38 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



to its surface, and to this it evidently owes its form. It is adjoined on the west 

 by a long tongue of similar lava, like that surrounding Heller Butte, and the 

 whole is surrounded by the friable Fraction dacite breccia. 



Farther west, just on the west side of the road, is a similar dome-like hill that 

 rises out of a limited irregular area of the same dacite and is surrounded by the 

 Fraction dacite breccia. Still farther west, a short distance, there is a projecting 

 ridge of Heller dacite, capped by a mamelon 5 feet high. The platy structure slopes 

 away from the ridge on all sides. 



These three similar cones are aligned in a northeast-southwest direction. 



Age of Heller dacite. The lava of Heller Butte has been faulted. It is thus 

 older than the later intrusive Brougher dacite, which makes up the important hills 

 of the district (see p. 44), and was erupted before the general faulting. It has been 

 found in normal contact only with the Fraction dacite breccia, which circumstance, 

 so far as it goes, favors an age either immediately before or after this formation. 

 On the southeast side of Heller Butte the lava of the butte is separated from the 

 Siebert tuffs by a fault contact. Examination of what is practically the same fault 

 (the California fault) a few hundred feet farther north, where the tuffs are brought 

 into contact with the Fraction dacite and the earlier andesite, both of which are 

 known to-be older than the tuff, shows that the tuff block is downthrown, and 

 that the Heller dacite belongs to a lower' horizon than the tuff. The fact that 

 nowhere has any of this Heller dacite been found between the Fraction dacite breccia 

 and the overlying formations would further restrict the probabilities; and the fact 

 that the dacite of the butte appears to dip under the Fraction dacite breccia near 

 Heller Butte and reappear beneath the breccia in the Tonopah City shaft favors 

 the final assignment of the Heller dacite to a period preceding the formation of 

 the Fraction dacite breccia. The inclusions of later andesite in the Heller dacite 

 again fixes the dacite as later than the andesite, and the place of the Heller dacite 

 may be held to be between the later andesite and the Fraction dacite breccia. 



Nature of Heller dacite. The glassy groundmass of the Heller dacite indicates 

 cooling at or not far from the surface, and the apparently waterworn pebbles 

 included in the dacite in the Tonopah City shaft suggest that this portion of the 

 lava was a flow. At the same time the presence of inclusions of granitic rocks 

 (sometimes in bowlders several feet in diameter), as well as of the later andesite 

 near Heller Butte, shows that the lava rose directly from depths below the granite 

 and passed through this rock and the already erupted andesites on its way up. A 

 vent or volcanic neck is thus suggested and the topographic forms of Heller Butte 

 and the similar smaller buttes described with platy structure parallel to their 

 surface offer the same suggestion. 



Summarizing the evidence and inferences, it appears that the eruption of 

 the later andesites was followed by an interval of rest and erosion; and that the 



