









THE ESSENTIAL CHARACTERS OF THE TERTIARY GRAVELS. 



297 





The first thing which docs excite the curiosity of the practical man in 

 regard to the Tertiary gravel deposits is their elevated position ; hence the 

 term which has often been applied to them, namely, "high." They are 

 accumulations of detritus far above the level of the present larger streams, 

 the difference of elevation being usually hundreds, and not unfrcquently 

 thousands of feet between the ordinary river-beds and the "high gravels." 

 It having been already stated that the topographical features of the Sierra,, 

 so far as the outline of the main drainage-basins is concerned, is essentially 

 the same now as it was during the gravel epoch, it becomes an interesting 

 problem to inquire why the Tertiary detrital deposits occupy this peculiar 

 position, for it would seem natural that the debris brought down by the 

 streams should continue to be accumulated along the same lines of drainage 

 so long as there was no orographic reason for a change in this respect. 



This, then, is one of the most important questions to be investigated : 

 namely, to show how it is that the gravel of the Tertiary period, as a general 

 rule, occupies a position far above that which is now being deposited by those 

 streams of the present day which are the representatives of the ancient rivers. 

 The next point claiming attention is the magnitude of the older deposits 

 of detrital material as compared with those accumulations which we perceive 

 to be directly connected with the present streams. This fact is the one 

 which, next to altitude, impressed itself most strongly on the minds of the 

 earlier placer miners; hence we find the deposits in question to have been 

 called "in the early days," "deep " as well as " high " gravels ; and, even at 

 the present time, both these terms are used indiscriminately by those who do 

 not wish to designate them as " hydraulic " ; that is, as being so situated as 

 to be suitable for exploitation by the hydraulic process, — a method of attack 

 which is entirely inapplicable to detrital materials which are not raised above 

 the ordinary valley levels, or which are of insignificant thickness. 



Thus we are presented with another problem : namely, to account for the 

 quantity of gravel of Tertiary age, which, as has been made abundantly 

 evident from the detailed descriptions given on the preceding pages, is often 

 several hundred feet in thickness and spread over wide areas. And in en- 

 deavoring to give a satisfactory explanation of the position and magnitude 

 of the older gravel deposits, we shall have brought up for consideration a 

 great numbfer of minor problems, the solution of which is involved with that 

 °t the other main questions. The nature of these problems will become 

 evident during a perusal of the following sections of the present chapter, and 



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