224 GEOLOGY OF THE COMSTOCK LODE. 



slates gave eighteen cents silver and twenty cents gold. The diorite from 

 Bullion Ravine also showed an indeterminably small trace of gold, while 

 the andesites carry about as much as the diabase. 



Silver traced to the augite. — It scemed probable from Professor Sandberger's 

 investigations that the augite of the diabase was the seat of its metallic con- 

 tents. To test this point, the feldspar and augite were separated by Thoulet's 

 method and separately assayed. It appeared that, for equal weights, the 

 augite was eight times as rich as the feldspathic material, and, as a per- 

 fectly clean separation by Thoulet's method is impracticable, this seems 

 substantially equivalent to a proof that the silver is a constituent of the 

 augite. 



Results of the assays. — By comparisou of the different assays it appears that 

 decomposed diabase carries somewhat less than half as much silver as the 

 fresh rock. Where the decomposed rocks are pyritous, the experiments 

 made do not indicate any essential diminution of the gold contents. This 

 fact, however, is quite possibly due to irregularity in distribution and the 

 minuteness of the quantities of gold to be determined. As the decomposi- 

 tion of the rock in question has proceeded at a great depth beneath the sur- 

 face, it is highly unlikely that silver .should have been extracted unaccom- 

 panied by gold. Much of the decomposed rock, too, is nearly free from 

 pyrite, and had the gold contents of such specimens been determined a 

 smaller percentage would probably have been found. The omission was 

 not detected until too late to resume the investigation. So far as quantita- 

 tive relations are concerned, only the silver can be relied on, though the 

 qualitative detection of gold as well is both interesting and important. 



Comparison with the yield of the Lode. If, then, the COM'-TOCK LoDE is SUppOSed 



to liave derived its precious metals from the diabase, we should expect to 

 find that it yielded dore silver containing a small quantity of gold. The 

 gold contents has actually been very variable, in some few cases exceed- 

 ing the value of the silver and in other instances amounting to only a fourth 

 of its value. The J.,ode has been pretty thoroughly explored to a depth of 

 2.500 feet, and the extent of diabase exposed may be put roughly at a 

 length of 8,000 feet and a thickness of 2,.'')00 feet. If about 13 cents per 

 ton, or, say, 1 cent per cubic foot, has been extracted from this mass, the 



