64 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
by the gorges or cafiions in which the present rivers run, in a most extraor- 
dinary manner; he will find it several hours’ work to descend into one 
of these and rise again to the general level on the other side, even if 
assisted by a well-beaten trail. All along the western slope of the Sierra, the 
streams have worn for themselves deep caions, and it is these tremendous 
gorges which form the leading feature of the topography of the region. If 
the streams ran nearly on a level with the general elevation of the surface, 
the whole character of the mountain slope would be changed. This was 
formerly the condition of the drainage of the Sierra slope, as will be seen 
farther on. At present, however, if we start from any point in the foot-hills, 
we find the general level of the ridges, between the streams, rising much 
faster than the streams themselves; so that, when we have reached an ele- 
vation of from 3,000 to 4,000 feet on the divide between any two streams, 
the bottom of the caiions on either hand are from several hundred to two 
thousand feet and even more below us. The section (Plate G), given farther 
on in this volume, illustrating the relative grades of the Pliocene and present 
channels of the American and Yuba rivers, well illustrates this, the broken 
lines indicating the levels of the beds of the rivers of the present day, be- 
tween the foot-hills and the crest of the Sierra, while the unbroken lines 
show the position of the Pliocene river beds in the same region; and, as will 
be explained hereafter, the level of the Pliocene rivers is, in general, that of 
the bed-rock surface on the divides between the streams. The diagram, 
however, only includes the various branches of the American and Yuba 
rivers, which are those draining the most important portions of the hydraulic 
mining region. If we had the data for making a similar profile for Feather 
River, the great depth of the present river beds below the general level of 
the country would be shown in a still more impressive manner. On com- 
paring, in the section given, the difference of elevation between the Pliocene 
Middle Fork of the American and the present bed of the same river, we see 
that at Michigan Bluff, for instance, this difference is about 1,800 feet, and 
that it remains about the same for several miles in either direction, gradually 
diminishing, however, as we descend the Sierra, and increasing as we ascend. 
At Iowa Hill, on the divide south of the North Fork of the American, the 
difference of elevation between the Pliocene and the present rivers is about 
1,500 feet. To arrive at the actual difference of level between the divide 
and the river cafion adjacent, we have to add to the figures given the thick- 
ness of the accumulated gravels and volcanic deposits piled upon the bed- 
