RIVERS. 63

 

matters which are found in the strata of nearly all the
geological formations. The reservoirs are of the same
nature as those of artesian wells, the oil being obtained
by boring.

Petroleum springs are numerous, The most extensive
regions in the world are found in the great oil districts of
Western Pennsylvania and the neighboring States.

Bituminous Springs, or those from which pitch or
bitumen issue. Their origin is the same as that of oil
springs, the decomposition, however, occurring in a some-
what different way. The famous pitch lake on the island
of Trinidad, north-east of South America, probably owes
its origin to the large quantities of trees and other vege-
table matters, which have been rolled down the Orinoco
and buried in the delta formation on the eastern shores
of the island.

—-0205 00 ——_.

a CLAP Ere ls
a Rivers.

168. Definitions——The water that issues from
the ground as springs, that is derived from the
melting of ice or snow, or that drains directly
from the surface after rainfall, runs down the
slopes of the land and collects in the depressions
formed by the intersection of the slopes, forming
rills or rivulets, which at last combine in larger
streams called rivers.

The source of a river is the place where it
rises; the mouth, the place where it empties; the
channel, the depression through which it flows.
Rivers generally rise in mountains, where the
rainfall is greater than elsewhere, and where
vast beds of snow and ice occur.

In reality, all rivers have three mouths, or places where
they discharge their waters:

(1.) Where the river empties directly into some other
body of water;

(2.) Where the river empties by evaporation into the
air; that is, its entire upper surface ;

(3.) Where the river empties into the earth through the
porous strata of its bed or channel.

Since the downward motion of a river is caused by the
inclination of its channel from the source to the mouth, a
sorrect idea of the general inclination of any country can
be obtained by a careful study of a map in which the di-
rections of the rivers are represented. In studying the
various river-systems the student should endeavor to ob-
tain in this way clear ideas of the general directions of the
continental slopes.

The River-System is the main stream, with all
its tributaries and branches.

The Basin is the entire area of land which
drains into the river-system.

The Water-shed is the ridge or elevation which

 

separates two opposite slopes. The streams flow
in opposite directions from the water-shed.

The Velocity of a river depends on the inclina-
tion or pitch of the channel and the volume or
depth of the water. .

169. River-Courses-The river-channel, from
its source to its mouth, is, for ease of description,
conveniently divided into three parts or courses:
the upper, middle, and lower.

The Upper Course of a river is that part which
is situated in the mountainous or hilly country
near its source.. In this course the river has a
great velocity, and its channel is characterized by
sharp, sudden turns, alternating with long, straight
courses. In the upper course erosion occurs
almost entirely along the bottom of the channel,
so that the river runs between steep, and some-
times almost vertical, banks. In this way river-
valleys are formed, generally with narrow and
overhanging, precipitous sides. In the upper and
middle courses rapids and waterfalls occur.

Rapids and Waterfalls—During the erosion of
the channel, where harder rocks occur in the bed
of the stream, the softer strata, immediately adjoin-
ing them down stream, are rapidly worn away, and
the obstruction becomes at last the head of a
waterfall. The height grows rapidly from the
increased force of the falling water, and continues
until stopped by some similar obstruction below.

 

 

Fig, 61, Erosion of Waterfall.

Thus, suppose a a, Fig. 61, is the bed of a river, the di-
rection of flow of which is shown by the arrow. The softer
rock being worn away more rapidly, the bed reaches the
Jevel 1,1. A fall, and consequent increase in the velocity
of the river, soon causes the level of the bed to reach 2, 2,
3, 3, and 4, 4, successively. At the same time the falling
water eats away the vertical wall of the precipice, causing
the waterfall to move up stream. The water then cuts the
precipice away in steps, as shown at 5, 6, 7, thus changing
the fall into cascades. These are finally worn away, as
shown at 8, changing the cascades to rapids, when, finally,
the fall disappears entirely, or the erosion of the hard
rock is completed.

When the water falls perpendicularly—that is,
when it does not slip or slide—it forms a water-
fall or cataract; in all other cases of swift de-
scent it forms rapids.