188 J, LeConte—Old River-beds of California. 
subsequent erosion has been so great that the evidences are 
mostly obliterated. In Alpine County, about Silver Mountain — 
and about Markleeville, the Sierra crest is formed largely of 
voleanic rocks. From this region probably a large number of 
streams radiated. But over the larger portion of the high 
Sierra erosion has bitten so deep that the lava streams have 
been entirely removed. I have however observed many dio- 
ritic, doleritic and felsitic dikes in all the granite region above 
the lava flow. These I have thought are probably the exposed 
roots of the flow. 
rary. 
As as seen, the mammalian remains are a mixture of 
the characteristic Pliocene species still lingering and of charac- 
teristic Quaternary species just coming in. They undoubtedly 
therefore indicate a transition from Pliocene to Quaternary, and 
whether on these evidences we refer the gravels to late Plio- 
often compelled to linger beyond the epoch to which they 
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