OUTLINE OF PAPER. 23 



selenium; that they also contained carbonic acid and sulphur, with some chlorine and fluorine; but 

 that they were noticeably deficient in iron. 



The alteration by thermal waters of the later andesite is also discussed. By comparison of analyses 

 and by microscopic studies it is concluded that the waters which produced the alteration were highly 

 charged with carbonic acid and sulphureted hydrogen, and contained magnesia, iron, and lime. 

 The advent of the waters is believed to have followed the eruption of the white siliceous rhyolite 

 above referred to. 



The composition of the mineral waters in the two cases above referred to does not seem to 

 correspond with that of the volcanic rocks whose eruption their advent followed. The eruption of 

 andesite was followed by the advent of siliceous ad potassic waters, poor in iron; the eruption of the 

 rhyolite by waters rich in lime, magnesia, and iron. This antithesis may have some bearing on the 

 origin of these waters. There are two theories of the origin of hot springs atmospheric and 

 magmatic. In the dry Nevada region there are cold springs which give evidence of magmatic origin, 

 while most of the hot springs show no connection with atmospheric precipitation. The meaning of 

 the nature of the metals in the-Tonopah veins is also discussed. The conclusion is reached that the 

 waters which produced the veins were largely given off from the congealing lava below. 



The temperature in the Tonopah mines shows an abnormally rapid increase with depth, 

 comparable to that in the Comstock. 



The water encountered by underground workings is very irregularly distributed. Some of the 

 shafts have reached a depth of over 1,000 feet without encountering any general body of ground 

 water, yet along certain steeply inclined fracture zones water is found sometimes quite near the 

 surface. These water zones are widely spaced and occur only in brittle rocks. They are probably 

 reservoirs bottomed by impervious clay seams. The porous rocks, such as the volcanic breccias, 

 absorb the precipitation like a sponge, and no water has yet been encountered in them. 



The relief of the range of hills in which Tonopah lies is primarily due to the volcanic 

 accumulations. These Tertiary volcanic rocks have been eroded and much material has been 

 transported from the hills into the adjoining desert valleys. In arid climates erosion is more general 

 than in moist climates, and as a result the relief is determined to a much greater degree by the 

 relative hardness of the rocks. This feature is beautifully illustrated at Tonopah. The complicated 

 faulting has had very slight effect upon the topography. 



