346 RESUME AND THEORETICAL DISCUSSION, 
During the gravel period the streams were broad, the slope of the Sierra 
being more uniform than it now is, and the valleys between the ridges 
decidedly shallow as compared with the present cafons. These streams, 
when choked with débris, had room to make for themselves new channels 
to one side or the other. They refused to be confined within fixed limits, 
and thus wear down one narrow channel, because the volume of water which 
they carried was too large. 
Such were the conditions during the first portion of the gravel period. 
Vast quantities of débris must have been swept down to the very foot-hills 
and into the Great Valley itself. Even before the existence of this valley 
the western slope of the range was being eroded away; for, as already 
explained, it is necessary to look to this quarter for the source of supply for 
certainly a very considerable portion of the material out of which the Coast 
Ranges were built, these mountains having been uplifted while the gravels 
were depositing, and the detritus of which they are formed having been 
certainly accumulated, in large part, during the Cretaceous and early Tertiary 
periods. As the force of the eroding agents gradually slackened, more and 
more of the débris remained on the slope of the Sierra itself, where it accu- 
mulated, in favorable localities, to a depth of several hundred feet. The 
different river basins, however, were distinct from each other, the gravel not 
having filled them up so as to obliterate their boundaries, the erosive 
agencies having, previous to the deposition of the mass of the gravel on the 
flanks of the Sierra, worn out depressed areas more than large enough to 
hold all the débris afterwards accumulated. 
Next follows in order the outbreak of the voleanic epoch, which piled on 
the previously accumulated gravels, in places, a thickness of several hundred 
feet more of various kinds of eruptive rock. This material, although, as it 
appears, mostly emitted from near the summit of the range, must rapidly 
have found its way to the most accessible depressions, carried there by gray- 
ity, with or without the aid of water. Thus the surface was still more nearly 
brought to a general level; and this fact is rendered very apparent when 
the observer stands on the surface of one of the old lava-tables, and looks 
across the country in a direction parallel with the crest of the Sierra, so that 
the immense cajions are concealed from view. The gentle, uniform slope to 
the west, and the smooth, flat surfaces formed by lava-flows on the divides 
between the streams, are most striking features of the scenery.* 
* See ante, p. 63; and Geology of California, Vol. I. p. 244. 
