232 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
now revealed by erosion. It seems very probable, therefore, that this chief 
ore-bearing district has a greater vertical than lateral extent. 
AGENTS OF MINERALIZATION. 
From an economic point of view the deposition of metallic sulphides is 
the most important change which has come about in the rocks, but from a 
strictly scientific standpoint the formation of sulphides is only a minor 
phase in the general alteration which produced chiefly dolomite, quartz. 
and barite. Sulphides of the metals, where present, are always closely 
associated with these gangue materials, are generally inclosed by them, 
and occur along the same zones, these zones following faults and fractures 
which have evidently been watercourses. Microscopic study has shown 
that in places some of the sulphide was probably contemporaneous in for- 
mation with the magnesium and iron carbonate which replaced the lime- 
stone, forming ferriferous dolomite. The dolomite, quartz, and iron have 
been shown to be derived probably from heated and ascending waters, and 
this origin, therefore, is the one which, is most applicable to the associated 
sulphides. 
INFLUENCE OF DIFFERENT ROCKS ON ORE DEPOSITION. 
The chief ore deposits occur in limestone or dolomite, in the majority 
of cases close to the Silver and Contact faults. There is some deposition 
of the precious metals in the lower formations, but not to any great extent. 
In Queens Gulch some ore has been shipped from the Silurian dolomite, 
and in the Princess Louise shaft in Spar Gulch a small amount was taken 
from the Cambrian quartzite. Near the Castle Creek fault in Queens Gulch 
even the granite shows evidence of mineralization, being altered and partly 
filled with gangue materials, such as quartz and ferriferous dolomite. Above 
the Leadville limestone some mineralization has been noticed in the red 
Maroon beds on Red Mountain, these beds being impregnated to a small 
extent with copper and silver, but not sufficiently to form ore bodies. 
Within the chiefly mimeralized zone ore has been found extending through 
the whole thickness of the blue limestone, as in the Aspen, Durant, Camp 
Bird, Justice, and other mines, and in places nearly the whole thickness of 
Leadville dolomite is mineralized, as is the case in the Bonnybel mine 
