LAVAS IN THE GREAT BASIN REGION 633 



3. Acid (rhyolites with composition like 1) with occasional connected 



basalts. 



4. Basic intermediate (more basic andesites and aleutites). 



5. Basic (basic basalts), frequently with closely connected rhyolites. 



CORRELATION OF LAVA GROUPS IN POINT OF AGE 



The absolute age of the different Tertiary lavas is not easy 

 to find out, although in many special cases it may be done with 

 a fair degree of accuracy. The most recent eruptions are 

 naturally the most easy of determination, while those more 

 remote are more obscure. 



1. Acid. — In the Pine Nut and Sweetwater ranges the definite 

 age of the older rhyolites is uncertain, but they are separated 

 from the great bodies of massive hornblende-pyroxene-biotite 

 andesite, which itself antedates the Pliocene Shoshone Lake, by 

 a long period of erosion. These rhyolites are also affected by a 

 sheeting which is not found in the andesites, and which indicates 

 crustal disturbance between the eruption of the two lavas. 



The observations concerning the Pine Nut and Sweetwater 

 ranges are in general true, also, for the Reveille, Quinn Canyon, 

 and Grant ranges. 



In Meadow Valley Canyon the* basal rhyolite, with its over- 

 lying tuffs, is folded and separated by a marked unconformity 

 from the andesitic lavas and tuffs which succeed. The rhyolite 

 tuffs are roughly estimated at 4000 feet thick, and mark a long 

 period of Tertiary sedimentation, which intervened between the 

 rhyolites and the andesites. 



In the Silver Peak region rhyolites are interbedded in por- 

 tions of the Tertiary lake sediments, which are probably late 

 Eocene or early Miocene. 1 



In the Panamint Range, in the Randsburg district, and near 

 Daggett in the Mojave Desert, lake beds which are probably in 

 part at least Upper Eocene overlie the basal rhyolite. 



In general, therefore, it may be said that the age of the 

 rhyolite, which is the first member of the general succession, 



1 H. W. Turner : The Esmeralda Formation. Am. Geo]., Vol. XXX, March 

 1900, p. 168. 



