Notes on a Great Silver Camp. 405 
ings down below this horizon, or in the quartzites, ore is 
being now mined along faults where it was first deposited. 
As to the vital questions where and how does the min- 
eral occur, in the early days of this district the ore was 
mined in the ‘‘ Blue” limestone just below and in contact 
with the overlying ‘“‘ white porphyry,” and these ores for 
several years comprised mostly carbonates and oxides 
formed by the decomposition, by circulating surface waters 
carrying carbonic acid and other solvents, of the original 
sulphides, which in due course, since greater depths have 
been reached and mining been carried on below the natural 
drainage-level, or below the influence of these waters, are 
found as first formed and are the chief source of supply 
at the present; for a positively authentic case of the ori- 
ginal deposition of lead, silver or zine as oxides or carbon- 
ates, and not as sulphides, is yet to be recorded. Emmons, 
in the early days of this camp, when but comparatively 
little work underground had been done, made a careful and 
successful examination here for the United States Geological 
Survey, and from the abstracts of his very valuable reports, 
published in 1882,! we glean some of his convictions as to 
the genesis of these ore bodies. He concludes that the ore 
was deposited when these formations lay at a depth of 
10,000 feet from the surface, and he believes “‘ in the occur- 
- rence, on an enormous scale, of intrusive bodies of eruptive 
rock of Secondary or Mesozoic age, which are so regularly 
interstratified as to form an integral part of the sediment- 
ary series; and yet which never reach the surface, but 
were spread out and consolidated before the great dynamic 
movement or mountain-building period at the close of the 
Cretaceous. 
“The original ore deposition took place after the intru- 
_ sion of the eruptive rocks, and before the folding and fault- 
ing occasioned by the great dynamic movement. 
“That the minerals contained in the principal ore deposits 
? Annual Report of the Secretary of the Interior, vol. iii, 1882, 
p- 208. 
