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 to a 

 limited extent interstratified with, these detrital beds of purely aqueous 

 origin, there exist heavy deposits of volcanic materials, some solid, others 

 brecciatcd 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 canons 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 pala> 

 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. 



