OUTLINE OF PAPER. 



Ore deposit* were discovered in the Tonopah mining district, Nevada, in April, 1900, by James L. 

 Butler. The town of Tonopah soon had a population of several thousand. The climate is arid 

 and the water supply scanty. 



The rocks of the mining district are all of immediate volcanic origin, with the exception of a series 

 of water-laid tuffs, which represent the accumulations of fine volcanic detritus in a Tertiary lake. 

 All the rocks are of Tertiary age, probably Miocene-Pliocene. 



The first eruptions of this volcanic epoch, as displayed at Tonopah, were andesite. Two andesites 

 have been distinguished the younger or earlier andesite and the later andesite, which is slightly 

 more basic than the earlier andesite. Subsequently rhyolite and dacite eruptions occurred at inter- 

 vals for a long time and produced several of the formations mapped, which include tuffs and flows. 

 The rhyolite and dacites are closely connected in every way. In one of the latest periods of eruption 

 these lavas produced the volcanoes whose necks, left in relief by the erosion of the surrounding softer 

 material, now form the hills around Tonopah. 



The water-laid fine tuffs were deposited in this rhyolite-dacite volcanic epoch at a time when the 

 eruptions had ceased temporarily. The lake basin may have been formed by a sinking of the crust 

 consequent upon the long-continued volcanic outpourings. The epoch of the deposition of the lake 

 beds was closed by an uplift accompanied by regional tilting. A little basalt was then thrown out 

 from volcanic vents, and cones of agglomeratic dacitic material were formed, whose once liquid necks 

 are now represented by the isolated hills. 



The area occupied by the dacitic volcanic necks is coextensive with the region of observed com- 

 plicated faulting. Study leads to the conclusion that this faulting was initiated chiefly by the intrusion 

 of these necks. After the intrusion and subsequent eruption there was a collapse, a sinking of the 

 various vents. The still liquid lava in sinking dragged down with it adjacent blocks of the intruded 

 rock. ( 



The silica content of the lavas shows a fairly regular transition between the different types, but 

 there is a marked break in general composition between the andesite-basalts on the one hand and the 

 rhyolite-dacites on the other. In some of the most siliceous rhyolites there appear to be numerous 

 pseudomorphs after hornblende, which consist of fresh rhyolitic groundmass and indicate that the 

 hornblende had been dissolved and replaced by the magma. In the dacitic phases of the rhyolite-dacite 

 fresh hornblende is occasionally found. In the audesites, especially the earlier phase, hornblende is 

 abundant. In the basalt there is abundant hornblende, but it is often pseudomorphosed by magmatic 

 action into aggregates marked by crystals of iron oxide. It is concluded that in both the highly 

 siliceous (rhyolitic) and in the least siliceous (basaltic) magmas, hornblende was developed as a first 

 crystallization, which was unsuited to later conditions. A change of magmatic composition since the 

 first crystallization is inferred, arid the original magma is thought to have been intermediate or 

 andesitic. This theory of magmatic segregation is tested by comparison of analyses, and bears the 



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