Correlation of Lavas and Gravels with Eastern Deposits. 



Recency of Glaciation in California. — It is usual to make the tacit assump- 

 tion that the Sierra Nevada of California Avas glaciated contemporaneously 

 with the northeastern portion of North America, and, when it is suggested 

 that the glaciation in the two regions may be independent phenomena, the 

 answer is that some great change in terrestrial or cosmical conditions would 

 seem needful to restore the glacial epoch. So far as the Sierra is concerned, 

 it seems to me that the history of the past twelve months sufficiently dis- 

 proves this argument. The snowfall last winter in the Sierra was excep- 

 tionally large, about 2? times the average precipitation having fallen. Much 

 of this snow remained unmelted through the season, and when I left the 

 mountains, on October 1, there were still thousands of snow banks where in 

 ordinary seasons none remains even far earlier in the season. Many of these 

 banks were also of great depth, say 100 feet, more or less. It is clear, there- 

 fore, that were this and succeeding winters to be as wet as the last, the range 

 would show glaciers in great numbers, much as the Alps now do ; in short, 

 the glacial period of the Sierra would recur in a moderate way. Now, no 

 one doubts that there was some cause for the unusual snowfall of 1889-'90, 

 but no one has any suspicion what it was. No sensible change in cosmical 

 or terrestrial conditions has occurred, the weather of the world at large was 

 not remarkable, and, excepting as to precipitation, the year was not extra- 

 ordinary even in California. In short, some very minute change, seemingly 

 local in character, modified very delicately balanced conditions sufficiently 

 to produce results which if repeated would be of great geological importance. 



I regard this as proof that the glaciation of the Sierra may possibly have 

 been local, and that it will be reasonable to pronounce it local if good con- 

 firmatory evidence to that effect can be adduced. 



No one who has examined the glaciated regions of the Sierra can doubt 

 that the great mass of the ice disappeared at a very recent period. The im- 

 mense areas of polished surfaces fully exposed to the severe climate of from 

 say 7,000 to 12,000 feet altitude, the insensible erosion of streams running 

 over glaciated rocks and the freshness of erratic bowlders are sufficient evi- 

 dence of this. There is also evidence that the glaciation began at no very 

 distant geological date. As Professor Whitney pointed out, glaciation is the 

 last important geological phenomenon and succeeded the great lava flows. 

 There is also much evidence that erosion has been trifling since the commence- 

 ment of glaciation, excepting under peculiar circumstances. I have discussed 

 the deepening of the canons of the gold belt in another paper * and need say 

 no more here than that I attribute it to somewhat narrowly restricted local 



* This volume, ante, p. 64. 



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