AGES OF THE BOCKS. 69 



At Tonopah the succession of lavas, as above worked out, may be expressed 

 as follows: 



(a) Hornblende-biotite-andeaite. 

 Biotite-augite-andesite. 



(b) Dacites and rhyolites, with a little basalt. 



These may be assumed to coincide with 2 and 3 of the above general grouping. 



Probable Neocene age. In the comparative study above referred to" available 

 data were accumulated for determining roughly the age of the different groups 

 with reference to the standard divisions of geologic time and to the different 

 periods of Tertiary lakes as defined by King in his .-ummary of the results of 

 the Fortieth Parallel Survey. The eruption of group No. 2 (the hornblende- 

 biotite-pyroxene-andesites, followed by the dacites) occurred between the end of 

 the Eocene and the latter part of the Miocene, and was contemporaneous with 

 the Miocene lakes, while that of No. 3 (rhyolites, sometimes accompanied by 

 basalts) extended from the latter part of the Miocene well into the Pliocene, to 

 the time of the beginning of the Pliocene Shoshone Lake. On the assumption 

 that the correlation of the Tonopah lavas above given is correct, the andesites, 

 both earlier and later, would belong to the first half of the Miocene and to the 

 Miocene lake period; while the dacites, rhyolites, and basalts would extend from 

 near the middle of the Miocene into the Pliocene, and would be partlv 

 contemporaneous with the latter part of the Miocene lake. 



INFUSORIA IN THE SIEBERT TUFFS. 



In the white tuffs at the east base of Siebert Mountain a stratum, not 

 distinguished in the field from the more ordinary white rhyolitic or dacitic tuff, 

 was shown by the microscope to be entirely made up of minute diatoms or infu- 

 soria. These were recognized by the writer as probably similar to species described 

 by Mr. King as occurring in the deposits of the Miocene lakes of Nevada. At 

 the time the recognized succession of lavas did not seem compatible with this 

 idea, and the thin section was referred to Dr. Rufus M. Bagg, jr., for examina- 

 tion. Subsequently, it is proper to add, new discoveries as to the lava succession 

 removed the difficulties in the way of considering the deposits Miocene. 



Doctor Bagg's report follows: 



"The material submitted me from Tonopah, Nev., for examination consists of 

 innumerable diatoms which belong almost exclusively to two species, Helosira 

 granulala, L. W. Bailey, and Melosira varians, Ag., the latter being considerably 

 less abundant than the former. 



"Jour. Geol., vol. 8, No. 7, p. 637. 



