320 RESUME AND THEORETICAL DISCUSSION. 
opments of life, which we know could not have been accomplished without a 
corresponding lapse of time. 
From the very nature of the case, there can be no such large and 
complete collections of fossil remains made in the gravel region as have 
been obtained from the lacustrine deposits farther east in the Cordilleras. 
The turbulent river channels, with their hundreds of feet in thickness of 
rolled gravels and boulders, are of quite a different character from those 
undisturbed deposits of fine sediment which have in so many places in the 
Cordilleras tranquilly filled up the gradually desiccated areas. All this, 
however, has been sufficiently explained in a previous chapter: it only re- 
mains here to emphasize the conclusions which have been previously drawn. 
We may say then, in brief, that the whole Tertiary period must be repre- 
sented in the gravel accumulations of the Sierra; but only a small part of 
the life of that period. The nearer we come to the present epoch, the more 
complete the record; because, as will be shown hereafter, there was a 
gradual slackening of the forces inimical to life. The existence of the 
Eocene period can hardly be recognized at all in the gravel period ; Miocene 
types occasionally present themselves; but forms which may most properly 
be referred to the Pliocene are by far the most numerous. There is no 
possibility of drawing any lines, however, in the field between one and the 
other of these divisions of the Tertiary. No section allows us to trace a 
succession of life in the various beds: it is only here and there a few frag- 
ments —a single tooth, perhaps — that we are lucky enough to secure. 
That the mass of the gravels underlying the volcanic formations contains 
no remains of animals of existing species, is an established fact, so far as our 
present observations extend. The only exception to this would be in the 
case of man, the evidence of whose existence in the strata beneath the 
basalt has been laid before the reader in the preceding pages. It is, how- 
ever, not to be forgotten that the mastodon, which also existed contem- 
poraneously with man at an epoch anterior to the eruption of the basalt, 
continued to live until very recent times. There is a great body of evidence 
showing that this proboscidean was extremely abundant in California, as 
well as over a large part of the remainder of North America, during post- 
Tertiary times; and it was then contemporaneous with some other animals 
which have not yet disappeared. Thus the mastodon, which lived through 
a portion at least of the volcanic epoch in California, perished long after- 
wards under those subtle and but little understood influences which bring 
