90 SILVER-LEAD DEPOSITS OF EUREKA, NEVADA. 



the period of the rhyolite eruptions near Eureka may have covered cen- 

 turies. The character of the decomposition of the rhyolite is familiar, and 

 consists largely in the extraction of the heavy bases and alkalies, leaving 

 siliceous clay as a residue. ' Sulphureted hydrogen is almost invariably an 

 accompaniment of volcanic action, and the alkalies in solution were in part 

 converted thereby into sulphides. As most of the sulphides of the metals 

 are soluble in solutions of the alkaline sulphides, a vehicle was thus formed 

 for the transportation of any of those metallic sulphides which might be 

 present. Those sulphides, or compounds which would yield them, might 

 have formed constituents of the rhyolite. 



Rhyolite as a source of the ore. — As a matter of fact there is no rhyolite in the 

 immediate vicinity of the ore which contains sufficient gold, silver, or lead 

 to admit of its being regarded as the source of these minerals in the ore. 

 It is barely possible, though not likely, that the rhyolite body of which the 

 dikes in the mines are the upward continuation, may have carried a notably 

 larger percentage of heavy metals than that now to be found in a fresh 

 state on the surface. In fact, near the mines, it is almost completely decom- 

 posed, and it cannot be obtained in a tolerably fresh state above ground at 

 considerable distances from the workings. 



Quartz-porphyry as a source of the ore. The l'hyolite is UOt the Only ei'Uptive l'Ock 



met with in the mines. Quartz-porphyry also occurs, but only in the neigh- 

 borhood of Adams Hill. This rock, however, contains considerable quantities, 

 relatively speaking, of gold and silver, particularly of the former. The 

 explorations in the mines in which it is found are not sufficient to give any 

 definite idea of its extent, but it is possible that it is much more extensive 

 than its croppings suggest. The result of the assays made of this por- 

 phyry, which are described in the chapter on assays, indicates that this 

 rock contained silver and gold, and perhaps lead, after it solidified and before 

 any solfataric action could have affected it. Though the age of the quartz- 

 porphyry cannot be proved from this district, there can be no doubt, from 

 its lithological character and its mode of occurrence in innumerable other 

 localities, that it is pre-Tertiary and far older than the rhyolite. That the 

 solfataric action incident to this eruption had an effect upon this porphyry 

 is extremely probable; at any rate, changes of a solfataric character were 



