur singly, but in groups of three or more, which follow parallel paths. As many as forty have been reported from one locality in one day. The av- erage number in the United States is about 150 per year. They are most frequent in Kan- sas, Iowa, Missouri, Illinois, and Georgia, and are almost unknown north of the forty- fifth parallel and west of the one hundredth meridian. Spouts. — A tornado at sea takes the form of a whirling column which extends from the clouds to the water surface and is called a waterspout. It is formed by the usual tapering funnel, which descends from the clouds and is met by rising water below. The greater part 3d 71 £9. 6^29,5 3 C \ jHIGH 30.1 4- \J^^^^^^^l {(n^^^^^^x nNs- ^/Tn^^^lffiu ^.V^tfrAr— 50° J %dS- 60 ° ''\^^kr-3^^A Jr ^ 70 ° , j ^^.\*2iL^4 •^ Jit- 30.0 90.0 V ->, Fig. 298. Location of tornadoes in a cyclone. (Shown thus: x x x.) of the column, however, is composed of cloud and rain. When it passes over a ship, as occasionally happens, there is a deluge of fresh water. In the desert columns of whirling sand are of frequent occurrence and are maintained for several hours. The small whirlwinds common on dry, warm days are worthy of careful observation, since they present many of the essential features of cyclones on a small scale. Thunderstorms. — The rapid condensation of water vapor is often accompanied by the generation of electricity, and when this occurs to such a degree as to produce frequent discharges of lightning from cloud to cloud or between the clouds and the earth, the disturbance is called a thunder- storm. A local ascending current of warm, moist air de- velops at its summit a cumulus cloud with a flat base. This increases in height and area for several hours, until rain begins to fall. The air is cooled and pressed downward until the current in the central portion is reversed. The column of descending air spreads out at the bottom and 326 THE ATMOSPHERE becomes surrounded by a current of ascending air which continues to supply moisture to the cloud above. At the center the pressure is high and the temperature low ; at the circumference these conditions are reversed, and be- tween the two there is a zone of strong contrasts and steep gradients marked by squalls of violent wind and rain. lllll nil —Hill fTlm Fig. 299. —Clouds and winds in a thunderstorm which is moving toward the right. Progressive Thunderstorms. — A thunderstorm is very apt to take on a progressive movement in the direction of the general air current (east- ward in the United States), and to' broaden out so as to present a convex front, which increases in length. The cloud mass may attain a length of 100 miles and a breadth of 30 miles, and reach to a height of 5 miles. Its front edge of cirro-stratus, with rolling festoons of cloud below, extends from 10 to 50 miles in advance of the rain. The rate of movement is from 20 to 50 miles an hour, and it may continue from 2 to 12 hours. As the storm approaches, the sky is gradually overcast, the air is hot, breathless, and oppressive, the barometer falls, and the distant thunder is heard. In front and below there is a strong out- rush of cool air which lasts but a few minutes and is followed by the dash of rain. The temperature falls rapidly, sometimes as much as twenty degrees in a half hour, the barometer jumps suddenly upward, and the darkness of the downpour is broken by vivid flashes of light- ning. The rainfall seldom lasts more than an hour unless a second storm follows close upon the first. Cloudbursts. — In tornadoes or thunderstorms strong ascending cur- rents may carry up and sustain the rain or hail until an excessive quantity has accumulated aloft, which sooner or later falls in an almost solid mass of water. Such events are popularly known as cloudbursts. CHAPTER XXVIII RAINFALL Causes of Rainfall. — The general causes and conditions which promote condensation of water vapor and the fall of rain and snow have been discussed in Chapter XXIII. The distribution of rainfall over the earth remains to be con- sidered. On account of the absence of permanent observ- ing stations at sea, the rainfall has never been accurately measured there. The facts as observed upon land are shown on maps, pp. 328, 330, 331. The conditions neces- sary for considerable rainfall anywhere are (1) a large body of water from which sufficient evaporation may occur, (2) air currents to transport the vapor over the land, and (3) some agency for cooling and condensing the vapor. The first requisite is supplied almost solely by the sea, the second by prevailing winds, and the third by winds and by elevations of the land. On account of the intimate relations between the winds and the supply of moisture, each wind belt is characterized by a peculiar type of rainfall, while the varied relief of the land breaks up the belts into more or less distinct and contrasted por- tions. Hence the patchwork appearance of the maps. Equatorial Rains. — In the region between the tropics the rainfall is generally large and is the result of two proc- esses : (1) the rising of the air in the equatorial calm belt, and (2) the flow of the trade winds from the ocean over the land. In the belt of equatorial calms, heavy rains are of almost daily occurrence, and are produced by the mechanical cooling of rising air. 327 ft «• RAINFALL 329 The mornings are usually clear, but cumulus clouds soon begin to form, which continue to grow until afternoon, when thunderstorms occur, often succeeding one another into the night. These conditions accompany the equatorial belt of low pressure in its migrations and therefore pass over the regions within its range twice a year. Near the northern and southern limits reached by the belt the two rainy seasons merge into one and occur in summer when the sun is nearly vertical. At the middle of the belt, rainy seasons occur in the spring and fall. Those months during which the trade winds blow without interruption are dry except where winds from the sea strike the side of a plateau or moun- tain range and are compelled to ascend. In January, on account of the low pressure over the southern land masses, the rains extend beyond the southern tropic in South America and Africa and reach the northern coast of Australia. In July they cross Central America, the West Indies, and central Africa, and are carried by the monsoons to the coast lands of Asia from India to Japan. Over the regions between these limits, with few exceptions, the rainfall ranges from 40 to over 80 inches per year. In South America there is an excess on the northeast and southeast coasts due to the highlands, which extend across the course of the trade winds and act as very efficient condensers. The wide extent of heavy rainfall over the lowlands of the Amazon basin is probafbly due to the dimin- ished speed of the trade winds by friction in passing over the land. The currents from the ocean supply more air than can pass in a layer of uniform thickness, and the air is compelled to rise, as the surface of a stream of water is raised in passing over an obstruction in its bed. On the west coast of South America there is a deficiency of rainfall, be- cause the lofty chains of the Andes permit little moisture to pass over them. In Africa there is a deficiency in the eastern peninsula where the monsoons blow from the land. In southeastern Asia there is a large excess of rainfall in summer, but on account of the varied relief it is unequally distributed. The southwest coasts of India and Indo- China and the Ganges and Brahmaputra basins receive excessive rain (80-400 inches), m<5st of which falls in'The summer; while over the Dgkka'n plate|u*there is a deficiency. The Khasia hills, north of the Tiead qi^he Day of Bengal, receive the heaviest rainfall in the world, ^a^praging 493 inches per ^ear and amounting in some years to 600 inches. Over 438 incnes falls in five summer months, and more than 40 inches has^ieen recorded in a single day. w 33° 332 THE ATMOSPHERE Tropical Rains. — In the tropical belt of high pressure the air is warmed by compression as it s descends, and hence brings little rain. As it shifts north and south, it is followed by the trade winds on the equatorial side and by the temperate cyclones on the polar side. The northern tropical belt is much less regular and well-defined than the southern. In summer it nearly ceases to exist over the land, and its place is taken by the trades. Where these blow from the ocean, against highlands, as in Central America, they bring a wet season ; but where they blow from the land, as in the Mediterranean region and southwestern Asia, they bring a dry season. By a combination of these conditions, southwestern United States, north Africa, Arabia, Persia, and the region of the Caspian and Aral seas are perennially dry. Of these regions, the Sahara and Arabian deserts are the largest and driest in the world. The southern tropical belt in winter crosses southern South America, south Africa, and central Australia, causing a deficiency of rainfall at that season. A narrow strip on the coast west of the Andes in northern Chile forms the desert of Atacama, and a larger area in southwest Africa the Kalahari desert, while central Australia contains the largest desert in the southern hemisphere. In summer the tropical belt lies to the south of all the land except South America. Rains of Middle Latitudes. — Beyond the tropical belts of high pressure rains are brought either by the westerly anti- trades or by temperate cyclones ; consequently the rainfall varies more along east-west than along north-south lines. In winter the west coast of North America has abundant rainfall as far south as 35 , but on account of the mountains near the coast it extends but a few hundred miles inland. The coast of southern Alaska, being reached by perennial southwest winds, has rain at all seasons, but south of 40°, on account of the cessation of southwest winds in summer, rain is scant or wanting in that season. Central North America, from Mexico to the Arctic Ocean, is dry, partly from being too far inland, and partly on account of the mountains along the Pacific coast. The Rocky, Wasatch, and other mountains and plateaus which rise above 8000 feet receive a moderate rainfall. Eastern North America from the Gulf of Mexico to Hudson Bay enjoys a rainfall above 30 inches, RAINFALL 333 increasing to 60 inches on the Gulf coast. This is due chiefly to the cyclonic storms which bring moisture from the south and southeast, and to the absence of elevations sufficient to shut it out. The Gulf of Mexico appears to furnish the larger quantity, but the southerly winds are fed in summer by the northeast trades from the Atlantic and sweep far inland toward the continental center of low pressure. The rainfall is well distributed throughout the year, but is generally heavier in spring and summer than in autumn and winter. The increased frequency of cyclones in winter nearly compensates for the lower absolute humidity of the air. The largest mean annual rainfall in North America is at Sitka, Alaska, 112 inches, and at Neah Bay, Washington, 105 inches; while the Mohave desert, California, has the smallest recorded rainfall in the world, 1.85 inches. The rainfall of Europe and northern Asia is chiefly supplied by the westerly winds from the Atlantic. It therefore decreases from west to east and is greatest in summer and autumn, when the center of low pressure over Asia causes the winds to penetrate farther inland. North- western Europe has a rainfall well distributed throughout the year, with a slight excess in autumn and winter, due to the center of low pressure in the north Atlantic and the greater frequency of cyclonic storms. The rainfall gradient is steep in the British Isles and Norway, some places on the west coast having seven times as much rain as places one or two hundred miles to the east. The high table-lands of central Asia are chiefly desert on account of the surrounding rim of mountains. In southern South America the west winds bring ample rainfall to a narrow strip on the Pacific slope of the Andes, but it diminishes rapidly eastward, and the plains of Patagonia are arid. During the southern summer (January) the southeast trades bring to southeast Africa and to Australia moisture, which is condensed by the highlands near the coast. General Laws of Distribution. — In spite of the great irregularity of distribution of rainfall, four general laws may be observed. (1) Rainfall decreases from the equator toward the poles. (2) Rainfall is generally less in the interior of a con- tinent than on the coasts. DR. PHYS. GEOG. — 20 334 THE ATMOSPHERE (3) In trade-wind regions (between 35 south and 40 north) rainfall is greater on east coasts than on west. In antitrade-wind regions the reverse is true. (4) Rainfall increases with altitude up to a certain ele- vation, which varies in different regions, and then dimin- ishes. On mountain ranges the rainfall is greater on windward slopes. The position of high ranges is well marked on the rainfall map. Although no known regions are absolutely rainless, about 20 per cent of the land surface has less than ten inches of rain per annum, and is consequently in a desert condition, while nearly 50 per cent has less than twenty inches, and is generally unfitted for agriculture without irrigation. CHAPTER XXIX WEATHER AND CLIMATE Weather. — The conditions of the atmosphere at any- given time constitute the weather. It includes pressure, temperature, humidity, state of the sky, precipitation, and direction and force of the wind. The most prominent characteristic of weather is its changeableness, the exact continuance of any given combination being hardly more than momentary. Weather changes are chiefly of three classes : ( i ) daily changes due to the rotation of the earth upon its axis, (2) yearly seasonal changes due to the revolution of the earth around the sun, and (3) irregular changes due to the passage of storms. The daily changes bring relatively warm days and cool nights, the yearly changes bring more or less decided variations of average daily and monthly temperature and rainfall, and the irregu- lar changes bring alternations of fair and stormy weather. These various kinds of changes are of very different prominence and value in different parts of the earth. Climate. — The average succession and distribution of weather conditions at any given place constitute the cli- mate of that locality. The climate of a place can be determined only by taking into account a period of at least ten years. The factors of climate and the different methods of grouping them are very numerous. The most important factors are (1) the mean annual temperature, (2) the annual range of temperature, (3) the mean annual rainfall, and (4) the distribution of rainfall through the year. 335 336 THE ATMOSPHERE The mean annual temperatures of New York and of London are nearly the same, but the range at New York is 40 , while at London it is only 20 . The rainfall of Portland, Ore., is forty-seven inches and of Portland, Maine, forty-two inches ; but in Oregon two thirds of it falls in five winter months, while in Maine it is almost equally dis- tributed through all the months. Weather and climate are so closely related that they are studied to the best advantage together. The distribution of temperature and rainfall has been discussed in previous chapters, but it remains to consider their combinations with each other and with subordinate factors which deter- mine the various climates of the globe. Zones of temperature are well defined by isotherms as given on p. 295. Zones of rainfall bear close relation to the belts of pres- sure and prevailing winds, but their boundaries are vague and shifting, and each zone is far from presenting uniform conditions throughout. To map out zones which represent the distribution of the still more complex phenomena of climate is a difficult matter. The continents and oceans extending north and south cut across all zones and break them up into strongly contrasted portions. In discussing climatic zones sharp distinctions, definite boundaries, and uniform conditions must not be looked for. The Trade or Constant Zone. — Figure 270 shows that the northern and southern limits of the swing of the isotherm of 70 , or the limits of hot summers, correspond quite closely with the tropical belts of high pressure and the limits of the trade winds. If the January isotherm of 30 is substituted for the July isotherm of 70 across North America and from central Europe to Japan, these boun- daries will be near the parallels of 30 south and 40 north latitude, and mark out a zone of tolerably uniform climatic conditions. The zone bounded by these parallels is characterized as a whole by high and relatively uniform insolation amounting everywhere to more than 80 per cent of that at the equator, by prevailingly high temperatures WEATHER AND CLIMATE 337 generally above 6o°, by constant winds, by excessive rain- fall, by an absence of strong seasonal contrasts, and by uniformity of weather conditions from day to day. While the trade winds blow the daily changes are more prominent than any other, and the daily range of temperature may be greater than the yearly. The curves of temperature and pressure show their daily maximum and minimum with scarcely any variation from week to week. The sky is clear or partly cloudy in the daytime, and rain falls, if at all, in the latter part of the day. Cyclones rarely occur, but are extremely violent. The lowlands of the trade-wind belts are largely desert or semi-arid except on east coasts, and include the Sahara, Arabian, Australian, and South African deserts. The wind in these deserts is excessively dry and laden with dust. On account of the absence of cloud and moisture radiation is unchecked. The temperature at mid- day may rise to uo° or 120°, but at sunset the wind subsides and the air soon cools to temperatures not far above freezing. The trade-wind belts are separated and modified by the belt of equatorial calms. As it swings back and forth it carries with it a calm, hot, moist air with abundant cloud and daily rainstorms. These conditions continue at any given place for two or three months, constituting the rainy season. Their most important results appear in the heavy rainfall and dense forests of equatorial Africa, equatorial South America, and the East Indian archipelago. The Asiatic monsoon region is a peculiar and divergent offset from the trade-wind zone, subject to both extremes of wet and dry. In one season of the year the climate resembles that of the Sahara, and in the other that of the Amazon basin. In winter the weather is cool and dry, the northerly trade winds being occasionally interrupted by a mild cyclonic storm. The spring is dry and hot, but by June the southerly monsoon has become established and brings a copious rainfall which continues until October. Transition Areas. — Certain limited areas along the borders of the trade-wind zone are included in that zone in summer and in the anti- 338 THE ATMOSPHERE trade zone in winter. Among these are southern California, the coast lands of the Mediterranean, and southern Australia and Africa. They have a summer temperature between 65 and 8o°, and a winter tempera- ture between 50 and 6o°, the annual range being less than 30 . They are too far from the equator to receive the equatorial rains and too near it to be visited by the cyclonic storms and cold waves of middle latitudes. In summer they have uninterrupted fair weather and in winter a light rainfall. Their climate is among the most enjoyable in the world, and in the northern hemisphere they have long been famous as health resorts. The Southern Antitrade Zone. — Between the tropical belt of high pressure and the Antarctic circle the face of the earth is occupied by the sea, interrupted only by New Zealand and the narrow extremity of South America. This zone presents, therefore, in a high degree, the characteristic oceanic climate of middle latitudes. The westerly winds are almost as steady as the trades, but are much stronger. The ocean currents wheel perpetually with the winds around the polar center, and their circuit is joined in only two or three places by streams of air or water from the north. Cloud, fog, and storm are the rule at all seasons. The mean annual temperature is low and the daily and yearly ranges are small. The chief difference between the seasons is the greater frequency and strength of cyclonic storms in winter. There is neither summer nor winter, in the northern sense of the words, but an alternation, at intervals of two or three days, of more and less stormy weather which may bring rain or snow in any month of the year. The southern zone is truly temperate in that its climate is singularly free from extremes of any kind and uniform in its distribution over the whole zone. It is also truly variable in that weather changes are frequent although not of great magnitude. Here great variations of insola- tion are almost overcome by the equalizing effect of a large body of water. WEATHER AND CLIMATE 339 The Northern Antitrade Zone. — The middle latitudes of the northern hemisphere are occupied by the largest land masses on the globe, and consequently possess a climate in strong contrast with that of the corresponding south- ern zone. The northern zone is truly intemperate, in that its climate is characterized by extremes of all the factors and by a want of uniformity in their distribution. Extremes UNITED STATES 43'/2°LAT. EURASIA 52° LAT. ^ fa UI \ if S z> S .„° % \~- < N ./ s \\ \ A . ul a. -- — „„° 2 1- _i u. / N -- r^ < ^. \ -— -- — -"' N - West Coast Interior — fast Coast Fig. 300. prevail through so large a part of the year that annual averages lose their value. The seasons of maximum and minimum temperature are separated by well-defined tran- sition periods. The occurrence of four distinct seasons of nearly equal length is peculiar to this zone. Daily changes are most prominent in summer, and irregular cyclonic changes in winter. On account of the different heat relations of land and water (see p. 296), the isotherms over the land bend far southward in winter (see Fig. 268) and far northward in summer (see Pig. 269). Over the 340 THE ATMOSPHERE oceans reverse bends are equally decided. This irregular- ity is intensified by a cold current on the west side of each ocean and a warm current^ on the east side. West coasts of the land are swept by winds from the ocean and have an oceanic climate. East coasts are swept by winds from the continents and have a climate similar to that of the continental interiors. Under the combined influence of these conditions the isotherms near either coast extend almost parallel with it, especially in winter, and the tern- j perature varies more rapidly along east-west than along north-south lines. Another result is the great range of temperature in the interior of the land masses (see map, p. 299). The differences of climate between the west coasts and the interiors rival in magnitude those between the equator and the poles. West Coast Climate. — On the western coasts of North America and Europe, in the antitrade zone, the climate is almost as equable as that of the trade wind belt and less stormy than that of the south temperate zone. The air is damp and more than half the days are cloudy. In winter there is an almost constant fog and drizzle of rain, with snow on 3 the higher elevations. The winds are more northwesterly in summer and more southwesterly in winter. There is an excess of rainfall in winter. The winds from the ocean and the prevailing cloudiness com- bine to prevent great or sudden changes of temperature. The lines of equal annual range run parallel with the coast, the range slowly increas- ing northward from 20° to 40 in America and from 15 to 30 in Europe. Eurasia. — On account of the strength and constancy of the south- west winds over the north Atlantic and the great mass of warm water driven by them in the Gulf Stream, the British Isles and Norway enjoy winters of unparalleled mildness for their latitude, and in strong con- trast with the east coast of America on one side and the interior of Eurasia on the other. To the eastward there is a gradual change from these moist and temperate conditions to the dry steppes traversed by the Volga and Ural rivers, the arid plains of the Caspian and Aral de- pression, the bleak wastes of the Tarim and Gobi deserts, and the sever- ity of east Siberian climate, where the lowest winter temperature and WEATHER AND CLIMATE 341 the greatest annual range on the globe occur in the same province. The great ranges of temperature extend with some mitigation to the Pacific coast, where the winters are nearly as dry and cold as in the in- terior. The summers are cool and rainy, with a southeast monsoon. North America. — The region of equable temperature and copious winter rains on the west coast of North America is confined to a nar- row belt between the sea and the mountains. A day's journey by rail would carry the traveler from the mild climate of the coast to the parched and burning deserts of Nevada, or the frozen and almost equally dry plains of the Mackenzie basin. The interior of North America is sec- ond only to central Asia in the intemperate character of its climate. The isotherm of 6o° swings from near the tropic in January almost to the Arctic circle in July, where the summers are long and warm enough to ripen wheat. Summer temperatures above ioo° are common over a great part of the region, and the winter temperature falls in some places to — 50 , the absolute range reaching 170 on the northern boundary of the United States. On the dry and elevated plateaus radiation is rapid and the daily range of temperature is very large. In summer violent local storms bring the greater part of the rainfall, and in winter the whole country from the Arctic Ocean to the Gulf states is liable to be flooded for a time with air below zero. North of latitude 50 the sever- ity of these conditions extends to the Atlantic coast, but south of that parallel they are gradually mitigated toward the southeast. •■' ' _" VV'NTEF V.'E .'-HEP CHANGES AT TE „,, ' . |a THREE CVCLONE^N^WE^JAI Fig. 301. The eastern half of the United States lies open to the northwest winds from the interior center of cold and high pressure in winter, and to the regular southwest antitrades from the heated tropical center of Mexico and Arizona in summer. Cyclonic storms are more frequent than in any other part of the northern hemisphere, and while they impress upon the climate its peculiar variability, they carry moisture from the Gulf far inland. From October to May the weather may be 342 THE ATMOSPHERE Fig. 302— A summer weather map. said to be controlled by a succession of cyclones and anticyclones, which alternate with each other on an average as often as twice a week, and bring each its characteristic type of weather. In summer, cyclonic and anticyclonic conditions are much less fre- quent and pronounced, the distribution of pressure is often nearly uni- form, and the isobars have no definite form or trend. The regular southwest antitrade wind blows with considerable strength, bringing a steady, moderately high pressure and clear, hot days. The nights are calm, clear, cool, and dewy. The barograph and thermograph curves show their daily maxima and minima with a regularity equal to that of the trade-wind region. These conditions may continue for weeks, but are liable to interruption by the passage of moderate cyclones. rrr^^^ -Of TEMPERATURE AND. PRESSURE AT. T.ERSE HAUTE, IND. \ ■ IN SUMMEIR, JUNE 27 TO JULY 4-, IS38i Fig. 303. WEATHER AND CLIMATE 343 Summary of the Climate of the United States. — The United States may be divided into three regions which exhibit three strongly marked types of climate peculiar to a continental area in middle latitudes. (i) The Pacific coast enjoys the mild, windward coast climate due to the prevailing winds from the ocean. ' The winters are warm and the summers cool except in the south- Fig- 304. —Cyclone paths and climatic regions of United States. ern part. The wet season occurs in the winter and the dry season in summer. The rainfall increases rapidly from south to north. (2) The plateau region is bounded on the west by the Pacific mountains and on the east by the rainfall line of 20 inches and the contour line of 2000 feet, both of which lie near the 100th meridian of west longitude. Its width is about 1 500 miles, and its average elevation about 5000 feet. It has a truly continental climate intensified by its altitude. The winters are cold and the summers 344 THE ATMOSPHERE Fig- 3°5- — Average temperature for January in the United States. (After Greely. ) hot. In winter the cold is continuous and in summer the daily range of temperature is great. This region contains the coldest part of the United States, in Montana, and the hottest, in Arizona. Great and sudden changes of tern- Fig. 306. — Average temperature for July in the United States. (After Greely.) WEATHER AND CLIMATE 345 7CfV S\\ / 6^" ll r \/^ } l^- ^)Tjf — — *~j M ( lY JBtf K%\ 4 > oS 03 MS '5 -O S-3 2 MS | fe b ■* CJ *—? , u ^ rt '^ ui : ^ cS Oh c -.W> two - is 5.S « >, .£ en -B JS B ]B_ c -2 rt S2-1 co * ._ cci cti -B -B "3 CQ CO W CO S-B ~ O -2 "5 oj '0 3 4-t J-l OS . 1 (/I -l-» • — C oj -tj m "O ca rvi oS ca e " ts cj ' M ^ ^r 1 -B B CJ CS 5 T5 pj nj <-> is 5 b a, X <3 tn cci o ^ Ph 2 < u -B M-B-B ti « cd C c C C JS J3 ioco lack, or s ong rojec tiick road UPQ JPnHfl CJ — 1 M cj C O rj H 4J "q^ U Oi, «) £; CO -t-J cj 5 ^ H -B o oS cj S n csi v< ■— ' O 2 Cut) P-. - CU £ B •- OS ^ p — ■ - 3-; o tuo o '35 o ° t;-B >-b O - ■ — 1 CJ w CJ cu ? B S 3 cu ■ COH ,. r 3. C •2.2 ^ - tn 3~ 05 en 03 cu hB-a"? u --; cj oj B B •n i^ ^ '" § d -5 S-b^o 390 LIFE Civilized Men. — No people of Ethiopian race has ever risen, without help from some other race, above a condition of barbarism. The same is true of the American race with the exception of the Aztecs of Mexico and the Incas of Peru. Native civilization belongs chiefly to the Cauca- sian race, and in a lesser degree to the Mongolian. As in the case of other animals, man has attained his highest development in the north temperate zone, within which all the great civilizations of ancient and modern times have sprung up. The great centers of civilization have been located upon lowlands which either were traversed by large rivers, or were easily accessible from the sea, or both. This is true of China, India, Babylonia, Egypt, Greece, Italy, Great Britain, and the countries of western Europe. Man can live in the north frigid zone, but it furnishes only the bare necessities of life, and the whole of human energy must be expended in obtaining them. In the torrid zone the climate is oppressive and hardly permits prolonged exertion. Clothing and shelter are scarcely needed, and food can be procured without much effort or forethought. The luxuriance of plant and animal life is so great that man is over- whelmed by it and remains insignificant. In temperate climates food and clothing may be obtained in abundance, but only by the exercise of industry and invention. The inclement and unproductive winter makes it necessary to provide beforehand substantial shelter and a sup- ply of food. These conditions stimulate men to exert their physical and mental powers, and their efforts are rewarded with comforts and luxuries. Well-watered lowlands are very productive, especially of the cereal grains. The presence of navigable rivers or the sea renders travel and transportation easy, leading to commerce and that inter- change of ideas characteristic of enlightened peoples. Influence of Physical Features. — The degree of civiliza- tion, power, and influence attained by a state or people depends upon numerous physical factors belonging to the territory which it occupies. The latitude, distance from the sea, and relief of a country largely determine its THE GEOGRAPHY OF MAN 391 temperature, rainfall, and soil, consequently its products and the occupations of its people. Switzerland, the Scotch highlands, and Norway produce a different type of people from those which inhabit the low plains to the south of them. A long and irregular coast line, with arms of the sea extending far into the land, constitutes one of the most favorable conditions for human occupation. The contrast between Europe and Africa, and between Great Britain and Australia, is in this respect very great. Mountain ranges act as barriers to rainfall, making it unequal upon opposite slopes, form the natural boundaries of states, and in their disturbed and dislocated structure expose veins of coal, iron, and other minerals. A network of rivers and lakes affords opportunities for internal commerce and fur- nishes water power for manufactures. The size or area of a country exerts no small influence upon the condition of its people. The area of the United States is so large that it includes all temperatures from sub-tropical to frigid, all rain belts from the heavy rainfall of the Pacific and Gulf coasts to the almost rainless deserts of the Great Basin, all relief forms from the low plains of the Atlantic coast and Mississippi basin to the high plateaus and mountains of the western half. Hence its agricultural and mineral products are so varied and abundant as to render it largely independent of the rest of the world, and to constitute the largest resources of natural wealth belonging to any one people in the world. Natural Resources. — ■ Civilized men are learning more and more how to modify the conditions of their environ- ment and to turn them to their own advantage. They are everywhere engaged in developing the natural resources of their country. These consist of three classes : (1) agri- cultural products, both vegetable and animal, which depend 392 LIFE upon the soil, but also upon the energy supplied by the sun in the form of heat and light, and upon water vapor in the air ; (2) mineral products, such as coal, iron, copper, lead, tin, zinc, gold, silver, salt, and building stones, which are con- tained in the crust of the earth, and the quantity of which can not be increased ; (3) resources zvhich furnish pozver, such as coal, petroleum, natural gas, water power, and wind power, used to run machinery. Of all these, agricultural resources furnish the foundation of the state. While the quantity is not unlimited, it can be increased by skillful management so as to support a population more dense than now exists anywhere in the world except in China and India. Of min- eral resources, coal and iron ore — the indispensable mate- rials of modern civilization — are by far the most important. These are not now being formed, and the exhaustion of the supply seems to be a certainty of the remote future. The more extensive use of water power, as at Niagara Falls, will make the consumption of coal less rapid. The increasing use of machinery involves the use of greater quantities of iron and other metals and of fuel, the growth of manufac- ture, and the extension of commerce. The building of rail- ways has rendered most other means of land transportation useless or of less importance. The use of large and swift steamships has changed the sea from an impassable barrier to a means of easy communication between peoples. Within a century the world has shrunk for purposes of human intercourse to practically one tenth its former size. The most enterprising nations are extending their lines of com- merce and influence in every direction, and the more pro- lific peoples, like the British, Germans, Russians, and the people of the United States, are expanding by an- nexation and colonization to occupy, control, and develop nearly every portion of the habitable globe. APPENDIX I THE EQUIPMENT OF A GEOGRAPHICAL LABORATORY 1 Geography can not be learned without suitable material and appli- ances any more profitably than physics or chemistry. The apparatus required consists of models, maps, pictures, specimens, and instruments for work in meteorology. Models. — Many models of especially instructive regions which have been adequately surveyed, are now available, but really good models are necessarily rather expensive. Those of crude and inaccurate construc- tion, and with vertical heights greatly exaggerated, are liable to teach more error than truth, and are worse than useless. The following mod- els (sometimes called relief-maps) are among the best and most useful. Fig 335. —Howell's model of the United States. The United States, Gulf of Mexico, and portions of the Atlantic and Pacific oceans, constructed as a section of a sphere 16 feet in diameter. Horizontal scale, 40 miles to 1 inch : vertical scale, 8 miles to 1 inch. Size 4 ft. 2 in. x8 ft. $125.00. 1 See Journal of School Geography, 2, 170. 393 394 APPENDIX I A copy of the above on a scale of 120 miles to 1 inch. Size 1 ft. 6 in. x 2 ft. 10 in. Easily portable. $25.00. The Uinta and Wasatch mountains, showing folded and faulted mountains, canyons of Green River, escarpments, and dip slopes. Scale 4 miles to 1 inch. Vertical heights exaggerated 2 to 1. Size 4 ft. X4 ft. 2 in. $125.00. The Grand Canyon of the Colorado and the plateaus of southern Utah. Horizontal and vertical scales 2 miles to 1 inch. Size 6 ft. x6ft. $125.00. The Henry Mountains and vicinity. Scale 2 miles to 1 inch. Size 3 ft. x 3 ft. 6 in. $30.00. Stereogram of the Henry Mountains showing the same region as the preceding as it would be if the eroded material were restored. $12.00. Southern New England. Scale 2 miles to 1 inch. Size 5 ft. 7 in. x8 ft. 4 in. $135.00. The Chattanooga district. Scale 1 mile to 1 inch. Size 3 ft. 4 in. x 3 ft. 10 in. $50.00. New York. Horizontal scale 12 miles to 1 inch; vertical heights exaggerated 5 to 1. Size 2 ft. 1 in. x 2 ft. 10 in. $25.00. Mt. Shasta. Size 3 ft. 3 in. x 3 ft. 4 in. $40.00. Mt. Vesuvius. Size 2 ft. x 2 ft. 6 in. $10.00. These models are made and sold by Edwin E. Howell, 612, 17th St. N.W., Washington, D.C. Mr. Howell also furnishes a set of five models of the continents at $150.00. Professor John F. Newsom, Leland Stanford Junior University, Cali- fornia, furnishes the following six models : Morrisons Cove, Penn., showing anticlinal and synclinal folds. Size 1.9 ft. x 2.3 ft. $35.00. Allamakee County, Iowa, showing topographic forms in a region of horizontal strata. Size 2.3 ft. x 2.5 ft. $20.00. Marysville Buttes, Cal., showing volcanic cone surrounded by sedi- mentary strata. Size 1.8 ft. x 1.8 ft. $12.00. Ideal Restoration of- Marysville Buttes, showing maximum develop- ment of a volcano. $5.00. ■ Crater Lake, Oregon. Size 1.1 ft. x 1.4 ft. $7.00. Sectioned model of the Leadville region, Col., showing intense fold- ing, faulting, and igneous intrusions. Size 2.6 ft. x 3.2 ft. $85.00. EQUIPMENT OF A GEOGRAPHICAL LABORATORY 395 Harvard Geographical Models, a set of three, each 25 x 19 inches, showing (1) Mountains Bordering the Sea, (2) Coastal Plain and Mountains, (3) Embayed Mountains. Ginn & Co., Boston. Per set, ,$20.00. Jones's New Model of the Earth, mounted as a globe, 20 inches in diameter. Vertical scale exaggerated 20 times. A. H. Andrews & Co., Chicago. $50.00. Maps. — Large-scale maps for the wall or table are indispensable for the class room, and can now be obtained at small expense. 1 Map of the Alluvial Valley of the Mississippi River. Scale 5 miles to 1 inch. 8 sheets. Per set, $1.00. Map of the Alluvial Valley of the Upper Mississippi River. 4 sheets. Per set, 70 cents. Map of the Lower Mississippi River in 32 sheets. Scale 1 mile to 1 inch. Per set, $1.60. Map of the Upper Mississippi River in 30 sheets. Per set, $1.50. Address Secretary Mississippi River Commission, St. Louis, Mo. Map of the Missouri River from its mouth to Three Forks, Mont. Scale 1 mile to 1 inch. 96 sheets, 5 cents per sheet. Address Secre- tary Missouri River Commission, St. Louis, Mo. Survey of the Northern and Northwestern Lakes. Price list may be obtained from United States Engineer's Office, Detroit, Mich. The Niagara Falls and Lake St. Clair charts are of especial value. United States Coast and Geodetic Survey Charts. An illustrated catalogue of charts may be obtained on request from the Superintendent, Washington, D.C. Old charts which have been superseded, but are not less valuable for teaching purposes, may often be obtained free. Topographical Atlas of New Jersey ; 20 sheets at 25 cents each, or the set, $5.00. Geological Survey of New Jersey, Trenton. Topographical Atlas of Massachusetts ; 54 sheets at 5 cents each, or the set $4.25. Topographical Survey of Massachusetts, Boston. Topographical Map of Rhode Island, $2.00. Topographical Survey of Rhode Island, Providence. Topographic Atlas of the United States. Published in sheets, many of which are accompanied by geological maps, pictures, and descriptive text, the collection being called a Folio (in the following pages folios are marked thus: *). Relief shown by contour lines. Single sheets 1 Consult Governmental Maps for Use in Schools, Henry Holt & Co., N.Y. 30 cents. Also Journal of School Geography, 1, 200. 396 APPENDIX I 5 cents, or $2 00 per 100. Folios, 25 cents each. Price list sent on appli- cation to the Director, U. S. G. S., Washington, D.C. Out of several thousand the following are especially useful : — Physiographic Types, Folio 1 : Ten maps with descriptive text : A Region in Youth, A Region in Maturity, A Region in Old Age, A Rejuvenated Region, A Young Volcanic Mountain, Moraines, Drum- lins, River Flood Plains, A Fiord Coast, A Barrier Beach Coast. Folio 2 : A Coast Swamp, A Graded River, An Overloaded Stream, Appalachian Ridges, Ozark Ridges, Ozark Plateau, Hogbacks, Volcanic Peaks, Plateaus and Necks, Alluvial Cones, A Crater. A single sheet map of the United States. Relief shown by nine shades of brown color; also with relief shown by contours. Marine Plains : Glassboro, N.J. Fluviatile Plains : Marysville,* Cal. Lacustrine Plains: Sierraville, Lassen Peak,* Cal.; Tooele Valley, Utah ; Disaster, Paradise, Nev. Glacial Plains : Marion, la. Dissected Plains : Spottsylvania, Farmville, Palmyra, Va. ; McCor- mick, Ga. ; Clanton, Ala. Upland Plains : Springfield, Bolivar, Tuscumbia, Fulton, Mo. ; Iola, Kan. Plateaiis : Fort Defiance, Ariz. ; Las Animas, Kit Carson, Lamar, Granada, Col. Dissected Plateaus : Mesa de Maya, Col. ; Marsh Pass, Ariz. ; Cold- water, Meade, Kan. ; Hazard, Salyersville, Warfield, Ky. ; Kanawha Falls, Nicholas, Huntersville, Hinton, W. Va. ; Scottsboro, Ala. ; Se- wanee, Tenn. ; Marshall, Ark. ; Gaines, Pa. Trenched Plateaus, Cliffs, Suites, Canyons : Kaibab, Echo Cliffs. Ariz. ; Escalante, Price River, Kanab, Utah. Denuded Plateaus, Escarpments, Outliers, Mesas : Watrous, Corazon, N. Mex. ; Sewanee,* Tenn. ; Kaaterskill, N.Y. ; East Tavaputs, Utah ; Tusayan, Ft. Defiance, Ariz. ; Abilene, Brownwood, Tex. Basin Ranges : Tooele Valley, Utah ; Disaster, Nev. ; Alturas, Cal. Rocky Mountains : Canyon City, Huerfano Park, Pikes Peak,* Platte Canyon, Telluride,* Col. ; Livingston,* Mont. ; Yellowstone National Park,* Wyo. Wasatch and Uinta Mountains : Salt Lake, Uinta, Utah. Black Hills: Rapid, S.D. * A folio (see p. 395). EQUIPMENT OF A GEOGRAPHICAL LABORATORY 397 Appalachian Mountains: Lykens, Pottsville, Harrisburg, Hummels- town, Pa.; Monterey,* Franklin,* Estillville,* Va. ; Piedmont,* W.Va. ; Mt.Mitchell, Asheville, Pisgah, N.C. ; Briceville,* Cleveland,* Loudon,* Pikeville,* Kingston,* Chattanooga,* Tenn. ; Ringgold,* Atlanta, Ga. ; Gadsden,* Stevenson,* Ala. Mountain Highlands : Hawley, Chesterfield, Granville, Becket, Mass. ; Winsted, Bridgeport, Cornwall, Derby, Conn. ; Hackettstown, N.J. Volcanoes: Shasta, Marysville,* Cal. ; San Francisco Mt., Ariz.; Mt. Taylor, N. Mex. Lava Plains: Modoc Lava Bed, Cal.; Bisuka, Boise, Silver City, Nampa, Ida. Laccolitcs: San Rafael, Henry Mountains, Utah. Volcanic Dikes, Mesas, and Plugs: Absaroka,* Wyo. ; Elmoro,* Col. ; Greenfield, Holyoke,* Mass. Flood Plains: Donaldsonville, Mt. Airy, Pointe a la Hache, Gibson, Houma, La. ; Fort Payne, Ala. ; St. Louis (east sheet), Independence, Marshall, Mo. ; Junction City, Kan. ; Minden, Neb. Meandering Valleys : Versailles, Tuscumbia, Mo. ; Palo Pinto, Gran- bury, Tex. Transverse Valleys and Water Gaps: West Point, Tarrytown, Har- lem, N.Y. ; Harpers Ferry,* Va. ; Harrisburg, Delaware Water Gap, Pa. Filled Valleys : Lake, Wyo. ; Independence, Marshall, Mo. ; Disaster, Granite Ridge, Long Valley, Nev. Migrating Divides and Trellised Drainage : Doylestown, Pa. ; Dahlonega, Gainesville, Walhalla, Ga. ; Franklin, Pocahontas, Va. Hudson River : Hoosick, Troy, Albany, Coxsackie, Catskill, Pough- keepsie, N.Y. Cataracts and Gorges : Rochester (special), Niagara Falls (special), N.Y. ; Minneapolis, Minn. ; Great Falls, Mont. ; Yellowstone National Park,* Wyo. Moraines, Drumlins : Madison, Sun Prairie, Waterloo, Watertown, Oconomowoc, Wis. ; Charlestown, R.I. ; Stonington, Conn. Cirques: Anthracite and Crested Butte,* Col. Glacial Lakes : Webster, Mass. ; Madison, Geneva, Wis. Finger Lakes : Ithaca, Elmira, N.Y. Volcanic Lakes : Ashland, Crater Lake (special), Ore. Old Lake Outlets : Ottawa, Marseilles, Lasalle, Calumet, Des Plaines, 111. ; Oneida, Oriskany, Schenectady, Cohoes, N.Y. * A folio (see p. 395). 398 APPENDIX II River Terraces : Springfield, Mass. ; Hartford, Conn. Drowned Valleys and Fiords : Wicomico, Md. ; Fredericksburg,* Nomini,* Mt. Vernon, Va. ; Norwich, New London, Conn. ; Portland, Casco Bay, Boothbay, Me. Bays and Bars : Duxbury, Nahant, Boston, Mass. ; Ontario Beach, N.Y. ; Duluth, Minn. ; San Francisco, Cal. ; Seattle,* Tacoma,* Wash. Barrier Beaches and Spits : Sandy Hook, Asbury Park, Barnegat, Long Beach, N.J. ; Marthas Vineyard, Gay Head, Provincetown, Mass. Foreign Maps. — Most of the European countries have published governmental maps on a large scale, many of which are models of the cartographic art. Consult " Large-Scale Maps as Geographical Illustra- tions," by Davis, Journal of Geology, 4, 484. Pictures and Lantern Slides. — Pictures are now so common and cheap that a very good collection can be made from magazines, railroad advertisements, and newspapers. A few collections of large photo- graphic or autotype pictures containing many illustrations of geographical features have been made. Scenes from Every Land, 500 photographs, J. W. Jones, Springfield, Ohio, $5.00; America Photographed, 210 views, Donahue & Hennebery, Chicago, $1.00; and Our Own Country, 500 pictures with descriptive text, The National Co., St. Louis, Mo., $3.50, may be recommended. The lantern for projection has become the common adjunct of school instruction. It is now supplied by all dealers in scientific instru- ments. E. E. Howell, Washington, D.C., supplies a list of slides selected by Professor Davis. The American Bureau of Geography, Winona, Minn., has undertaken to supply good photographs and slides. Announcements are made in its Bulletin, quarterly, $1.00 per year. Suggestive exercises in laboratory work in geography will be found in Journal of School Geography, 1, 172, 204; 3, 368. APPENDIX II METEOROLOGICAL INSTRUMENTS 1 The Measurement of Temperature. — Standard Thermometer ($2.75). Temperature is measured by a thermometer. This instrument consists of a small glass tube with a bulb at one end. The bulb and part of the * A folio (see p. 395). 1 See Journal of School Geography, 3, 241. METEOROLOGICAL INSTRUMENTS 399 tube are filled with mercury or alcohol, the air is removed and the tube closed. The bulb is then placed in melting ice and the point at which the top of the column stands is marked 32 and called the freezing point. The bulb is then placed in the steam above boiling water, and the point at which the top of the column stands is marked 212 and called the boiling point. The space between is divided into 180 equal degrees and the graduation is ex- tended below 32 on the same scale. This is called the Fahrenheit scale. The Centigrade scale, which marks the freezing point o° and the boiling point ioo°, is also much used. In determining the error of a common ther- mometer by comparison with a standard, the comparison should be made at as many different points in the scale as possible. Maximum and Minimum Thermometers ($8.25). These instruments should be mounted together upon a board as shown in Fig. 337. The tube of the maximum is bent and constricted just above the bulb. As the temperature rises, the mercury passes up the tube. When the temperature falls, the column breaks at the constriction and remains at the highest point reached. After reading, the instrument should be set by rotating it rapidly around the pin at its upper end. The minimum is filled with alcohol and contains a steel index. When the temperature falls, the index is dragged downward by the surface tension of the alcohol. When the temperature rises, the index Fig. 336. Standard thermometer. Fig. 337- is left behind at the lowest point reached. The instrument is set by raising the bulb until the index slides down to the surface of the alcohol. Shelter. Thermometers should be exposed in a latticed shelter in an open space away from buildings and four to ten feet above the ground. 400 APPENDIX II If a shelter is not available, they may be placed outside a north window in such a position that they may be read without open- ing the window. The Measurement of Pressure. — The Mercurial Barom- eter ($5.75 to $30.- 00) consists of a glass tube and cup containing mer- cury, inclosed in a metal tube for protection and pro- vided with devices for convenient and accurate reading. The distance to be measured is the Fig- 338. —A latticed shelter. difference between the level of the mercury in the tube and its level in the cup. As the mercury falls in the tube it rises in the cup, and vice versa : therefore it is necessary to bring the mercury in the cup to a certain fixed level before reading. The cup has a leather bot- tom which is pressed by the screw C, Fig. 339. By turning this screw, the level of the mercury is adjusted so that its surface just touches the point of an ivory pin at B. This is the zero point of the scale. The zero is sometimes marked by a black line on the outside of the cup. In the upper part of the metal tube two slots are cut so that the mercury column can be seen. Two metal pieces slide in the slots and are moved by the screw E. Placing the eye in a position where the lower edge of the front piece just hides the lower edge of the back piece, move both pieces until their lower edges just cut off the light between themselves and the sur- face of the mercury at the center of the tube. To the metal tube is fastened a scale graduated into inches and tenths. The hundredths are read from the vernier, or Fig. 339. — Mercu- rial barometer. METEOROLOGICAL INSTRUMENTS 401 @ sliding piece, being indicated by the line on the ver- nier which coincides with a line on the fixed scale. The reading in Fig. 340 is 29.25 inches. Some barometers are read without a vernier. The mercury of the barometer is expanded by heat, and when the temperature is high it requires a longer column to balance the air pressure than when the temperature is low. It is therefore neces- sary to read the attached thermometer at D, and to correct the barometer reading for temperature. In drawing isobaric maps the observed pressures are generally reduced to what they would be if the observing station were at sea level. This is done by adding the length of a column of mercury which would balance a column of air extending from sea level up to the station. Tables for the reduction of the barometer reading to 32 F. and to sea level are given on pp. 406, 407, and in Ward's Practical Exer- cises in Elementary Meteorology. The Aneroid Barometer ($6.00 to $15.00) indi- cates pressure by the expansion and contraction of a vacuous metal box, the movement of which is communicated to a pointer like a clock hand, which revolves over a circular scale. If com- pensated for temperature, this instrument is accurate and convenient. The Measurement of Humidity. — The Hygrometer ($6.50) consists of two simi- lar thermometers mounted upon a board. The bulb of one is kept wet by being cov- ered with a lamp wick which dips into a cup of pure water. The evaporation of the water cools the mercury and makes it stand lower than in the dry thermometer. If the air is dry, evaporation is rapid, and the dif- ference between the two thermometers may be 15 or 20 . If the air is damp, evaporation is slow and the difference is small. The indications of the hygrometer are made definite by refer- ence to tables. See pp. 408, 409; or Psychrometric Tables, published Fig- 340/ Fig. 34 1 -— Aneroid barometer. DR. PHYS. C.EOG. 24 402 APPENDIX II by the U.S. Weather Bureau (price 10 cents) ; or Ward's Practical Exercises in Meteorology. The wet bulb should be fanned before reading, to prevent the accumulation of vapor near the instrument. Any thermometer may be made to serve as a hygrometer by covering its bulb with wet muslin and swinging it around in the air by an attached cord until the mercury ceases to fall. Its reading should be com- pared with that of a similar dry thermom- eter. This instrument is called the sling ftsydirometer. The Measurement of Precipitation. — The Rain Gauge ($1.25 to $5.25) is a metal cylinder having an inside diameter of 8 inches. The receiver is funnel-shaped and carries the water into a measuring tube whose area of cross section is one tenth that of the receiver. Thus one tenth of an inch of rainfall gives a depth of 1 inch of water in the tube. The depth is measured by a stick graduated in inches and tenths. The gauge should be mounted in .a verti- cal position several feet above the ground in an open space at a distance from build- Fig. 342. — Hygrometer. ings and trees, and read emptied every morning. Snow is estimated as water after melt- ing. The Tliermograph and Baro- graph ($30.00 each) are instru- ments which make continuous records of temperature and pres- sure upon a strip of paper. They are indispensable for a thorough and comprehensive study of the weather. Instruc- tions for their use are furnished by the dealers. lii-iil Section. Scale Fig. 343. -Rain gauge. METEOROLOGICAL INSTRUMENTS 403 Fig. 344. — Thermograph. Measurement of the Wind. — The direction of the wind can not be determined with accuracy without the use of a vane. The best vane is Fig- 345— Barograph, made of wood 6 feet long, and has a divided tail, the two parts making an angle of 22|°. It should be placed in a position above all trees and buildings. & Vertical Sec. Fig. 346. — Vane. 4°4 APPENDIX II The Anemometer is a windmill with cup-shaped arms which records by the number of its revolutions the wind velocity. For purposes of elementary study it is suffi- cient to estimate the wind velocity according to the fol- lowing scale, o. Calm. i . Light, 2-5 miles per hour, moving leaves. 2. Moderate, 7-10 miles, moving branches. 3. Brisk, 1 8-20 miles, swaying branches, blowing up dust. 4. High, 27-30 miles, sway- ing trees, blowing up twigs. 5. Gale, 45-50 miles, break- ing branches, loosening bricks, signs, etc. 6. Hurricane, 75 miles, de- fig- 347- — Anemometer. straying everything. Meteorological instruments are furnished by many firms : Queen & Co., Philadelphia, Pa.; H. J. Green, 1191 Bedford Ave., Brooklyn, N.Y. ; L. E. Knott Apparatus Co., Boston, Mass.; Julien P. Friez, Balti- more, Md. Form for Meteorological Record 3 O 3 ft Temperature. i ft Wind. Cloud. Precipit. Date. >> Q c g u Q >> V > 3 c S < -a c 2 c E < The signs used on the U. S. Weather Maps may be used for wind direction, amount of cloud, and kind of precipitation. A laboratory course in elementary meteorology is outlined in the Journal of School Geography, 1, 41 ; 2, 2, 56, 96, 104, 139. METEOROLOGICAL INSTRUMENTS 405 Use of the Tables. — The tables on pp. 406-409 are almost self- explanatory ; but students not accustomed to the use of such tables should study the following explanations of their use. (1) Temperature corrections for barometer readings (p. 406), some- times called "tables for reducing barometer readings to 32 ." In the first column, in heavy-faced type, are temperatures from o° to ioo°, and on the same line with each temperature are printed, in ordinary light- faced type, eight different numbers in as many columns. The correc- tion to be applied is the number in the column with the heading (in heavy-faced type) which is nearest the barometer reading. For instance, if a barometer reads 28.21, and the attached thermometer 70 , the tem- perature correction is found in the column headed 28, and on the line with 70 . Applying the correction, .11, we have 28.10 as the corrected reading. For temperatures below 28 the correction is to be added, but for temperattires above 28 the correction is to be subtracted. (2) Table for reducing barometer readings to sea level (p. 407). As in the other tables, the known data are printed in heavy-faced type, and the quantities given by the table in light-faced type. For any particular elevation, the amount to be added varies with the temperature. For instance, if the elevation is 1200 feet, the amount is 1.43 at o° tempera- ture, 1.40 at io°, and so on. If the barometer reading at elevation 1200 feet, corrected for temperature, is 28.10, and the air temperature is 70 , the amount to be added is found from the table to be 1.24. The read- ing as reduced to sea level is therefore 29.34. (3) Table for finding relative humidity (pp. 408, 409). The various columns in the three parts of this table are headed by numbers in heavy- faced type from 1 to 42, each representing a possible difference between the reading of the dry thermometer and that of the wet-bulb ther- mometer at the same time, the reading of the wet-bulb thermometer being always the lower. On each line of the table are printed, in light-faced type, the various percentages of relative humidity for a certain tempera- ture (printed in heavy-faced type at the left of the table) as given by the dry thermometer. For instance, if the temperature (dry thermom- eter) is 70 , a difference of i° in the readings of the dry and wet-bulb thermometers indicates a relative humidity of 95 per cent (p. 408) ; a difference of 2 , a relative humidity of 90 per cent ; a difference of 15 , a relative humidity of 37 per cent (p. 409) ; and so on. The table on pp. 408, 409, is correct for a barometrical pressure of 29 inches, and approximately correct for all other ordinary pressures. 406 APPENDIX II Temperature Corrections for Barome- ter Readings: Amounts to be Added 14" i6° i8° 24" 26 28° Barometer reading, inches 24 25 26 27 28 29 30 31 06 .07 •07 .07 .07 .08 .08 06 .06 .06 .07 .07 •07 •07 0=; .06 .06 .06 .06 .07 .07 •05 •05 ■o.S .06 .06 .06 .06 05 •05 •05 •05 •05 •05 .Ob 04 .04 .04 •OS •OS •OS •OS 04 .04 .04 .04 .04 .04 •OS 03 •03 .04 .04 .04 •04 .04 °3 •03 •03 •03 •03 •03 •03 02 .02 •03 ■03 •03 ■03 •03 02 .02 .02 .02 .02" .02 .02 01 .02 .02 .02 .02 .02 .02 01 .01 .01 .01 .01 .01 .01 01 .01 .01 .01 .OI .01 .01 00 .00 .00 .00 .00 .00 .00 Temperature Corrections for Barome- ter Readings: Amounts to be Subtracted 29" 30° 3I o 32 33° 34° 35° 3&° 37° 38 39° 40 41° 42 43° 44° 45° 46° 47 48 49° 50 5i° 52 53° Barometer reading, inches 24 25 26 27 28 29 30 31 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .01 .02 .01 .01 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 .02 •03 •03 .02 .02 .02 •03 •03 •03 •03 •03 •03 •03 •03 •03 •03 •03 •03 •03 ■03 ■03 •03 •03 •03 •03 •03 •°3 •03 •03 .04 .04 •03 •03 ■03 •04 .04 .04 .04 •03 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 .04 •OS .04 .04 .04 .04 .04 •OS •OS .04 .04 .04 •05 •05 •OS •OS .04 .04 •OS •OS •05 •OS •o.S .04 •OS •05 •OS •05 •°5 .ob •OS •OS •OS •OS •OS .06 .06 •OS •OS .os .ob .ob .ob .ob .os •OS .06 .06 .ob .06 .ob •OS .06 .06 .06 .06 .06 .07 .02 •03 ■03 ■03 .04 .04 .04 •04 •OS ■05 •OS •05 .06 .06 .ob .07 I .07 1 Temperature Corrections for Barome- ter Readings: Amounts to be Subtracted a. S SB 54° 55° 5°° 57 n 58 59° 6o° 6i° 62 63 64° 65 66° 67° 68° 69 70 7i° 72- 73° 74° 75 o 76 o 77° 78° 79° 8o° 8i° 82° 83° 84° 85° 86° 87° 90 u 9 i° 92° 93° 94° 95° 96° 97° 98° 99° 100° Barometer reading, inches 24 25 26 27 28 29 30 31 .06 .06 .06 .06 .ob •07 •07 .06 .06 .06 .06 •07 •07 •07 .ob .ob .ob •07 .07 .07 •07 .ob .ob .07 •07 •07 .08 .08 .ob .07 .07 •07 •°7 .08 .08 .07 .07 .07 .07 .08 .08 .08 .07 .07 •07 .08 .08 .08 .09 •07 .07 .08 .08 .08 •09 .09 .07 .08 .08 .08 .09 .09 .09 .08 .08 .08 .08 .09 .09 .09 08 .08 .08 .09 .09 .09 .10 08 .08 •09 .09 .09 .10 .10 08 .09 .09 .09 .10 .10 .10 08 .09 .09 .09 .10 .10 .10 .09 •09 .09 .10 .10 .10 .11 •09 .09 .10 .10 .10 .11 .11 ■09 .09 .10 .10 .11 .11 .11 .09 .10 .10 .10 .11 .11 .12 .09 .10 .10 .11 .11 .11 .12 .10 .10 .10 .11 .11 .12 .12 .10 .10 .11 .11 .12 .12 .12 .10 .11 .11 .11 .12 .12 •13 .10 .11 .11 .12 .12 .12 • IS .11 .11 .11 .12 .12 •13 •13 .11 .11 .12 .12 •1.3 •1.3 •1.3 .11 .11 .12 .12 •13 •13 .14 .11 .12 .12 •13 •13 •14 •14 .11 .12 .12 ■ z 3 •13 .14 ■14 .12 .12 ■13 •13 .14 •14 •i.S .12 .12 •13 •13 .14 •14 •is .12 •13 •1-3 .14 •14 •is •is 12 •13 •13 .14 .14 •i.S •i.S 12 •13 ■14 •14 •i.S •i.S .ib 1.3 ■13 •14 •14 •is •is .ib M •13 ■14 ■i.S •i.S .ib .ib 13 .14 .14 •IS •is .ib .ib 13 .14 .14 •IS .16 .ib •17 14 .14 •i.S •i.S .ib .lb ■17 14 .14 •IS ■i.S .lb •17 •17 H •IS ■IS .ib .lb •17 •17 14 •15 •15 .ib ■17 •17 .18 14 •is .16 .16 •17 •17 .18 i.S •i.S .ib .ib •17 .18 .18 IS •is .ib •17 •17 .18 .19 i.S .lb .ib ■17 .18 .18 •19 is .ib •17 •17 .18 .18 •19 is .16 •17 ■17 .18 •19 ■19 METEOROLOGICAL INSTRUMENTS 407 TABLE FOR REDUCING BAROMETER READINGS TO SEA LEVEL: Amounts to be added Elevation Temperature in feet 0° 10° 20° 30° 40° 50° 6o° 70 8o° 90 IOO .12 .12 .12 .12 .11 .11 .11 .11 .10 .10 200 .24 .24 •23 ■23 .22 .22 .22 .21 .21 .20 300 •36 •36 •35 •34 ■34 ■33 ■32 •32 •31 •30 400 .48 •47 .46 .46 ■45 ■44 43 .42 .41 .40 500 .60 •59 •58 •57 •56 •55 •54 •53 •52 •5i 600 .72 •71 .69 .68 .67 •65 .64 ■63 .62 .61 700 .84 .82 .81 •79 .78 .76 •75 ■73 •72 •71 800 .96 •94 •92 .90 .88 .87 •85 .84 .82 .81 900 1.08 1.06 1.03 1. 01 •99 •97 .96 •94 .92 .90 1000 1.20 1. 17 i-i5 1. 12 1. 10 1.08 1.06 1.04 1.02 I. CO IIOO i-3i 1.29 1.26 1.23 1. 21 1. 19 1.16 1. 14 1. 12 1. 10 1200 i-43 1.40 1-37 1-34 1.32 1.29 1.27 1.24 1.22 1.20 1300 i-55 i-5i 1.48 1-45 1.42 1.40 i-37 i-35 1.32 1.30 1400 I.b6 1.63 i-59 1.56 i-53 1.50 1.47 1-45 1.42 1.40 1500 1.78 1.74 1.70 1.67 1.64 1.61 1.58 i-55 1-52 1.49 l600 1.89 i-8 S 1.81 1.78 1.74 1.71 1.68 i.6 S 1.62 i-59 1700 2.00 1.96 1.92 1.89 1.85 1.81 1.78 i-75 1.72 1.69 1800 2.12 2.07 2.03 1.99 i-95 1.92 1.88 1.85 1.82 1.78 1900 2.23 2.19 2.14 2.10 ■ 2.06 2.02 1.98 i-95 1.91 1.88 2000 2-34 2.30 2.25 2.21 2.16 2.12 2.08 2.05 2.01 1.97 2100 2.46 2.41 2.36 2.31 2.27 2.22 2.18 2.14 2.10 2.07 2200 2-57 2.52 2-47 2.42 2-37 2-33 2.28 2.24 2 20 2.16 23OO 2.68 2.63 , 2 -57 2.52 2.47 2 43 2.38 2-34 2.30 2.26 24OO 2.79 2-73 2.68 2.63 2.58 2-53 2.48 2.44 2.40 235 2500 2.90 2.84 2.79 2-73 2.68 2.63 2.58 2-54 2.49 245 2600 3.01 2-95 2.89 2.84 2.78 2-73 2.68 2.63 2.58 2-54 270O 3.12 3.06 3.00 2.94 2.88 2.83 2.78 2-73 2.68 2.63 2800 3-23 3.16 3.10 3-04 2.98 2-93 2.88 2.82 2.77 2-73 29OO 3-34 3- 2 7 3.21 3-15 3-«9 3-o3 2.97 2.92 2.87 2.82 3000 3-45 3-38 3-3i 3-25 3-19 3-13 3-«7 3.02 2.96 2.91 3100 3-5° 3-49 342 3-35 3-29 3-23 3- J 7 3-n 3.06 3.00 3200 3.66 3-59 3-52 345 3-39 3-32 3.26 3.21 3-i5 3.10 33°o 3-77 3-69 3.62 3-55 349 342 3-36 3-3o 3-24 3-i9 3400 3.88 3.80 3-72 3-65 3-59 3-52 346 34° 3-34 3.28 35oo 3-98 3-9C 3.82 3-75 3.68 3.62 3-55 349 343 3-37 3600 4.09 4.01 3-93 3-85 3-78 3-7i 3-65 3-58 3-5 2 • 346 3700 4.19 4.1 1 4-«3 3-95 3.88 3.81 3-74 3-67 3.61 3-55 3800 4-3° 4.21 4-i3 4-°5 3-98 3-9° 3-83 3-77 370 3- 6 4 3900 4.40 4-32 4-23 4-15 4.08 4.00 3-93 3.86 3-79 3-73 4000 4-51 4.42 4-33 4-25 4.17 4.10 4.02 3-95 3-89 3.83 4100 4.61 4-S 2 443 4-35 4.27 4.19 4.12 4-«5 3-98 3-91 4200 4.71 4.62 4-53 445 4-37 4.29 4.21 4.14 4.07 4.00 4300 4.82 4.72 4.63 4-54 4.46 4-38 4-3o 4-23 4-15 4.08 4400 4.92 4.82 4-73 4.64 4-56 447 4-39 4-3 2 4.24 4.17 4500 5.02 4.92 4.84 4-74 4-65 4-57 449 4.41 4-33 4.26 4600 5-12 S-02 4-93 4.84 4-75 4.66 4-58 4-5° 4.42 4-35 4800 5-32 5.22 5.12 5.02 4-93 4-85 4.76 4.68 4.60 4-52 5000 5-52 542 5-32 5- 2 2 5.12 5-°3 4.94 4.86 4-77 4.69 4o8 APPENDIX II TABLE FOR FINDING RELATIVE HUMIDITY: Percentages Dry therm. Difference between Dry and Wet-bulb Thermometers (air temp.) 1 2 3 4 5 6 7 8 9 10 11 12 13 r 4 68 35 3 2 7i 41 12 4 73 46 19 6 75 50 25 1 8 77 54 3i 9 10 79 57 36 .15 12 80 60 4i 21 3 14 82 63 45 27 10 16 83 66 ■ 49 33 16 18 84 68 53 38 22 7 20 85 70 56 42 28 14 22 86 72 59 45 32 19 7 24 87 74 61 49 36 24 12 26 88 75 64 52 40 29 18 7 28 88 77 66 55 44 33 23 12 2 30 89 78 68 57 47 37 27 17 8 32 90 79 69 60 50 41 3i 22 13 4 34 90 81 72 62 53 44 35 27 18 9 1 36 9i 82 73 65 56 48 39 3i 23 14 6 38 9i 83 75 67 59 5i 43 35 27 19 12 4 40 92 84 76 68 61 53 46 38 3i 23 16 9 2 42 92 85 77 70 62 55 48 41 34 28 21 14 7 44 93 85 78 71 64 57 5i ■ 44 37 3i 24 18 12 5 46 93 86 79 72 65 59 53 46 40 34 28 22 16 10 48 93 87 80 73 67 60 54 48 42 36 3i 25 19 14 50 93 87 81 74 68 62 56 50 44 39 33 28 22 17 52 94 88 81 75 69 63 - 58 52 46 41 36 30 25 20 54 94 88 82 76 70 65 59 54 48 43 38 33 28 23 56 94 88 82 77 71 - 66 61 55 50 45 40 35 3i 26 58 94 89 83 77 72 67 62 57 52 47 42 38 33 28 60 94 89 84 78 73 68 63 58 53 49 44 40 35 31 62 94 89 84 79 74 69 64 60 55 50 46 41 37 33 64 95 90 85 79 75 70 66 61 56 52 48 43 39 35 66 95 90 85 80 76 7i 66 62 58 53 49 45 41 37 68 95 90 85 81 76 72 67 63 59 55 5i 47 43 39 70 95 90 86 81 77 72 68 64 60 56 52 48 44 40 72 95 91 86 82 78 73 69 65 61 57 53 49 46 42 74 95 9i 86 82 78 74 70 66 62 58 54 5i 47 44 76 96 9i 87 83 78 74 70 67 63 59 55 52 48 45 78 96 9i 87 83 79 75 71 67 64 60 57 53 50 46 80 96 9i 87 83 79 76 72 68 64 61 57 54 5i 47 84 96 92 88 84 80 77 73 70 66 63 59 56 53 SO 88 96 92 88 85 81 78 74 71 67 64 61 58 55 52 92 96 92 89 85 82 78 75 72 69 65 62 59 57 54 96 96 93 89 86 82 79 76 73 70 67 64 61 58 55 100 96 93 90 86 83 80 77 74 7i 68 °S 62 59 57 METEOROLOGICAL INSTRUMENTS 409 TABLE FOR FINDING RELATIVE HUMIDITY : Percentages {Continued) Dry therm Difference between Dry and Wet-bulb Thermometers (air temp.] 15 16 17 18 19 20 21 22 23 24 25 26 27 28 46 4 48 8 3 50 12 7 2 52 15 10 6 54 18 14 9 5 50 21 17 12 8 4 S« 24 20 15 11 7 3 60 27 22 18 14 10 6 2 62 29 25 21 J 7 13 9 b 2 04 3i 27 23 20 ib 12 9 5 2 66 33 29 2b 22 18 15 11 8 5 1 68 35 3i 28 24 21 17 14 11 8 4 1 70 37 33 30 26 23 20 17 13 10 7 4 1 72 39 35 32 23 25 22 19 16 13 10 7 4 I 74 40 37 34 30 27 24 21 18 15 12 9 7 4 I 76 42 3« 35 32 29 2b 23 20 17 H 12 9 b 4 78 43 40 37 34 31 28 25 22 19 ib 14 11 9 b 80 44 4i 38 35 32 29 27 24 21 18 16 13 11 8 82 4 b 43 40 •37 34 3i 28 25 23 20 18 15 13 10 84 47 44 4i 3« 35 32 30 27 25 22 20 17 15 12 86 48 45 42 39 37 34 3i 29 2b 24 21 19 17 14 88 49 46 43 4i 38 35 33 30 28 25 23 21 18 ib 90 50 47 44 42 39 37 34 32 29 27 24 22 20 18 92 5i 48 45 43 40 38 35 33 30 28 2b 24 22 19 94 52 49 4 b 44 4i 39 3^ 34 32 29 27 25 23 21 90 53 50 47 45 42 40 37 35 33 3i 29 2b 24 22 98 53 5i 48 4 b 43 41 39 3b 34 32 30 28 2b 24 100 54 52 49 47 44 42 40 37 35 33 3i 29 27 25 TABLE FOR FINDING RELATIVE HUMIDITY: Percentages (Continued) Dry therm. Difference between Dry and Wet-bulb Thermomete rs (air temp.) 29 30 31 32 33 34 35 3<5 37 3« 39 4° 4* 42 76 1 78 4 1 80 b 4 1 82 8 6 4 1 84 10 8 6 4 2 86 12 10 8 b 4 2 88 14 12 10 8 6 4 2 90 16 14 12 10 8 6 4 2 92 17 15 13 11 9 8 b 4 2 94 19 17 15 1.3 11 9 8 b 4 2 1 96 20 18 17 1.5 13 11 9 7 6 4 3 I 98 22 20 18 ib 14 1.3 11 9 7 6 4 3 I 100 23 21 19 18 ib 14 12 11 9 7 6 4 3 1 APPENDIX III THE CONSTRUCTION OF A WEATHER MAP The student will learn to read a weather map more rapidly and under- stand it more thoroughly by first making one. The table on p. 411 gives the data sent into the United States Weather Bureau from all the stations on the morning of March 15, 1899. Blank maps, form DD, giving the location of the stations, may be obtained from the Bureau at $1.55 per thousand. Let the student write below the circle indicating each station upon the map the temperature at that station, and then proceed to draw the isotherms for each ten degrees. Draw first the isotherm of 30 , which passes through all sta- tions having a temperature of 30° and separates those having a higher temperature from those having a lower. Starting a little north of Boston, it runs westward north of Albany, south of Parry Sound, Sault Ste. Marie, and Marquette, east of St. Paul and Des Moines, through Concordia, Oklahoma, and El Paso, and thence passes northward east of Grand Junction. In a similar manner draw isotherms at ten-degree intervals from 70 to — 30 . Upon another blank map write the pressure at each station, and pro- ceed to draw the isobars for each tenth of an inch. First find the area of low pressure, which appears from inspection of the table to be the Lake region, with Grand Haven as a center. Inclose the center with the isobar of 29.60 inches, which passes south of Sault Ste. Marie, east of Dubuque and Davenport, north of Indianapolis, and west of Detroit. Locate the area of high pressure in Northwest Territory, and inclose it with the isobar of 30.70 inches. These will indicate the general pattern of the isobars. When the isobars are completed and numbered, draw upon the same map at each station a small arrow flying with the wind, as given in the table. Transfer the isotherms to the map of isobars. The two sets of lines may be drawn in different colors. Attach to each arrow the symbol used upon weather maps for clear, fair, cloudy, rain, or snow, as the case may be at each station ; or the area where the table indicates cloud, rain, or snow may be shaded lightly, and the areas where rain or snow is falling shaded more deeply. Observe upon the map thus drawn : (1) The pressure slopes between Grand Haven and Boston, Norfolk, Montgomery, Valentine, and Min- nedosa ; between Ou'Appelle and San Antonio ; between Swift Current 410 THE CONSTRUCTION OF A WEATHER MAP 411 Observations taken at 8 A.M., 75th Meridian Time Si a 00 -a 3 — .- •a -O o_c c . T3 u_n c . s.s P. TJ-2 u « c S.S Q. •0.2 M a a ^ E 1^' in 2 c s H c t> ij CO Atlantic Coast. Upper Miss. Val. Boston .... 30.46 34 S E. 12 cloudy Cairo .... 29.88 54 S.W. 20 clear Albany .... 3°-34 32 S.E. 20 " St. Louis . . . 29 84 40 w. 28 cloudy New York . 30.32 33 E. 34 " Springfield, 111. . 29 74 38 W. ZO " Philadelphia . . 30.24 40 S.E. 14 rain Keokuk .... 29 84 36 w. 26 " Washington 30.16 36 N. Lt " Davenport . . , 29 62 34 W. 16 rain Lynchburg . . . 30.12 36 N.E. Lt. " Des Moines . . 29 96 28 N.W. 20 cloudy Norfolk . " . 30. 12 42 N. Lt. " Dubuque . . 29 64 32 N.W. 20 snow Jacksonville 30. 12 s. 8 clear St Paul .... 29 5:- 24 N.W. 16 fair Tampa .... 30.14 >>3 S.E. T2 cloudy Missouri Valley. Gulf States. Kansas City . 30.08 32 N.W. 12 cloudy Atlanta .... 30.02 5° N.E. 6 rain Springfield, Mo. . 3° 10 28 N.W, 26 clear Mobile . . . 30.06 70 s.w. Lt. fair Concordia . . . 30 30 3° N.W. 24 " Montgomery . 29.98 70 S. 12 cloudy Omaha .... 30 10 24 N.W. 16 cloudy Vicksburg . . . 30.08 62 N.W. 12 fair Sioux City . 30 T2 18 N.W. 28 " New Orleans . 30.06 70 S.w. 6 " Huron .... 30 20 10 N.W. 30 " Shreveport . . 30.14 54 N.W. 8 clear Bismarck 3058 - 4 N W. 8 clear Fort Smith . . . 30.16 ;S W. 12 Moorhead . . . 30.30 8 N.W. 20 cloudy Little Rock . . Galveston . . 30.06 30.06 4 S 66 W. 12 N.W. 6 fair cloudy Northwest Ter. Palestine . . 30.18 52 N.E. 6 " Calgary .... 30.62 -16 O clear San Antonio 30.04 62 N. 14 " Minnedosa . 30.70 — 12 s.w. Lt " Fort Worth . . . 30 24 4 2 N. 8 fair Prince Albert . 30.68 —32 fair Ohio Val. andTen. Swift Current . . Qu'Appelle . . 30.72 30.64 — 12 — 6 Lt. cloudy Indianapolis . . Pittsburg . . . 29.64 29.88 56 S.W. 28 s. 6 clear Rocky Mt. Slopes. 42 rain Cincinnati . . . 29 74 58 S. 14 cloudy Havre .... 30.42 zero N.E. 8 clear Columbus . 29.72 54 S. 12 " Helena .... 3° 40 2 w. Lt snow Louisville . 29.76 58 S. 14 " Miles City . . . 3° 5° 2 N. Lt fair Chattanooga . . 29.98 5° S.E. Lt. rain Rapid City . . . 30 5° 4 N.E. 6 " Memphis . . 30.02 56 w. 14 fair Valentine . . . 3° 4 S 4 N.W. 12 clear Nashville . . . 29.92 60 w. 8 cloudy North Platte . . 3° 48 8 N.W. 14 " Parkersburg 29.82 5° S.E. 14 " Cheyenne . 30 40 8 s. 8 " Lake Region Lander .... Salt Lake City . 30 3° 36 00 4 40 s.w. Lt S.E. 6 cloudy Chicago .... 29.54 40 S.W. 36 cloudy Denver ... 30 38 14 N.E. 18 clear Detroit .... 29.64 42 S. IO " Pueblo .... 3° 3° iS E. Lt. fair Grand Haven . . 29.50 40 S. 12 " Santa Fe . . 3° 18 24 clear Marquette 29.6S 24 W. 12 snow El Paso .... 3° 14 30 N.E. Lt. " Sauk Sie. Marie 29.62 24 E. 14 " Abilene ... 30 26 36 N. 6 " Duluth .... 29.90 26 N.W. 18 " Amarillo . . '. 3° 3° 22 N. 14 " Cleveland 29.68 48 S.E. 30 rain Oklahoma . . . 3° 24 30 N. 14 Buffalo .... 29.78 40 S. 18 cloudy Dodge City 3° 38 18 N.W. 12 Parry Sound . . 29.74 28 S.E. 36 " Wichita . 30 3° 24 N.W. 14 White River . . 29.80 18 N. Lt. snow Grand Junction . 30 14 32 E. 20 cloudy 412 APPENDIX IV and Salt Lake City. (2) The direction of the wind compared with that of the isobars and of the pressure slopes ; the general air movement in the cyclone. (3) The wind velocities near the center of low pressure; near the center of high pressure. (4) The course of the isotherms across the cyclone; across the anticyclone. (5) The position of the areas of cloud and of rain or snow in relation to the cyclone. Account for all these conditions. Make a forecast of the weather for March 15 and 16, 1899, at the place where you live. This exercise may be repeated as often as desired, by giving the students data obtained from other weather maps. Consult Ward's Practical Exercises in Elementary Meteorology. Daily weather maps may usually be obtained on request from any Weather Bureau Station. APPENDIX IV 1 REFERENCE BOOKS The following list of standard books and periodicals is not intended to be a complete bibliography of the subject, but it comprises a large part of the literature in English, other than regular text-books on physi- cal geography, available for the student and teacher of that subject. As a rule, the best books are named first under each topic. See Hints to Teachers and Students on the Choice of Geographical Books, Mill, $1.25, Longmans, Green, & Co. General References The International Geography. $3.50. D. Appleton & Co., N.Y. Physiography, Huxley. $1.80. Macmillan. Our Earth and Its Story, Brown. 3 Vols., $9.75. Cassell & Co., N.Y. 1 Abbreviations used in this appendix: A. G., American Geologist (Minneapo- lis) ; A. J. S., American Journal of Science (New Haven) ; B. A. G. S., Bulletin (Journal) of the American Geographical Society (N.Y.) ; B. G. S. A., Bulletin of the Geological Society of America (Rochester); G. J., Geographical Journal (Lon- don); G. S., Geological Survey (Washington) ; J. G., Journal of Geology (Chicago); J. S. G., Journal of School Geography (Lancaster, Pa.) ; N., Nature (London) ; N. G. M., National Geographic Magazine (Washington); N. G. Mon., National Geographic Monographs; P. S. M., Popular Science Monthly (N.Y.) ; S., Science (Lancaster, Pa.); S. G. M., Scottish Geographical Magazine (Edinburgh) ; S. R., Report of the Smithsonian Institution (Washington). REFERENCE BOOKS 413 Outlines of the Earth's History, Shaler. $1.75. Appleton. Annual Reports, Monographs, and Bulletins of the United States Geological Survey. Apply to the Director, Washington, D.C. (Abbrev. G. S.) Reports of the Geological Surveys of the various states. Annual Report of the Smithsonian Institution. Apply to the Secre- tary, Washington, D.C. (Abbrev. S. R.) Periodicals National Geographic Magazine. $2.50. Washington, D.C. (Abbrev. N. G. M.) Bulletin (Journal) of the American Geographical Society. $4.00. N.Y. (Abbrev. B. A. G. S.) Geographical Journal, $6.00. London, Eng. (Abbrev. G. J.) Scottish Geographical Magazine. $5.00. Edinburgh, Scotland. (Abbrev. S. G. M.) Journal of School Geography. $1.00. Lancaster, Pa. (Abbrev. J. S.G.) Bulletin of the American Bureau of Geography. $1.00. Winona, Minn. Journal of Geology. $3.00. Chicago, 111. (Abbrev. J. G.) Bulletin of the Geological Society of America. $5.00. Rochester, N.Y. (Abbrev. B. G. S. A.) American Geologist. $3.50. Minneapolis, Minn. (Abbrev. A. G.) American Journal of Science. $6.00. New Haven, Conn. (Abbrev. A. J. S.) Science. $5.00. Lancaster, Pa. (Abbrev. S.) Nature. $6.00. London, Eng., and New York. (Abbrev. N.) Popular Science Monthly. $3.00. McClure, Phillips & Co., N.Y. (Abbrev. P. S. M.) Book I Chapter 1. — A New Astronomy, Todd. $1.30. Am. Book Co. Chapter 2. —Manual of Geology, Dana." $5.00. Am. Book Co. Text-Book of Geology, Dana. $1.40. Am. Book Co. Text-Book of Geology, Geikie. $7.50. Macmillan. Common Minerals and Rocks, Crosby. 40 cents. D. C. Heath & Co., Boston. Story of Our Planet, Bonney. 414 APPENDIX IV Geological Studies, Winchell, $2.50. Scott, Foresman & Co., Chicago. N. 34, 400; 46, 348, 372; 59, 330. B. G. S. A. 11, 61. S. R. 1896, 233. G. J. 13, 225. Book II The Earth and Its Story, Heilprin. $1.00. Silver, Burdett & Co., Boston. A Text Book of Geology, Brigham. $1.40. Appleton. Elements of Geology, Le Conte. $4.00. Appleton. Introduction to Geology, Scott. $1.90. Macmillan. National Geographic Monographs, 10 numbers, 20 cents each. Bound $2.50. Am. Book Co. (Abbrev. N. G. Mon.) Handbook of Physical Geology, Jukes-Browne. $175. Macmillan. Aspects of the Earth, Shaler. $2.50. Scribner's. Fragments of Earth Lore, Geikie. John Bartholomew, Edinburgh. Earth Sculpture, Geikie. $2.00. G. P. Putnam's Sons, N.Y. Scenery of Scotland, Geikie. $3.50. Macmillan. Any text-book of geology. Chapter 4. — 'Rocks, Rock Weathering, and Soils, Merrill. $4.00. Macmillan. Origin and Nature of Soils, Shaler. G. S. 12th Rep. 1, 219. Rivers of North America, Russell. $2.00. Putnam's. Geology of the Uinta Mountains, Powell, p. 181. Dept. of the Interior, Washington. Geology of the Henry Mountains, Gilbert, p. 93. Dept. of the In- terior, Washington. G. S. 14th Rep. 2, 149. Chapters. — Principles of Geology, Lyell, -I, 435. 2 vols. $800. Appleton. A. J. S. 116, 417; 152, 29. P. S. M. 25,594. Scribner's Monthly, 22, 420. North American Review, 136, 212. Forum, 24,325. Chapter 6. — History of the Grand Canyon District, Dutton. G. S. Mon. II. $10.00. Canyons of the Colorado, Powell. $10.00. Flood & Vincent, Meade- ville, Pa. A. J. S. 112, 16, 85. P. S. M. 7, 385, 531, 670. Chapter 7. — Niagara Falls and Their History, Gilbert. N. G. Mon. B. G. S. A. 1, 66, 563 ; 9. 59. B. A. G. S. 31, 101. A. J. S. 128, 123 ; 140, 425 ; 148, 455. S. R. 1890, 231. P. S. M. 49, 1. REFERENCE BOOKS 415 Chapter 8. — Geology, Prestwich, I, 155. $6.25. Clarendon Press. Celebrated American Caverns, Hovey. $2.00. Robert Clarke Co. Cincinnati. Yellowstone National Park, Chittenden. $1.50. Robert Clarke Co. J. S. G., 1, 133. Chapter 9. — Illustrations of the Earth's Surface: Glaciers. Shaler and Davis. $10.00. Houghton, Mifflin & Co., Boston. Glaciers of the Alps, Tyndall. $2.50. Longmans, Green & Co., N.Y. Forms of Water, Tyndall. $1.50. Appleton. Glaciers of North America, Russell. $1.75. Ginn & Co. Ice Work, Past and Present, Bonney. $1.50. Appleton. First Crossing of Greenland, Nansen. $1.25. Longmans. Northward over the Great Ice, Peary. $6.50. Fred. A. Stokes, N.Y. Greenland Ice Fields, Wright. $2.00. Appleton. Glaciers of the United States, Russell. G. S. 5th Rep. 309. Second Expedition to Mt. St. Elias, Russell. G. S. 13th Rep. 2, 7. Glacier Bay and Its Glaciers, Reid. G. S. 16th Rep. 1, 415. B. G. S. A. 6, 199. J. G. 1, 219 ; 2, 649, 768 ; 3, 61, 198, 469, 565, 668, 833, 875 ; 4, 582, 769, 912 ; 5, 229. A. J. S. 133. 1 ; 143, 169. P. S. M. 29, 660; 46, 1. B. A. G. S. 28, 217. Century Magazine, 41, 865; 44, 190; 50, 235. Cosmopolitan Magazine, 17, 296, 411. N. G. M. 4, 19. Chapter 10. — Ice Age in North America, Wright. $5.00. Appleton. Man and the Glacial Period, Wright. $1.75. Appleton. The Great Ice Age, Geikie. $7.50. Appleton. The Canadian Ice Age, Dawson. $2.00. Scientific Pub. Co., N.Y. Studies in Indiana Geography, Dryer. $1.25. Inland Pub. Co., Terre Haute, Ind. Climate and Time, Croll. $2.50. Appleton. Cause of an Ice Age, Ball. 75 cents. Kegan Paul, Trench, Trlibner & Co., London. Island Life, Wallace. Chaps. VII-IX. $1.75. Macmillan. Terminal Moraine of the Second Glacial Epoch, Chamberlin. G. S. 3d Rep. 291. Rock Scorings of the Great Ice Invasions, Chamberlin. G. S. 7th Rep. 147. The Glacial Gravels of Maine, Stone. G. S. Mon. XXXIV. 416 APPENDIX IV The Illinois Glacial Lobe, Leverett. G. S. Mon. XXXVIII. Eighteenth Rep. Indiana State Geologist, 83. Indianapolis. J. G. 1, 52, 129, 246, 255; 2, 123, 517, 613, 708,837; 3, 70; 4, 129, 948 ; 6, 147. B. G. S. A. 1, 287, 395 ; 4, 191 ; 7, 17, 31. B. A. G. S. 30, 183, 217. S. R. 1893, 277. A. J. S. 128, 407 ; 135, 401. A. G. 13, 397 ; 14, 12 ; 17, 16 ; 24, 93, 157, 205. Chapter ir. — Lakes of North America, Russell. $1.50. Ginn & Co. Present and Extinct Lakes of Nevada, Russell. N. G. Mon. Lake Bonneville, Gilbert. G. S. Mon. I. Lake Lahontan, Russell. G. S. Mon. XI; 3d Rep. 195. Glacial Lake Agassiz, Upham. G. S. Mon. XXV. Studies in Indiana Geography, Dryer. Mono Lake, Russell. G. S. 8th Rep. 1, 265. G. J. 1, 481 ; 6, 46, 135. S. G. M. 11, 60; 16, 193. B. A. G. S. 25, 1 ; 30, 226; 31, 1, 101, 217. B. G. S. A. 1, 71, 297, 563 ; 2,243, 465; 5,339; 7 > 327, 423- P. S. M. 45, 40, 224; 49,157. J.G.I, 394. A. G. 14, 289; 18, 169. N. G. M. 8, 33, in, 233. A. J. S. 133, 278 ; 140, 443 ; 141, 12, 201 ; 144, 290 ; 147, 105 ; 149, 1 ; 153, 165. N. 43,203; 57,2ii. Forum, 5, 417. J. S. G. 1,65; 2,291. Chapter 12. — Rivers of North America, Russell. Chapter IX. Geology, Scott. Chap. XVIII. G. S. Mon. XXIII, in. N. G.M.I, 203. G. J. 5, 127. B. G. S.A. 7,505. A. J. S. 112, 88. J. G. 4, 567, 657. S. 12, 131. Chapter 13. — Structure and Distribution of Coral Reefs, Darwin, $2.00. Appleton. Corals and Coral Islands, Dana. $5.00. Dodd, Mead & Co., N.Y. The Bermuda Islands, Heilprin. A. Heilprin, Philadelphia. Animal Life, Semper, p. 224. $2.00. Appleton. N. 22, 351 ; 35, 7 y ■ 37, 98, 393,414, 546; 39,236,424; 40, 53,203, 222, 271 ; 41, 300; 42, 29, 162; 51, 203; 55, 373, 390. A. J. S. 130, 89,169. G.J. 5, 73. P. S. M. 32, 241. Chapter 14. — Geology of the Uinta Mountains, Powell. Fragments of Earth Lore, Geikie, p. 36. The Northern Appalachians, Willis. N. G. Mon. The Southern Appalachians, Hayes. N. 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Flood and INDEX PAGES Ablation 113 Adaptation, of animals 366 of plants 361, 362 Agassiz, Lake 133, 146, 147 Age, of mountains 190 of rivers 155, 161, 162 Air 2 73 _2 79 composition of 273 cooling of 282 density of 276 moisture in 280-282 temperature of 276-279, 348 visibility of 275 weight of 275 Air pressure 275, 276, 288 distribution of 3 OI -3 r 1 in a thunderstorm 326 in a tornado 324 in cyclones 313, 317, 320 measurement of 275, 400, 401 relation to temperature 302, 304 Alaskan glaciers 117-119 Alkali plains 173 Alluvial cones and fans i7r Alluvial deposits 170, 171 Alluvial lakes 149 Alluvial plains 171, 172 Alpine glaciers 109 Alps, influence on man 225, 226 structure of 187 Amphitheaters 221 Animal geography 364-382 Animal realms and regions 367-379 Animals, assist weathering 59 domestication of 384 earth habitable by 53-56 food of 364 lime deposits by 174, 175, 177 on mountains. . . .,. 224, 225 species of 365 struggle for existence 364 PAGES Antarctic drift 265, 266, 267 Antarctic ice cap 120 Antecedent stream 162 Anthracite coal 36 Anticline 179 eroded 218, 186 Anticyclones 306, 315-317 Antitrade winds 307 Antitrade zones, of climate 338-346 Appalachian Mountains 184-187 extent of folding in 192 Aqueous rocks 33. 36 Arctic-Alpine carpets 357 Arctic Ocean 246 Argon 273, 274 Artesian wells 103 Ashes, volcanic 198, 207 Atlantic Ocean 245, 246 currents in 265, 266 Atmosphere 26, 273-348 thickness of 273 (see Air) Atolls 176 Avalanche 113 Bad Lands 212 Banner cloud 283 Bar 232 Barograph 402, 403 Barometer 275, 276, 400, 401 Barometric gradient 288 Barrier basins 147, 148 Barrier beach 231 Barriers, to animal migration , 365-367, 369, 37i, 372, 373 to plant migration 360 Basalt 35 Base-level 79, 154 Basin 61, 66 Basins, of lakes 135-151 Bay 228 421 422 INDEX PAGES Bayou 77, 75 Beach 231, 232 Bed rock 31, 32 joints in 222, 223 Bends, or horseshoe curves 62, 75, 76 Biosphere 26 (see Life) Bituminous coal 33 Black Hills 205 Blizzard 315 Block mountains 181, 182 Block picture 53 Bombs, volcanic 197 Bonneville, Lake 136 shore lines of 238 Border seas 246, 245 Bores 264 Boulder clay no Boulders 30, 31, 122, 123 Breaker 260 Breccia 33 Buttes 214, 212 sea-made 231 C Calderas 200 Calms 307 Canoe valley 184 Canyons 84-91 formation of 88, 89 Capacity for vapor '. 281 Capes 228 Capture, by streams 157, 158 Carbon dioxide 273, 274 agent of weathering 57 Caspian Sea 137 Cataracts or falls 99-101, 153 Caves or caverns 104, 105 spouting 205 Cayuga Lake 141, 142 Celestial sphere 14 Centrosphere 26, 27-29 Chain of mountains 181 Chalk 33 Channel, of a stream 62 Chasms 205 Chimney, volcanic 197 Cinders, volcanic 198 Cirques 221 Cirrus clouds 284 PAGES Civilization 384, 385, 390 Classification of land forms 242 Clay 30, 59 Cliffs 137, 138, 213, 223 sea 230 Climate 335-348 effect on man 390 in Eurasia 340 in North America 341 in the United States 341-346 zones of 336 -340, 346 Cloudbursts 326 Clouds 282-284 effect on temperature 278 Coal 33. 36 Coast forms 227 238 Coast line, effect on man 391 of United States, eastern 235-238 Coast shelf 44, 172 Coastal plain, Atlantic and Gulf 235 Coasts, rising 227 sinking 228 Cold waves 315 Colorado River system 81-91 as source of knowledge 167 Compressed folds 179 Concretionary limestone 33 Condensation 282 Cone, volcanic 195, 197 slope of 202 Cones, alluvial 171 Conglomerate 33 Consequent streams 153, 162 Continental block 39 Continental climate ... 343, 344, 340 Continental deposits, on sea floor 248 Continental glaciers. 121, 124 Continental islands 45, 228 Contours 49 Contraction theory 48 Convection, atmospheric 287 Coral reefs and islands 174-177 Corrasion 65 curve of 156 Coulee lake 148 Crater 195, 197 Crater Lake 148, 149 Crevasse, in a levee 77 Crevasses, in glaciers no Culture, of man 383-385, 389 INDEX 423 PAGES Cumulus clouds ■ 283 Currents, alongshore. 231, 232 Currents, ocean 264-270 cause of '■ 267-269 effect on isotherms 296, 297 effects of 269, 270 map of 256, 257 Cusp 232 Cut-off 76 Cyclones 31 2-325 causes of 320, 321, 322 effect on rainfall 313, 320, 321, 333 form of 323 movement of air in 306, 312 paths of 318,319 tornadoes 323-325 tropical 318-321 D Day and night, length of 22, 23 Dead Sea 137, 138 Deeps 43, 40, 41 Deflection of winds 289-292 Degradation 66 Degree, length of 25 Delta 172, 234, 235 of the Mississippi 78 Density, of air 276 of centrosphere 27 of earth 27 of sea water 255 of water 255 Denudation 216 Deposition or sedimentation 168-177 by animals and plants 174-177 by evaporation 173, 150 by glaciers 170, 115-117, 126-131 by springs 106 by streams 170-172 by winds 168, 169 Deposits on sea floor, 248, 249 Depression , area of 39, 43 Depression or subsidence 47, 48, 190-193 Dew 285, 286 Dew-point 282 Diastrophic basins 135-138 Diastrophism 47 Dikes 203, 204, 205 Dissected plateau 217 Distributary 78 PAGES Divides 61 development of 155, 156 migration of 157, 158 Domestication 384, 385 Drainage 163 Drainage systems, development of .. .152-163 Drift 60 glacial 114, 115-117, 122 134 shore 231, 232 Drift plain 132, 131 Drouth plants 357 Drowned valleys 95 Drumlins 129, 130 Dunes 169 Dust deposits 207 E Earth, attitude of 20-22, 53 axis of 17, 20 curvature of 10 density of 27 face of 38-48 orbit of 18, 23 Plan of 39-44, 55 revolution of 1 8, 54 rotation of 17, 54 shape of 10-12 size of 12, 54 structure of 26-37 surface of 38-46 temperature of 28, 53 why habitable 53 - 56 Earth-crust 26, 29-37 acted on by ground-water 102-107 acted on by internal and external forces 239-242 acted on by streams 63 density of 27 movements in 47, 48, 199-193 structure of 37, 29 -34 Earthquakes 190, 191 East coast climate, in northern antitrade zone 340 Ebb tide 261 Edentates 370 Elevation, area of 39. 43 Elevation or upheaval 47, 48, 190-193 effect on streams 160, 90 Ellipse 18 Equatorial calms 307 424 INDEX PAGES Equatorial currents 265, 266 Equatorial rains 327, 329 Equinoxes 23, 15 note Erosion 66, 57-67 effects on land surface 212-224 of mountains 182-190, 218-220 Erratics 123 Eruptions, volcanic 195-203 Eruptive rock 34 Escarpment 214, 97 Eskers 129 Estuary 228 Evaporation 280, 281 Eye of a storm 320 F Falls, or cataracts 99-101, 153 Fan fold 179 Fans, alluvial 171 Fault 178 Faulting, cause of 191-193 Ferrel's Law 290 Filled valley 161 Finger Lakes 140-142 Fiords 229, 133 Floe 270 Floebergs 270 Flood plains 62, 73-79, 154 and man 164 Floods 76, 77 Flow, of tide 260 Focus, of an earthquake 190 Fog 282 Folded mountains 182-187 Folding, cause of 191-193 Folds, rock 1 78, 1 79 Food, of animals 364 of man 384, 385 of plants 350 Foreland 232 Forests 357-359. 35 2 "354 effect on drainage 163 Fossils 365, 369 Frost 285, 286 agent of weathering 57 G Geocentric theory 17 Geography 27 Geysers 106 PAGES Glacial drift 114, 115-117, 122-134 Glacial sculpture 220, 221 Glaciated basins 138 147 Glaciation 115, 220, 221 effect on streams 160, 161, 132, 133, 145 fiords caused by , 229 of Europe 133, 134 of North America 130-133 Glaciers 109, 108-121 abrasion by .... 115 effect on animals 367 effect on plants 363 effect on valleys 220 formation of 108 forms of deposits by 115-117, 126-131 melting of 113, 112 movement of 109-112, 120 Gneiss 35 Gobi, desert, lakes in 136 Gorges 153, 97, 101 (see Canyons) Graded plain 234 Granite 35,58 Gravel 30, 65 Gravity, agent of land sculpture 210 assists weathering 58 Great Basin 135, 136, 138 mountains in 181, 182 Great Lakes 92, 93, 143-146, 267 Great Rift Valley 138 Great Salt Lake 136 Green River 81-85 Greenland ice cap 119 Ground swell 259 Ground-water 102-107 Gulf Stream 265, 266 H Habitable region 349 Hachures 51 Hailstones 285 Hanging valleys 221 Hawaiian volcanoes 198-202 Heat (see Temperature) Heredity, law of 361 , 364 Hoarfrost 286 Hogbacks 206 Hook 232 Horseshoe curves, or bends 62, 75, 76 Horseshoe lakes 76 INDEX 425 PAGES Humidity 281, 282 controls distribution of plants 354 _ 359 measurement of 282, 401, 402 Humus 30, 163 Hurricanes 319-322 Hydrographic basin 62 Hydrosphere 26 (see Sea) Hygrometer 282, 401, 402 I Ice 235, 270 (see Glaciers) Icebergs 120, 118, 271 transportation by 170, 271 Ice-dammed lakes 145, 146 Igneous rocks 34-36 Indian Ocean 246 currents in 266 Inland seas 246 Insolation 293 Intermediate plants 357~359 Intermorainic lakes 148 Intrusive rock 35 Irrigation 164 Islands 45, 228 animals on 378 Isobars 288 Isostasy 47, 48 Isotherms 294 J Jetties 78 Joints, in bed rock 222, 223 Jura Mountains 183 K Karnes 129 Kettle holes 129, 128, 139 Kilauea 200 Knobs 128, 220 Krakatoa 198 Kurosiwo 265 L Labrador current 266 Laccolites 205 Lacustrine plains 172 Lagoon, in an atoll 176 Lagoons, shore 148, 231 PAGES Lahontan, Lake 136 Lakes, and lake basins 135-151 destruction of 150 effects on a river 93, 101, 150 Great 143 horseshoe or crescent-shaped 76 ice-dammed 145, 146 mountain 142 relation to rainfall and drainage 150 Land, height of 45, 40, 41 once covered by the sea 47 surface of 44 Land forms, classified 242 Land sculpture 210-224 Land surface, effect of erosion 212-224 temperature of 53, 294, 296, 300 Landscapes 210 Landslide 168, 191 Lapilli 198 Latitude 23, 24 Laurentian Lakes 143-146 Lava 195, 197-202 Lava flow3 203, 204 Levees 77 Life 349"39 2 (see Plants and Animals) Lime, deposits of *73 -1 77 in sea water - 250 Limestone 33 formation of 176, 177 ground-water in 102-105 Lithosphere 26 (see Earth-crust) Llanos 357 Loam 30 Loch Katrine 143 Loess 124, 169 Longitude 24, 25 Lost rocks 123 M Malaspina Glacier 119 Man, ascent of 383-385 earth as home of 53-56 food of 384, 385 geography of 383~39 2 influence of mountains 224-226 influence of streams 163-167 influence of the sea 27T, 272 races of 386-389 426 INDEX PAGES Mantle rock ; 29, 30 formation of 59, 6° removal and deposition of 168-177 Maps 48-51, S95-39 8 Marl 30 Marsh plants 355, 357 .Marshes 132, 163 tidal 231 Marsupials 368 Mature stream 154, 162 Mauna Loa 198-202 Meanders, or bends 62, 75, 76 development of 158 Mechanical cooling of air 282 Meridians 24, 25 Mesas 214, 212 Metamorphic rocks 35, 36 Mica schist 35 Migration, of animals 365-367, 376, 378 of men 386, 387 of plants 359, 360 Mississippi River system 6S-80 deposits by 172, 173 Missouri River 69-71 Models 51, 52, 393-395 Moisture in the air 2S0-286 Monotremes 368 Monsoons 307 Moon, causes tides 261-263 Moraines 113-115, 116 in North America 126-128 Moulin 113 " Mound and sag " surface 128 Mountain climate 347, 348 Mountain lakes 142 Mountain range 180, 181 Mountains 180, 190, 178-226 age of 190 block 181, 182 complexly folded 183-187 erosion of 182-190, 218-220 formation of 191-193 influence on life 224-226, 391 plateau 189 relict 188, 189 simply folded , 182 volcanoes 194-209 Muck 30 Mud rock 32 Muir glacier 117, 118 N PAGES Natural bridge 105 Natural resources 391, 392 Neap tide 263 Neck, volcanic 207 Nev£ 108 Newer drift 126 Niagara Falls 98-100, 166 Niagara Gorge 97-100 Niagara River 95-101 Night and day, length of 22, 23 Nile River 164 Nimbus clouds 284 Nitrogen 273, 274 Noah's brush heap or barnyard 124 O Ocean basins 24 5-247 Ocean currents 264-270 cause of 267-269 effect on temperature 269, 270, 296, 297 map of 256, 257 Oceanic climate 340 Oceanic islands 45 Oceanography 244 Ohio River 69, 73 Old stream 162, 155 Older drift 124 Oolitic limestone 33 Ooze 248, 249 Orbit, of earth 18, 23 Organic deposits, on sea floor 248, 249 Outcrop 32 Oxbow or horseshoe-shaped lakes 76 Oxygen 273, 274 agent of weathering 57 P Pacific Ocean 245 currents in 265 Pack 270 Pampas 357 Parallels 24 Passes, at mouth of Mississippi 78 Passes, in mountains 187, 219 Peaks 187, 219, 128 Peat 30 Pebbles • 30 Peneplain 218 Peninsulas 228 INDEX 427 PAGES Physiographic cycle 242, 241 Pipe, volcanic 197 Piracy, by streams 157, 158 Plain, alluvial .' -- 172 coastal -152, 235 drift 132, 131 graded 234 lacustrine 172 peneplain 218 Plant geography 349-363 in the animal regions. .368, 371, 372, 374, 376 Plant societies 354 Plants, adaptation of 361 assist weathering '. 59 control by humidity 354 _ 359 control by temperature 35 I_ 354 domestication of 385 earth habitable by 53-56 food of 350 lime deposits by 174, 177 migration of 359, 360 on mountains 224, 225 species of 362 struggle for existence 360-362 Plateau 152, 178 Plateau mountains 189 Plateaus, sculpture of 212-218 Playas 136 Plug, volcanic 206 Polar regions, climate of 346 Polar whirls 307-3 1 1 Polaris, or Polestar 13.14 Population of world 387, 388 Prairies 357 Precipitation 284, 285 (see Rainfall) Pressure in centrosphere 27 Pressure of atmosphere 275, 276, 288 distribution of 301-31 1 in a thunderstorm 326 in a tornado 324 in cyclones 313, 317, 326 measurement of 275, 400, 401 relation to temperature 302, 304 Pressure of sea water 255 Pressure slope 288 Prevailing westerlies 307 Profiles 52, 53 Promontories 228 Psychosphere 26 PAGES Pudding stone 33 Pumice 198 Purgatories 205 Pyramid Lake 136 R Races, caused by tides 264 Races, of men 386-389 Rain gauge.. 285, 402 Rainfall 3 2 7-334 agent of land sculpture 210, 222 agent of weathering ■ 57 caused by cyclones 313, 320, 321, 333 causes of 327 factor in climate 335 _ 347 in United States 343-346 measurement of 2S5, 402 Rain-wash, curve of 156 Range, of mountains 180, 181 Range of temperature 298-300 Rapids 153 Red clay, on sea floor 249 Reefs , 174-177 Regelation in Rejuvenated stream 161 Relict mountains 188, 189 Relief 44 causes of 46-48 range of 46 representation of 48-53 Residual soil 60, 168 Revolution, of earth 18 Ridge, of mountains 180 Rift basins 137, 138 Rivers, action of 61-67 blocked 235 falls in 99-101, 153 life history of 152-154 three typical 68-101 (see Streams) Rock sphere 26 (see Earth-crust) Rocks, classified 36, 29 35 formation of sedimentary 177 (see Weathering, Erosion, Sculpture, Glaciation) Rotation, of earth 17 effect on winds 289-292 Run-off 61, 62 of mantle rock 168 428 INDEX s PAGES St. Lawrence River system. . .92-101, 143-146 Salt, deposits of 173, 174 in sea water 250 Sand 30 Sand bars 62, 63 Sandstone 32, 33 formation of 177 Saturation 280 Scoriae 198 Sculpture, land 210-224 coast 230 Sculptured forms, development of . . . . 216-218 Sea 243-272 action on coast lines 227-238 currents in 231, 232, 264-270 depth of 43, 40, 41 figure of 243-247 influence on man 271-272 1'fe in 379-382 movements of 258-271, 254 surface of 244, 245 temperature of 251-254 tides in 260-264 volume of 46, 243 waves in 258-260 Sea cliff 230 Sea floor, or bottom 247-249 study of 244 temperature of 251-254 Sea ice 270, 271 density of 255 Sea level 244, 245 Sea water 250-255 composition of 250 pressure and density of 255 Seasons 19-23 in different climatic zones. .339, 337, 338, 346 Sediment, in streams 63-65 Sedimentary or aqueous rocks 33, 36 formation of 177 Sedimentation, forms of 168-177 Shale 32 formation of 177 Shore drift 231, 232 Sills 205 Sinkholes 104, 105 Skerries 230 Slate 32 Snow 285 PAGES Snow line 108 Soapstone 32 Soil 29, 30 Solstices 23, 15 note Solution 1 73 basins formed by 144 caves formed by 104 deposits from 173 177, 105, 106 rock carried in 64 Southern Ocean 247 Spit 232 Spouts 325 Spring tide 263 Springs 102, 103 hot 106 mineral 105, 106 Spurs 219 Stacks 231 Stalactites 105 Stalagmites 105 Stars, apparent movement of 13, 14, 19 Steppes 357 Stereogram 53 Storm paths 318 Storms 312-326 Strata 32 formation of 171 Stratification 171 Stratified rocks 34 formation of 177 Stratus clouds 284 Streams, action of 61-67 agents of land sculpture 211-219, 224 classified 161, 162 corrasion by 65 development of 152-163 effect of elevation 160, 90 effect of glaciation. . .160, 161, 132, 133, 145 effect of lakes 93, 101, 150 effect of subsidence 160, 95 influence on man 163-167 transportation by 63-65 underground 103 (see Rivers) Striae 115 Stromboli 194, 195 Struggle for existence 360-362, 364 Subsequent streams 159, 160, 162 Subsidence, or depression 47, 48, 190-193 effect on streams 160, 95 INDEX 429 PAGES Sun, apparent movement of I 5 -I 7. 22 causes tides 263 heat from 19, 23, 293 size of 18 Superimposed stream 162 Superior, Lake 143-146 Survival of the fittest 362, 364, 366 Suspension, rock carried in 64 Swamp plants 355. 357 Syenite 35 Syncline 179 eroded 218, 185 System, of mountains 181 T Talus 60, 168 Temperature, agent of land sculpture. 210, 218 agent of weathering 58 controls distribution of plants 35 I_ 354 distribution of 294-300 effect of clouds 278 effect of elevation 347, 348 factor in climate 335~348 in United States 343"346 influence of ocean currents 269, 270, 296, 297 measurement of 279, 398, 399 of air 276-279, 348 of centrosphere 28 of the sea 251-254 range of 298-300 zones of 297, 298 Terrace, wave-built 23 1 , 232 wave-cut 230 Terraces 84, 85, 160, 161 Terrane 216 Thermal equator 297 Thermograph 402, 403 Thermometer 279, 398, 399 Throw i7g Thunderstorms 325, 326 Tidal marsh 231 Tides 260-264 effect on coast lines 235 Till 116 Tilted blocks 181 Tornadoes 323-325 Towers 212, 213 Trade winds- 306 Trade zone, of climate 336, 337 PAGES Transportation, by glaciers n 3-1 15 by icebergs 170, 271 by streams 63-65 by winds 168, 169 Trap rock 35 Trellised drainage 160, 185 Tributaries 61 Tropical calms 307 Tropical cyclones 318-322 Tropical rains 332 Tufa 33 Tundras 357 Typhoons 319-322 U Uinta Mountains 182, 183 Underground waters 102-107 Undertow 230 Upheaval, or elevation 47, 48, 190-193 V Valleys, development of 155 drowned 95 effect of glaciers on 220 filled 161 formation of 65-67 hanging 221 in mountains 219 Vane, wind 403 Vapor 280-282 quantity in air 273, 275, 280 Variable zone 227 Variation, law of 361 , 364 Vegetation (see Plants) Vesuvius 196, 197 Volcanic basins, of lakes 148, 149 Volcanic cone 195, 197 slope of 202 Volcanic rock 34 Volcanoes 194-209 causes of 208, 29 distribution of 207 W Warm waves 315 Water gap 160 Water plants 355 Water sphere 26 (see Sea) Water surface, temperature of.. .294, 296, 300 430 INDEX PAGES Waterspout 325 Waves 258-260 erosive action of 230, 231, 232, 260 Waves of temperature 315 Weather 335 -348 in the United States 341-346 Weather maps 318, 314, 316 construction of 410-412 Weather observations 279 Weathering 57-60 differential 223 Wells ro2, 103 West coast climate, in northern anti- trade zone 340 Wind deposits 168, 160 Wind gaps 160 Wind sculpture 221, 222 PAGES Wind velocity 289 in tropical cyclones ■ . . . 319, 320 in tornadoes 323, 324 measurement of 404 Winds 287 cause of ocean currents 267-269 deflection of 289-292 distribution of 304 311 effect on isotherms 296, 297 erosive effect of 59 factors in climate 335~34° in storms 312-326 relation to pressure 288, 304-311 Y Yellowstone River 70, 71 Young stream 161 ■h