THE EROSION OF THE BED-ROCK DONE BY WATER. 295 
the whoie of the mass of the upper portion of the chain was somewhat more 
elevated than it now is cannot, of course, be denied, since erosive agencies 
have been continually bringing down materials from higher and depositing 
them in lewer regions. That volcanic agencies also have played an important 
part during the gravel epoch, and especially towards its close, is also perfectly 
clear, for the innumerable sections presented in the hydraulic mining region 
show the presence of eruptive materials almost everywhere, while an inspec- 
tion of the various maps accompanying this volume would bear witness to 
the same fact. We may, however, with propriety discuss the bearing and 
importance of the volcanic deposits, as connected with the other occurrences 
of the gravel epoch, without its being necessary to enter upon any investi- 
gation of the nature of volcanic phenomena in general,—a subject, as is well 
known, of the greatest obscurity and difficulty. The effect of a heavy cover- 
ing of lava over the gravels, acting as a mechanical protection of that which 
is beneath; the blocking up of old channels by masses of eruptive material 
borne down from above, either directly in a melted condition, or inairectly 
through the aid of water; the impossibility of getting at the gravel, except 
by means of drifting, where it has been too deeply covered, — all these con- 
ditions are of extreme simplicity, and without theoretical difficulty. 
Again, it is perfectly clear that the shaping of the surface of the bed-rock 
and all the erosion which has taken place since the beginning of the gravel 
epoch have been exclusively the work of water. Of course it is not intended 
by this to exclude the idea that aqueous causes have acted independently of 
all others. On the contrary, the descending currents have taken advantage 
of every favorable condition to do their work more quietly and more thor- 
oughly. Chemical decomposition of the bed-rock, under the influence of 
volcanic agencies ; heavy grades, giving the necessary velocity to the down- 
moving waters; and, above all, copious precipitation, producing streams of 
large volume,—these are among the favorable conditions which would insure 
a rapid performance of the erosive work. It can be set down, however, as 
established beyond any possibility of doubt, that ice had nothing to do with 
any part of the erosion of the gravel period. It was not until the whole 
mass of gravels and other detrital deposits had been accumulated, and the 
volcanic strata piled over these, often to a thickness of several hundred feet, 
in short, not until the whole of Tertiary time had passed, that the higher 
portions of the Sierra Nevada were occupied by glaciers. More than this, 
it is clearly established that the present river cations had been cut down into 
