CHAPTER I. 

 GENERAL GEOLOGY. 



DESCRIPTION OF THE ROCK FORMATIONS. 



PRE-TERTIARY LIMESTONE AND GRANITE. 



In the immediate vicinity of Tonopah the rocks are all Tertiary volcanics or 

 tuffs. Eight or 9 miles south of the camp, however, there is limestone, very likely 

 of Cambrian or Silurian age, which is intruded by granitic rock. Limestones and 

 granites occur also several miles north of Tonopah, and at intervals between 

 Tonopah and Belmont. At Belmont the limestone, which is intruded by granite, 

 is known to be Silurian. From 20 to 40 miles west of Tonopah, on Lone 

 Mountain and the Silver Peak Range, both Cambrian and Silurian limestones are 

 cut into by granite. 



At Tonopah occasional limestone and qnartzite fragments and more abundant 

 blocks of granite (often pegmatitic in structure) occur in the volcanic breccias. 

 Their position shows them to be blocks which were hurled out from volcanoes. 

 Thus it is shown that at an uncertain depth below the present surface the ascend- 

 ing lavas broke through rocks of this character. In every case noted these inclu- 

 sions were in extremely glassy, generally light-yellow volcanic breccia having the 

 composition of rhyolite-dacite." Three out of four localities are also on the borders 

 of areas of a peculiar dacite, considered probably the oldest dacite of the region 

 (Heller dacite), though whether this fact has any further significance is not clear. 



At the northeast base of Heller Butte in this glassy Heller dacite there are 

 inclusions of angular granitic blocks, often several feet in diameter. At the west 

 base of the butte another bowlder of siliceous granitic rock was found in the 

 dacite. A fragment of the same rock was found on the borders of the Heller 

 dacite in the southeast part of the area mapped, southwest of the fork in the 

 road that runs southeastward from Tonopah. A similar fragment was found in 

 glassy rhyolite-dacite at the south base of Ararat Mountain. All these fragments 

 were probably derived from a single underlying granitic mass. 



Fragments of altered limestone were noted in dacite breccias, especially in the 

 vicinity of the New York Tonopah shaft. 



a These two rocks are intimately allied and associated in the Tonopah district, and in their glassy phases are often not 

 easily distinguishable one from another. 



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