354 University of California Publications in Geology [Vol.8 



Mountain and in the Pilot Range, and presumably also of Jurassic 

 age. The limestone in the Gabbs Valley Range is similarly folded 

 and crumpled. 



Overlying the limestones unconformably are basic lavas, agglom- 

 erates and tuffs. Around Table Mountain, and along the east flanks 

 of the Gabbs Valley Range from Table Mountain northward to Gabbs 

 Valley, the pre-lacustral lavas are andesites, usually intercalated with 

 agglomerates and tuffs of andesitic composition. The lavas present 

 a great variety of colors. They usually contain an abundance of 

 soda-lime feldspar phenocrysts. The intercalated agglomerates are 

 often very coarse, masses of porphyritic andesite twenty feet or more 

 in diameter being not uncommon. They are rudely, or not at all, 

 stratified. The liner material consists largely of ash. Thin beds of 

 nearly pure white ash also occur in the series. The Esmeralda strata 

 of Stewart Valley lie unconformably upon these lavas along the east 

 Hanks of the Gabbs Valley Range, the lavas dipping under the valley 

 more steeply than the lake beds. At Table Mountain andesites also 

 overlie the Esmeralda beds, but whether conformably or otherwise 

 was not determined. Table Mountain is capped by a flow of basalt, 

 which is believed to overlie the post-lacustral andesites unconformably. 



The lavas of the Gabbs Valley Range, in the limited number of 

 localities Avhere they were observed, are therefore apparently of the 

 following ages. The andesites dipping under the Esmeralda beds 

 along the west side of Stewart Valley and the pre-lacustral andesites 

 of Table Mountain are post-Jurassic and pre-upper Miocene in age. 

 The Cretaceous and most of the early Tertiary were very probabh' 

 periods of erosion in at least this part of the Great Basin, as is evi- 

 denced by the removal of the cover of the post-Jurassic batholiths 

 which invaded the Jurassic rocks, and by the absence of Cretaceous, 

 Eocene. Oligocene, and lower and middle Miocene sedimentary de- 

 posits so far as known. It would appear therefore that these lavas 

 are not Cretaceous or earliest Tertiary in age. They antedate the 

 upper Miocene, however, by a period of time of sufficient length to 

 permit their deformation previous to the deposition of the upper 

 Miocene Esmeralda beds. It is probable therefore that these rocks 

 were extruded in lower or middle Miocene time. 



The post-lacustral andesites have been deformed and greatly eroded 

 and are probably early Pliocene in age. The basalt capping Table 

 Mountain, probably unconformable on the post-lacustral andesites, yet 

 little deformed or eroded, is probably late Pliocene in age. 



