1922] Vaughan: Geology of Sa?i Bernardino Mountains 383 



east limit of the basalt is an escarpment, but to the west and south it 

 disappears under fanglomerate. At its eastern extremity it is imme- 

 diately overlain by a thin stratum of coarse sandstone above which 

 lies fanglomerate. The basalt is compact and very dark grey, nearly 

 black, in color. In thin section it is seen to contain large euhedral 

 augites, the outer portions of which have a violet tint and are slightly 

 pleochroic. Smaller euhedral crystals of olivine with hyalosiderite 

 rims are more abundant. The ground mass is composed of small laths 

 of basic labradorite, considerable magnetite, and very small grains of 

 colorless augite. 



Fig. 8. a, Negro Butte; 6, three low hills two miles east of Negro Butte, 

 showing probable position of faults. 



Three miles north of Old Woman Springs a series of basalt flows 

 rests on granite which is exposed on an escarpment overlooking a 

 playa on the north side. Two and a half miles to the west there is a 

 small patch of basalt on a hill of granite. Just south of this hill there 

 are several small areas. 



Negro Butte is a small granite hill, the southwest slope of which 

 is covered with basalt dipping 30° to the southwest (fig. 8a). Because 

 of the generally flat-lying attitude of the basalt in the region this 

 slope is believed to be due to faulting. About two miles to the east 

 there are three similar areas of basalt also tilted to the southwest 



Fry Mountain is capped with basalt which slopes 15° to the north 

 and is continuous with a flat area more than two miles in length. At 

 the base of this basalt cap there is a vesicular layer which has been 

 oxidized to a deep red and this is offset by a north-south fault. 



Three important points are to be noted regarding the basalt : 

 although the areas are small, it is widely distributed ; wherever there 

 is evidence of the nature of the surface on which it rests, this is found 

 to be flat or of very low relief; the basalt is now found at places of 

 considerable difference in elevation and there is abundant evidence of 

 its having been faulted. It is therefore evident that the basalt was 



a 



b 



(fig. 8b). 



