REVIEWS 3 2 5 



phism and hydro-metamorphism of augite-porphyrite. The rocks of 

 the auriferous slate series comprise, besides the Milton formation just 

 noted, an older set of rocks probably largely of Carboniferous age, as 

 fossils of that age only have been found in them. These Carboniferous 

 rocks are divided into two formations, the Robinson formation of late 

 Carboniferous or possibly of Permian age, and the Calaveras formation, 

 comprising rocks probably older than the Robinson formation. 



The superjacent series consists of river slake deposits, moraines, 

 and volcanic material. All of the rocks of the bed-rock series, both 

 slates and igneous rocks, were greatly eroded during Cretaceous time 

 and upon this old eroded surface there has been deposited by rivers 

 of Tertiary age an extensive series of gravels, known as the auriferous 

 gravel series. The river system of Tertiary time appears to be divisi- 

 ble into two distinct drainage systems with a divide between. This 

 divide is represented by the high ridge of which the Sierra Buttes is 

 the culminating point. The rivers to the west of this high north-south 

 ridge drained southerly or southwesterly in Tertiary time as they do 

 now. The deposits to the east of this Neocene divide appear to have 

 been deposited by one large river which flowed north, draining into a 

 basin to the north of the quadrangle. Superimposed upon these 

 gravels, and at other points upon the older bed-rock surface, are 

 extensive deposits of lavas, mostly in the form of breccias which have 

 been very largely eroded. At one time these lavas probably covered 

 nearly the entire surface of the quadrangle. They consist chiefly of 

 andesites and basalts. The latest basalts, notably the kind forming 

 the larger portion of Mount Ingalls, are possibly of early Pleistocene 

 age. After the volcanic forces had subsided, portions of the region 

 were occupied by glacial ice, resulting in the formation of very exten- 

 sive moraines, which are finely seen on the east slope of the Sierra 

 Buttes, about Gold Lake, and in the neighborhood of Johnsville. 

 All of the lakes that lie on the east slope of the high ridge extending 

 from the Sierra Buttes to Eureka Peak, are of glacial origin. 



There are evidences of faulting in Tertiary and later time at various 

 places in the district. The largest fault zone is that lying immediately 

 west of Mohawk Valley. The faulting along the south and west sides 

 of the American Valley, may perhaps be correlated with the same fault 

 zone. As a result of this faulting the country to the east of the fault 

 zone has subsided, resulting in the formation of the Mohawk and 

 American valleys. Mohawk Valley, during early Pleistocene time, 



