66 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



(No. 388) from the south side of Golden Mountain. If each of these is combined 

 separately with the later andesite analysis, No. 349, the result is as follows: 



Comparison of mean analyses of daeites and andesites. 



1, 2. Averages of later andesite and dacite. 



3. The average of the fresh later andesite specimens 349 and 225 is averaged with the average 

 of the three dacite analyses, 359, 368, and 388. 



STATEMENT OF DIFFERENTIATION THEORY. 



These considerations suggest that an original magma of composition similar 

 to that of the earlier andesite has split up by differentiation, first into a more 

 basic andesite (later andesite) and a siliceous dacite, and later, by continuation of 

 the process, into a siliceous rhyolite and a basalt, as follows:" 



Intermediate andesite. 

 Basic andesite. Siliceous dacite. 



Basalt. Siliceous rhyolite. 



SUMMARY OF GEOLOGICAL, HISTORY. 



Previous to the Tertiary period, Paleozoic limestone, intruded by granitic 

 rocks, occupied this region. With the Tertiary began a period of volcanism, 

 attended by the accumulation of lake sediments and subaerial deposits in inclosed 

 basins. These deposits began in the Eocene, and beds belonging to this epoch 

 are found near Tonopah, though not within the area mapped. 



About 8 miles north of Tonopah and 1 mile west of the little mining camp 

 of Ray the writer found a series of folded gravels, tuffs, lavas, and some white, 

 thin limestones carrying numerous Eocene fossils. These were sent to Dr. 

 W. H. Dall for determination, who remarks: 



"According to the literature the fresh-water beds from which these fossils 

 came have been referred by Doctor White and Meek to the Wasatch, or Bear 

 River Laramie, Eocene, which is believed to be nearly the equivalent of the 

 lower Eocene or Chickasawan marine Eocene (Lignitic of old authors) of our 

 southeastern coastal plain. The species are: 



"This corresponds with the scheme for the general succession of IMVMS In the Orcat Basin, as outlined by the 

 writer (Jour. Qeol., vol. 8, p. 643), and reaches the same conclusion that is already arrived at from independent 

 considerations. It coincides, as the writer has previously pointed out, with the luw previously deduced by Iddings 

 from ntudy of the volcanic* of the Oreat Basin and other regions (Bull. Phil. Soc. Wash., vol. 12, p. 145). 



