242 THE AURIFEROUS GRAVELS OF THE SIERRA NEVADA. 
associated voleanic rocks. The extent and nature of this erosion, and the 
character of the agencies by which it has been brought about, will be still 
further discussed in a future chapter, and the subject is only alluded to here, 
on account of its intimate connection with the determination of the position 
of the various fossils occurring in the formations described. 
It so happens that the close of volcanic action in the part of the Sierra 
now under discussion seems, almost beyond a doubt, to have been marked 
by the eruption of a kind of lava differing from those which had been pre- 
viously emitted from the interior. The material referred to as, in a measure, 
closing the volcanic epoch, is the basalt ; which, wherever the series is com- 
plete, is found overlying the rhyolitic and andesitic flows, and over which 
no extensive deposits of gravel, so far as known to the writer, have ever 
been found. 
A peculiar character of the basaltic masses is their solidity and unbroken 
condition. The underlying volcanic masses are very largely — indeed, al- 
most exclusively —brecciated and fragmentary; and it is clear that they 
have been in large part brought into their present position by the agency 
of water. The basaltic flows, on the other hand, are exclusively igneous: 
they rest where they descended in a molten condition. This solidity of the 
basalt connects itself, also, with its indestructibility. The material is very 
slowly acted on by the ordinary erosive agencies, as has already been ex- 
plained in describing the way in which it has protected the underlying in- 
fusorial strata. Hence it forms the capping of the various “Table Moun- 
tains,” of which class of flat-topped elevations the one in Tuolumne County 
is so remarkable an example. 
As a consequence of the conditions above described, we are justified in 
feeling confident that deposits found under the basaltic masses remain in their 
original position; or, at least, that they must have been accumulated in the 
place where we find them previous to the cessation of the period of volcanic 
activity. The gravels which have not been protected by a capping of basalt, 
if only thinly or not at all covered by erupted materials, may in some places 
have been overlain by recent deposits in such a way that the line between 
volcanic and post-voleanic cannot be distinetly drawn. Mention has already 
been made, in the preceding pages, of cases where slides have taken place on 
the steep walls of caiions, mixing older and more recent formations in inex- 
tricable confusion. And it must not unfrequently have happened that fossils 
have been washed out of the less coherent detrital beds, belonging to the 
