140 ECONOMIC GEOLOGY OE SILVERTON QUADRANGLE, [bull. 182. 
pay, and it is not always possible to determine the actual downward 
limit of the enrichment. But the numerous assays made of the 
unworkable pyrite in the deepest levels of the Guston, Silver Bell, and 
National Belle mines all show the presence of a fraction of an ounce 
of gold and a few ounces of silver. Lead has not been determined. 
It is probable that in the assays made for practical purposes no 
attempt was made to determine its presence in small amounts. It 
should further be borne in mind that the enrichment in question is 
not of necessity due solely to waters which have descended from the 
upper portion of the selfsame ore deposit. The descriptions of the 
mines indicate that some of the second concentration was effected by 
waters which may have traveled laterally, from some distance, through 
rocks which are themselves mineralized and may contain considerable 
bodies of ore as yet undiscovered. 
The theory of secondaiy enrichment, which is regarded as giving 
the most probable and satisfactory explanation of some of the pecu- 
liar features of the Red Mountain deposits, has, if its application is 
correct, certain very important and practical consequences. It indi- 
cates that the rich ores formerly mined have a working lower limit 
which is probably less than 1,000 feet in depth. While small bodies 
of rich ore may occur deeper than this, it is probably not worth while 
prospecting for them when once the depth has been reached at which 
the ore is practically all low-grade pyrite. On the other hand, as 
some pyrite occurs at all depths, and as the vertical limits of the 
zones of enrichment are necessarily irregular and overlapping, it is 
unwise to suspend operations too quickly, merely because the ore falls 
off locally in value and becomes pyritic. 
Passing from the Red Mountain stock deposits to the lodes occur- 
ring in other portions of the quadrangle, the evidence of secondary 
enrichment is less easily read. It seems veiy probable that the rich 
silver minerals, such as polybasite, proustite, and argentite, are the 
result of secondary enrichment similar to that described by Weed 1 
in the Neihart district in Montana. In the San Juan region, however, 
the actual steps of the alteration of lower-grade ores into those rich 
in silver can not be traced. The evidence for a secondary origin con- 
sists in the fact that these rich minerals occur largely in vugs, and 
that in several cases they pass at moderate depths into low-grade ores. 
These features, however, do not in themselves preclude the idea that 
the minerals are primary, and resulted from original deposition in a 
zone where ascending and descending waters mingled. 
In the Tomboy mine the undoubted association of the richest gold 
ore with shattering and movement in the lode, accompanied by some 
oxidation far below the level of general weathering, is suggestive of 
secondaiy enrichment, but it is hardly safe to assume that this process 
1 Geology of the Little Belt Mountains, Montana: Twentieth Ann. Rept. U. S. Geol. Survey, 
Pt. Ill, 1900, pp. 421-422. 
