abstracts: geology 317 



Part II (by V. C. H.): — Gives history and production from 1869 to 

 1916, including production by ore zones and by kinds of ore; also the 

 history of smelting and milling. 



Part III (by W. L.) : — Discusses relations of deposits to fractures in 

 igneous and sedimentary rocks, showing selective replacement of lime- 

 stone ; underground water ; mineralization, with particular attention to 

 silicification of limestone and dolomite, paragenesis of ore and gangue 

 minerals, horizontal and vertical zones of deposition, and processes of 

 oxidation of different ores, genesis of the ore deposits; future of the dis- 

 trict. Detailed descriptions of mines follow, and reconnaissance re- 

 ports on the East Tintic and North Tintic districts are included. 



G. F. L. 



GEOLOGY. — The Genesis of the ores at Tonopah, Nevada. U. S. Geol. 

 Survey Prof. Paper 104. Edson S. Bastin and Francis B. 

 Laney. Pp. 47, 16 plates, 22 figs. 1918. 



This investigation supplements the work of Spurr and Burgess by 

 applying to the ores methods of microscopic study not in general use 

 when these reports were prepared. The Tonopah district is underlain 

 by a thick series of rocks that are products of volcanic activity and are 

 believed to be of Tertiary age. In spite of complicated faulting most 

 of the volcanic formations are rather flat-lying. In 19 15 the Tonopah 

 production of silver was exceeded in the United States only by that of 

 Butte. 



The bulk of the metal production of the district has come from ore 

 bodies lying wholly within the Mizpah trachyte. Following are the 

 more important conclusions: 



1. The hypogene or primary ores have been modified in places by 

 oxidation and enrichment through the agency of the air and oxygenated 

 solutions originating at or near the surface. The high silver content of 

 much of the ore obtained in the past and of some ore now remaining is 

 unquestionably due in part to these processes. 



2. There is evidence not only of recent oxidation of the ores but also 

 of at least one period of ancient oxidation, and supergene sulphide en- 

 richment was probably an accompaniment of each of these periods. 



3. The rich silver ores now being mined at Tonopah are probably 

 in the main of hypogene or primary origin. 



4. Mining has shown that in certain veins the primary sulphides 

 become less abundant with increasing depth, though the same species 

 are present; mere increase in depth may account for this change in 



