SUMMARY OF GEOLOGICAL HISTORY. 67 



" Vivipara, close to if not V. couesi; Planorbis utahensis Meek; Ancylus 3 sp. ; 

 and a small bivalve, probably a Corbicula, but which I suspect to be the same as 

 Sphserium idahoense Meek. The specimens are merely internal casts, but if they 

 are really Corbicula may prove to be C. occidetitalis Meek. Their condition is 

 too imperfect to be certain even of the genus, but the form closely approaches 

 that of the figures of S. idahoense.'''' 



These overlie the Paleozoic limestones near Ray. Similar beds were noted 

 at several places between Ray and Sodaville. They are probably continuous with 

 a part of the Tertiary deposits of the Silver Peak and Monte Cristo mountains." 



The oldest of the Tertiary rocks within the area of the Tonopah map are 

 probably early Miocene, and so far as known the volcanic manifestation began 

 with an eruption of andesite. In this andesite were formed fracture zones, along 

 which heated waters ascended and deposited the valuable veins of the region. 

 Another extensive eruption of similar but slightly more basic andesite followed, 

 and then there was probably a period of volcanic rest and of denudation. 

 Eruption was resumed by the outbreak of volcanoes, which alternate!}- ejected 

 siliceous dacite and poured out volcanic mud and frequently pumiceous lava. Some 

 of the material may have been accumulated in water; most of it was probably 

 deposited upon the land. Later, more glassy dacite of a slightly different 

 composition ascended from below in irregular channels and poured out on the 

 surface as thin sheets, or exploded and formed tuffs. Heated ascending waters 

 followed the intrusive contacts of this lava and formed a group of quartz veins 

 which contain gold and silver, but which are less important as regards strength 

 and values than the veins formed after the eruption of the earlier andesite. 



As these dacite-rhyolite eruptions quieted down a lake was formed in a basin, 

 which may have been due to a depression of the crust consequent upon the 

 previous copious eruptions. In this lake there accumulated quietly several 

 hundred feet of sediments, with occasional light showers of ash from volcanoes, 

 and, in the lower portions, some thin flows of dacite lava. Then the beds were 

 lifted and became dry land. This uplift may have been due to the accumulation 

 of additional volcanic material beneath this portion of the crust. Streams began 

 to cut into the lake beds, the uplift was continued, and the whole district was 

 tilted bodily to the west at an average angle of 20. After this there were 

 renewed outbursts, from probably new vents, which doubtless, corresponded, in 

 part at least, to the present mountains. On Brougher and Butler mountains 

 explosive eruptions occurred, the material being dacitic, like that immediatelv 

 preceding the lake deposits. Cones of ash, cinders, and bombs were built up, 

 and there were occasional very thin and scant}' glassy flows. On Siebert 



a Turner, H. W., Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2, pp. 192-244; Spurr, J. E., Bull. U. S. Geol. 

 Survey No. 208, PI. 1, and pp. 105-106, 185. 



