296 RESUME AND THEORETICAL DISCUSSION. 
their present form, and that the whole topography of the Sierra was, even 
down to its minutest details, just what it now is, before ice had any exist- 
ence in the range. 
The main features of the gravel epoch, therefore, as it can be unhesitat- 
ingly stated, have been recognized with clearness. There has been an 
accumulation of water-worn detrital material in the beds and along the sides 
of a system of rivers, which flowed down the slopes of the Sierra, the prin- 
cipal divisions of whose drainage-areas are essentially the same at the present 
time as they were at the beginning of the gravel epoch. Upon, and toa 
limited extent interstratified with, these detrital beds of purely aqueous 
origin, there exist heavy deposits of volcanic materials, some solid, others 
brecciated and fragmentary, and others water-worn, their whole aspect being 
such as to warrant the belief that the epoch during which igneous agencies 
prevailed was one of long duration. After the close of the period of volcanic 
disturbances and overflows, water continued its work; and the result is visi- 
ble in the form of the present river cafions and in the character of the 
detritus deposited in their bottoms. 
Such being the admitted facts, we now direct our attention to an inquiry 
into the difficulties presented by the phenomena of the gravels, in the course 
of which the whole series of events which took place during that period will 
be passed in review, the principal object being to throw light on the physi- 
cal conditions prevailing while this geological work was being done. And 
in introducing this discussion it will be necessary, first, to state briefly what, 
from a geological point of view, are the fundamental differences between the 
present epoch and that of the gravels. | 
The difference in geological age between the high gravel deposits and 
those in the beds of the present streams, in so far as it is a matter of pale- 
ontological evidence, presents no other essential difficulty than that of draw- 
ing the line between Tertiary and Recent. That the epoch of the gravels 
represents a large portion of Tertiary time cannot be doubted, and the en- 
deavor will be made in a subsequent section to show why, perhaps, there is 
so imperfect a representation of the earlier stages of that epoch among the 
fossils found in these deposits. The question of geological age, as determined 
by fossil remains, is not one, however, which much interests the miners, who 
almost invariably consider that they recognize the leaves and wood so fre- 
quently found inbedded in the gravels as being the same as those now living 
in the Sierra. 
* See the Climatic Changes of Later Geological Times, passim. 
