AMERICAN GEOLOGY DECADE OF 1870-1879. 579 



The time of deposition was put down by him as during the latei 

 Pliocene epoch, and not as late as the drift of the Diluvial period. 



Many pages were devoted to the vertebrate fossils found in the 

 auriferous gravels, and particularly to the evidences of man, and the 

 Calaveras skull. The question of the contemporaneous deposition 

 of this skull with the gravels has been too frequently discussed by 

 ethnologists to need more than a brief mention here. Suffice for the 

 present to state that Whitney himself seems to have been fully con- 

 vinced of the genuineness of the find, and regarded it as establishing 

 beyond doubt the existence of Tertiary man in California. 



In discussing the source of the gold and its distribution in the gravel 

 Whitney took occasion to express the opinion that, as a whole, the 

 occurrence of metalliferous ores is rather a surface phenomenon than 

 a deep-seated one, and that this is due to the favorable conditions for 

 fissure formation and deposition from solution controlled by tempera- 

 ture and pressure. 



The source of the gold he thought to be undoubtedly the quartz 

 veins which traverse the Jurassic slates, a fact to his mind refuting 

 Murchison's theory of the occurrence of gold exclusively in Paleozoic 

 rocks. He found no evidence to support the opinion that the large 

 size of the nuggets in the gravel was due to a gradual growth through 

 chemical precipitation." He regarded such as more or less dentritic 

 and branching masses which have been liberated from the gangue and 

 reduced to pebble form by the pounding they received in the moving 

 gravels. 



Whitney's work on the auriferous gravels was followed in 1880 and 

 1882 by an equally comprehensive work on climatic changes of later 

 geological times, the discussion being based largely on observations 

 made during the work of the California survey. 

 changed 88oTf882 Whitney was struck by the appearance of recent desic- 

 cation in the West, as illustrated by the lake regions 

 of the Great Basin, and from a study of the phenomena here met with 

 as compared with those in other regions he was led to conclusions 

 radically at variance with those commonly accepted by his co-workers. 

 It will be remembered that in the work on the auriferous gravels he 

 took the ground that there had been no appreciable change in eleva- 

 tion of the Sierras since the close of Cretaceous times, and he argued 

 that the Tertiary auriferous gravels were laid down by the rivers of 

 that period which flowed through broad channels, the present deep-cut 

 V-shaped channels being due to the smaller volume of water which 

 was the result of a decrease in annual precipitation. 



Concerning such, he wrote: 



It is, as a general rule, safe to assume that where U-shaped valleys exist the per- 

 pendicular walls have an orographic origin and that those of V form have had that 



« Recent work of A. Liversidge, the Australian mineralogist, confirms this view. 



