254 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



manganese, and silica. They have produced silicitication, and have deposited silica 

 in fissures, but the silica is usually greatly exceeded by the calcite (figs. 14, 15). 

 These waters then were also characterized by the materials of basic rather than of 

 acidic igneous rocks. 



Along the contact of the dacitic rocks there has frequently been profound 

 alteration of the later andesite, but the process has not been studied sufficiently to 

 give definite conclusions. A specimen from the later andesite near the Molly shaft, 

 at the contact with the Golden Mountain dacite, is entirely altered to calcite and 

 quartz, the former unusually abundant, with siderite and pyrite, etc. At the Belle 

 of Tonopah shaft specimens of the later andesite near the contact with the glassy 

 Tonopah rhyolite-dacite are largely altered to calcite, together with quartz and 

 probable sericite; other specimens near here are more plainly silicified, but are 

 ferruginous. The glassy rhyolite-dacite itself, near the contact, is often silicified, 

 but shows frequently considerable epidote. Calcification as well as silicitication is 

 therefore suggested in all these instances. 



Omitting, therefore, as without sufficient data, the consideration of the solu- 

 tions accompanying the rhyolite-dacites and referring only to the Oddie rhyolite 

 and the earlier andesite, the conclusions, if correct, may have a bearing on the 

 source of these solutions. 



THEORY OF ATMOSPHERIC ORIGIN OF HOT SPRINGS. 



There are two possible explanations of hot springs in general. One is that 

 atmospheric water, of which such a large quantity sinks below the surface, 

 becomes warmer in depth by the natural increment of temperature or in volcanic 

 regions by the residual heat of the rocks, and on finding a channel ascends toward 

 the surface as hot water, carrying with it materials which it has dissolved out of 

 the rocks on its passage. A physical objection to this theory is that surface 

 water could hardly work its way downward against pressure, to the depths neces- 

 sary to become highly heated. This has been met by the experiment of Daubre'e, 

 which showed that water would work itself downward through a solid marble 

 slab by capillarity, in spite of the resistance offered by a strong pressure on the 

 underside of the slab. It has been argued that by such capillary circulation 

 the supplies of hot springs may be replenished. 



THEORY OF MAGMATIC ORIGIN OF HOT SPRINGS. 



The other explanation goes back to the hypothetical origin of the atmos- 

 pheric or surface water at the period of the consolidation of the globe. Accord- 

 ing to the commonly accepted theory, when the molten or fluid earth stuff cooled 

 and was consolidated, a large part of the contained water was separated, and by 

 reason of its great mobility formed the oceans. Processes similar to those which 



