Butler's physical geography

Preface

The great advances made in geographical research during the past few years have placed Physical Geography among the essential studies.

The volume which the publishers herewith present embraces the latest discoveries in geographical science. In the chapters on Vulcanology, Ocean Hydrography, and Meteorology, there is much new material which has never before appeared in a school text-book.

The contents have been so arranged that the leading principles are found in the text, while other relevant matter of instructive nature has been placed in the notes. ^ By this arrangement clearness of statement has been obtained, which makes the volume adaptable to schools of all grades.

Acknowledgments are due to the U. S. Coast and Geodetic Survey, the U. S. Geological Survey, and the U. S. Hydrographic Office, for valuable information.

Contents

Introduction ......•• 5

The Earth regarded as a Planet 6

The Structure of the Earth .... 9

Introduction

-

During past ages the surface of the Earth has undergone many and great modifications, and the same agents which then wrought are still actively at work.

Physical Geography, or Physiography, is the science which treats of the surface of the Earth and the manner in which if has been changed by the forces of nature.

The outlines of this science are not closely defined, and the following chapters treat not only oi those subjects which are properly included in the study of Physical Geography, but also of certain principles which are common to Astronomy, Geology, and Meteorology.

As considered in this volume, Physical Geography is embraced under the following divisions:

. I. The Earth Regarded as a Planet.

II. The Structure and Formation of the Earth.

III. The Land.

IV. The Water.

V. The Atmosphere and its Phenomena; or, Meteorology. VI. The Distribution of Life.

PHYSICAL GEOGRAPHY.

Part One. the earth regarded as a planet.

Chapter I

1. The Earth is a planet, 1 and, so far as known, it is the third in order of distance from the sun.

2. Form of the Earth. — To an observer who stands on an elevated position, the earth seems to be an immense plain diversified with mountains and valleys, and, for the greater part, covered with a vast expanse of water. The apparent flatness is an illusion, for various experiments prove beyond doubt that the earth has a globular form.

I. When a vessel approaches land, the topmasts are first seen, then the lower sails appear, and finally the hull of the vessel comes into view. 2

THE CURVATURE OF THE EARTH'S SURFACE.

II. Navigators sailing in a given general direction have at length reached the ports from which they sailed. This has been accomplished repeatedly.

in. The visible portion of the earth's surface is always circular. For instance, if a slice be cut from a spherical body, as an orange, the rounded surface of the slice will represent that part of the earth which is seen by the observer. The bounding line of this surface is the circumference of a circle. If the slice be cut from an oval body, as a lemon, the bounding line will not always be the circumference of a circle, but usually that of an ellipse.

iv. The shadow of the earth which is projected on the moon during a lunar eclipse is always a circle. No solid except a sphere will always project a circular shadow.

Although the earth is globular in form, it is not a perfect sphere. If it were a perfect sphere, the length of a degree, angular measure, would be the same at all parts of its surface. Careful measurements show, on the contrary, that the length of the degree varies, being somewhat greater at the poles than near the equator. From this and from other observations, it has been shown that the earth is slightly flattened at the poles — a form to which the name oblate spheroid 3 is given.

3. Size of the Earth. — According to recent measurements, the equatorial diameter of the earth is 7925.6 miles,' and the polar

1 From the Greek word planet.es, a wanderer ; a heavenly body that revolves about the sun.

'A similar and very striking illustration may be seen from the railway trains which traverse the Gila Desert in Arizona. Just after the westbound train leaves Tucson, the top of a certain peak protrudes above the horizon which hounds the perfectly flat expanse of desert. As the train approaches it, more and more of the peak appears, and finally a mountain several thousand feet high comes wholly into view.

3 A series of interesting observations, made recently under the direction of the U. S. Coast Survey, show that the eartli lias a somewhat "squareshouldered" form, and that it is not a true oblate spheroid, as has been commonly supposed. This result has been reached by noting the time diameter, 7899.2 miles — a difference of 26.4 miles. The circumference of the earth at the equator is 24,899 miles. The surface of the earth is about 197,000,000 square miles in area; its volume is about 260,000,000,000 cubic miles. Compared with the sun, the earth is about ll25 o,ooo ^ s s ' ze '

4. Density. — The density of the whole earth is five times that of water. At the surface, however, the density is only two and a half times that of water. It is inferred from this that the interior of the earth consists largely of metallic substances.

5. Axis of the Earth. — The axis of the earth is the line about which it spins or rotates. This line is also the polar diameter. The northern extremity of the axis is called the north pole ; its southern extremity, the south pole.

The position of the earth's axis determines that of each of the other imaginary lines.

6. Circles. — In order to describe the positions of places, geographers conceive the surface of the earth to be divided by

certain imaginary lines. These lines, although imaginary so far as the earth is concerned, are real upon maps and globes. They are called meridians and parallels.

Great Circles are those which pass through the centre of the earth and divide it into equal parts called hemispheres ; small circles are those which divide the earth into unequal parts.

The circumference of every circle may be divided into three hundred and sixty parts, called degrees ; each degree is divided into sixty minutes, and each minute into sixty seconds. Degrees, minutes, and seconds are designated respectively by the characters ° ' and "—thus, 34° 25' 58".

The Equator is a great circle perpendicular to the axis. It divides the earth into the northern and soidhem hemispheres.

NORTH POLE

NORTH POLE

SOUTH POLE PARALLELS.

SOUTH POLE

MERIDIANS.

Parallels of Latitude are small circles parallel to the equator. Two of these are known as polar circles, and two others as tropics. They are important because they mark the limit of the

in which a pendulum of given length performed its vibrations. If the earth were perfectly spherical, the rate of vibration of such a pendulum would be the same at all parts of the earth's surface. It is found by careful observation that the rate of vibration varies with latitude, being somewhat greatest in latitudes where the flattening is greatest. By means of the pendulum experiments, it is found that a diameter extending from about 28° north to the corresponding degree south latitude is a little longer than it would be if the earth were a true oblate spheroid.

4 The statute mile is 5820 feet long; the geographical mile is one-sixtieth of a degree of latitude (one minute of arc), and is "075 feet long. Unless otherwise designated, statute miles are referred to in this book. The geographical or nautical mile is used chiefly by sailors.

THE EARTH REGARDED AS A PLANET.

sun's rays at certain seasons of the year. One, the Arctic circle, is 23° 27' from the north pole; the other, the Antarctic circle, is 23° 27' from the south polo. The tropic of Cancer is a small circle 23° 27' north of the equator; the tropic of Capricorn is a small circle 23° 27' south of it.

Meridians are great circles passing through the poles'of the earth. They are perpendicular to the equator.

7. Zones. — Zones are belts encircling the earth. They are included between the tropics, the tropics and the polar circles, ami the polar circles ami the pules. They are five in number. The space between the tropics is called the Inn-id zone; thai included between the tropic of Cancer and the Arctic circle is the north temperate :<>nr. The south temperate tone is included between the tropic of Capricorn and the Antarctic circle. The two frigid zones lie between the poles and the polar circles.

The zones have received their respective names on account of the different degrees of heat they receive from the sun.

8. Latitude and Longitude. —The position of a place on the earth's surface is determined by its latitude and longitude.

The latitude of any place is its distance -th or south of

the equator, measured in degrees, minutes, and second.-. Latitude is reckoned from the equator to the poles.

Regions north of the equator are in north latitude; those south of the equator are in south latitude. Two or more places situated, for instance, on the 20th degree of south latitude, are said to he on the same parallel, because the same circle passes through them. Degrees of latitude increase in length from the

equator to the poles, the length of a degree at the equator being 68.7

miles, hut at the poles, till. 4 miles.

The longitude of any place is its distance east or west, measured in degrees, minutes, and seconds, from a meridian taken as a standard. < >u maps, the figures designating degrees of longitude are placed usually at the top and at the bottom. 1

1 Iiuil recently, in most el' the kingdoms and states of Europe the meridian which passed through the principal observatory was taken as a standard. Within a lew years, however, most of the civilized nations liave agreed upon the meridian of Greenwich as the standard from which io reckon longitude.

2 This was taught by Pythagoras, a Greek philosopher, live hundred years before the Christian era, Imt the theory was rejected by succeeding astronomers, who regarded the earth as the centre of the universe, around which all the heavenly bodies revolved every twenty-four hears. The true theory was revived by Copernicus, in the sixteenth century.

Longitude is reckoned cast and west from the standard meridian, being measured in each direction from 0° to 1<S0°. Places lying west of the standard meridian are said to he in uit -t longitude, up to 180°; those to the east, in east longitude, to the same extent.

This is shown in tin- accompanying figure, where the circle CEDB represents the equator, and A the pole of the earth.

Because the meridians converge toward the poles, degrees of longitude vary in length at different latitudes. At the equator the length of a degree is ahout (>!) miles; at the 20th parallel, nearly 64.8 miles; at the 40th, about 52.8 miles; and at the 80th parallel, only 12 miles. At the poles it becomes nothing.

9. Motions of the Earth. — The earth has two motions: first, a daily rotation ahout its axis from west to east, causing the succession of day and night: and, secondly, a yearly revolution

around the sun, which gives rise lo the change of seasons.'

10. Daily Motion. — The fact that the earth revolves upon its axis rests upon the following proofs ;

i. ll is impossible, from the laws of centrifugal motion, that the heavenly bodies should revolve daily about the earth ; but

it is perfectly consistent to consider their motion as apparent, and due to a real motion of the earth.

11. The flattening of the earth at its poles is an effect of rotation. Newton proved its figure to be that of an oblate spheroid

before the fact was shown by actual measurements.

ill. Balls falling from the summit of a high tower strike the ground east of the point whence they started. The top of the lower moves more rapidly eastward than the base, as it is farther from the axis of the earth, and consequently has a greater centrifugal force. In 1804, thirty balls were dropped from Michael's Tower, in Hamburg, from the height of 235 feet. The deviation from a perpendicular was found to be one- //o'/'i/of an inch. The annexed cut illustrates this. A ball dropped from B docs not take the direction of the perpendicular KB', but that of the inclined line III).

IV. The diminished weight ' of a body at I he equator indicates a centrifugal force resulting from the rotation of the earth.

A body removed from the poles to the equator would weight.

v. All other planetary bodies, so far as known, revolve on their axes; and it is reasonable to infer that the earth also revolves on its axis.

11. Annual Motion. — The revolution of the earth around the sun' is performed in 365 days, 5 hours, 48 minutes, and 46

se< il- a period of time called a year. The path or orbit in

which the earth whirls around the sun is an oval-shaped figure or ellipse. The orbit of the earth, and, indeed, the orbits of the

' A body removed from the equator to the poles gains T ?,i of its entire weight ; .',„ is due to the flattening of the earth at the poles. The difference between , ,', , and -.',,,, equal to .1,,, still remains to be accounted tor; and the rotation of the earth dues account for it.

* Besides the daily and the yearly motion, the earth, together with the rest of the solar system 1 , is constantly sweeping on through -pace. Neither the rate of motion, nor the direction in which the solar system is moving, is with certainty known. There is much evidence to show that the direction is towards the constellation Hercules, from the fact that the stars of that constellation are apparently spreading out.

use by this force . J ,-,- of its

PHYSICAL GEOGRAPHY.

other planets, are so slightly elliptical as to be hardly distinguished from circles. The orbit of the earth differs from a circle by about one-sixtieth of its diameter.

The sun is not situated at the centre of the ellipse, but at a point called the focus? The earth is, therefore, nearer the sun at one time than at another. When nearest the sun it is said to be in perihelion ; when farthest away, in aphelion.

The Ecliptic is the name of the earth's orbit. The earth is in perihelion about December 21, and in aphelion about June 21. At perihelion the earth is about 90,000,000 miles from the sun ; at aphelion, about 93,000,000.

12. Velocity. — The velocity of the earth in its annual motion varies, being greatest at perihelion. The average velocity is about 19 miles a second, or 66,000 miles per hour.

The rotation of the earth on its axis gives to a point situated at the equator a velocity of 25,000 miles a day, or a little more than 1000 miles an hour. But, by reason of the spherical shape of the earth, a point between the equator and either pole will move more slowly, because, moving in a smaller circle, it has not so far to travel. 2

This difference in velocity, as will be seen in a following chapter, has much to do with the direction of winds and ocean currents.

13. Length of Day. — During the summer season the length of the day increases with the latitude ; in winter it decreases as the latitude increases. The longest days are always in summer ; the shortest in winter.

"Within the torrid zone, the longest day is about loj hours in length ; the shortest, 101 hours. In the temperate zones, the longest day is 24 hours ; the shortest, a few minutes in length. In the frigid zones, the days and the nights each vary from a few minutes to six months in length. 3

On the 15th and 16th of March, to an observer at the north pole the sun appears to sweep around the horizon. Each succeeding day it rises higher and higher, until, on the 21st of June, it is 23° 27' above the horizon. After that time it gradually sinks. By the 15th of September it again skims the horizon, and by the 21st of December it is 23° 27' below it.

14. The Seasons. — The change of seasons is due to the following causes :

i. The sun illumines but half of the globe at a time.

ir. The direction of the earth's axis is always the same, being inclined 23° 27' from a perpendicular to the ecliptic.

On the 20th of March the sun's rays are perpendicular to the earth's axis, illuminating the half of the earth on which they fall from pole to pole. As the earth revolves on its axis, every place on its surface is now twelve hours in darkness and twelve in light. It is then spring in the northern hemisphere and autumn in the southern.

As the earth moves on its course, the circle of illumination gradually overlaps the north pole and recedes from the south pole.

1 An ellipse may lie considered as having two centres, one on each side of the true centre. The centres of the ellipse are ha foci. The eccentricity of the ellipse is the distance between its foci. The eccentricity of the earth's orbit is about 3,000,000 miles.

2 The velocity of rotation in different latitudes is seen in the following :

In the northern hemisphere, the days increase and the nights decrease in length ; in the southern hemisphere, the reverse occurs.

On the 21st of June the circle of illumination reaches its limit. The rays of the sun then extend 23° 27' beyond the north pole to the Arctic circle, and fall short of the south pole by the same amount, reaching to the Antarctic circle. Within the former circle continual day reigns, and within the latter unbroken night.

In the northern hemisphere it is the beginning of summer, and the time when the days are longest and the nights shortest. In the southern hemisphere it is the period of winter, when the nights are longest and the days shortest.

As the earth recedes from the summer solstice, 4 the circle of illumination returns by degrees to the north and south poles, lengthening the nights in the northern hemisphere and the days in the southern.

On the 22d of September the sun's rays again reach from pole to pole, making the days and nights once more equal in length. This is the autumn of the northern hemisphere and the spring of the southern.

The earth still moving forward in its course, the circle of illumination gradually withdraws from the north pole, and extends beyond the south pole, lengthening the nights in the northern hemisphere and the days in the southern.

On the 21st of December this change attains its full extent : the rays of the sun then extend beyond the south pole to the Antarctic circle. The regions within this circle now enjoy a continual day; but in the northern hemisphere the sun's rays reach only to the Arctic circle, and this zone is veiled in the gloom of constant night.

It is then winter in the northern hemisphere and summer in the southern, the longest day at that time occurring in the southern hemisphere and the shortest in the northern.

LATITUDE.

VELOCITY PER HOUR.

LATITUDE.

VELOCITY PER MO

0°

Him miles.

70°

354 miles.

30°

MP iso "

40°

703 "

60°

517 "

90°

These changes are illustrated in the accompanying cut, where the twelve globes represent the position of the earth in its orbit in the successive months of the year, DCL the equator, and NCS the axis — N being the north pole and S the south.

3 The following table shows the length of the longest day in various latitudes:

LATITUDE.

LENGTH OF DAY

0° 0'

40° 2'

58° '27'

ish.

03° '23'

LATITUDE.

LENGTH OF DAY.

65° 18'

1 mo.

611° 51'

2 mos.

90° 0'

G mos.

4 From the Latin sol, sun, and stare, to stand ; because the sun appears to be stationary in the heavens, as regards its apparent motion north or south.

THE STRUCTURE OF THE EARTH.

Paet Two. the structure of the earth.

IGNEOUS ROCK— BASALTIC COLUMNS, GIANTS' CAUSEWAY, IRELAND

CHAPTER I.

THE CENTRAL MASS OF THE EARTH, AND ITS CRUST.

1. The Earth consists of an outer shell, called its crust, and an intensely heated central mass, either solid or plastic 1

That tin' interior of the earth is intensely heated is inferred from the following facts:

i. In all deep mines the temperature of the rock increases one degree 3 for every fifty or sixty feel of descent, after t lie first hundred feet. 3 This is reasonably true in all parts of the world ; hut the rate of increase differs according to the kind of rock through which the mine passes.

ii. Water flowing from an Artesian 4 well increases in temperature with the depth of the well. The water from the well of Grenelle, at Paris, 1798 feet deep, lias a constant temperature of nearly 82° ; while the mean temperature of the air in that city is only 53°. The temperature of the water, therefore, increases, on an average, 1° for every sixty-two feet of descent. 5

m. Hot springs, whose sources are very deep below its surface, also prove the existence and increase of internal heat.

fractory rocks would be melted at the distance of about thirty miles below the surface of the earth, if the temperature eon Stantly increased to this depth at the rate stated. 8

2. The Earth's Crust. — Deep mines and borings have been sunk indifferent parts of the globe, revealing its structure; hut none have pierced farther than a \\'\\ hundred feet below the sea-level. The greatest depth attained is, at present, at the salt-works

at Sperenburg, Prussia, where the boring sinks 4190 feet beneath

the level of the ocean. 7

Our knowledge of the earth's structure extends beyond the depth of two thousand feet, for in many places the rocky strata have been tilted and bent so that their edges have been forced to the surface in inclined and curved positions. Geologists have thus been enabled to study the nature of the globe to the depth of many miles. By a careful study of the position of these strata, we learn that the earth's crust consists of various kinds of rocks, arranged in consecutive order.

I nder the name rock is included everything that enters into the structure of the earth's surface. It includes not only compact masses of stone, hut clay, soil, gravel, sand, etc.

Chemistry teaches us that all matter with which we are acquainted is composed of about seventy elementary bodies. Sixteen

TILTED STRATA OF RED SANDSTONE, LAKE SUPERIOR.

proportions, form of these, united in different combinations : nearly all the known matter of the globe.

These important elements are silicon, aluminum, potassium, iv. Volcanoes exhibit striking proof of the intense heat of the sodium, magnesium, calcium, iron, manganese, oxygen, hydrogen, central portions of the earth. According to Lyell, the most re- nitrogen, carbon, sulphur, chlorine, fluorine, and phosphorus.

1 In its relation to other heavenly bodies, the earth behaves, not as a solid shell surrounding a fluid central mass, but as a solid body.

* The degrees of temperature are those of Fahrenheit's thermometer, unless otherwise designated.

3 It must he kept ill mind that these figures represent an average. The rate of increase depends largely on the conductivity of the rock. It is also controlled more or less by the chemical decomposition of the roek.

1 By boring into the earth deep enough to reach a subterranean ho.lv of water, whose sources are higher than the place of the perforation, a spring or well is obtained, called an Artesian well, because such wells were Hrst made in the province of Artois, France.

6 Observations on this point were made by Walferdin on two Artesian wells at Creuzot, in France. The borings were a mile apart, one descending 1900 feet, ami the other -±;~;s feet. The results showed that, to the depth of 1800 feet, the temperature rose at the mean rate of 1° for even fifty-live feet of descent, alter which the change was more rapid, heing 1° for every forty-four feet.

6 It is by no means certain that the molten matter ejected from volcanoes comes from the "molten centre" of the earth. Such authorities as Mallet. Judd, and Le * onte incline to the belief that the ejections of volcanoes are derived from local reservoirs of matter heated by chemical action.

'Great depths have been reached in mines beginning at higher places : thus, the Comstock silver mines in Nevada, among the deepest in the world, descend to the depth of nearly 4000 feet.

PHYSICAL GEOGRAPHY.

Oxygen, combined with one or inure of these elements, composes the greater portion of the rocks. It constitutes in itself from a half to a third of the whole crust of the globe. United with hydrogen it forms water, and mixed with nitrogen, the atmosphere. By weight it constitutes eight- ninth* of all the waters of the earth, and one-fifth of the atmosphere.

Carbon forms a considerable portion of all vegetation. Coal consists of it, and it enters largely into the composition of limestone and marble. The diamond is nothing more than pure, crystallized carbon. To silicon we are indebted for glass ; and chlorine, in combination with sodium, gives us that most useful

article, salt.

ROCKS.

Rocks are divided into three great classes — stratified, 1 unstratified, and metamorphie. 1

CURVED STRATA.

3. Stratified Rocks. — Stratified rocks occur in layers, usually parallel to one another.

They have been formed by the action of water, which has deposited the sediment in layers, or strata. Such rocks have, therefore, received the name of aqueous rocks. Sandstone, clay, and limestone are included in this class.

INCLINED STRATA.

Throughout past ages the elements have been incessantly at work wearing away the land. Frost and heat have crumbled

1 From the Latin word stratum, a bed.

- From the Kreek word metamorphosis', a transformation.

3 From the Latin word fossilis, that which is dug out of the earth.

4 From the Latin words fossifa and fe.ro, to bear; i.e., fossil-bearing.

5 The amount and extent of these organic remains, microscopic and otherwise, are truly amazing; for each cubic inch of chalk is estimated to contain a million distincl structures. Such organisms form nearly half the bulk of the chalk of Northern Europe, and exceed this ratio in that of Southern Europe. The inorganic portion of the chalk consists of decomposed shells, which cement the fossils into a compact mass. The beds of marble in the Pyrenees, and the limestone range at the head of the Adrithe rocks ; rivers and glaciers have ground them to powder ; rains and rivers have carried down the sediment and deposited it at the bottom of lakes and seas, where it has become consolidated into beds of rock.

NUMMULITE MARBLE.

HORIZONTAL STRATA.

But these have not remained at rest ; for the contraction of the earth's crust has bent them into vast folds, again to be subjected to the action of the elements, and to pass through the same changes.

Stratified rocks abound in the fossil 3 remains of plants and animals, and especially in marine forms. Rocks of this kind are called fossiliferous 4 rocks.

The fossils are sometimes found so perfectly preserved that in the remains of a fish not a scale is wanting ; and shells of unknown age have retained not only their shape, but even their colors. These relics have been discovered even on the tops of the loftiest mountains, proving that the highest peaks were once buried beneath the ocean. 5

The central part of North America is rich in fossiliferous strata, and the valley of the Mississippi abounds in rocks filled with corals, crinoids, 6 and shells.

Not only are the bones of gigantic animals entombed in the stratified rocks, but fossil organisms so minute exist that forty-one thousand millions are contained in a cubic inch of stone.

So widely diffused are the fossiliferous rocks that they are supposed to constitute two-thirds of the surface of the present continents. The study of these rocks is full of interest ; for the fossils they enclose reveal to us the former life of our globe.

4. Unstratified Rocks. — These, as their name implies, are not formed in successive layers, but occur in masses, which are supposed to have been produced by the action of heat. For this reason they are also called igneous'' rocks. They are included in three divisions — viz.: Granitic, Trap, , 8 and Volcanic' Rocks.

atic Sea, are composed of fossil shells termed nummulites. The nummulitie formation is often thousands of feet in thickness, and is traced from Egypt, through Persia, to the Himalaya Mountains.

6 So called from the Greek words krinos, a lily, and eidos, form ; because this family of animals, which are nearly extinct, have the shape of a lily with a jointed stem.

7 From the Latin word ignis, fire.

8 From the Swedish word trappa, a stair. It often presents the appearance of a series of steps.

9 From Vulcan, who, according to classic mythology, forged the thunderbolts of Jupiter.