18 p. B. KING 



regionally metamorphosed, then were invaded by successive masses 

 of granitic rocks — a sequence which began later than deposition of 

 the youngest Jurassic rocks of the Sierra, and must have continued 

 for a long time thereafter. Moreover, radiometric determinations on 

 the granitic rocks indicate that they themselves were injected over a 

 period of more than 60 milHon years, or between Middle Jurassic and 

 Middle Cretaceous times, with the oldest to the west in the Klamath 

 Mountains and western edge of the Sierra Nevada, and the youngest 

 near the east edge of the Sierra Nevada (Evernden et al., 1957). 

 Elsewhere in the eugeosynclinal belt where evidence is available, the 

 climax of the orogeny varies in age. In the Hawthorne-Tonopah area 

 of southwestern Nevada thrust faulting was in progress during 

 deposition of Lower Jurassic sediments (Ferguson and Muller, 1949, 

 p. 13), whereas in northern Baja California Lower and Middle 

 Cretaceous rocks are involved in the orogeny, and are unconformably 

 overlain by Upper Cretaceous rocks (Woodford and Harriss, 1938, 

 pp. 1328-1330). 



Evidence is inconclusive as to the nature of the lands produced by 

 the Nevadan orogeny. Sediments laid down east of the deformed 

 belt suggest that the climate there was arid during Triassic and Early 

 Jurassic times, and became more humid later, but these conditions 

 were influenced by so many unknown factors that they are difficult 

 to relate to local topography. As mentioned earlier, a land barrier 

 existed on the site of the Great Basin during Triassic and Jurassic 

 times, but during initial phases of the Nevadan orogeny the eugeo- 

 synclinal belt west of the barrier probably remained low; meta- 

 morphism and plutonism of the rocks of the belt indicate that they 

 were deformed at a considerable depth, so that first orogenic move- 

 ments may have been more downward than upward. Thereafter, 

 during forcible injection of the younger plutonic bodies, the surface 

 of the deformed belt may have risen, and its erosion may have con- 

 tributed to the large volumes of sediment laid down east and west 

 of it (described under succeeding headings). Such erosion products 

 do not necessarily imply very great relief in the belt ; they might as 

 plausibly suggest that leveling by erosion nearly kept pace with 

 uplift. 



Be that as it may, it is worth emphasizing that the topographic 

 forms produced by deformation of the eugeosynclinal rocks had little 



