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Jacques W. (Jacques Wardlaw) Redway.

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In very many instances these rocks are largely composed
of the remains of animals so small that several thousand of
them together would not be so large as the head of a pin.

The sea, especially iu regions of warm water, cont;iius
many thousand species of such animals; moreover tbry
multiply with great rapidity. But the animals are short-
lived, and as soon as tbey die their bodies sink to ihe
bottom. The mineral remains of these organisms consist
mainly of lime or silica, and in time the thick Liyer that
accumulates fin.-dly becomes cemented into rock. The
growth of rock in this Avay is slow, it is true, but time alone



30 PHYSICAL GEOGRAPHY

is reqniit'd to niakt; such layers of very great thickness.
The clialk cliffs of England and France were formed in this
maimer, and they aggregate nearl}' half a mile in thickness.
The limestones of the Mississippi Valley also accnnuilated
on sea-bottoms and have about the same thickness.

Metamorphic Rocks. — There are many instances in
which the character of sedimentary rocks has been sub-
sequently changed. Thus, by pressure and heat in the
presence of moisture, beds of clay have been transformed
into layers of gritty slate; chalk and limestone have be-
come crystalline marble ; and bituminous coal has become
anthracite. Certain kinds of granitic rock, especially
gneiss or " stratified granite," are metamorphic. Older
granitic rock has crumbled, and the rock waste has been
cemented into firm rock again with l)ut little alteration.

One might infer, therefore, that the older and deeper
stratified rocks would be thus changed.'^ This is usually
the case. The weight of the overlying rock produces im-
mense pressure, and the changes resulting from the moist-
ure within them greatly alter their appearance. Many of
the older rocks, indeed, are much like igneous rock in ap-
pearance. Rocks that form a part of mountain folds are
apt to be metamorphic on account of the pressure that
results from the folding and crumpling.

EXERCISE.— Procure specimens of clay and slate, chalk (not crayon)
and marble, bituminous (soft) coal and anthracite. Examine each pair
with reference to hardness, foliation, crystalline appearance, and den-
sity (weight of pieces of equal size). Make a list of the rocks occurring
in the neighborhood in v/hich you live, and classify them as igneous,
sedimentary, or metamorphic.

Order of the Strata. — Most of the sedimentary rocks
were deposited in horizontal layers, but, on account of the
vertical movements of the rock envelojoe, they are often



THE STRUCTURE OF THE. EARTH



31



found in ()l)luiue positions. Sometimes they occur in gen-
tle folds ; l)ut in mountainous regions they are much
crumpled and broken. In some of the old sea-beds now
raised above the surface the strata are undisturbed.




SEDIMENTARY ROCK: SECTION THROUGH THE CANON OF THE COLO-
RADO RIVER

The level of the strata l}as not been dislid he,i.

It is by studying the upturned edges of broken and tilt-
ed strata that the story of the earth has been read. Each
stratum is a chapter by itself, and to read the history prop-
erly it is best to begin with the lowest. It is not always
easy to tell the relative position of strata at some distance
from one another, but as each stratum has fossils, or ani-
mal remains peculiar to itself, the position is usually de-
termined by the kind and character of these.




SEDIMENTARY ROCK : TIl.THD STRATA

The total thickness of the stratified rocks is estimated
at upward of twenty miles. There is no locality known,
and none exists, in which all the various strata are found —
no locality is known in which even any considerable num-
Ijcr occur. Not infrequently very old rocks are overlaid
])V those; of the most recent formations ; all the intermedi-
ate strata are missing.''

To tTi(! lowest strata, that do not differ much from the
grauitic rocks and possibly include some of them, the name



32



PHYSICAL GEOGRAPHY




Archcvaa^^ is given. They seem to be the foim elation of

the coutinents and the floor of the oceans. The decay

^g2^^^E£^ ^^^^ wearing away of these has

^~~f~^ - -:-^. formed the material of which

-j^-^S^fr^!^^ nearly all the sedimentary rock

p3?utt^p^^|p~r— "^ is composed. '' The waste of ilie

old land is tlie material of the
neicT

The remaining strata are
named in accordance with the
character of the life forms that
existed when the rocks that
compose them were undergoing
formation. Upon the Archaean,
rest the rocks of the Palceozoic
era — the age of the earliest life
forms. Then follow the rocks
of the Afesozoic, or middle-life
era ; the Cenozoic, or era of re-
cent life ; and, last of all, the
era of man.

Archaean Era. — In Archaean times North America con-
sisted mainly of a narrow, V-shaped strip of land south of
Hudson Bay. The crests of the Appalachian Mountains
were just above the sea level ; the Black Hills and one or
two peaks of the Rocky Mountains had also just emerged.
The general form of the American continent was outlined
in Archaean times. With the possible exception of a few
species resembling the sponge, no forms of life are found
in Archaean rocks. {See illustration, jo. 34.)

Palaeozoic Era. — -The Palaeozoic era was of very long
duration. The sediments composing it are 25,000 feet
thick in places. The greater part of Europe and North
America were above sea level during this period, but the



UNCONFORMABLE STRATA:
CANON OF THE COLORADO RIVER

The tilted strata, originally horizontal,
were deposited on the surface of the
igncoui rock: Subsequently the upper
layers were deposited on the broken
surface of the tilted layers.



THE STRUCTURE OF THE EARTH



33



land was many times
iipheaveJ and sub-
merged. In North
America the greater
part of the INIississip-
pi Valley was a shal-
low inland sea, that
later became an im-
mense marsh.

In the variety and
extent of life forms
the Palaeozoic era is
the most noteworthy
of all the geological
periods. It began
with the lowest form
of sponges and closed
with the advent of
mammoth reptiles.
During this period an-
imals with backbones
appeared for the first
time. Insects Avere
numerous, and toward
the close reptiles ex-
isted. Fishes and
mollusks seem to have
been the prevailing
forms.

The climate wns
warm and moist. The
vast accumuliitious of
vegetable; matter that
are now the coal fields




ORDER OF STRATA



34



PHYSICAL GEOGRAPHY



~T-T7"



■^ih



1v '"^ -^



were found in swamjas of this age.'"' In North America
these swamps covered much of the area that is now the
central United States.

Mesozoic Era. — During the Mesozoic era both North
America and Europe had grown to about their present
shape. In the former division the Gulf of Mexico reached
as far north as the mouth of the Ohio, and a ncjrth-

western branch of it ex-
tended nearly to the 50th
parallel. In Europe all
tlie principal mountain
ranges '" and the higher
elevations of land most
probably had been raised
permanently above sea-
level.

It was an age of gigan-
tic reptiles. The animals
of some species were from
sixty to eighty feet in
length. For the first time
birds appeared. They
were very much like rep-
tiles, however, and in
some species they had, instead of horny beaks, heavy jaws
with socket teeth.

Cenozoic Era. — This era was largely one of uplift and
mountain-making, although both in North America and
Europe the various ranges and systems had received defi-
nite forms. The former was a continent of vast fresh-water
lakes ; the latter of inland seas.

Most of the life forms that flourished in preceding ages
were common, but one great step in advance maybe noted
— the appearance of mammals. Their genera included




NORTH AMERICA IN ARCH^AN TIMES

The shaded area shows the part of the conUneitt
above sea-level.



THE STRUCTURE OF THE EARTH



35




the elephant, camel,'' rhiuoceros, M^olf, deer, and horse.^^
There was a cousideralile advance in plant-life, and the
forest trees both of North
America and Europe in-
cluded most of the spe-
cies found to-daj.

Quaternary Age. —
The abrupt close of the
Cenozoic era was prob-
ably due to an elevation
of a large part of North
America and Europe from
1,000 to 2,000 feet, and a
decided lowering of tem-
perature. The ice and
snow of the north polar
regions crept southward
until it enveloped nearly north America in cenozoic times

all of Europe and the The unshaded area ^hows the part of the continent
1 above sea -level.

greater part of the United

States.' This accession of ice is commonly known as the
(jlacial epoch. It is marked on a stupendous scale by a
movement of drift similar to that which marks the gla-
ciers of the present time.

The changes of the Quaternar}^ age were disastrous to
life. In the area covered by glacial ice most of the spe-
cies of larger mammals perished. The cave bear, horse,
wolf, and reindeer survived. Many species of plants were
destroyed, but many escaped.

That man existed before the close of the glacial epoch
seems certain. In the caverns of Belgium, Germany, and
Italy the bones of man have been found in caves along
witli the skeletons of animals and various implements of
the chase. From the few scra[)S of unwritten history it



PHYSICAL GEOGRAPHY



seems that primitive man was a savage of the lowest type,
He lived in caves and obtained his food by hunting aud
He did not cultivate the soil nor did he have any



fishing.




THE UNITED STATES AT THE BEGINNING OF THE QUATERNARY AGE
The shaded area ihows the part added in recent times.

domestic animals. He had learned the use of fire, hoAV-
ever, and from that moment his intellectual development
was a question of time only.

QUESTIONS AND EXERCISES— A mixture of iron filings, sand,
and meal is gently shaken in a glass : what position will the compo-
nents take when they come to rest ? Explain why.

It is sometimes assumed that the rock envelope is about forty miles,
and the atmosphere about two hundred miles, in thickness. Construct
a diagram on the blackboard or on paper, showing the relative thick-
ness of each on scale in the ratio of 4000 : 40 : 200.

Obtain specimens of iron ore, marble, and dry clay, and compare
the weight of pieces of the same size. If possible find the specific
gravity of each. Determine, or judge by " hefting," the relative weight
of the various kinds of rock in the neighborhood in which you live.

Note and describe any instances within your personal knowledge of
the action of water on the rock envelope ; explain the nature of the
changes and how they have been brought about.

Study the various rock formations in the neighborhood in which you
live and classify them according to their origin— that is, as sediment-
ary or igneous.

Make a collection of them for future use.



THE STRUCTURE OF THE EARTH 37

A stream flows over a bed of limestone rock that is slightly soluble,
into a lake without an outlet ; what changes in the formation of
rock are likely to occur ? Will the rock formed be stratified or un-
stratified ? In what way may it become fossiliferous ?

From the official State reports find the order and distribution of rock
strata in the State in which you live, and from the information given
construct a geological map.



COLLATEEAL READING

Powell.— Physiography of the United States, pp. 22-29.
Le Coxte. — Elements of Geology, pp. 127-182.
Mill.— Realm of Nature, pp. 211-230, 249-261.
Shaler. — First Book of Geology, pp. 107-124.



NOTES

'That is, the substances specifically heaviest are nearest, and
the lightest are farthest, from the centre.

* Iron and its compounds form one of the most abundant con-
stituents of the earth, and it is likewise one of the most al)undant
substances of the sun and of some of the fixed stars. All tlie me-
teorites that have landed on the earth contain it, and in most of
them it is the chief element present.

' It must not be inferred from this, however, that the heated in-
terior is in a liquid condition ; on the contrary, the eai'th be-
haves like a solid but somewhat elastic body. The melting or
fusing of a substance depends not on temperature alone, but on
j>ressure as well. With increase of pressure, the temperature of
fusion is also raised ; and the great weight of the overlying rock
may possibly produce a pressure great enough to pi*event lique-
faction.

* The increase varies not only in different localities, but in dif-
ferent kinds of rock, the average being one degree for each si.xty
or seventy feet. In a certain boring in Upper Silesia, 6,700 feet
deep, there is a slight decrease in the ratio, but a nuirked in-
crease in the actual temperature at the greater depths.

'The crystalline form of many rocks is due to the water tliey
contain in chemical coml)ination, and there are but few rocks of



38 PHYSICAL GEOGEAPHY

which water does not form a consi<?erable part. It is by no means
impossible that the wateivs of tlie eartli, in time, may be aljsorbed
in this way, disappearing as free water, to reappear in cliemical
combination.

* About one-fifth of the atmosphere consists of free oxygen, an
element that forms also about one-half the weight of the earth's
crust, so far as can be estimated. In time, possibly, all the free
oxygen will be absorbed, entering into chemical combination with
other substances.

' In most instances the rate of sinking is about equal to the
depth of the layer of sediment annually spread over the surface.
The amount of sediment carried into the Gulf of Mexico is enor-
mous, but it does not apparently raise the level to any great ex-
tent ; few parts of the made-land surrounding the gulf are more
than ten or fifteen feet above sea-level.

" At San Pedro. California, the upward movement has been un-
usually rapid. Several layers of shells uiixed with sand are found
one above another, at heights vai-ying from five to fifteen feet or
more. The shells belong to species some of which are not now
extinct, and most of them have been preserved in their natural
state. The highest beach is nearly three hundred feet above sea-
level. The various beaches are so slightly weathered that they
seem scarcely altered. (Hee illustration, p. 24.)

" According to this principle the rock envelope of the eai'th
always maintains a state of lialance, adjusting itself to the load
it carries. It is readily illustrated by putting an ounce weight
on an inflated toy balloon. The surface of the balloon is de-
pressed by the weight, but if the latter be removed the surface
again rises ; or if the weight be moved from one part of the bal-
loon to another the surface at the one part rises while at the other
it sinks.

'" Normally, granite is a mixture of mica, felspar, and quartz.
If it contains hornblende instead of mica it is called syenite ; if
both mica and hornblende are present it is syenitic granite. If
the felspar contains soda the granite is diorite. If the rock
shows layers it is then called gneiss.

" An interesting example of rock-formation occurs at Sweyney
Cliffs, Shropshire, England. A small stream of water pours over
a red sandstone cliflf, mainly in the form of a rapid. The water
contains a considerable proportion of lime and magnesia ; and a



THE STliUOTURE OF THE EARTH 39

species of coarse moss grows freely in the saturated earth about
the stream-bed. The mineral salts of the water are depoV^ited co-
piously on the moss, and little by little the latter, together with
tiie other matter entangled, has become so completely incrusted
that it forms a dyke about twenty feet wide. The dyke stands
out, having built itself from the edge of the cliff a distance of ten
feet or more. Aljout thi-ee cubic yards are added each year.

'- Substances ordinarily insoluble in water are quickly changed
when subjected to water under a high temperature. If a thick
steel tube, filled with water and fragments of granite, be intensely
heated for several hours, the larger part of the rock will be dis-
solved. Hot alkaline water will also dissolve granitic rocks, the
dissolved matter being precipitated when the water cools.

'' Thus, the rocks of the Mississippi basin belong to a very old
and remote geological period. They are overlaid by a thin cover
of rock waste that l)elongs chieHy to the most recent period.

"The word Archaaii means "the beginning"; Falaozoic is
derived from two Greek words meaning " early life " ; Mesozoic,
similarly, is "middle life"; and Cenozoic, "recent life." The
Silurian age was named from "Silures. " a former name for the
people of Wales; Devonian comes from "Devon," England;
Huroii.iiiii., from "Huron"; and Laurentian from " 8t. Law-
rence." All the.se names are derived from the localities in which
the rocks were first studied.

'^ Coal measures are not confined to the Carboniferous age ; they
occur in all geological ages. Thus, the coal fields of the Pacific
coast belong to the Tertiary age. Those of the Carboniferous
age, however, are so vast in extent that they overshadow all
other features.

'"The uplift of the Pyrenees i\Iountains did not occur until
nearly the end of Mesozoic times.

" There were several species of came] during these times. It
is interesting to note that this aiMnial, now coidined to the east-
ern continent, was a native of tlu; west.

'" 'V\u' earliest species of horse had, instead of one, five toes. In
subsequent times two of these gradnalls disappeared. Tlie horse
of modern geological times has but one toe, but the "splint
bones" just above tlie hoof are the toes of the Quaternary horse.




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CHAPTER III



LAND AND WATER, AND THEIR OUTLINES



The surface of tlie rock envelope is not smooth, nor is
any considerable part of it perfectly level, as the word is
commonly used. More than three-fourths of its surface is
covered by the sea, but the remaining part consists of very
irregular areas that are
higher than the level of
the water. The great
body of water that covers
so much of the rock en-
velope is the sea ; ^ the
areas above sea-level
constitute the land. The
lowest part of the rock
envelope below sea-level
- — that is, the lowest part
of the sea-bottom — is

about five and one-half miles, and the highest point above
it is just about the same distance. The average elevation
of the land is not far from '2,000 feet, but the average
depth of the sea is about 2,000 fathoms.

The land aggregates about 53,000,000 square miles. It
clusters around the north pole, and from this circumpolar
region it radiates toward Cape Horn, toward the Cape
of Good Hope, and toward Tasmania. In which hemi-
sphere is the greater part V AVhich of the two temperate

41



OCEANIC AREA




C ONTI NE NT/VL
AREA


















1 3 L A N I5S





RhLATlVE AREAS OF LAND AND WATER



42



PHYSICAL GEOGRAPHY



zones iucludes tlie greater area ? How many great land
masses, each surrounded by water, are there ? The two
largest masses are divided nearly in twain, each at the
central part,^ and the smallest is separated by an arm of
the sea which seems to have severed it from the largest.
The three largest land masses are called continents ; ^ the
smaller ones islands. The line along which the land and
the sea meet is the shore ; the narrow strij) of land next
the shore, the (xxist.

The Continents. — The continents are so called on ac-
count of certain features of their structure. Each one, for
convenience, is divided into grand divisions, and the latter



4








LJ '




















a 1












Z


SOUTH


NORTH




1












1


AMER ICO.


AMERICA


AF RICA


(r El


u


R


A


S


A


u








D ]





















Ui 1













RELATIVE AREAS OF THE CONTINENTS AND GRAND DIVISIONS

are also couvenientl}' called continents. In general, the
continents have a high border on one side and a lower one
on the opposite side. They are variously named, but they
are usually styled the Eastern, or Asian ; the Western, or
American ; and the Australian. The shore of a great
bbdy of land in the south circumpolar regions is known
to exist, but practically nothing is known of its extent.

In a previous chapter it has been noted that changes in
elevation, especially along the shore, are taking place.
The real extent of the continents, therefore, is not appar-
ent ; in man}' places it comprises an area somewhat greater
than the part above water. Each is surrounded by a
margin, varying from a few rods to one hundred miles or
more, upon which the sea is comparatively shallow ; be-



LAND AND WATER 43

yond this margiu the surface sloj^es rather abruptly into
deep water.

The submerged margiu is very generally considered a
part of the continent. The depth of water along its extent
varies, and in places the margin itself reaches above sea-
level. The margin of each continent is more or less con-
tinuous, and forms a high surface in comparison with the
surrounding sea-bottom. It is usually called the continental
shelf. ^ The map on p. 45 shows both the highland and the
lowland regions of each continent and also its submerged
shelf : facing what ocean are the highlands ? — the lowlands ?
AVhere is the continental shelf widest ? — on which side of
North America has it the greatest width ? The highlands
are represented by the area above the level of 2,000 feet :
compare the extent of highlands and lowlands in each con-
tinent ; in North America. Are the highlands continuous
or broken ? Each one is a great plateau rimmed and trav-
ersed by lofty mountains. About one-fifth of the Austra-
lian, two-fifths of the American, and three-fifths of the
Asian continent are above the 2,000-foot contour.

The altitude of the highest regions of the continents
differs much. The greater elevations of North America
are from one to one and a half miles above sea-level ;
those of South America, about two miles ; and the highest
parts of Asia are more than three miles above sea-level.
The mountains that rim or surmount the highlands are
much higher — in many instances about twice as high.

The slopes toward the Arctic and Atlantic Oceans are
long and gentle ; how does this fact compare with the
slopes of the Pacific and Indian Oceans ? As a rule, the
lowland regions are more nearly level than the highlands.
On which side of the eastern continent are its principal
lowlands ? On which side of the American continent are
they situated?



THE WORLD

showing

ELEVATION OF LAND

and

DEPTH OF WATER.



TJevation ul' Land

fJ^^'-'A 20,000 Feet or over

E^^3 10,000 - 20,000 Feet

1,000-10,000 "

0- 1,000 "

below sea level
Depth of Water

- 1,000 Fathoms

1,000- i2,000 "

2,000- 4,000 "

4,000 or over



46



PHYSICAL GEOGRAPHY



The mean elevation of the land varies considerably in
the various continents. If their surfaces were levelled off
Australia and Europe would be not far from one thousand
feet higli ; North America and Africa about two thousand
feet ; and Asia nearly three thousand feet. Africa would
be probably a little higher, and South America not quite
so high as North America.

In a few instances there are depressions in the land be-
low sea-level. The surface of the Caspian Sea is eighty-four
feet below that of the Mediterranean ; the Dead Sea, situ-
ated in a gash north of the Red Sea, is thirteen hundred feet
below sea-level. There are two small dej^ressions in North
America, north of the Gulf of California ; and two or three
in Africa, south of the Atlas Mountains. It is not unlikely
that these were former arms of the sea that were severed

from the main body.

Islands.— The islands
have an aggregate area
of about three million
square miles, or about
one-seventeenth of the
entire land sm-face of the
earth. The majority of
them are situated on the
continental plateau, and
are at no great distance
from the continents to
which they belong. Many
of them are partly sub-
merged ranges of moun-
tains that are parallel to
the maritime ranges of
the continent, or that ex-
tend from it. Find two




Jtalatrom *(/' West , §,



j»>#= *,






ROST •: •




THE M.-M. CO.



A STRETCH OF THE COAST OF NORWAY

The coast, deeply indented with fjords-, is bordered
by many thousand rocky islets.



LAND AND WATER 47

such chains near the American continent, two near the
Asian continent. Ishimls of this character are usually
called continental islands ; and the reason is obvious.

In a few instances, here and there, are islands far distant



Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 3 of 25)