THE ESSENTIAL CHARACTERS OF THE TERTIARY GRAVELS. 297 
The first thing which does 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 unfrequently 
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 débris 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, 
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- 
a method of attack 
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 number of minor problems, the solution of which is involved with that 
of the other main questions. The nature of these problems will become 
evident during a perusal of the following sections of the present chapter, and 
