Elementary physical geography

II

1 ' 772g waste of the Old Land is the material of the New ' '

NEW YORK

CHARLES SCRIBNER'S SONS

QOFY *' 2. d fr 7 3/

Copyright, 1900, 1908, by JACQUES W. REDWAY

Preface

The science of Geography sets forth the relations of life and its environment to the earth, and it is the function of both the writer and the teacher of geography to explain these relations. In the elementary geography the pupil studies the various peoples of the earth and the countries in which they live; in the advanced geography there is presented in addition a discussion of the industries of life and their geographic distribution. The present volume, is designed to show that the distribution of life is governed very largely by the conditions of geographic environment, and that human history and industries are alwaj^s closely connected with geographic laws — in many instances the direct resultants of them.

The science of Geography as now understood includes something more than a mere description of topographic forms — it comprehends the gradual and progressive development of these forms and their results as regards life, as well. It includes also the effects of temperature and moisture, for life and its activities depend also on them. That is, it naturally involves the principles of descriptive geography, physiography, and economics; and this volume is designed to show their interrelation.

In scope this book contains all the principles recommended by the Committee of Fifteen, and such other features as have suggested . themselves to the author. It is designed to be used in the junior grades of the High School, vi PREFACE

and in Normal Schools. The arrangement of the subjects is logical, but the teacher may readily organize a course of study in the subject without reference to the present arrangement. To make this more easily accomplished, the principles of the subject are set forth in the larger type; relevant matter that is illustrative but non-essential is presented in smaller type. In general, the teacher should not hesitate to omit a topic, the discussion of which is too difficult for the class.

In this edition the notes are inserted in connection with the text and a brief laboratory manual is added to the appendix. There is no change in the arrangement of chapters, and none in the sequence of topics except as noted.

The Questions and Exercises are designed to stimulate observation and independent thought.

In the preparation of the new edition I take pleasure in acknowledging the most valuable counsel of Principal Myron T. Pritchard, Everett School, Boston.

The books designated for reference and collateral reading are intentionally few in number, and those most commonly cited should be in the school library. The teacher will also find it very advisable to keep in close touch with the publications of the United States Geological Survey, and the Weather Bureau.

J. W. It.

Contents

PAGE

CHAPTER

I. The Earth Among Planets ...".. 9 II. The Structure, of the Earth . . . . . 21

Chapter I

The Solar System. — The cluster of heavenly bodies called the solar system is one of many groups in space. The members of this group revolve about a common centre of gravity, however, and for this reason they form collectively a system.

The members of this system vary greatly in size. The largest is about 886,000 miles in diameter, and the smallest are probably too minute to be measured by ordinary standards. Eight of them are about three thousand miles, more or less, in diameter; about four hundred vary approximately from ten to five hundred miles in diameter.

The largest member of the solar system, the sun, is about eight hundred times as large as all the other members together, and the common centre of gravity around which the various members revolve is very near to or within it. The eight bodies next in size are called planets, and all but two of them are attended by one or more satellites or m.oons. The four hundred or more small planets are called asteroids, or planetoids. There are also several comets and groups of meteors which have a permanent place in the solar system. ,

The asteroids move in orbits in the space between Mars and Jupiter. Many of them do not exceed twenty or thirty miles in diameter, and the largest probably does not exceed five hundred miles. Their combined volume is less than one four-thousandth part of the mass of the earth. Eros, one of the recently discovered asteroids, has an orbit so eccentric that it crosses that of Mars, and at times is nearer to the earth than is Mars.

But little is known about the nature and structure of comets, but some of them are composed largely of gaseous matter. One comet, Tempel's, undoubtedly consists of a vast swarm of meteors; hut it is probable that comets are constituted of different kinds of matter. Several of them belong to the solar system, but many are temporary visitors, coming from unknown regions of space, whirling around the sun and again vanishing into space.

Meteors, or shooting stars, are small bodies that seem to exist generally throughout space. In sweeping through space, the earth and the other planets encounter them without number. Most of them on reaching the earth's atmosphere are heated to whiteness and are dissipated as white-hot vapor. Many of the larger ones reach the earth, and sonic of them have been analyzed. They consist mainly of iron and nickel in a metallic form, or else of matter not differing materially from lava. No element has yet been found in a meteor that does not occur in the earth. In one instance gold, in another minute diamonds, were found in a meteorite.

The planets are composed of the same kinds of matter as the earth, but they are unlike one another in physical condition. Some, bulk for bulk, are but little heavier than water, others are about as heavy as iron ore. Some of the planets have apparently lost the greater part of their heat ; still others are very hot. The sun, for instance, is a glowing mass at white heat.

The Sun and the Planets.— The similarity between the sun and the planets is very marked. They whirl from west to east around a common centre of gravity, and each turns or spins on its axis in the same direction. Bach is nearly spherical in shape, and is more or loss flattened at its poles. So far as is known, each is surrounded by an atmosphere.

According to the nebular theory of La Place, which is held by many scientists, the members of the solar system formerly existed as a body of gaseous matter, or, perhaps, of minute masses. The force of gravity drew the particles together, toward the centre of gravity, and a rotation

THE SOLAR SYSTEM The space within (he orbit oj Jupiter shows the relative size oj the Sun of the mass around the centre of gravity resulted. Finally, parts of the mass were thrown off, one after another in the form of rings, a planet developing out of each ring. In a similar manner, the rapid rotation of each planet threw off portions of its mass forming the satellites.

The researches of Chamberlain and Moulton, however, make it probable that solar systems develop from spiral nebulae, the plaints having been formed around nuclei of considerable size. The nuclei, in turn, attracted the scattered matter near which it passed. This theory, known as the " planetesmal hypothesis," accords with many facts not explained by any other theory.

So far as is known, matter exists in at least three physical forms —

A PORTION OF THE MOON'S SURFACE solid, liquid, and gaseous — and nearly every chemical element and many of their compounds may assume each of these forms. In the solid form the molecules are bound by a strong cohesion ; in the liquid form they are slightly cohesive; in the gaseous form they repel one another. Most of the substances that in the earth are solids, in the sun exist as white-hot vapors.

Although the assumed formation of the solar system by either process is a matter of theory, both theories are supported by evidence. The telescope reveals many such masses of gaseous matter showing planetary

A SPIRAL NEBULA— CANES VENATICI formation. The spectroscope, an instrument for analyzing a substance by the kind of light which it emits, reveals not only the mutter of which the nebulae are composed, but also that the matter is in rapid motion. Calcium, hydrogen, iron, and sodium, the substances of greatest abundance in the sun, are also among the most abundant substances of the earth.

The Form of the Earth. — The earth is one of the planets. From Table I: (Appendix), find how it ranks among the other planets in size; — in distance from the sun. Like the other planets, it is nearly spherical, but slightlyflattened at the poles. It is usually said to be an oblale spheroid — that is, a flattened sphere. As it deviates slightly from this form, the term geoid is sometimes used to designate its irregular shape.

The spherical form of the earth is shown in various ways; it is best demonstrated by surveying a horizontal straight line along a level surface, such as that of a pond. The line thus projected is not parallel to the surface of the pond; the latter curves away from it, and the curvature is such as corresponds to the surface of a spherical body.

This m?y be illustrated in the following manner: Three stakes are set in line, or as nearly in line as is practicable, one mile apart, along the shore of a canal, a pond, or the sea shore. Sighting marks are then

EXPERIMENT TO SHOW THE EARTH'S CURVATURE

made on the stakes each at a uniform distance above water-level. An engineer's level is then placed so that the cross-wires cut t he sighting marks of the first and third stakes. If the telescope of t he level 1 >e t urnci 1 upon the middle stake it will be found that the cross-wires cut the stake at a point eight inches below the sighting mark. The head and shoulders of a man standing in a boat five miles from shore are visible, but the boat itself is not. Among other facts that serve to establish the spherical shape of the earth are: The circular shadow of the earth projected upon the moon at the time of an eclipse ; the variation of time with respect to longitude; and the more practical fact of circumnavigation.

Were the earth a true sphere, the weight of a body would be the same at every part of its surface. This, however, is not the case; a given body weighs a little more in polar than in equatorial latitudes, and from the difference in weight the amount of flattening at the poles has been determined.

A pendulum consisting of a ball weighing about one hundred pounds swinging on a wire of fixed length is allowed to oscillate freely. When all errors are corrected the rate of vibration will be the same at all.points of the earth's surface equally distant from the centre. At any place on the earth's surface that is nearer to its centre, as the poles, the rate of vibration is slightly faster ; at any place more remote it will be slower. The United States Coast and Geodetic Survey has carried on a series of pendulum observations covering a period of many years with the results as noted below. Professor Ferrel has shown that, theoretically, the level of the sea between the 20th and 27th parallels is about thirteen metres (40 ft.) higher than it would be if the earth were a true spheroid.

Size of the Earth. — The following are the earth's dimensions :

Polar diameter 7,901 .5 miles

Equatorial diameter 7,926.6 miles

Circumference at equator 24,912.2 miles

Surface (approximate) 197,000,000 square miles

What is the difference between the polar and the equatorial diameter? Compare the diameter of the earth with that of the sun (Table I., Appendix). Large as the earth seems to us, it would require about one and a quarter million bodies of its size to make a globe of the same size as the sun.

Motions of the Earth.— Tho earth has several distinct

motions. It revolves about the sun in an elliptical path, making a complete journey of about 585,000,000 miles in very nearly 365j days — a period of time called a year. It also rotates, or spins on its axis, the time required for a complete rotation being called a day. The axis of the earth also swings or oscillates in a path which resembles that of the peg of a "sleeping" top.

The revolution of the earth around the sun, together with the inclination of the axis, causes the successive change of the seasons and the varying length of sunshine and darkness. Rotation upon its axis causes the succession of day and night. The oscillation of the axis causes the precession of the equinoxes.

In long intervals of time it is thought that this motion is connected with certain changes of climate. It is a subject, however, that belongs to the science of astronomy, and not to physical geography.

The Inclination of the Axis. — The axis of tho earth is not perpendicular to the plane of the earth's path, called the plane of the ecliptic, but inclines about 231 degree's, as shown in the accompanying figure. The degree of inclination varies in long intervals of time, but practically this change may be neglected; practically the axis is always parallel to itself. The north end of the axis if prolonged would extend nearly in the direction of a star named Polaris; this star is called the north, or pole star.

If the earth's axis were perpendicular to the plane of its orbit, each place on the earth's surface would have an unvarying season. It would be always mild in mid-latitudes, and equally cold in the same latitudes of polar regions.

With an inclined axis, however, the case is different. In June (see diagram below) the sun's rays are almost vertically on mid-latitude parts of the Northern Hemisphere, while in the corresponding latitudes of the Southern Hemisphere they are very oblique. At this season, therefore, the Northern Hemisphere receives more light and more heat than the Southern. Six months later the conditions are reversed; the vertical rays are oh the Southern Hemisphere, while on the Northern they are oblique. In

INCLINATION OF THE EARTH'S AXIS

The unshaded hemisphere shows the position of the light circle at each of the Jour seasons.

December, therefore, the Southern Hemisphere receives its greatest warmth.

Thus each of these hemispheres has a period of warm and long summer days, alternating with one of short days and cooler temperature. In equatorial latitudes the difference is not great, but beyond the tropics it is the difference between winter and summer. In polar latitudes the sun is shining on the greater part for six months. Each hemisphere, therefore, is alternately in light and darkness during this period. As a result, the season of sunshine, or summer, may become oppressively hot, while the season of

darkness, or winter, is very cold.

Any change in the inclination of the earth's axis would produce decided changes of climate. For instance, were the inclination increased, the limits of the frigid zones would be pushed farther toward the equator. If the inclination of the axis were forty degrees, the polar circles would each be forty degrees from the poles, and the tropics would be each forty degrees from the equator.

The Effects of Rotation. — The rotation or spinning of the earth on its axis causes the succession of daylight and darkness. One-half the surface, being always toward the sun, is illuminated; the opposite side is in darkness. The rotation of the earth presents every part of the surface successively toward the sun, lighting all parts in turn. Were the axis of the earth perpendicular to the ecliptic, day and

night would be of equal length at all parts. of the earth's surface. On account of its inclination, the relative length varies, not only in different latitudes, but with the changes of the seasons in the same latitude.

In the torrid zone the period of daylight and darkness does not vary much in length, and at the equator the days and nights are each twelve hours long. In the Icin-

RELATIVE LENGTH OF DAY AND NIGHT

The shaded part <tj each parallel shows the length o) the night; the unshaded part, the proportionate length oj the day.

perate zones the days are longest near the polar circles and shortest near the tropics, varying from thirteen to twenty-four hours. Within the frigid zones day and night correspond practically to summer and winter. There, both the day and the night vary from a few moments to six months in length.

The relative length of daylight and darkness and the changes of the seasons have much to do with food crops. The long days and short nights of summer in the temperate zones make possible the cultivation of plants that would not mature were the day only twelve hours in length.

Only a few species of animals and plants thrive in regions of long-continued darkness, and the}' are mainly the lower forms; the higher species require an environment in which light and darkness follow one after the other in periods of short duration. Most plants fail to mature and fructify unless exposed to strong light, and many species will not live at all.

QUESTIONS AND EXERCISES. — Make a circle one inch in diameter on the blackboard, and from the centre of this circle, with a radius fifty-five inches long, draw as much of the arc of a circle as the size of the blackboard will permit. The two circles represent the relative size of the earth and the sun.

In the diagram, p. 17, the axis of the earth is inclined 23*° from the dotted line; which of these positions represents summer in the Northern Hemisphere? — In the Southern? Copy the diagram, p. 18, and mark the point the sun's rays reach beyond the north pole; how many degrees from the pole to this point? What circle passes through this point? Mark the point on the circumference where the rays are vertical. What circle passes through this point? From each pole to the equator the angular distance is 90 : find the distance in degrees from the Arctic Circle to the Tropic of Cancer; this distance is the width of the Temperate Zone. If the inclination of axis were 28 , what would be the width of each light-zone? If

32 ? Ninety degrees less twice the angle of inclination equals the width of the Temperate Zone.

From the datum given on p. 15, calculate the speed of the earth per second at the equator; compare this with the speed of a train scheduled at sixty miles per hour. With reference to the earth's rotation on its axis, what is the speed per hour of a point on the equator? In latitude 6o° it is about half as great; how great is it at the poles?

In the diagram, p. 18, the proportionate length of the longest day and shortest night are shown by the shading; determine by measurement the length of the longest day in latitude 40 ; in latitude 6o°. Subdivide the parallel into twenty-four parts by halving it three times and dividing the last subdivisions each into three parts; each of the smallest subdivisions has practically an hour value.

Mill. — Realm of Nature, pp. 63-81.

Redway — Manual of Geography, pp. 64-78.

Howe. — Elements of Astronomy. Problems, a-g, p. 83. Jackson. — -Astronomical Geography.

Todd. — New Astronomy, chaps, v-vi.

Newcomb. — Popular Astronomy, pp. 88, 397.

Museum op Natural History, New York City. — Collection of meteorites, photographs of planets, nebula? and other celestial bodies.

Chapter II

In the long period of time that has elapsed since the earth was glowing with intense heat, the substances composing it seem to have adjusted themselves in accordance with the laws of gravitation — that is, the heaviest kinds of

IDEAL SECTION THROUGH THE EARTH

The thickness of the various envelopes is greatly distorted.

matter are nearest the centre. Structurally the earth consists of a dense and practically solid globe, the lithosphere, nearly covered with a comparatively thin layer of water, the hydrosphere, the whole being surrounded by an envelope of air, or atmosphere.

The shape of the lithosphere and the condition of the substances composing it, show that in times past it was intensely heated, and that much of the rock composing it has been in a molten condition. The globular form is the only one that would naturally result from the action of gravitation on a plastic or a fluid body; and the flattening at the poles is most reasonably explained by a hypothesis that, while it was still plastic, the earth spun, or rotated on its axis.

The density of the lithosphere, together with the waters, is about that of iron ore — that is, bulk for bulk, it is about five and one-half times as heavy as water. At the surface, however, the density of the rocks is about half as great as that of the whole globe; it is certain, therefore, that the substances forming the interior are much heavier than those at the surface.

The outer part of the lithosphere, called the rock envelope or, popularly, the "crust of the earth," surrounds an intensely heated interior, the centrosphere.

That the centrosphere is very hot cannot be doubted; for in every place where the rock envelope has been penetrated by deep borings, a constant increase of temperature is observed — the greater the depth the higher the temperature. The thickness of the rock envelope is not known, but at a depth of less than forty miles it is thought that the temperature is high enough to fuse the most refractory substances.

It must not be inferred from this, however, that the centrosphere is liquid; on the contrary, the earth behaves like a solid hut somewhat elastic body. The melting or fusing of a substance depends not alone on temperature, but also on pressure. With increase of pressure, the fusing point is also raised; and the great weight of the overlying rock produces pressure enough to prevent liquefaction. The increase of temperature varies in different kinds of rock and in different localities, the average being one degree (F.) for every sixty or seventy feet.

Four-fifths of the surface of the rock envelope is covered by a layer of water, the hydrosphere, averaging a little more than two miles in depth. The water not only exists in a free state, at the surface, but also as a chemical constituent of various kinds of rock.

The crystalline form of many rocks is due to the water they contain in chemical combination, and there are but few rocks of which water does not form a considerable part. It is by no means impossible that the free waters of the earth, in time, may be absorbed in this way, to reappear in chemical combination.

Water forms a most important constituent of the earth. It is essential to the existence of life; for not only does it form the greater part of every plant or animal, but it is also the chief means by which nutrition is distributed throughout the various parts of the body of the animal or the plant.

Within a range of a very few degrees of temperature, water exists in one or another of three forms — a solid, ice; a liquid, water; and a vapor, steam. In one or the other of its forms water is the chief agent by which the surface of the rock envelope has been sculptured; therefore it has an important place in the science of physiography.

The Atmosphere. — The atmosphere consists of a mixture of gaseous substances, namely — nitrogen, oxygen, water vapor, and carbon dioxide. Oxygen is required in the respiration of animals; carbon dioxide, the gas formed when coal burns, is essential in the breathing of plants; nitrogen forms a part of the structure in both animals and plants; and water vapor is the form in which the fresh water is carried from the sea to the land. The atmosphere, therefore, is just as essential to life as the water envelope.

The thickness of the atmospheric envelope is not known. Various estimates place it between one hundred and two hundred miles. At the latter estimate, on a globe three feet in diameter, the proportionate depth of the atmosphere would be about one-half an inch.

At the plane where the atmosphere rests upon the land and the sea the physiographic processes which are most noticeable are continually in action.

The Rock Mantle. — The three envelopes of the earth are constantly acting and reacting upon each other. Movements of the rock envelope have divided the waters from the land. Moreover, the rock envelope has been crumpled, and folded so as to form plateaus, ranges, and valleys. The heat of the sun causes a part of the ocean waters to take the form of vapor, and the latter, mingled with the air, flows over the land. Being chilled, the vapor again takes the form of rain or of snow, and falling on the land wears away its surface. The water, gathering into channels, carries the particles of rock waste mingled in its flood to the sea, and there deposits them.

As a result, almost every part of the rock envelope is covered with a layer of loose rock that has been worn from its surface. This layer is called the rock mantle, or sometimes the waste mantle. It is usually thick in the valleys and along the coasts to which it has been carried by running water; it is apt to be thin, or absent altogether on mountain slopes. The rock mantle is composed of about every mineral substance that enters into the structure of the rock envelope. The top of the rock mantle is commonly mixed with the remains of decayed vegetation. It contains the elements of plant food and constitutes soil.

For convenience, the constituents of the rock mantle are classified as follows: Clay, a silicate of the metal aluminium, is derived from the mineral felspar. Sand consists of grains of uniform size derived from the mineral quartz. As a rule, sand is a seashore product, and the uniform size of the grains results from the sorting power of the waves. Sand is also formed by the action of the wind. The name is applied loosely to any deposit of finely sorted grains of rock. Gravel is a term loosely applied to any accumulation of small pieces of rock. For the greater part, gravels are found along stream beds, in glacial drift and on shores. Peat is a black, slimy muck, composed mainly of carbon, that results from the decay of vegetable matter. It occurs chiefly in swamps, and old lake bottoms. The name is also applied to the stems of certain swamp plants. Marl is a limy substance, usually mixed with clay, that results from the decay of minute living organisms, shell-fish, etc. It is valuable as a fertilizer. The rock mantle consists chiefly of a mixture of sand, clay, and the products of decayed vegetation.

Movements of the Rock Envelope. — The most apparent changes in the surface of the rock envelope are the wearing away of the' rock from the higher surfaces and the transportation of the rock waste to lower levels. That is, water in its various forms loosens particles of rock, and the streams carry it seaward. If the land were everywhere level, the run-off of water could wear away but little of it. But vertical movements of the rock envelope are taking place, and these, by making new slopes, give the run-off waters increased wearing power.

These never-ceasing changes make a fairly complete cycle. Thus, vertical movements of the rock envelope form mountains and plateaus. By the action of water in its various forms, these are gradually worn down, and the rock waste that once composed them is transported to the shores of the continents and there spread out in the form of a long margin of sediment. The sediments, in turn, become layers of rock; in time these are folded into mountain ranges or uplifted in the form of plains and plateaus. Practically every part of the earth's surface has undergone changes of this sort.

In areas to which extensive sediments are being carried, evidence of sinking is usually apparent; while, as a rule, areas that arc being denuded are rising. It is probable, therefore, that vertical movements of the rock envelope in many instances are connected with the wasting of the land and the transfer of sediment.

The causes of these earth movements are not known, but it is believed that the gradual contraction of the rock envelope to fit itself around a more rapidly shrinking interior is the chief factor. There is evidence, too, that gravitation is a factor. The removal of great amounts of rock waste — from one locality to another, relieves weight at one place and increases it at the other. Therefore it is inferred that a sinking occurs at the latter place, and an uplift at the former.

According to this principle the rock envelope of the earth always maintains a state of balance, adjusting itself to the load it carries. This condition, called the isostatic balance, is regarded as an important factor in the explanation of the various movements of the rock envelope.

In general the vertical movements of the rock envelope are of two kinds: the uplift, folding, crumpling, and breaking which may be observed in the formation of mountain ranges; and the gentler movements observed in the slight uplift or the depression of areas of considerable size. Between the two kinds there is no broad distinction. Usually such movements are slow, covering periods of many centuries in duration. Occasionally, however, it takes the form of a sudden break of the rock envelope, thereby producing an earthquake.