284 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
Whether any of the species occurring in the gravels are not at present ex- 
tinct is not to be decided from the material thus far collected. The tapir 
found in the gravel, for instance, may or may not be identical with a living 
species. But, taking the whole evidence together, it is certain that the fauna 
of the gravel deposits is almost exclusively made up of extinct species, and 
we are justified in saying that it is far more Pliocene in its aspects than Post- 
pliocene or Recent. 
On the whole, then, it seems reasonable, in the light of the evidence thus 
far collected, to assert that the conclusions drawn by Mr, Lesquereux from the 
study of the fossil plants of the gravel series are strengthened by the indi- 
cations afforded by the fossil animal remains. The Miocene may be repre- 
sented by some of the lowest deposits, while a portion of the series lying 
under the basalt is probably of Pliocene age. The passage from Pliocene 
through Post-pliocene, if such a division can hereafter be maintained in 
this region, has been a gradual one, and some of the Pliocene animals have 
certainly lived on close up to the Recent period. That a portion of the 
human remains and implements described in the preceding pages are as old 
at least as Pliocene, it seems hardly possible to doubt. 
It may be proper to add a few remarks in regard to the amount of time 
which has elapsed since the human race began to exist in California, as 
shown by the evidence which has been brought forward on the preceding 
pages. | 
In the first place, the total change in the fauna and flora of the region, 
and we may say, in fact, the succession of changes, will always be admitted 
by geologists as having required an immense amount of time for their com- 
pletion. Nothing that we know of the history of the past will justify the 
assumption of a sudden extinction of the animal and vegetable creation within 
an extensive region and the immediate introduction of a new creation. The 
more zodlogists and palzontologists have studied the record of the past, the 
more they have become convinced that such changes have taken place step 
by step, and that time has been an essential factor in their occurrence. 
The necessity of the grant of time demanded for the changes in the or- 
ganic life of the region is vouched for by the phenomena of erosion which 
the region presents, and which has already been sufficiently insisted on. 
We know, for instance, that after the fossiliferous strata under the various 
basalt-capped mountains in California were deposited there were repeated 
