THE EARTH.
In beginning the study of the earth it is important that the student
should at once form the habit of keeping in mind the spherical form of
the planet. Many persons, while they may blindly accept the fact that
the earth is a sphere, do not think of it as having that form. Perhaps
the simplest way of securing the correct image of the shape is to
imagine how the earth would appear as seen from the moon. In its full
condition the moon is apt to appear as a disk. When it is new, and
also when in its waning stages it is visible in the daytime, the
spherical form is very apparent. Imagining himself on the surface of
the moon, the student can well perceive how the earth would appear as
a vast body in the heavens; its eight thousand miles of diameter,
about four times that of the satellite, would give an area sixteen
times the size which the moon presents to us. On this scale the
continents and oceans would appear very much more plain than do the
relatively slight irregularities on the lunar surface.
With the terrestrial globe in hand, the student can readily construct
an image which will represent, at least in outline, the appearance
which the sphere he inhabits would present when seen from a distance
of about a quarter of a million miles away. The continent of
Europe-Asia would of itself appear larger than all the lunar surface
which is visible to us. Every continent and all the greater islands
would be clearly indicated. The snow covering which in the winter of
the northern hemisphere wraps so much of the land would be seen to
come and go in the changes of the seasons; even the permanent ice
about either pole, and the greater regions of glaciers, such as those
of the Alps and the Himalayas, would appear as brilliant patches of
white amid fields of darker hue. Even the changes in the aspect of the
vegetation which at one season clothes the wide land with a green
mantle, and at another assumes the dun hue of winter, would be, to the
unaided eye, very distinct. It is probable that all the greater rivers
would be traceable as lines of light across the relatively dark
surface of the continents. By such exercises of the constructive
imagination--indeed, in no other way--the student can acquire the
habit of considering the earth as a vast whole. From time to time as
he studies the earth from near by he should endeavour to assemble the
phenomena in the general way which we have indicated.
The reader has doubtless already learned that the earth is a slightly
flattened sphere, having an average diameter of about eight thousand
miles, the average section at the equator being about twenty-six miles
greater than that from pole to pole. In a body of such large
proportions this difference in measurement appears not important; it
is, however, most significant, for it throws light upon the history of
the earth's mass. Computation shows that the measure of flattening at
the poles is just what would occur if the earth were or had been at
the time when it assumed its present form in a fluid condition. We
readily conceive that a soft body revolving in space, while all its
particles by gravitation tended to the centre, would in turning
around, as our earth does upon its axis, tend to bulge out in those
parts which were remote from the line upon which the turning took
place. Thus the flattening of our sphere at the poles corroborates the
opinion that its mass was once molten--in a word, that its ancient
history was such as the nebular theory suggests.
Although we have for convenience termed the earth a flattened
spheroid, it is only such in a very general sense. It has an infinite
number of minor irregularities which it is the province of the
geographer to trace and that of the geologist to account for. In the
first place, its surface is occupied by a great array of ridges and
hollows. The larger of these, the oceans and continents, first deserve
our attention. The difference in altitude of the earth's surface from
the height of the continents to the deepest part of the sea is
probably between ten and eleven miles, thus amounting to about two
fifths of the polar flattening before noted. The average difference
between the ocean floor and the summits of the neighbouring continents
is probably rather less than four miles. It happens, most fortunately
for the history of the earth, that the water upon its surface fills
its great concavities on the average to about four fifths of their
total depth, leaving only about one fifth of the relief projecting
above the ocean level. We have termed this arrangement fortunate, for
it insures that rainfall visits almost all the land areas, and thereby
makes those realms fit for the uses of life. If the ocean had only
half its existing area, the lands would be so wide that only their
fringes would be fertile. If it were one fifth greater than it is, the
dry areas would be reduced to a few scattered islands.
From all points of view the most important feature of the earth's
surface arises from its division into land and water areas, and this
for the reason that the physical and vital work of our sphere is
inevitably determined by this distribution. The shape of the seas and
lands is fixed by the positions at which the upper level of the great
water comes against the ridges which fret the earth's surface. These
elevations are so disposed that about two thirds of the hard mass is
at the present time covered with water, and only one third exposed to
the atmosphere. This proportion is inconstant. Owing to the endless
up-and-down goings of the earth's surface, the place of the shore
lines varies from year to year, and in the geological ages great
revolutions in the forms and relative area of water and land are
brought about.
Noting the greater divisions of land and water as they are shown on a
globe, we readily perceive that those parts of the continental ridges
which rise above the sea level are mainly accumulated in the northern
hemisphere--in fact, far more than half the dry realm is in that part
of the world. We furthermore perceive that all the continents more or
less distinctly point to the southward; they are, in a word,
triangles, with their bases to the northward, and their apices,
usually rather acute, directed to the southward. This form is very
well indicated in three of the great lands, North and South America
and Africa; it is more indistinctly shown in Asia and in Australia. As
yet we do not clearly understand the reason why the continents are
triangular, why they point toward the south pole, or why they are
mainly accumulated in the northern hemisphere. As stated in the
chapter on astronomy, some trace of the triangular form appears in the
land masses of the planet Mars. There, too, these triangles appear to
point toward one pole.
Besides the greater lands, the seas are fretted by a host of smaller
dry areas, termed islands. These, as inquiry has shown, are of two
very diverse natures. Near the continents, practically never more than
a thousand miles from their shores, we find isles, often of great
size, such as Madagascar, which in their structure are essentially
like the continents--that is, they are built in part or in whole of
non-volcanic rocks, sandstones, limestones, etc. In most cases these
islands, to which we may apply the term continental, have at some time
been connected with the neighbouring mainland, and afterward separated
from it by a depression of the surface which permitted the sea to flow
over the lowlands. Geologists have traced many cases where in the past
elevations which are now parts of a continent were once islands next
its shore. In the deeper seas far removed from the margins of the
continents the islands are made up of volcanic ejections of lava,
pumice, and dust, which has been thrown up from craters and fallen
around their margin or are formed of coral and other organic remains.
Next after this general statement as to the division of sea and land
we should note the peculiarities which the earth's surface exhibits
where it is bathed by the air, and where it is covered by the water.
Beginning with the best-known region, that of the dry land, we observe
that the surface is normally made up of continuous slopes of varying
declivity, which lead down from the high points to the sea. Here and
there, though rarely, these slopes centre in a basin which is occupied
by a lake or a dead sea. On the deeper ocean floors, so far as we may
judge with the defective information which the plumb line gives us,
there is no such continuity in the downward sloping of the surface,
the area being cast into numerous basins, each of great extent.
When we examine in some detail the shape of the land surface, we
readily perceive that the continuous down slopes are due to the
cutting action of rivers. In the basin of a stream the waters act to
wear away the original heights, filling them into the hollows, until
the whole area has a continuous down grade to the point where the
waters discharge into the ocean or perhaps into a lake. On the bottom
of the sea, except near the margin of the continent, where the floor
may in recent geological times have been elevated into the air, and
thus exposed to river action, there is no such agent working to
produce continuous down grades.
Looking upon a map of a continent which shows the differences in
altitude of the land, we readily perceive that the area is rather
clearly divided into two kinds of surface, mountains and plains, each
kind being sharply distinguished from the other by many important
peculiarities. Mountains are characteristically made up of distinct,
more or less parallel ridges and valleys, which are grouped in very
elongated belts, which, in the case of the American Cordilleras,
extend from the Arctic to the Antarctic Circle. Only in rare instances
do we find mountains occupying an area which is not very distinctly
elongated, and in such cases the elevations are usually of no great
height. Plains, on the other hand, commonly occupy the larger part of
the continent, and are distributed around the flanks of the mountain
systems. There is no rule as to their shape; they normally grade away
from the bases of the mountains toward the sea, and are often
prolonged below the level of the water for a considerable distance
beyond the shore, forming what is commonly known as the continental
shelf or belt of shallows along the coast line. We will now consider
some details concerning the form and structure of mountains.
In almost any mountain region a glance over the surface of the country
will give the reader a clew to the principal factor which has
determined the existence of these elevations. Wherever the bed rocks
are revealed he will recognise the fact that they have been much
disturbed. Almost everywhere the strata are turned at high angles;
often their slopes are steeper than those of house roofs, and not
infrequently they stand in attitudes where they appear vertical. Under
the surface of plains bedded rocks generally retain the nearly
horizontal position in which all such deposits are most likely to be
found. If the observer will attentively study the details of position
of these tilted rocks of mountainous districts, he will in most cases
be able to perceive that the beds have been flexed or folded in the
manner indicated by the diagram. Sometimes, though rarely, the tops of
these foldings or arches have been preserved, so that the nature of
the movement can be clearly discerned. More commonly the upper parts
of the upward-arching strata have been cut off by the action of the
decay-bringing forces--frost, flowing water, or creeping ice in
glaciers--so that only the downward pointing folds which were formed
in the mountain-making are well preserved, and these are almost
invariably hidden within the earth.
[Illustration: Fig. 7.--Section of mountains. Rockbridge and Bath
counties, Va. (from Dana). The numbers indicate the several
formations.]
By walking across any considerable mountain chain, as, for instance,
that of the Alleghanies, it is generally possible to trace a number of
these parallel up-and-down folds of the strata, so that we readily
perceive that the original beds had been packed together into a much
less space than they at first occupied. In some cases we could prove
that the shortening of the line has amounted to a hundred miles or
more--in other words, points on the plain lands on either side of the
mountain range which now exists may have been brought a hundred miles
or so nearer together than they were before the elevations were
produced. The reader can make for himself a convenient diagram showing
what occurred by pressing a number of leaves of this book so that the
sheets of paper are thrown into ridges and furrows. By this experiment
he also will see that the easiest way to account for such foldings as
we observe in mountains is by the supposition that some force residing
in the earth tends to shove the beds into a smaller space than they
originally occupied. Not only are the rocks composing the mountains
much folded, but they are often broken through after the manner of
masonry which has been subjected to earthquake shocks, or of ice which
has been strained by the expansion that affects it as it becomes
warmed before it is melted. In fact, many of our small lakes in New
England and in other countries of a long winter show in a miniature
way during times of thawing ice folds which much resemble mountain
arches.
At first geologists were disposed to attribute all the phenomena of
mountain-folding to the progressive cooling of the earth. Although
this sphere has already lost a large part of the heat with which it
was in the beginning endowed, it is still very hot in its deeper
parts, as is shown by the phenomena of volcanoes. This internal heat,
which to the present day at the depth of a hundred miles below the
surface is probably greater than that of molten iron, is constantly
flowing away into space; probably enough of it goes away on the
average each day to melt a hundred cubic miles or more of ice, or, in
more scientific phrase, the amount of heat rendered latent by melting
that volume of frozen water. J.R. Meyer, an eminent physicist,
estimated the quantity of heat so escaping each day of the year to be
sufficient to melt two hundred and forty cubic miles of ice. The
effect of this loss of heat is constantly to shrink the volume of the
earth; it has, indeed, been estimated that the sphere on this account
contracts on the average to the amount of some inches each thousand
years. For the reason that almost all this heat goes from the depths
of the earth, the cool outer portion losing no considerable part of
it, the contraction that is brought about affects the interior
portions of the sphere alone. The inner mass constantly shrinking as
it loses heat, the outer, cold part is by its weight forced to settle
down, and can only accomplish this result by wrinkling. An analogous
action may be seen where an apple or a potato becomes dried; in this
case the hard outer rind is forced to wrinkle, because, losing no
water, it does not diminish in its extent, and can only accommodate
itself to the interior by a wrinkling process. In one case it is water
which escapes, in the other heat; but in both contraction of the part
which suffers the loss leads to the folding of the outside of the
spheroid.
Although this loss of heat on the part of the earth accounts in some
measure for the development of mountains, it is not of itself
sufficient to explain the phenomena, and this for the reason that
mountains appear in no case to develop on the floors of the wide sea.
The average depth of the ocean is only fifteen thousand feet, while
there are hundreds, if not thousands, of mountain crests which exceed
that height above the sea. Therefore if mountains grew on the sea
floor as they do upon the land, there should be thousands of peaks
rising above the plain of the waters, while, in fact, all of the
islands except those near the shores of continents are of volcanic
origin--that is, are lands of totally different nature.
Whenever a considerable mountain chain is formed, although the actual
folding of the beds is limited to the usually narrow field occupied by
these disturbances, the elevation takes place over a wide belt of
country on one or both sides of the range. Thus if we approach the
Rocky Mountains from the Mississippi Valley, we begin to mount up an
inclined plane from the time we pass westward from the Mississippi
River. The beds of rock as well as the surface rises gradually until
at the foot of the mountain; though the rocks are still without
foldings, they are at a height of four or five thousand feet above the
sea. It seems probable--indeed, we may say almost certain--that when
the crust is broken, as it is in mountain-building, by extensive folds
and faults, the matter which lies a few score miles below the crust
creeps in toward those fractures, and so lifts up the country on which
they lie. When we examine the forms of any of our continents, we find
that these elevated portions of the earth's crust appear to be made up
of mountains and the table-lands which fringe those elevations. There
is not, as some of our writers suppose, two different kinds of
elevation in our great lands--the continents and the mountains which
they bear--but one process of elevation by which the foldings and the
massive uplifts which constitute the table-lands are simultaneously
and by one process formed.
Looking upon continents as the result of mountain growth, we may say
that here and there on the earth's crust these dislocations have
occurred in such association and of such magnitude that great areas
have been uplifted above the plain of the sea. In general, we find
these groups of elevations so arranged that they produce the
triangular form which is characteristic of the great lands. It will be
observed, for instance, that the form of North America is in general
determined by the position of the Appalachian and Cordilleran systems
on its eastern and western margins, though there are a number of
smaller chains, such as the Laurentians in Canada and the ice-covered
mountains of Greenland, which have a measure of influence in fixing
its shore lines.
[Illustration: _Waterfall near Gadsden, Alabama. The upper shelf of
rock is a hard sandstone, the lower beds are soft shale. The
conditions are those of most waterfalls, such as Niagara._]
The history of plains, as well as that of mountains, will have further
light thrown upon it when in the next chapter we come to consider the
effect of rain water on the land. We may here note the fact that the
level surfaces which are above the seashores are divisible into two
main groups--those which have been recently lifted above the sea
level, composed of materials laid down in the shallows next the shore,
and which have not yet shared in mountain-building disturbances, and
those which have been slightly tilted in the manner before indicated
in the case of the plains which border the Rocky Mountains on the
east. The great southern plain of eastern and southern United States,
extending from near New York to Mexico, is a good specimen of the
level lands common on all the continents which have recently emerged
from the sea. The table-lands on either side of the Mississippi
Valley, sloping from the Alleghanies and the Cordilleras, represent
the more ancient type of plain which has already shared in the
elevation which mountain-building brings about. In rarer cases plains
of small area are formed where mountains formerly existed by the
complete moving down of the original ridges.
There is a common opinion that the continents are liable in the course
of the geologic ages to very great changes of position; that what is
now sea may give place to new great lands, and that those already
existing may utterly disappear. This opinion was indeed generally held
by geologists not more than thirty years ago. Further study of the
problem has shown us that while parts of each continent may at any
time be depressed beneath the sea, the whole of its surface rarely if
ever goes below the water level. Thus, in the case of North America,
we can readily note very great changes in its form since the land
began to rise above the water. But always, from that ancient day to
our own, some portion of the area has been above the level of the sea,
thus providing an ark of refuge for the land life when it was
disturbed by inundations. The strongest evidence in favour of the
opinion that the existing continents have endured for many million
years is found in the fact that each of the great lands preserves many
distinct groups of animals and plants which have descended from
ancient forms dwelling upon the same territory. If at any time the
relatively small continent of Australia had gone beneath the sea, all
of the curious pouched animals akin to the opossum and kangaroo which
abound in that country--creatures belonging in the ancient life of the
world--would have been overwhelmed.
We have already noted the fact that the uplifting of mountains and of
the table-lands about them, which appears to have been the basis of
continental growth, has been due to strains in the rocks sufficiently
strong to disturb the beds. At each stage of the mountain-building
movement these compressive strains have had to contend with the very
great weight of the rocks which they had to move. These lands are not
to be regarded as firm set or rigid arches, but as highly elastic
structures, the shapes of which may be determined by any actions which
put on or take off burden. We see a proof of this fact from numerous
observations which geologists are now engaged in making. Thus during
the last ice epoch, when almost all the northern part of this
continent, as well as the northern part of Europe, was covered by an
ice sheet several thousand feet thick, the lands sank down under their
load, and to an extent roughly proportional to the depth of the icy
covering. While the northern regions were thus tilted down by the
weight which was upon them, the southern section of this land, the
region about the Gulf of Mexico, was elevated much above its present
level; it seems likely, indeed, that the peninsula of Florida rose to
the height of several hundred feet above its present shore line. After
the ice passed away the movements were reversed, the northern region
rising and the southern sinking down. These movements are attested by
the position of the old shore lines formed during the later stages of
the Glacial epoch. Thus around Lake Ontario, as well as the other
Great Lakes, the beaches which mark the higher positions of those
inland seas during the closing stages of the ice time, and which, of
course, were when formed horizontal, now rise to the northward at the
rate of from two to five feet for each mile of distance. Recent
studies by Mr. G.K. Gilbert show that this movement is still in
progress.
Other evidence going to show the extent to which the movements of the
earth's crust are affected by the weight of materials are found in the
fact that wherever along the shores thick deposits of sediments are
accumulated the tendency of the region where they lie is gradually to
sink downward, so that strata having an aggregate thickness of ten
thousand feet or more may be accumulated in a sea which was always
shallow. The ocean floor, in general, is the part of the earth's
surface where strata are constantly being laid down. In the great
reservoir of the waters the _débris_ washed from the land, the dust
from volcanoes, and that from the stellar spaces, along with the vast
accumulation of organic remains, almost everywhere lead to the
steadfast accumulation of sedimentary deposits. On the other hand, the
realms of the surface above the ocean level are constantly being worn
away by the action of the rivers and glaciers, of the waves which beat
against the shores, and of the winds which blow over desert regions.
The result is that the lands are wearing down at the geologically
rapid average rate of somewhere about one foot in five thousand years.
All this heavy matter goes to the sea bottoms. Probably to this cause
we owe in part the fact that in the wrinklings of the crust due to the
contraction of the interior the lands exhibit a prevailing tendency to
uprise, while the ocean floors sink down. In this way the continents
are maintained above the level of the sea despite the powerful forces
which are constantly wearing their substance away, while the seas
remain deep, although they are continually being burdened with
imported materials.
[Illustration: Fig. 8.--Diagram showing the effect of the position of
the fulcrum point in the movement of the land masses. In diagrams I
and II, the lines _a b_ represent the land before the movement, and
_a' b'_ its position after the movement; _s_, _s_, the position of the
shore line; _p_, _p_, the pivotal points; _l_, _s_, the sea line. In
diagram III, the curved line designates a shore; the line _a b_,
connecting the pivotal points _p_, _p_, is partly under the land and
partly under the sea.]
It is easy to see that if the sea floors tend to sink downward, while
the continental lands uprise, the movements which take place may be
compared with those which occur in a lever about a fulcrum point. In
this case the sea end of the bar is descending and the land end
ascending. Now, it is evident that the fulcrum point may fall to the
seaward or to the landward of the shore; only by chance and here and
there would it lie exactly at the coast line. By reference to the
diagram (Fig. 8), it will be seen that, while the point of rotation is
just at the shore, a considerable movement may take place without
altering the position of the coast line. Where the point of no
movement is inland of the coast, the sea will gain on the continent;
where, however, the point is to seaward, beneath the water, the land
will gain on the ocean. In this way we can, in part at least, account
for the endless changes in the attitude of the land along the coastal
belt without having to suppose that the continents cease to rise or
the sea floors to sink downward. It is evident that the bar or section
of the rocks from the interior of the land to the bottoms of the seas
is not rigid; it is also probable that the matter in the depths of the
earth, which moves with the motions of this bar, would change the
position of the fulcrum point from time to time. Thus it may well come
about that our coast lines are swaying up and down in ceaseless
variation.
In very recent geological times, probably since the beginning of the
last Glacial period, the region about the Dismal Swamp in Virginia has
swayed up and down through four alternating movements to the extent of
from fifty to one hundred feet. The coast of New Jersey is now sinking
at the rate of about two feet in a hundred years. The coast of New
England, though recently elevated to the extent of a hundred feet or
more, at a yet later time sank down, so that at some score of points
between New York and Eastport, Me., we find the remains of forests
with the roots of their trees still standing below high-tide mark in
positions where the trees could not have grown. Along all the marine
coasts of the world which have been carefully studied from this point
of view there are similar evidences of slight or great modern changes
in the level of the lands. At some points, particularly on the coast
of Alaska and along the coast of Peru, these uplifts of the land have
amounted to a thousand feet or more. In the peninsular district of
Scandinavia the swayings, sometimes up and sometimes down, which are
now going on have considerably changed the position of the shore lines
since the beginning of the historical period.
There are other causes which serve to modify the shapes and sizes of
the continents which may best be considered in the sequel; for the
present we may pass from this subject with the statement that our
great lands are relatively permanent features; their forms change from
age to age, but they have remained for millions of years habitable to
the hosts of animals and plants which have adapted their life to the
conditions which these fields afford them.