BASALT. 55 



BASALTIC ERfPTIOXS. 



The conditions thus suggested could not have lasted for a long time, for at a 

 short distance from the conglomerate, at about the same horizon (on the east side of 

 Siebert Mountain near the summit), the white tuffs are overlain by l>eds of yellow 

 pumice breccia, full of fragments of black, slaggy basalt, a rock not known to have 

 been previously erupted. Small hollow spheres of pumice (lava bubbles) are 

 present. Some layers are made up entirely of large, angular fragments of scori- 

 aceous basalt. Over this lies a bed of black basalt 40 or 50 feet thick. This rude 

 accumulation of pumice and scoria; appears to lie unconformably on the tuffs, for it 

 is nearly horizontal, while the tuffs have a decided dip to the west; and the same 

 breccia appears at several other points on the mountain in contact with different 

 horizons of the tuffs. The uplifted tuff's of the same age as the river conglomerate 

 were probably tilted bodily to the west by a continuation of the disturbing uplift, 

 and after this tilting new volcanic vents were opened and there occurred a violent 

 explosion which scattered a relatively slight amount of basaltic material. This 

 explosion was followed in the neighborhood of Siebert Mountain b\- the welling 

 out of a thin sheet of slaggy basalt. On Brougher Mountain also a volcanic breccia 

 overlies the tuffs, but here no basalt is exposed. 



KKdlOXAL TIl.TIXli ACCOMPANYING UPLIFT. 



The uplift which preceded the explosions was not local. The westward dip of 

 the tuffs on Siebert Mountain is not essentially different from their general attitude 

 wherever found in the area mapped. There is a notably persistent north-south 

 strike, and a westward dip averaging perhaps 20, independent of local phenomena 

 These local phenomena bring about variations in the attitude; for example, near the 

 great Butler Mountain neck, where, as will be presently explained, the rocks have 

 been faulted and dragged down at the contact, there are places where the tuff is 

 locally folded so that it dips toward the mountain. 



HASALT. 

 LOCATION. 



Basalt in place occurs in only one small area within the district mapped 

 near the top of Siebert Mountain (PI. XI), although it was observed in three other 

 places, close to the area. Near Tonopah, on the road from Sodaville, low hills of 

 vesicular lava stand on the edge of the wash-covered desert valley. This lava is 

 an augite-olivine-basalt, containing augite and reddish altered olivine in a micro- 

 litic groundmass consisting of feldspar, augite, olivine, and magnetite. 



The top of a broad, black mountain just north of Ararat Mountain is alos 

 covered with basalt of the kind just mentioned. A determination of one of the 

 feldspars here showed anorthite. Similar lava forms the hill east of Golden 

 Mountain and overlies the Fraction dacite breccia. 



