54 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



as is usually the case in the southern half of the area mapped. These facts show- 

 that before the deposition of the sediments considerable active erosion stripped 

 off the debris of the earlier dacite-rhyolite eruptions and bared the underlying 

 andesites. It is not unlikely, however, that the land which adjoined the lake and 

 which contributed the sediment was vigorously worn away, and that the sediments 

 were extended over this eroded region as a result of a rise in the lake or of a 

 shifting of its boundaries due to crustal movements. This idea is strengthened 

 by the fact that a careful macroscopic and microscopic study of the materials in 

 the tuffs proves that they were derived mainly from the erosion of the glassy 

 dacites and rhyolites. Pebbles in the tuffs, besides those of the rock,s just men- 

 tioned, are frequently of the later andesite, well rounded. 



EXPLOSIVE ERUPTIONS OF THE LAK PERIOD. 



It is probable that the quiet of the lake's existence was occasionally slightly 

 disturbed by small local eruptions of rhyolitie or dacitic material. Stratified 

 beds composed of rounded waterworn pumice fragments are sometimes found 

 between fine-grained strata. The imperfect bedding shows that they were 

 deposited more hastily than most of the strata, and each bed probabty represents 

 an explosive eruption. Sometimes angular fragments of obsidian occur in the 

 pumice. Moreover, thin sheets of Tonopah rhyolite-dacite, similar to the main 

 masses, are sometimes found within the tuff series. 



UPLIFT TERMINATING LAKE PERIOD. 



At one point on the northeast side of Siebert Mountain the tuff at its contact 

 with the Siebert dacite body, which is here intrusive, contains a conglomerate 

 f xmi which may be made significant inferences as to the conditions prevailing at 

 the time of its formation. This conglomerate is made up of rounded pebbles up 

 to 4 inches in diameter, most of which are composed of the Tonopah rhyolite-dacite, 

 but some are of later andesite. In it was found a fragment of silicified wood 

 over a foot long. This conglomerate is exposed for only about 50 yards. It dips 

 with the inclined tuffs, but is not continuous; in fact, it occupies a channel in 

 the tuffs. The change between the tuff and the conglomerate is abrupt and 

 complete, indicating a sudden change of conditions. All this suggests that these 

 pebbles are old river gravels. If this is true, the tuffs were uplifted at the close 

 of the lake period and became land. Immediately thereafter important outbreaks 

 of lava occurred, and the hypothesis may be formulated that the accumulation of 

 the lava beneath tin- future vents produced the uplift. A river, probably flowing 

 from the north (where the later andesite is now and was then exposed), brought 

 down the pebbles to this bed. That the banks of the stream were wooded is 

 shown by the now silicitied fragment. 



