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Jacques W. (Jacques Wardlaw) Redway.

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An inspection of the accompanying diagram shows that
during the month of June the sun's rays fall almost verti-
call}' on mid-latitude parts of the Northern Hemisjihere,
while in the corresponding latitudes of the Southern
Hemisphere the rays are very oblique. At this season,
therefore the Northern Hemisphere receives more light
and more heat than the Southern.

Six mouths later the conditions are reversed; the belt
of vertical and nearly vertical rays is in the Southern Hem-
isphere, while in the Northern, the rays of light nnd heat
are very oblique. At this season, therefore, the Southern
Hemisphere receives its greatest \varmth. Thus, it is seen,
the amount of light and warmth received by each hemi-
sphere varies. In equatorial latitudes the difference is not
great, but beyond the tropics, in higher latitudes, it is tbe
difference between winter and summer. In polar latitudes
the sun is shining the gi-eater part of the time for six
months alternately in each hemisphere, the other hemi-
sphere being in darkness. As a result the season of sun-
shine, or summer, becomes oppressively hot at times, while
the season of darkness, or winter, is intensely cold.

The rotation or spinning of the earth on its axis causes
the succession of day-light and darkness, or, popularly,
" day " and " night." One-half the surface, being always
toward the sun, is therefore illuminated, while the opposite
side is in darkness. The rotation of the earth, however,
presents every part successively toward the sun, lightir.g
all parts in turn. Were the axis of the earth perpendicu-
lar to the direction of the light-rays, day and night Avould
be of equal duration in all parts of the earth's surface ; but
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.



16



PHYSICAL GEOGRAPHY



In the torrid zone the period of daylight and darkness
does not vary much from twelve hours each, and at the
equator each is twelve hours long through the year. In the
temperate zones the da3's are longest near the polar circles
and shortest near the tropics, varying from thirteen to
twenty-four hours. Witiiin the frigid zone day and night
correspond practically to summer and winter. There, both
the day and the night vary from a few brief moments

to six months in length.
The relative length of
daylight and darkness and
the changes of the sea-
sons liave much to do with
the subject of physiogra-
jDhy. For their vitality
almost all the forms of
life depend not only on
the presence of sunlight,
but on the time and man-
ner of distribution as well.
Onl}^ a very few species
of animals and plants
thrive in regions of long-
continued darkness, and
they 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. With few exceptions, plants fail to mature
and fructify unless exposed to strong light, and many spe-
cies will not live at all. Plants that are forced into blos-
som in darkened rooms have usually pale or white flowers,
and the leaves of growing plants are apt to be yellow
instead of green.




RELATIVE LENGTH OF DAY AND NIGHT

The shaded part of eaeh parallel shows, the length
of the night: the unshaded part, the proportion-
ate length of the day.



THE EAKTH AMONG PLANETS 17

QUESTIONS AND EXERCISES.— Make a circle one inch in diam-
eter on the blackboard, and from the centre of this circle, with a ra-
dius 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. 14, the axis of the earth is inclined 23! degrees
from the dotted line ; which of these positions represents summer
in the Northern Hemisphere ?— In the Southern ? Copy the diagram,
p. 16, 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 ninety degrees :
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 in-
clination of axis were 28 degrees, what would be the width of each
light-zone ? If 32 degrees ? Ninety degrees less twice the angle of in-
clination equals the width of the Temperate Zone.

In the diagram, p. 16, 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 60 . Sub-
di'vide the parallel into tiventy-four parts by halijing it three times and
di-vidinp the last subdi'visions each into three parts ; each of the smallest
subdivisions has practically an hour "value.

COLLATERAL READING AND REFERENCE.

Mill. — Realm of Nature, pp. 63-81.

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

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

Jackso>". — Astronomical Geography.

NOTES

* 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 live hundred
miles. Their combined volume is less than one four-thousandth
I)art of the mass of the earth. Eros, one of the recently discov-
ered asteroids, bas an orbit so eccentric that it crosses that of
Mars, and at times is nearer to the earth than is Mars.



18 PHYSTCAL GEOGRAPHY

^ But little is known about the nature and structure of
comets, but it is thought that the chief part of their masses, in
most instances, is gaseous matter. One comet, Tempel's, un-
doubtedly consists of a vast swarm of meteors, but it is probable
that the various comets are differently constituted. 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.

^ Meteors, or shooting stars, are small bodies that seem to ex-
ist very generally throughout space. In a few instances they are
seen in clusters, as in the case of Tempel's comet. The earth,
and probably the other planets, encounter many thousands of
them daily, in sweeping through space. By far the greater num-
ber on reaching the earth's atmosphere are heated to whiteness —
partly by compression of the atmosphere in front of it, and
partly by friction against it— and are dissipated as white-hot
vapor. Some of the larger ones reach the earth, and many of
these have been analyzed. Some consist mainly of iron and
nickel in a metallic form ; others are composed of matter not
differing materially from lavas. No element has yet been found
in a meteor that does not occur in the earth, but in a few in-
stances chemical compounds, of iron, nickel, and phosphorus,
and certain crystalline forms, have been found in meteors that
have never been met with naturally in terrestrial substances. In
one instance gold, in another diamonds, were found in a mete-
orite.

* So far as is known, matter exists in three physical forms —
solid, liquid, and gaseous— and nearly every chemical element
and many of their compounds may assume each form. In the
solid form the molecules are bound by a strong cohesion ; in the
liquid form they are very slightly cohesive ; in the gaseous form
they strongly repel one another. Most of the substances that in
the earth are solids, in the sun exist as white-hot vapors.

^An interesting experiment is suggested by Professor Edward
Jackson (Astronomical Geography, p. 3). Three stakes are in
line, or as nearly in line as is practicable, one mile apart, along
the shore of a canal or a pond. On these, sighting marks are made
at a uniform distance above water-level. An engineer's level is
then placed so that the cross- wires cut the sighting marks of the
first and third stakes. If the telescope of the level be turned



THE EARTH AMONG PLAXETS 19

upon the middle stake it will be found that the cross-wires cut
the stake at a point eight inches below the sighting mark.




1 MILE



1 Ml LE




EXPERIMENT TO SHOW THE EARTH'S CURVATURE.

^It is by measurements depending on this principle that the
exact shape of the earth lias been ascertained. A pendulum of
absolute uniform length, weighted by a cannon-ball weighing
about one hundred pounds, 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 part nearer the centre, as the poles, the rate of vibration is
slightly faster ; at any place more remote they 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 noted on p. 13. Professor Ferrel has shown that, theo-
retically, 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 .

^ Any change in the inclination of the earth's axis would have
the effect of producing decided changes of climate. For instance,
if the inclination were increased, the limits of the frigid zones
would be pushed farther toward the equator. That is, if the in-
<;lination of the axis were forty degrees instead of twenty-three
and one-half, the polar circles would each be forty degrees from
the poles, and the tropics would be each forty degrees from the
e(iuator.



CHAPTER II

THE STRUCTURE OF THE EARTH

In the lone; period of time that has ela])sed since the
earth was glowing with intense heat, the substances com-
posing it seem to have adjusted themselves in accordance
with the laws of gravitation ^ — that is, the heaviest kinds of




IDEAL SEC! ION lllROUoH IHE EARTH

The tbiilini-':^ of ll)r i\iiioHS envelopes in girat/y distorted.

matter are nearest the centre. Structurally the earth con-
sists of a dense and practically^ solid globe, the Jitliospliere,
nearly covered with a comparatively thin layer of water,

30



THE STRUCTURE OF THE EARTH 21

the liydroi^phcre or water envelope, the whole surrouutled
by an envelope of gaseous matter, the atmosphere.

The shape of the lithosphere and the condition of the
substances composing it, all go to 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 fluid body ; and the flattening
at the poles is most reasonably explained by the supposi-
tion of a rotation on its axis Avliile it was still plastic.

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 not much more than
half as great; it is certain, therefore, that the substances
forming the interior are much heavier than those occur-
ring at the surface.

The Rock Envelope. — The outer part of the litho-
sphere is a shell of more or less friable material called the
rock envelope, or, popularly, the " crust of the earth." It
surrounds an intensely heated interior.^ The rock enve-
lope itself has lost so much of the heat it once had that it
is comparatively cold ; the amount of heat it radiates is
about equal to that which it receives from the sun.

That the interior of the lithosphere is very hot, however,
cannot be doubted ; for in every place where the rock en-
velope has been penetrated by deep borings, a constant
increase of temperature is observed — the greater the depth
the liigher the temperature.^ The thickness of the rock en-
velope is not known, but at a depth of less than forty miles
it is thonglit that the temperature is high enough to fuse
the most refractory su]>stances. The broken folds of the
outer surface have revealed something of its character to
the depth of several miles. Borings have been made to a



22 PHYSICAL GEOGRAPHY

depth of a little more than a mile (Table II., Jpjyendix),
but beyond the slight knowledge obtained from these,
nothing ])ositive is known about its interior.

The Water Envelope. — About four-tifths of the sur-
face of the rock envelope is covered b}^ a comparatively
thin layer of water, the hydrosphere. The water not only
exists in a free state, at the surface, but in chemical com-
bination it is a constituent of various kinds of rock ^ that
occur at or near the surface.

The waters of the earth form a most important constitu-
ent so far as life is concerned. Water is an essential ele-
ment 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 vehicle 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 an invisible vapor, often called "steam."
AYater in one or the other of its forms is the agent by
which, more than any other, the surface of the rock enve-
lope has been sculptured ; therefore it has a very impor-
tant part in the science of physiography.

The Atmosphere. — The atmosphere consists of a mixt-
ure of gaseous substances — namely: nitrogen, oxygen,
water vapor, and carbon dioxide. Of these oxygen is the
substance required in the breathing of animals ; carbon
dioxide, the gas formed when coal or carbon " biirus," is
essential in the breathing of plants ; nitrogen forms a part
of the body structure in both animals and plants ; and
water vapor is the form in Avhicli 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



THE 8TRUCTUEE OF THE EARTH 23

hundred miles. At the hitter estimate, on a globe one yard
in diameter, the depth of the atmosphere in proportion
would be about one-half an iueh.'^ Illustrate by diagram.

Keeping" Nature's Balance. — The three envelopes are
constantly acting and reacting upon each other, and at the
same time each has certain movements of its own. The
movements of the rock envelope have changed the level of
its surface so that the waters are divided from the land, and
the surface of the land has been wrinkled, crumpled, and
folded so as to form the plateaus, ranges, and valle3s. 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 gathers into channels and,
carrying the mingled particles of rock waste in its flood,
flows back to the sea and there deposits them.

And so the cycle of change ever goes on. At the plane
where the atmosphere rests upon the land and the sea the
physiographic processes that modify the earth's surface are
ever in action.

Vertical Movements of the Rock Envelope. — The
changes in the sujface of the rock envelope that are most
noticeable are the wearing away of the land and the trans-
portation of the rock waste to lower levels. That is,
water falling as rnin loosens particles of rock, while
streams carry it seaward. If th(^ land were everywliere
l<!vel, the I'un-ofl" of water could wear away but little of it;
but vertical movements of tin- surface that are apparent
only after long intervals of time are taking place, and
these, making new slopes, have given the run-oft' waters
increased wearing power.

Mov<!ments of the rock (uiv(;loi)c in times past have di-
versiiied its surface with higlilauds and lowlands, moun-



24 PHYSICAL GEOGRAPHY

taius and valleys, and similar movements are going on at
the present time. Probably no part of the earth is free
from them, bnt they are most clearly observed along sea-
shores. Thus, the coast of New Jerse}^ is sinking ;" and




AN UPLIFTED COAST, SAN PEDRO, CAL.

From a siirviy inade by Mcrick Rcyiioltis, Jr. The successively formed beaches are shown

by the strata of shells and sand.

SO also is much of the coast around the Gnlf of Mexico, the
Zuyder Zee, and the delta of the Ganges-Brahmaputra.
The coast of the New England Plateau has subsided until
the sea has flooded the coast plain and the lower valleys,
and has buried most of the old river mouths. The multi-
tude of bays and fjords that characterize this coast are
examples of "drowned" valleys. On the other hand,
parts of the Mediterranean basin, of the California coast,^
the Scandinavian peninsula, and the basin of Great Salt
Lake are rising.

In nearly every instance in areas to which extensive
sediments are being carried there is evidence of sinking ;
while, as a rule, areas that are being denuded are rising.
It is evident, therefore, that vertical movements of the
rock envelope — that is, uplift and subsidence — are defin-
i-ely connected with the wasting of the land and the trans-
fer of sediment.

The cause or 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 move rapidly shrink-



THE STRUCTURE OF THE EARTH



25



ing interior is the chief cause. There is evidence, too, that
gravitation is a factor. The removal of great amounts of
rock Avaste — often many cubic miles in volume — from one
locality to another, relieves weight at one place and in-
creases it at the other.'-* Therefore it is inferred that a
sinking, because of the increased load, occurs at the latter
place, and an uplift at the former, on account of the less-
ened weight.

The effects of these earth movements are very far-reach-
ing. The gi'eat highland regions of the earth, with their
ridges and folds, are probably direct results, and it is not
improbable that the uplift of the continents themselves
was also due to them.




IGNEOUS ROCK: A FLOW OF LAVA



Rock and Its Formation. — To almost every mineral
substance that forms a part of the earth, the term rode is
ap])lied. Thus, clay, sand, gravel, limestone, quartz, gran-
ite, lava, and even the fine, wind-blown rock waste, are



26 PHYSICAL GEOGRAPHY

each called rock ; and so also is a combination or any mixt-
ure of them. In many instances there is no donbt at all
how the rock has been formed, or whether it has been al-
tered or not, because the whole ]n-ocess of its formation
has been carried on in plain sight. Thus, when a volcano
or a fissure pours out a flood of molten lava there is no
question about how the rock got into place, or whence it
came. Tiie lava, Avlien it has hardened, may be glassy, or
metallic in appearance, or it may be like cinder or furnace
slag ; but there are always qualities about it that deter-
mine its origin.

Beyond a depth of a few thousand feet from the surface,
nothing positive is known about the substances of which
the rock envelope is com])osed. It is certain, however,
that most of the rock now at the surface consists of sedi-
ments carried into place by running water and deposited
in the form of layers or strata that afterward hardened into
compact rock. But these sediments must have come from
somewhere, and there is but one place from which they
could be derived — namely, from the rock envelope itself.

Now, no one knows what the 'primitive or first rock that
formed the crust of the earth may have been, but certain
kinds of rock have been found underlying the water-formed
sediments from which the latter seem to be derived. Or-
dinary granite is an example of this kind of rock, and
granitic rocks are very abundant. There are various
kinds of granite, but the most common varieties contain
minerals of which nearly all the elementary rocks them-
selves are composed.

One of these minerals is silica, of which quartz and sea
sand are the best examples. Another is felspar, a mineral
which, decomposed, yields clay, potash, lime, and soda.
Another mineral is Jiornhlende, which decomposes mainly
into iron, lime, and silica. Still another constituent usually



THE STRUCTURE OF THE EARTH 27

present is mica, popularly called " isinglass ; " like felspar
it also decomposes into clay, silica, lime, and a number of
other substances.

EXERCISE.— Procure one or more specimens of granite,'" and with
th° aid of a magnifying-glass observe the following directions. Look
for small clusters of foliated or " leafy " mineral ; it may be whitish
or, perhaps, green or brown ; this mineral is mica. If no mica is
found, look for jet black crystals or masses ; this is hornblende ; it is
usually opaque, but sometimes translucent. Find the white, trans-
lucent mineral with glassy lustre ; it is quartz, or silica, and it is apt
to form the chief bulk of the rock. Look also for an opaque mineral
varying from yellowish-white to pink in color ; possibly it will break
into fragments having flat sides, or cleavage planes ; this mineral is
felspar ; it has different crystalline forms accordingly as it contains
lime, potash, or soda.

Igneous Rocks. — There are certain surface rocks that
have cooled from a molten condition, and of these the
lavas of volcanoes, though not the most abundant, are per-
haps the best known. The Hawaiian Islands are mainly
great piles or domes of lava, and this kind of rock is com-
mon in most mountainous regions. In man}^ instances the
molten rock has been ejected from long fissures and has
cooled slowly ; in this form it is usuall}' known as basalt,
or, if it breaks into regular blocks, trap. The Palisades of
the Hudson, Fiugal's Cave, and the Giant's Causewa}- are
examples.

All the foregoing are commonl}^ called vulcanic or igne-
ous rocks ; consult a good dictionary and learn why these
names are applied. Igneous rocks are usuall}^ found in
mountainous regions, or in localities from Avhich the sedi-
mentar}^ rock has been removed. Granite rocks prevail in
the New England Plateau ; igneous rocks are abundant in
the Western Highlands.

Sedimentary Rocks. — Altiioiigh the sedimentary rocks
that prevail in such a great extent of the laud are derived



28 PHYSICAL GEOGRAPHY

from the granitic aud other vulcanic rocks, there is nothing
about them to indicate their ch)se relation to the latter.
The making of tirm rock out of loose sediments is a some-
what complex process. Let us follow the forujation of
sandstone. In the first place the grains of quartz are
rounded, and in the second place they are Tiniform in size.
The rock from which they came, probably granite, has
crumbled, and water has sorted the various minerals from
one another. The waves, beatiug the fragments of quartz
and rubbing them against one another, have not only
rounded the grains, but they have also sorted them ac-
cording to size, and piled them in a nearly flat layer.
True sand, therefore, is nearly always a formation of beaches
or of water in motion.

In time the beach is lifted up above sea-level and cov-
ered deep Avitli vegetable remains mixed wdtli loam. Water,
in one form or another, flows over or stands upon the
surface ; and if the water contains lime in solution it wall
leach through the layer of sand and cement the grains,
forming sandstone.

In most instances, clay banks are derived from granitic
and similar rocks. Felspar decomposes into clay, and the
latter, being very light and fine, is carried ofl" by the water,
settling b}^ itself, while the heavier materials remain. In
many instances the clay is spread over large areas. Pos-
sibly it remains in the stiff, pasty form by which it is
commonly knowai ; more likely pressure, heat, and moisture,
acting together, convert it into slate.

It is not difticult to understand how rivers and other
running waters are active workers in making rock, because
one can almost always find clay-banks, gravel-beds, and other
sediments that have been brought dow^n stream and dis-
tributed by the water.^' It is not so easy to understand
how rocks are found at the bottom of the sea; as a matter



THE STRUCTURE OF THE EARTH



29




SEDIMENTARY ROCK, NEAR OLEAN, N. Y.
The face of the cliff is one side of a channel of the river.

of fact, however, proljably more sedimentary rock has been
formed iu oceau aiul lake beds than in any other places.



Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 2 of 25)