Page:The Journal of geology (IA journalofgeology21894univers).pdf/56

ably reviewed by Professor F. H. Knowlton, who studied extensive collections from the auriferous gravels of Independence Hill, Placer county, California. He concludes that the gravels are probably upper Miocene in age.^[1]

On stratigraphic grounds the auriferous gravels are regarded as contemporaneous with the Ione 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 development 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 contained 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 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