42 THE JOURNAL OF GEOLOGY. 



ably reviewed by Professor F. H. Knowlton, who studied exten- 

 sive collections from the auriferous gravels of Independence 

 Hill, Placer county, California. He concludes that the gravels 

 are probably upper Miocene in age/ 



On stratigraphic grounds the auriferous gravels are regarded 

 as contemporaneous with the lone formation of the Sacramento 

 valley, but here, too, as in the earlier auriferous gravels, the 

 fossil plants and shells appear to indicate that they belong to the 

 Miocene. 



That the approximate baselevel reached its greatest develop- 

 ment about the time the earlier auriferous gravels were deposited 

 is indicated by the fact that they lie in the broad shallow valleys 

 of that plain. The present tendency of the organic evidence con- 

 tained in the flora of these gravels is to indicate that their deposition 

 took place during the Miocene, most likely later Miocene. The 

 erosion necessary to develop the baselevel out of the topography 

 resulting from the uplift at the close of the Shasta-Chico period 

 must have occupied a long interval of time, possibly beginning 

 in the latter part of the Cretaceous and continuing through the 

 Eocene and earlier portion of the Miocene, but as the plain 

 appears to have attained its maximum extent during the Miocene, 

 it may be referred to as the Miocene baselevel. 



THE ELEVATION INDICATED BY THE FLORA OF THE AURIFEROUS 



GRAVELS. 



The flora of the region indicated by the remains found in the 

 earlier gravels is of special interest on account of its bearing on 

 the topography. Numerous fossil leaves have been found in the 

 early auriferous gravels about the northern end of the Sierra 

 Nevada at Mountain Meadows, near the summit of Spanish Peak 

 and elsewhere on the very crest of the Sierra, at altitudes ranging 

 from 2,900 to 6,350 feet above the sea. These plants were 

 studied by Professor Lesquereux, who recognized among them 

 three kinds of figs and a large number of lauraceous plants, with 

 other forms of similar significance. Not a single species of pine 



' U. S. Geological Survey, Bulletin 108, page 104. 



