GIO DESCRIPTIVE GEOLOGY. 



Range, vvliicli, though too isohxted and obscure to show clearly their lines of 

 structure, give sufficient evidence that the Mount Neva mass is a pre- 

 existing elevation of Weber Quartzite, continuous with that of the Dalton 

 Peaks Ridge. 



Along the eastern flanks of this elevation, at Tuscarora, on the 

 borders of Independence Valley, is a body of propylite, now largely 

 covered by flows of rhyolite and Quaternary debris. Near the town of 

 Tuscarora, the surface of this propylite is decomposed to a depth of 3 to 4 

 feet, and covered by as many feet of soil, which has been worked with 

 considerable profit for placer gold. The propylite of Tuscarora has the 

 characteristic physical habit of this rock, ofi'ering no prominent outcrops, 

 and generally occupying a subordinate topographical position. It is a light 

 greenish-gray, porphyritical rock, having s.omething of an earthy texture, 

 showing macroscopical crystals of opaque, somewhat decomposed ortho- 

 clase, and fresher plagioclase in a groundmass, w^hich is characterized by a 

 large admixture of light-green fibrous hornblende. There are also larger 

 prismatic crystals of dark, fresh hornblende, usually characteristic of ande- 

 site, in the rock, but not in sufficient quantity to form an important element 

 in its composition. The microscope detects in it no augite or biotite. The 

 two varieties of hornblende, the green and the dark-brown, may be seen in 

 Vol. VI, Plate IV, fig. 4, which presents a thin section of this propylite 

 with its characteristic groundmass. 



Another darker green, somewhat decomposed, variety, obtained from 

 under the Quaternary accumulation of the slopes, has been classed by Zirkel,^ 

 from his microscopic examination, as an andesite, with the remark, however, 

 that it contains a great quantity of hornblendes in an advanced stage of 

 alteration into epidote, a rare occurrence' with andesites. These horn- 

 blendes can be distinguished macroscopically in the feldspar crystals. The 

 rock certainly has the physical habit and position of propylite rather than 

 andesite. 



A more well-defined andesite occurs on the foot-hills of the range, 

 however, north of the town, where it has apparently poured out between 

 the propylite and the underlying sedimentary rocks. It is a dark, com- 



' Microscopical Petrograpliy, vol. vi, 130. 



