ORE DEPOSITION. 235 
The most common method, however, by which sulphides are formed in 
nature and in the arts is by the addition of sulphureted hydrogen to sol- 
uble salts of the metals. If we suppose that the ascending solutions con- 
tained the metals in the form of these soluble salts, and that on reaching 
points where the channel along which they had risen was intersected by 
some different channel these solutions were mingled with others carrying 
an excess of sulphureted hydrogen, the ores would be precipitated just 
as we find them. These sulphureted waters, since they flowed along 
flatter faults, may have been derived more nearly from the surface. The 
proximity of most of the ore deposits to the Weber shales has already been 
mentioned. ‘These shales contain much organic matter, the decomposition 
of which might readily have produced sulphureted hydrogen, which would 
be taken up by waters flowing through them. The occurrence of pyrite in 
shales is quite general, and often apparently results from the precipitation of 
iron brought in in the form of soluble salts by sulphur given off from decay- 
ing organic matter. In the Mollie Gibson mine water flowing from the 
shales contains much sulphureted hydrogen, and deposits native sulphur. 
This, however, may be in part derived from the oxidation of pyrite and 
other already existing sulphides. 
ORIGINAL SOURCE OF METALS. 
The beginning of the series of changes, of which the deposition of 
precious metals was one, consisted in the uplifting of the rocks over a 
limited area. This was accompanied by faulting, and along the channels 
afforded by these faults there arose mineral-bearing solutions which depos- 
ited their burden under the conditions which have been described. The 
cause of the peculiar, domelike uplift has been suggested to be possibly a 
body of eruptive rock which for some reason accumulated immediately 
below this area. The connection of eruptive rocks with ore deposits, not 
only in Colorado but throughout the world, is well known, and does not 
need any comment. In accordance with the generally accepted views of 
hot-spring action, we may suppose that surface waters, on sinking, came in 
contact with a body of heated rock which underlay the uplifted area, and 
that in this way the water itself became heated and dissolved some of the 
rock materials. Eruptive rocks usually contain barium, although this 
