SILVER AND GOLD. 213 



south. At Silver Cliff an outbreak of pinkish rhyolite occurs, im- 

 pregnated with silver chloride. It affords a free-milling although 

 low-grade ore. This forms a unique deposit. There is a great 

 thickness of tuffs beneath it, as shown in the Geyser mine, and 

 some remarkable forms of spherulitic crystallizations. 



2.09.20. Example 39. Bull Domingo and Bassick. The first 

 named is two miles north of Silver Cliff, and the latter seven miles 

 east, near Rosita. The Bull Domingo is in Archaean, hornblende 

 gneiss, and consists of what appear to be pebbles or boulders of the 

 wall rock, which are coated with argentiferous galena and an out- 

 er shell of quartz. The ore body is 40 to 60 feet across. The 

 Bassick is in andesite, and likewise consists of what appear to be 

 boulders and pebbles of the country rock, coated by concentric 

 shells of rich ores, and in an elliptical chimney 20 to 100 feet across. 

 The first coat is a mixture of lead, antimony, and zinc sulphides, 

 and is always present. A second, somewhat similar, but of lighter 

 color and richer in lead and the precious metals, is sometimes seen. 

 A third is chiefly zincblende, rich in silver and gold, and is the 

 largest of all. A fourth, of chalcopyrite, sometimes occurs, and 

 lastly a fifth, of pyrite. Various other minerals are found, and r 

 curiously enough, carbonized wood on the outer limits. Both, 

 these deposits have been thought to be the tubes of geysers, in 

 which boulders have been tossed about, rounded, and finally ce- 

 mented together. Mr. Emmons has argued against this view, and 

 in a forthcoming monograph will present the results of extended 

 study. A brief account of these results has, however, been pub- 

 lished by Dr. Whitman Cross. The region is shown to be one 

 containing numerous, although not always large, volcanic out- 

 breaks. One of them furnished the sheet of rhyolite at Silver 

 Cliff, while others^had for their conduits the chimneys of the Bull 

 Domingo and the Bajssick. The ore bodies thus occur in volcanic 

 necks, and make a ne\Kform for the science. 



2.09.21. Humboldt-P^cahontas. These mines are near Rosita 

 on fissure veins in andesite, but of a different flow and kind from 

 the walls of the Bassick. They are filled with gray copper and 

 chalcopyrite, in a barite gangue. Other mines of less importance 

 occur in the district, but the three above cited are given prom- 

 inence because of their own intrinsic interest and because they 

 have often been referred to in discussions about the origin of ores. 1 



1 R. N. Clark, " Humboldt-Pocahontas Vein," M. E., VII, 21. " Sil- 



