FERRATION. DADS} 
Wherever there has been any rearrangement of the materials in the 
rock by circulating waters, with or without deposition of precious metals, 
a varying amount of iron has almost invariably been deposited. The 
Maroon calcareous sandstone in the Yopsie tunnel, in Queens Gulch, 
which has been cited as showing silicification, shows also an accompanying 
ferration. The granite which les next the Castle Creek fault below the 
quartzite in Queens Gulch shows, among the new minerals introduced by 
waters which have circulated along this fault zone, veins and idiomorphic 
crystals of a carbonate which turns brown on oxidation and is probably 
ferriferous dolomite. In localities where precious metals have been 
deposited iron is always present as a metallic gangue. Often this iron 
is in the form of sulphide, but ferriferous dolomite or ankerite is also very 
common. This latter mineral is generally in the form of idiomorphic 
crystals, and occurs closely associated with barite and other gangue 
minerals, and from this association it appears that they were all formed 
at one time. 
It appears from these facts that carbonate of iron crystallized at the 
same time and under the same conditions as quartz and dolomite, as well 
as certain other rarer minerals. On the evidence afforded by small areas 
of ferriferous dolomite containing pyrite crystals surrounded by unaltered 
limestone free from sulphide, it appears that iron carbonate and iron 
sulphide were probably at times deposited simultaneously. The alteration 
observed in the blue limestone at Glenwood Springs, which resulted from 
the action of ascending hot springs, and which has been discussed in 
connection with the question of dolomization, was characterized also by 
deposition of iron. In the unaltered rock iron is practically absent, while, 
as the process of alteration goes on, the amount of iron becomes very 
noticeable. This iron is doubtless deposited as carbonate, but, on oxida- 
tion, changes to ferric oxide, and so stains the rock brown. The fractures 
along which hot waters have formerly circulated at Glenwood Springs, but 
which are now dry, are prominent on account of this bright-colored oxide. 
It is possible, also, that some of the iron may at this place be deposited as 
sulphide, but this could not be determined accurately by analysis, on 
account of the sulphur which exists in the rock as sulphate, and no 
microscopic study was made. 
