44 PHYSICAL GEOGRAPHY.

 

 

 

Fig. 40, A Mountain-Pass,

107. Orology treats of mountains and their
formation.

The force which upheaved the crust into moun-
tain-masses and plateaus had its origin in the
contraction of a cooling globe. There are good

reasons for believing that no extensive mountains’

existed during the earlier geological ages, since
the crust was then very thin, and would have
been fractured before sufficient force could accu-
mulate to upheave it into mountain-masses.

The great mountain-systems of the world are
formed from sedimentary deposits that slowly ac-
cumulated over extended areas until they acquired
very great thickness. The deposits forming the
Appalachians, according to Dana, were, in places,
40,000 feet in depth, and covered the eastern bor-
der of the continent from New York to Alabama,
varying from 100 to 200 miles in breadth.

After the accumulation of these strata they
were, through the contraction of the crust, sub-
jected to the gradual effects of lateral pressure,
by which they were sometimes merely flexed or
folded, but more frequently crushed, fractured, or
mashed together, and thus thickened and thrust
upward. That side of the deposit from which
the thrust came would have a steeper slope than
the opposite side, which received a thrust arising
from the resistance.

 

 

This theory of mountain-formation, which is
generally accepted, explains the following facts:

(1.) All mountains have two slopes—a short
steep slope, facing the ocean, and a long gentle
slope, facing the interior of the continent.

(2.) The strata on the short steep slope are
generally highly metamorphosed; those on the
long slope are in general only partially metamor.
phosed, or wholly unchanged.

(3.) The mountain-systems are situated on the
borders of the continents where the sedimentary
strata collected.

(4.) Slaty cleavage, or the readiness with which
so many of the rocks of mountains cleave or split
in one direction, is a proof of these rocks having
been subjected to intense, long-acting, lateral pres-
sure, since such pressure can be made to develop
slaty cleavage in plastic material.

Isolated Mountains.—Nearly all high isolated moun-
tains were formed by the ejection of igneous rocks from

' the interior; that is, they are of volcanic origin and have

been upheaved by a vertical strain or true projectile force,
as in the volcanic range of Jorullo in Mexico.

108. Valleys in mountainous regions are either
longitudinal or transverse.

Longitudinal Valleys are those that extend in
the dire¢tion of the length of the mountains.

Transverse Valleys extend across the moun-
tain. | It is in transverse vaileys that most passes
occur,

Although valleys, like mountains, owe their origin to
the contraction of a cooling crust, yet their present shapes
are modified by the operation of other forces. By the
action of their water-courses, valleys are deepened in one
place and filled up in another. Extensive land-slides often
alter their configuration. During the Glacial Period many
valleys were greatly changed by the action of huge mov-
ing masses of ice. Fiord-valleys were formed in this
manner.

In level countries valleys generally owe their
origin to the eroding power of water.

109. Peculiarities of Continental Reliefs.—
The following peculiarities are noticeable in the
relief forms of the continents:

(1.) The continents have, in general, high bor-
ders and a low interior.

(2.) The highest border lies nearest the deep-
est ocean; hence, the culminating point, or the
highest point of land, lies out of the centre of the
continent.

' (8.) The greatest prolongation of a continent
is always that of its predominant mountain-sys-
tem.

(4.) The prevailing trends of the mountain-