994 RESUME AND THEORETICAL DISCUSSION. 
are made up had “been brought down from the mountain-heights above, 
and deposited in pre-existing valleys”; but whether these valleys could be in 
any way correlated with those occupied by the present streams — whether 
the drainage areas of Tertiary times had any analogy with those into which 
the Sierra Nevada is now divided — could not be positively asserted. The 
most that could be said was, that the work was done “ under the action of 
causes similar to those now existing, but probably of considerably greater 
intensity.” This statement, as the writer believes, has been now shown to 
be strictly true, and it only remains to give the reasons why it is so, in a 
somewhat more systematic form than has been possible in the descriptive 
chapters of this volume, which portion has necessarily been arranged in a 
geographical order. 
It can now be stated as having been clearly established that the great 
drainage systems of the Sierra were, during the gravel epoch, not materially 
different from what they are at present. Not only was there no river, or 
system of rivers, running in a direction parallel with that of the present 
crest of the range, but the divides between the principal streams descending 
the slope of the Sierra must, in several of the most prominent instances at 
least, have existed from the beginning of Tertiary times. This topic has been 
sufficiently enlarged upon by Mr. Goodyear, in his review (Appendix B), and 
his conclusions are also corroborated by the results of Professor Pettee’s work, 
so far as regards the entire separation of the Feather River gravels from 
those of the Yuba. Farther north the gravel deposits partake of the now 
existing irregularity of the drainage system. Beyond the North Yuba the 
ranges are broken and irregular, and there is no dominating one ; the courses 
of the streams are not only often parallel with the crest of the Sierra for long 
distances, but are sometimes just the reverse of what they usually are farther 
south. Add to this other disturbing conditions, the nature of which will be 
explained further on, and it appears quite reasonable that in the region in 
question it should be difficult to correlate the present streams with those of 
Tertiary age. 
The main results which have been attained in the exploration of the high 
gravel deposits of the Sierra Nevada are these: that these detrital masses 
are the work of rivers which are of Tertiary age, as will be more fully set 
forth in a succeeding section of this chapter; and that the principal topo- 
graphical features of the range existed when the gravels began to be deposited 
on its slope, possessing much the same character that they have now. That 
