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 ; bee* 



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- 



aitse, as will be shown hereafter, there was a 



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 brins>- 





