280 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



There appear, then, reasons for believing that the belt of very late Pliocene- 

 Pleistocene-Recent andesitic eruptions continues farther than suggested in the 

 writer's paper quoted above (p. 275), and even that they are characteristic of the 

 whole great "circle of fire;" and this uniformity seems to indicate a single 

 major petrographic province for this period, extending around the whole zone." 



In some cases the analogy of the less-known Asian and Australasian portions 

 of this belt with the North American part is known to extend back of the Pleis- 

 tocene. In the East Indian archipelago, according to Zirkel, there was a general 

 eruption of pyroxene-andesite at the end of the Eocene or beginning of the 

 Miocene, since the early Miocene sediments already contain some andesitic material. 

 This period would correspond to group No. 2 of the scheme of succession presented 

 on page 68.* 



In New Zealand the Hauraki Peninsula is made up almost wholly of Tertiary 

 igneous rocks, mostly andesites, with accompanying heavy deposits of volcanic 

 agglomerates; these andesites and accompanying tufl's and breccias are regarded 

 as of late Eocene and early Miocene age. In places they are covered by rhyo- 

 lites and rhyolitic tuffs of early Pliocene age. c These andesites and rhyolites, 

 respectively, fall into groups 2 and 3 of the scheme on page 68. 



It is also probable that the coextension of the metallographic and the petro- 

 graphic provinces is greater than above established, for at many other points along 

 the belt of the petrographic province, in the Andes of South America (for example, 

 in Peru*), veins are reported having, so far as can be made out, a mode of occur- 

 rence, age, and composition similar to those of Mexico. The mines at Quespasia in 

 that country are in highly altered augite-andesite. The ore minerals are pyrargy- 

 rite, polybasite, and other rich silver ores, with galena and blende, and a little 

 copper pyrite and iron pyrite. In their richest portions they contained on an 

 average 2 per cent silver/ These richest portions in the Peruvian mines of this 

 type are like the Mexican bonanzas, and are called, in Peru, tajos.f 



At Cerro de Pasco, also in Peru, the argentiferous formation is a metamor- 

 phosed Mesozoic sandstone intruded by altered andesite. The ore consists of 

 free silver, silver sulphides and antimonides, lead carbonate and sulphide, various 



oThese andesites. constituting the most recent lava of this province, appear to be a distinctly later group in the 

 volcanic succession than the youngest (No. 6) enumerated in the scheme on p. 68. They may be designated as group 

 No. 6, Pleistocene and Recent, and the recurrence of lava of this composition, similar to Nos. 2 and 4 (early Miocene and 

 late Pliocene andesites, respectively), suggests the beginning of a new cycle of magmatic differentiation. \\ host- continua- 

 tlnn will bring about, for the fourth time In the history of this volcanic epoch, the eruption of basalts and rhyolites 

 similar to Xos. 1, 3, and 5. (See Spurr, J. E., Jour. Geol., vol. 8, No. 7. pp. 637-646.) 



In the region near Tonopah there is one probable representative of these latest andesites. In Mono Lake, Cali- 

 fornia, 90 miles west of Tonopah, are ten or fifteen volcanic cones of very recent date, the lavas being in part hypersthene- 

 andeHite. In part rhyolltc. (Russell, I. C., Eighth Ann. Rept. U. S. Geol. Survey, pp. 374, 375, 377, 380.) 



6 See Spurr, J. E., Jour. Cieol., vol. 8, No. 7, p. 637. 



Park, James, elted by Lindgren, W., Eng. and Min. Jour., Feb. 2, 1906, p. 218. 



' FuchH ct de Launay, Gltes metal 11 feres, vol. 2, p. 829. 



Beck, Erzlagerstfitten, 2d ed., p. 277. 



/ Fucha et de Launay, op. cit., vol. 2, p. 831. 



