DATE OF THE QUATEENARY LAKES. 273 



of the winds for thousands of years without being destroyed is improbable, 

 to say the least. 



Other facts bearing on the determination of the length of time that has 

 elapsed since the close of the Grlacial epoch may be observed in the canons 

 of the High Sierra, and have been described in part in a previous essay.®® 

 We need not consult the moraines left by the ancient glaciers, as these, like 

 the gravel embankments mentioned above, are comparatively stable struct- 

 ures; but in the glaciated canons there are numerous bosses and domes of 

 granite and quartzite that have been exposed to the sky since the glacial 

 ice was melted from above them. The ice-polish on these ledges is still 

 conspicuous, and causes them to glisten in the sunlight as brilliantly as do 

 similar surfaces adjacent to the existing glaciers of the High Sierra and of 

 Switzerland. These smooth surfaces are still scored with fine hair- like 

 lines, and the eye fails to detect more than a trace of disintegration that 

 has taken place since the surfaces received their polish and striations. Here 

 again we meet with the difficulty of applying quantitative measurements; 

 but as there is a limit to the time that rock surfaces may retain their polish 

 it seems reasonable to conclude that in a severe climate like that of the 

 High Sierra it could not remain unimpaired for more than a few centuries 

 at most. 



The cumulative weight of tliese various lines of inquiry is such as to 

 lead to the opinion that the last desiccation of the Quaternary lakes of the 

 Great Basin certainly occurred centuries but probably not many thou- 

 sands of years ago. On the other hand, it might be argued that the pres- 

 ence of the bones of the mastodon, camel, and horse in the lacustral clays, 

 deposited during the last great rise of the lake, is abundant evidence of the 

 antiquity of that event. The date of the various fluctuations of Lake Lahon- 

 tan, as measured by the standard used in human history, thus remains an 

 open question. 



*> Existing Glaciers of the United States, Fifth Annual Report U. S. Geological Survey. 



MoN. XI 18 



