252 



GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



the vicinity of the large subsequent igneous intrusions. It is therefore likely 

 that the alteration of the later andesite was largely produced by waters which 

 followed later, chiefly rhyolitic," intrusions into it. 



ALTERATION OF THE ODDIE RIIYOLITE. 



Some partial anah'ses were made, to show the composition of the fresh and the 

 altered white Oddie rhyolite. As a rule this rock is quite fresh, even when close to 

 the intensely altered earlier and later andesites. Sometimes, however, especially 

 along faults and watercourses, the rhyolite disintegrates and the feldspar is partly dis- 

 solved out, leaving cavities, while the scant biotite of the fresh rock has disappeared. 



The partial analyses are as follows: 



Analyses of Oddie rhyolite. 

 [By Dr. E. T. Allen.] 



The first two analyses being of fresh rock, the difference in the chemical 

 composition is probably original. This difference was, indeed, noted in the field, 

 where the rhyolite of Rushton Hill (No. 1) was observed to have a slightly more 

 basic aspect than the rhyolite of Mount Oddie (No. 2), and to approach in 

 appearance the siliceous dacite of Golden Mountain near by. No. 3, however, is 

 Oddie rhyolite which was probably originally of a composition similar to No. 2, 

 and the chemical change undergone on alteration seems to have been a slight 

 increase in silica and a loss of the alkalies, especially soda. 



The microscopic description of No. 3 is as follows: 



3. (Specimen 227) Mizpah Extension shaft, 385 feet down. Hand specimen is 

 white and hard, but shows cavities due to the dissolution of feldspar phenocrysts. 

 There is no biotite. Under the microscope there are also no signs of biotite, and 

 the feldspars are entirely altered to a sericite aggregate, both in the phenocrysts 

 and in the groundmass. The phenocrysts consist of abundant quartz, with sericite 

 areas representing original feldspars, while the groundmass consists of an aggregate 

 of crystalline granular quartz, much coarser than in the fresh rock and sericite. 

 The size of the quartz grains in the groundmass is evidently due to enlargement by 

 the waters which produced the alteration, for crystal faces are frequent and such 

 idiomorphic grains frequently impinge upon the area of the original idiomorphic 

 feldspar phenocrysts, now altered to sericite. 



' 'l in' glassy Tonopah rhyollKMlaclte Is a rhyolitic variety (p. 69.) 



