898 W. LINDGREN~F. H. KNOWLTON 



and ridges just mentioned, and indicate with their level sky line 

 the extend of a far older eroded surface, uplifted and dissected 

 long before the auriferous gravels were deposited or the lava 

 flows extruded. 



The later Mioce?ie period {the Io7ie formation). — This is the 

 period of the auriferous gravels par excelle7ice. Stream gravels 

 had already formed to some extent in the lower reaches and 

 deepest parts of the river valleys, but the greatest masses were 

 accumulated during this period. 



The shore line again moved eastward up to an elevation of at 

 least 400 feet above the present sea level and a heavy series of 

 sediments was laid down along the foothills which rose with 

 decided relief above the waters of the gulf ; this may be referred 

 to as the lone transgression. Only in the northern and middle 

 part of the range did the gravels attain their maximum depth. 

 One of the probable causes leading to their deposition has been 

 indicated by Mr. J. S. Diller,^ who holds that disintegration 

 exceeded transportation at the close of the Eocene and beginning 

 of the Miocene, and that consequently the surface was covered 

 with a deep mantle of decomposed material ; during the Miocene 

 a slight uplift increased the erosive power of the streams and 

 swept the detrital matter into the river courses. This is very 

 probably a correct explanation, although perhaps not the only 

 one. It seems that the steep foothill ranges may have acted 

 the part of barriers restraining the gravels in the valleys of the 

 middle slopes. To this the heavy accumulations along the 

 American and Yuba River on the middle slope is in part due. 



Close of the Miocene ; begimmig of the Pliocene. The eruption 

 of rhyolite and andesite near the crest of the range closed 

 the gravel period. At some time during the volcanic period, 

 probably in the interval between the rhyoliiic and the andesitic 

 flows, occurred a new break along the eastern base of the Sierra. 

 The principal movement in this latitude took place along the old 

 fault-line extending from Verdi, Nevada (ten miles west of 

 Reno), down towards Markleeville, while along the fault-lines 



' 14th Ann. Report U. S. G. S., p. 427. 



