104 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



FISSURES DUE TO MOVEMENT AFTER CONSOLIDATION. 



These fissures and fractures, judging from their distribution and direction, 

 plainly resulted from the continuation of the driving upward of the plug after con- 

 solidation was practically complete. 



The movement thus indicated is like that which was manifested by the plug of 

 Mont Pele'e in Martinique subsequent to the late eruptions, when it was forced 

 upward after solidification, so as to tower several hundred feet in the air. Around 

 the base of such a plug as Pelee's, phenomena like those on Ararat must have taken 

 place. 



The fillings are evidently the result of ascending hot water which followed the 

 channels thus opened and cemented them. That such large open spaces due to 

 rending could have been formed indicates that the spot was not very far distant 

 from the surface. 



PARAGENESIS OF VEIN MATERIALS. 



The substances deposited in the openings also are simple, as compared with 

 those of other periods of vein formation in the district. The alteration of the 

 rhyolite is confined to silicification and slight bleaching of the biotite. Some 

 of the specimens from the Wingfield tunnel show feldspar phenocrysts completely 

 altered to microcrystalline and cryptociystalline silica. In many cases this silici- 

 fication seems to have preceded the deposition of the carbonates, for the latter 

 are deposited in cavities upon the silicified rhyolite. In other cases, however, 

 the jaspery and chalcedonic quartz, which is often part of the fissure filling, is 

 plainly later than the carbonates. In several cases white calcite was observed 

 to be later than the dark or ferruginous calcite in origin. 



COMPOSITION OF VEIN-FORMING WATERS. 



No sericite was observed to be developed in the wall rocks, hence it seems 

 probable that the waters did not contain fluorine (see p. 232), or that their temperature 

 was very moderate, or both. Indeed, they do not give evidence of having contained 

 anything beyond silica, lime, iron, and manganese carbonates. Their content of 

 gold was small, for the veins are generally practically barren. No larger amount 

 of this metal is likely to have been present than has been detected in many hot 

 springs issuing at the surface. The presence of iron is contrasted with the 

 probable absence of iron in the solutions which produced the earlier andesite. 



oHovey, E. O.. Am. Jour. Sci., 4th ser., vol. 16, pp. 269-281. Russell, I. C., Science, vol. 17, pp. 792-796; Am. Jour. 

 8cl., 4th ser., vol. 17, 1904. 



