40 P. B. KING 



lofty scarps that have been outlined mostly by faults, although the 

 faults are not continuous and are offset en echelon in many places. 

 Minor faults also occur within the range, but most of the range, west 

 of its summit, is a tilted block with remarkably even, westward- 

 tloping crest lines, below which the tributaries of the Sacramento 

 and San Joaquin rivers have cut impressive canyons. 



Early Tertiary (Paleocene and Eocene) Environments. By early 

 Tertiary time, the topography that developed on the orogenically 

 deformed miogeosynclinal and eugeosynclinal rocks had become 

 decadent, but the post-orogenic topography of basins and ranges 

 had not yet developed. 



At many places in the eastern half of the Great Basin the Mesozoic 

 and older rocks are overlaid by patches of calcareous mudstone, fine- 

 grained sandstone, and coarser sands and gravels. These appear to be 

 remnants of originally much more extensive deposits that formed in 

 floodplains, swamps, and lakes, probably in a warm, humid lowland 

 (Van Houten, 1956, p. 2819). Although the deposits have been dated 

 only by meager fossil evidence, they are probably westward exten- 

 sions of the Paleocene and Eocene deposits of the Colorado Plateau 

 and the central and southern Rocky Mountains, which formed in a 

 similar environment. 



The western half of the Great Basin seems to have been part of a 

 low highland which extended westward across the area of deformed 

 eugeosynclinal rocks to the western edge of the Sierra Nevada. Few 

 or no early Tertiary sedimentary units exist in the western Great 

 Basin, although the lower parts of some of its volcanic sequences 

 may be as old as Eocene. At the western edge of the highland the 

 marine lone formation of middle to late Eocene age overlaps widely 

 on the deformed Mesozoic rocks (Allen, 1929). It is composed of 

 clays of remarkable purity, with interbedded sands, and is traceable 

 up the slope into the older gold-bearing stream gravels of the Sierra 

 Nevada foothills. During lone time the site of the Sierra must have 

 been worn down to low relief, and was drained by sluggish streams 

 that headed well east of the present mountain crest. The clays were 

 derived from deeply decayed granitic rocks that were widely ex- 

 posed on this worn-down surface. 



Middle Tertiary {Oligocene and Early Miocene) Environments. In 

 Oligocene and early Miocene time lavas, agglomerates, and tuffs of 

 varied composition were spread widely over the western part of the 



