DACITES. 37 



HELLER DACITE. 



Location. Heller Butte, a small, steep manielon near the town of Tonopah 

 (PI. IV), is made up of a dacite containing numerous included fragments. At first 

 it was considered to be of the same class and age as the larger buttes, such as 

 Butler and Brougher mountains, and since the latter are denuded volcanic necks 

 it was thought to represent a smaller contemporaneous vent. Afterward, however, 

 it was recognized that the marked abundance of inclusions, the unusually abundant 

 glassy groundmass, and the fact that the porphyritic crystals are frequently larger 

 than those of the dacite of the larger mountains were characteristic features 

 of this particular rock. Later, other grounds favoring its assignment to a quite 

 different and earlier period were discovered. 



Heller Butte has a height of 150 to 200 feet and a steep conical form, elliptical 

 at the base. Its rock is vesicular glassy dacite, which contains inclusions of 

 pumiceous material, frequently of later andesite, and occasionally of coarse 

 siliceous granite. The inclusions of andesite and granite are sometimes large 

 angular bowlders, several feet in diameter. The form of the butte seems to be 

 governed by platy structure. It is steep and slopes away in curves on all sides. 

 On the northeast and southeast sides the lava is cut off from the Fraction dacite 

 breccia and the Siebert tuffs by faults, along which are intruded glassy dikes sent 

 off from the Mount Golden mass of Brougher dacite. On the western side the lava 

 of the butte seems to dip under the nearly horizontal Fi'action dacite breccia. 



The Tonopah City shaft. 800 feet west of the Heller dacite area last referred 

 to, passed thi-ough 300 feet of the partly fragmental, loose Fraction dacite breccia 

 to solid, glassy dacite of the Heller type, which continued for 200 feet more to 

 the bottom. The contact in the shaft could not be seen by the writer on account 

 of the tight lagging, but it seems most likely that the order is normal and that 

 the lower formation is the older. Rounded and subangular inclusions of the later 

 andesite, having the appearance of waterworn pebbles, are frequent in the Heller 

 dacite of this shaft, and are more abundant toward the bottom. There are found, 

 also, smaller rounded quartz pebbles, which are accounted for on the hypothesis 

 that this lava was a flow, which ran over and caught up pebbles from an older 

 surface-gravel deposit. 



Near the southeastern edge of the area mapped are other outcrops of lava, 

 which have the same peculiar phases the abundant glassy groundmass and the 

 numerous inclusions of foreign materials as the lava at Heller Butte. Here, 

 northeast of the main road that crosses the valley, running between Butler and 

 Golden mountains, is a small, smooth mamelon, or symmetrical cone, about 20 feet 

 high and 80 feet in diameter, that resembles in a general way Heller Butte, and is 

 composed of the same lava. This cone has a concentric, platy structure parallel 



