O24 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
that the blue limestone becomes continually thinner. Under the Roaring 
Fork Valley the Silver fault cuts out the blue limestone altogether, as may 
be observed in the Mollie Gibson workings; and from this point north 
through Smuggler Mountain to the edge of the area mapped the Leadville 
dolomite lies on the east side of the fault. 
Uxtending a short distance into Smuggler Mountain, north of the point 
where the blue limestone is cut out, is a narrow band of porphyry lying 
“next the fault on the west side, but this, too, is only a wedge left by the 
encroaching fault, and dies out completely: in Smugeler Mountain, so that 
it is not found at all m the northern part. Throughout the northern part 
of the area shown on the Aspen special map (Atlas Sheet IX), therefore, 
the amount of displacement appears to have been greater than im the south- 
ern, for in the southern part it has removed only a part of the Weber shale, 
which lies underneath the main sheet of porphyry, while in the northern 
part it has removed the whole of this shale, together with the porphyry 
sheet and the blue Leadville limestone, with part of the Leadville dolo- 
mite. Just beyond the northern limits of the Aspen special map the fault 
cuts still farther down, so as to remove the whole of the Leadville dolomite, 
the Parting Quartzite, and a part of the Silurian. 
The Silver fault everywhere shows evidence of great mineralization, 
and along it most of the ore thus far taken out has been discovered. It 
therefore belongs to the premineral set. All the other faults of Aspen 
Mountain, however, displace the Silver fault, when they cut it, in exactly 
the same proportion as they do the rock formations, so that they must have 
been developed at a distinctly later time than the Silver fault. 
RESUME OF STRUCTURE ON ASPEN MOUNTAIN. 
In the eastern part of the area the beds have the nearly uniform westerly 
dip which is persistent throughout a large part of this belt of ore-bearing 
rocks, while in its southwest part the rocks are uplifted toward the south 
so as to form a sort of dome, whose northern termination pitches steeply 
toward Roaring Fork Valley. The north face of this dome, as seen on 
Aspen Mountain, is bent into minor folds parallel with the longest axis 
of uplift, of which the chief is a shallow northerly pitching syncline, which 
occupies the space between Hast Aspen and West Aspen mountains. Hast 
of this syneline the beds flatten somewhat and tend to assume an 
