LITHOLOGICAL CHARACTER OF THE GRAVEL. o: 
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eS 
about changes in the nature of the organic life of various regions of the 
earth. It would be difficult to find a more striking example of the working 
of this mysterious cause than is presented in the entire disappearance of the 
mastodon and elephant, almost during the historic period, and very nearly 
at the same time, from over an area of some millions of square miles. 
The question whether any part of the auriferous gravel series can ever be 
definitely separated from the overlying portions of the formation and distin- 
guished as being clearly of Miocene age, is one which cannot at present be 
answered. Further light must be thrown on this subject, it would seem, if 
not by discoveries in the Sierra itself, at least by those made in adjacent 
regions. <A full exploration of the fossiliferous Tertiary beds of Oregon 
would undoubtedly be of value in this direction, as giving some clew to the 
development of animal and vegetable life on the Pacific coast during the 
later geological epochs. ‘The question would not be one of so much impor- 
tance, were it not for the very considerable body of evidence which has been 
brought forward in a previous chapter, showing the existence of man daring 
Tertiary times. Naturally it is asked, and especially by European geologists, 
What name shall we give to that division of the Tertiary in which these 
human remains are found? To this it may be answered, that the adoption 
of either the word Miocene or Pliocene in this case would not necessarily 
imply that the event designated by either of these terms precisely corre- 
sponded in time to the one similarly named in Europe. It will be safe to 
say, that the human race in America is shown by the evidence to be at least 
of as ancient a date as that of the European Pliocene; and to have an idea 
how far removed that epoch is from the present one, it is only necessary to 
recall the amount of erosion which has taken place since the cessation of 
voleanic activity in that part of the Sierra in which lie the formations which 
have been described in the present volume. 
Section V.— Lithological Character and Peculiarities of the Gravel. 
Certain peculiarities of the enormous mass of detrital material to which the 
comprehensive term “ gravel” has been applied in this volume are worthy 
of special consideration. The main fact in regard to this material is a very 
simple one, namely, that it is water-worn. It is essentially the product of 
the action of water; and this action has been in many cases of such a nature 
as to give rise to masses of detritus of a surprisingly uniform and homoge- 
