RL I a 
362 - DYNAMICAL GEOLOGY. [Boox III. | 
up within the rocks and is dispensed gradually ; in the other, it in 
great measure runs off at once.’ i 
III. Flow.— While, jn obedience to the law of gravitation, a river 
always flows from higher to lower levels, great variations in the rate 
and character of its motion are caused by inequalities in the angle 
of slope of its channel. A vertical or steeply inclined face of rock 
originates a waterfall ; a rocky declivity in the channel gives rise to 
rapids; a flat plain allows the stream to linger with a scarcely 
visible current ; while a lake renders the flow nearly or altogether 
imperceptible. Thus the rate of flow is regulated in the main by | 
the angle of inclination and form of the channel, but partly also by 
the volume of water, an increase of volume in a narrow channel in- 
creasing the rate of motion even without an increase of slope. 
The course of a great river may be divided into three parts :— 
1. The Mountain Track,—where, amidst clouds or snows, it takes its 
rise as a mere brook, and, fed by innumerable similar torrents, 
dashes rapidly down the steep sides of the mountains, leaping from 
crag to crag in endless cascades, and growing every moment in 
volume, until it enters lower ground. 2. The Valley Track—The 
river now flows through lower hills or undulations, and is found at 
one time in a wide fertile valley, then in a dark gorge, now falling 
headlong in a cataract, now expanding into a broad lake. This is 
the part of its career where it assumes the most varied aspects, and 
receives the largest tributaries. 3. The Plain Track—Having 
quitted the undulating region the river finally emerges upon broad 
plains, probably wholly, or in great part, composed of alluvial 
formations deposited by its own waters. Here winding sluggishly 
in wide curves, it eventually perhaps bifurcates, as it approaches the 
sea and spreads through its delta, enclosing tracts of flat meadow or 
marsh, and finally, amid banks of mud and sand, passing out into © 
the great ocean. In Europe the Rhine, Rhone, and Danube; in Asia 
the Ganges and Indus; in America the Mississippi and Amazon ; in 
Africa the Nile, illustrate this typical course of a great river. 
If we draw a longitudinal section of the course of any such river 
from its source, or from the highest peaks around that source to its 
mouth at the sea, we find that the line at first curves steeply from ~ 
the mountain crests down into the valleys, but grows less and less 
inclined through the middle portion, until it finally can hardly be 
distinguished from a horizontal line. Though characteristic of great 
rivers, this feature is not confined to their courses, but belongs to 
the architecture of the continents. 
It is evident that a river must flow, on the whole, fastest in the 
first portion of its course, and slowest in the last. The common 
method of comparing the fall or slope of rivers is to divide the 
difference of height between their source and the sea-level by their 
length, so as to give the declivity per mile. This mode, however, 
often fails to bring out the real resemblances and differences of rivers, 
| ? Prestwich, Q. J. Geol. Soc. xxviii. p. Ixy. ; 
