THE VALLEY OF CALIFORNIA A PLEISTOCENE GULF. 197 



conditions. East of the range, however, for example at Virginia City, 

 andesites, which there is every reason to suppose preglacial, have scarcely 

 suffered at all from erosion, so that depressions down which water runs at every 

 shower are not yet marked with water-courses, while older rocks, even of Ter- 

 tiary age and close by, are deeply carved. The rainfall at Virginia City is, 

 to be sure, only about 10 inches, so that rock would erode only say one-third 

 as fast as on the California coast ; but even when full allowance is made for 

 this difference, it is clear that these andesites must be much younger than 

 the commencement of glaciation in the northeastern portion of the continent 

 as usually estimated. So, too, the andesites near Clear lake, in California, 

 though beyond a doubt preglacial, have suffered little erosion, and one of the 

 masses, Mount Konocti (or Uncle Sam), has nearly as characteristic a vol- 

 canic form as Mount Vesuvius. 



Furthermore, it is certain that the great valley of California was formerly 

 a shallow and therefore also a warm gulf. The existence of this sheet of 

 water would unquestionably increase the precipitation on the Sierra and in- 

 deed by nearly the whole amount of the evaporation from the gulf. Such a 

 body of water might surely influence the climate as much as the imperceptibly 

 small change of conditions which led to last winter's great snowfall, or in 

 other words it may reasonably be supposed to have caused glaciation. 



Thus there is evidence that the glaciation of the Sierra began and ended 

 at a late date relatively to the glaciation of the northeastern states, and there 

 is an assignable and even probable local cause for glaciation. 



Survival of Pliocene Animals in California. — The reasonable hypothesis of 

 a local glaciation of the Sierra, confined to time limits later than those of 

 what is known as the glacial epoch, may be made to account for the extra- 

 ordinary association of neolithic implements with Pliocene bones in Cali- 

 fornia. When glaciation began in northeastern America the preexisting 

 mammalian fauna no doubt died out in part, but it is not probable that all 

 the beasts resigned themselves to death without effort. Many of them must 

 have sought congenial climates in the south and southwest, and no one can 

 doubt that their existence must have continued longest in the more genial 

 portions of the country. The climate of California was then, as now, inde- 

 pendent of the great storm-generating divide of the Rocky Mountains, for 

 in the northern hemisphere the prevailing winds are westerly. In Cali- 

 fornia the great expanse of the Pacific ocean must always have affected the 

 climate somewhat as it does now. It thus seems reasonable to suppose that 

 the waning species of vertebrates found a veritable sanitarium west of the 

 Sierra, and that they continued to exist there long after their congeners of 

 the east were extinct. 



In short, then, it is not necessary to suppose that man reached the neo- 

 lithic age in California earlier than in Europe, if one supposes that a rem- 



XXX— Bull. Geol. Soc. Am., Vol. 2, 1890. 



