72 JOHN A. REID 



that the Coso Range became a very distinct unit, one main fault 

 going to the west and forming the east wall of the Sierra, another 

 passing to the east and forming the west wall of the Darwin Range 

 (see Fig. i). The large lake in which the Wancobi and Coso beds 

 were deposited was formed at this time, and granitic detritus deposited 

 from the Coso Mountains in the arm north of Darwin. The occurrence 

 of the few fragments of schist indicates that the last remains of the 

 older rocks in the body of the range were removed just previous to, 

 and in part coincident with, the draining of the lake. The only 

 locality where the older rocks yet remain is in an east-west ridge 

 about eight miles north of Darwin and just east of the low granite 

 spur noted above as marking a south limit of the lake beds. On this 

 east-west ridge of older rocks is exposed a magnificent section about 

 a mile thick, the strata dipping uniformly east at about 45°. No 

 time could be given to an examination of these rocks. The older 

 lake basin was eventually drained by further movements along the 

 fault planes, and basalt eruptions covered the west and north flanks 

 of the Coso Mountains, the south end of the Inyo Range, and a large 

 area of the lake beds south of Keeler. As some of the basalt lies 

 upon unstratified sands which are above the lake strata, the lake must 

 have been partly drained previous to the initiation of the volcanic 

 flows. 



Still later faulting along the old lines produced further elevation 

 of the Inyo Range and differential movements of the lake beds along 

 this sierra. Through it all the Coso Range was faulted only by small 

 displacements, caused by its position between the two great converging 

 faults to east and west. Thus these comparatively insignificant 

 mountains stand as a monument of the older geologic time when the 

 western cordillera was composed of low mature ranges not character- 

 ized by the excessive and great faulting seen today. 



