78 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
common pyrite. Dr. Stokes has kindly furnished the following note 
in regard to his method of determination : 
Supposed marcasite from the Red Cloud mine, Colorado. — Carefully selected por- 
tions of the mineral, as free as possible from foreign substances, gave the •• oxida- 
tion number*' 65, that of pyrite being 60, and of marcasite 17. There can be no 
question, therefore, that the substance is pyrite, the excess of 5 over the normal 
figure being accounted for by small quantities of As, Pb. and Cu, which were found 
on analysis, the effect of these being to raise the "oxidation number.*' 
The "oxidation number'' may be thus defined: When a sulphide is boiled with 
a, solution containing 1 gram ferric iron per liter, Fe'" is reduced to Fe", and iron, 
if present in the mineral, goes into solution. At the same time more or less S is 
oxidized to S0 3 . In the case of FeS 2 this is expressed by 
G.333 b . , . - , , .,. , (60 for pyrite 
°D=per cent of sulphur oxidized—' 
a ~ ^ * [17 for marcasite 
where a=amount of iron dissolved, 
6=amount of Fe' produced by reduction. 
When other sulphides are present the effect is to contribute to b but not to a, 
thus increasing the apparent percentage of sulphur oxidized, as in the present case. 
Tetrahedrite (gray copper). — Sulphantimonite and sulpharsenite of 
copper. The composition of tetrahedrite proper is 4Cu 2 SSb 2 S 3 , cor- 
responding to 52.1 per cent of copper. For tennantite, the arsenical 
variety, it is 4Cn 2 SAs 2 S 3 , corresponding to 57.5 per cent of copper. 
These two varieties are connected by various intermediate compounds. 
Isometric, tetrahedral; commonly massive. Color, gray. Streak, 
brownish or reddish. Hardness, 3-4.5. Specific gravity, 4.4-5.1. 
This mineral rivals galena in this region in importance and abun- 
dance as an ore constituent. Its high percentage of copper, with the 
fact" that it often carries a large amount of silver replacing part of the 
copper, gives it its value. Most of the varieties seen contain both 
arsenic and antimony, but the latter predominates, especially in the 
highly argentiferous varieties sometimes known as freibergite (the 
typical freibergite from Freiberg contains over 30 per cent of silver), 
which is usually of a lighter gray than ordinary tetrahedrite. Mod- 
erately argentiferous tetrahedrite occurs in the North Star, Belcher, 
Empire, Little Dora, and other lodes on Sultan Mountain, in the 
Aspen mine on Hazelton Mountain, and in the Royal Tiger and other 
mines in Silver Lake Basin, associated with galena, chalcopyrite, 
sphalerite, pyrite, quartz, and barite. An antimonial variety rich in 
silver, probably to be referred to freibergite, constituted the principal 
ore of the North Star (King Solomon) mine. Tetrahedrite has been 
found in the Pride of the West, Philadelphia, and Highland Mary 
mines, in Cunningham Gulch, and usually constituted the richest ore. 
It occurs in most of the lodes of Picayune and California gulches, in 
the Tom Moore lode, in the Mickey Breen, Poughkeepsie, Bonanza, 
and other mines in Poughkeepsie Gulch, and in many other mines 
and prospects in various parts of the quadrangle. In some, such as 
the North Star (King Solomon), Tom Moore, and Mickey Breen, it is 
