Geol.— Vol. I.] TURNER— ORIGIN OF YOSEMITE VALLEY. 297 



Gilbert summarizes his conclusions as follows: — 



The evidence from the moraines is thus shown to be consistent with that 

 from the molluscan fauna, and they generally confirm the presumption 

 derived from the recency and exceptional nature of the lakes and glaciers, 

 that the two phenomena were co-ordinate and synchronous results of the 

 same climatic changes. 



Since there were two periods of lake expansion, Gilbert 

 likewise concludes: — 



That the Glacial Period of the Sierra Nevada, the Wasatch, and other 

 mountains of the Western United States was divided into two epochs 

 separated by an interglacial epoch, but this has not been independently 

 shown. 



Since the Wasatch glaciers extended into Lake Bonne- 

 ville apparently only during the second high-water epoch, 

 the conclusion is drawn that the accumulation of ice was 

 greater in the second than in the first period. 



The conditions that brought about the glaciation of part 

 of two hemispheres could hardly have ended abruptly at 

 the western and southern limit of the great ice sheet of the 

 great continental glacier; hence the two hypothetical epochs 

 of glaciation of the western Cordilleran region may be cor- 

 related with the two epochs of the great continental glacier 

 of the northern United States and perhaps with the same 

 epochs in Europe. 



Evidence of Two Ice Periods in the Sierra Nevada. — 

 Thus far, however, there has been no positive evidence 

 obtained in the range itself that will warrant us in dividing 

 the Glacial Period of the Sierra Nevada into two epochs. 

 Certain facts, however, may be said to point in this 

 direction. 



The comparatively gentle surface of late Tertiary time 

 began to be dissected by streams in the early Pleistocene. 

 Let us suppose that the first epoch of glaciation of the 

 Cordilleran region corresponded in time with the first epoch 

 elsewhere, and hence began before the close of the Sierran 

 Period or before the canyons had attained their present 

 great depth. Much of the higher portion of the Sierra 

 Nevada ma}' then have been covered by ice. An extended 



