
_ Parr IL. Sror. ii. §3.] RIVER DEPOSITS. 379 
in North America. The rivers there flow in ravines thousands 
of a ag a maces of can long, through vast table- 
lands of near orizontal strata. e , 3 
Grand Cafon (egee of the Colorado W//7 ( 
river is 300 miles long, and in some GGm Yyyyy 
: Biccs more than 600° feet in depth. | | Ud 
In many instances there are two canons, Y 
the upper being several miles wide, with 7 | LO 
vast lines of cliff walls and a broad lain ] _. Y Z L ]/, 
between them, in which runs the second | IY / 
ppt 
a 
cafion, as another deep valley with 
the river winding over its bottom. The 
country is hardly to be crossed except by 
birds, so profoundly has it been trenched by these numerous 
gorges. Yet the whole of this excavation has been effected by 
the erosive action of the streams themselves." Some idea of the 
vastness of the erosion of these plateaux may be 
formed from Fig. 115, and illustrations in Book VII. 
‘In the excavation of a ravine, whether by the 
recession of a waterfall or of a series of rapids, 
the action of the river is more effective than that 
of the atmospheric agents. The sides of the 
ravine consequently retain their vertical character, 
which, where they coincide with lines of joint, 4 
5 : ° ° ORGE IN Fis- 
is further preserved by the way in which atmo- Sunnis Suen 
spheric weathering acts along the joints. But where, 
from the nature of the ground or of the climate, the denuding 
action of rain, frost, and general weathering is more rapid than 
that of the river, a wider and opener valley is hollowed out, 
through which the river flows, and from which it carries away 
materials washed into it from the surrounding slopes by rain and 
brooks. 
3. Reproductive Power.—Every body of water which when in 
motion carries along sediment, drops it when at rest. The moment 
a current has its rapidity checked, it is deprived of some of its 
carrying power, and begins to lose hold upon its sediment, which 
tends more and more to sink and halt on the bottom the slower the 
motion of the water. In Fig. 116, the river in flowing from ¢ to b 
has a less angle of declivity and a smaller transporting power, and 
will therefore have a greater tendency to throw down sediment than 
in descending the steeper gradient from 0d to a. 
In the course of every brook and river there are frequent checks 
to the current. If these are examined, they will usually be found to 
be each marked by a more or less conspicuous deposit of sediment. 



Fig. 113.—River Gorce 
IN LINE OF FAULT. 

1-For descriptions and figures of this remarkable region, s:e Ives and Newberry, 
‘«* Bxploration of the Colorado River of the West,” 1861. J. W. Powell, “ Exploration of 
the Colorado River of the West and its Tributaries,” 1875, and postea, Book VII. 
~ 
