RHYOL1TES AND DAC1TES. 51 



LATEST RHYOLITE OR DACITE. 



Location. A few thin sheets of glassy rhyolite-dacite, which are of very 

 little importance, do not clearly seem to be correlatahle with the other volcanic 

 formations described. One of the small areas of this lies on the south side of 

 Mount Oddie. This rock is a black, very glassy, thin flow, overlying a coarse 

 stratified tuff made up of small fragments of glass. It also overlies the later 

 andesite in such a way as to indicate that the tuffs may have been eroded in 

 places from the andesites before the glassy sheet was poured out. 



Similar lava occurs around the base of Brougher Mountain. On the north 

 side, immediately overling the tuff, is a thin bed of such lava. There seems to 

 be a slight unconformity between the two. Near by, the glassy lava seems to 

 rest on the Tonopah glassy rhyolite-dacite, which normally underlies the Siebert 

 tuffs, suggesting again that the tuff was eroded before the advent of the lava. 



Age and origin. These flows may have been emitted from the volcanoes of 

 Butler, Brougher, and Oddie mountains during their earlier history, while the 

 cinder cones were being built up, or as the writer is inclined to believe, mainly 

 during their later history and so subsequent to the eruption of the Brougher 

 dacite. They are not observed to be more than a few feet thick. In places 

 small amounts of similar lava seem to have ascended as dikes, especiallv along 

 faults. Where it occurs as dikes, however, it may be difficult to distinguish it 

 from some of the glassy rhyolite-dacite lavas of other periods. 



Mineral composition. Microscopically the lava resembles closelv the Tonopah 

 rhyolite-dacite. In a groundmass of brown glass there are porphyritic crystals 

 of quartz, orthoclase, striated feldspar, and biotite. 



SIEBERT TOFF (LAKE BEDS). 

 LACt'STRIXE ORIGIN. 



The white stratified tuffs form a conspicuous feature "of the geology near 

 Tonopah. As a rule they are beautifully and uniformly bedded, and composed of 

 well-assorted material. Where beds of conglomerate occur the pebbles are per- 

 fectly rounded. Since these sediments do not vary in character for thicknesses 

 of several hundred feet, it is plain that they were laid down in a large body of 

 standing water that lasted for a considerable length of time. That this body 

 was a lake is indicated by numerous general considerations derived from the 

 study of the geology of the surrounding region and by the presence of numerous 

 fresh-water infusoria in some of the strata. In contrast to the general regular 

 stratification, cross-bedded strata may occasionally be found, and .at one place 

 markings like those made by rills on a sandy shore were noted. These are 

 probably shore and delta features. 



