274 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



The rich ores occur in irregularly outlined portions of the lode called 

 bonanzas. These bonanzas are of limited extent both horizontally and vertically. 

 They are believed to have arisen as a consequence of the irregular intersection 

 of transverse fractures or tissure.s with the main vein channel, producing maximum 

 deposition in these portions. Intervening portions may be low grade or barren. 



In the oxidized zone, silver chlorides and bromides, free gold, manganese 

 oxide, etc., occur. 



THE PETKOGKAPHIC PROVINCE OF THE GREAT BASIN. 



After a study of the lavas of the Great Basin region of Nevada in 1900 the 

 writer" came to the conclusion that the whole region "southward into the 

 Mojave Desert, together with a portion at least of the Sierra Nevada, constitutes 

 a petrographic province; that is to say, it is underlain by a single body of molten 

 magma, which has supplied, at different periods, lavas of similar composition to 

 all the different parts of the overlying surface. The limits of this subcrustal 

 basin, however, are not yet defined in any direction." 



The general sequence of lavas, roughly outlined, was concluded to be as follows: 



1. Rhyolite (Eocene). 



2. Andesite (Miocene). 



3. Rhyolite with occasional basalt (Miocene-Pliocene). 



4. Andesite (Late Pliocene-Early Pleistocene). 



5. Basalts and occasional rhyolites (Pleistocene). 



EXTENSION OF THE GREAT BASIN PETROGRAPHIC PROVINCE INTO 



MEXICO. 



Later in the same 3'ear, Ordonez, in a stud}- of the rhyolites of Mexico 6 over 

 a northwesterly trending belt extending from the northern boundary southward 

 past the City of Mexico, found that the author's conclusions were also applicable 

 to this province. He writes as follows: 



With very slight differences, which are without decisive importance, one may 

 say that everywhere the relative order of eruptions, judging from the composition 

 and structure of the rocks, has been the same. Let us here present the example of 

 the Great Basin of Nevada. Many ranges of that region show a succession strictly 

 comparable with that of Mexico. 



The general succession is found to correspond with that given by the writer 

 above, and the rhyolites occupy the same position and are of the same age (Miocene- 

 Pliocene) as those under No. 3. The andesites, which preceded the rhyolites, 

 correspond with No. 2, and are Miocene. <' 



ogpurr, J. E., Jour. Geol., vol. 8, 1900, p. (S38. 



fcOrdoflez, E., Boletln del Instiluco geo!6gieo de Mexico, No. 14, p. 66. 



"Op. oil., p. 67. 



