164 PHYSICAL GEOGRAPHY.

 

The original fluidity of the earth is rendered probable by
the following circumstances: ’

(1.) By the spherical shape of the earth.

(2.) The crystalline rocks, or those formed in the presence
of great heat, underlying all others.

(3.) The warmer climate of the earth during the geolog-
ical past.

Volcanoes eject from the interior of the earth—1.
Melted rock or lava; 2. Showers of ashes or cinders; 3.
Vapors or gases.

These materials are brought up from great depths into
the volcanic mountain by the force caused by a contract-
ing globe. They may escape from the crater—1. By the
pressure of highly-heated vapors; 2. By the pressure
exerted by a column of liquid lava.

All volcanoes are found near the coasts of the continents,
or on islands.

The movements of the earth’s crust produced by earth-
quake shocks are—1, A wave-like motion around the
centre of disturbance; 2. An upward motion; 3. A rotary
motion.

The following facts have been discovered as regards
earthquakes :

(1.) Their place of origin is not very deep seated.

(2.) The area of disturbance increases with the energy
of the shock and the depth of its origin.

(3.) The shape of its origin is that of a line and not
that of a point.

(4.) The shape of the area of disturbance varies with
the elasticity of the materials through which the shock
moves.

(5.) The earthquake motion travels as spherical waves,
which move outward in all directions from their point
of origin.

The most violent earthquake shocks continue but for a
short time.

Earthquakes are generally caused by the strain produced
by the contraction of the crust.

Earthquake shocks are of more frequent occurrence—1.
In winter than in summer; 2. At night than during the
day; 3. During the new and full moon, than during any
other phase.

Earthquake shocks may occur in any part of the world,
but are of most frequent occurrence in the neighborhood
of volcanoes.

Rocks may be divided, according to their origin, into three
classes: 1. Igneous; 2. Aqueous; 3. Metamorphic.

They may be divided according to their condition, into—
1. Stratified ; 2. Unstratified.

Unstratified rocks are either igneous or metamorphic.

Rocks which contain organic remains are said to be
fossiliferous; if destitute of these remains, non-fossil-
iferous.

Stratified rocks are sometimes called fragmental. Un-
stratified rocks are sometimes called fragmental. Aqueous
rocks are sometimes called sedimentary.

During the geological past extensive changes occurred in
the land and water surface of the earth, and in the plants
and animals inhabiting it.

The changes now occurring in the earth’s crust are
caused—l. By the winds; 2. By the moisture of the
atmosphere; 3. By the action of running water; 4. By
the agency of man; 5. By the action of the heated in-
terior.

Of the 197,000,000 square miles of the earth’s surface,
144,000,000 square miles are covered by water, and
53,000,000 by land. The proportion between the land

and water is very nearly as the square of three is to the
square of five.

The continents extend farther to the north than to the
south; they are crowded together near the north pole.
Their southern projections are separated from each other
by extensive oceans.

Nearly all the land masses are collected in one hemi-
sphere, and a large part of the water in another.

There are two great systems of trends or lines of direc-
tion, along which the shores of the continents, the moun-
tain-ranges, the oceanic basins, and the island chains
extend.

The main prolongation of the eastern continent is in the
direction of the north-eastern trend; the western, in that
of the north-western trend.

The coast lines of the northern continents are very irreg-
ular, the shores being deeply indented with gulfs and bays,
while those of the southern continents are comparatively
simple and unbroken.

Of the 53,000,000 square miles of the land, 3,000,000, or
about one-seventeenth, is composed of islands.

Islands are either continental or oceanic.

Continental islands are detached portions of the neigh-
boring continents.

Oceanic islands are the summits of submarine mountain-
chains. They are either high or low: the high oceanic
jslands are generally of volcanic formation ; the low islands
are of coral formation. Mangrove islands occur off the
coasts of Florida.

There are four varieties of coral formations: 1. Fringing

- reefs; 2. Barrier reefs; 3. Encircling reefs; 4. Atolls.

A peculiar variety of coral reef occurs off the coasts of
Florida.

The subsidence of the ocean’s bed is proved—i. By the
exclusive occurrence of volcanoes on the shores of the con-
tinents or on islands; 2. By the occurrence of atolls or
coral islands; 3. By the general direction of the continen-
tal island chains.

The earth’s surface is composed of high lands and low
lands. The dividing line is 1000 feet above the level of
the sea.

High lands are either mountainous or plateaus.

Low lands are either hills or plains.

About one-half of the land surface of the earth is occu-
pied by plains.

Plains are 1. Undulating; 2. Marine; 3. Alluvial.

Mountains were formed by the contraction of the earth’s
crust, producing a lateral pressure on extended, thick de-
posits of sedimentary rocks. Slaty cleavage was caused
by this lateral pressure.

The following peculiarities are noticeable in the relief
forms of the continents:

1, The continents have, in general, high borders and a
low interior.

2. The highest border lies nearest the deepest ocean;
hence, the culminating point, or the highest point of land,
lies out of the centre of the continent.

3. The greatest prolongation of a continent is always
that of its predominant mountain-system.

4, The prevailing trends of the mountain masses are the
same as those of the coast lines, and are, in general, either
north-east or north-west.

Water acquires its maximum density at about the tem-
perature of 39.2° Fahr.

Water requires more heat to warm it, and gives out more
on cooling, than any other common substance.

During the constant washings to which the continents