346 



E&SUME 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 canons. These streams, 

 when choked with debris, 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 debris 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 debris 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 debris afterwards accumulated. 



Next follows in order the outbreak of the volcanic 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 grav- 

 ity, with or without the aid of water. Thus the surface was still more nearlv 

 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 canons 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 mU, p. 63 ; and Geology of California, Vol. I. p. 244 



