204 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
intervening valley. The Castle Creek fault crosses the gulch near the 
point where it appears to branch, not far from the center of the plate, and 
here is situated the Dubuque tunnel. The ore in this district is found along 
the Castle Creek and its dependent faults. 
Dubuque tunne—The Dubuque tunnel (see Pl. XLIII, (@) starts in the 
yellow and brown sandstones of the Maroon formation on the west side of 
the Castle Creek fault, and runs east through the gray siliceous limestone 
which forms the base of the Maroon. At some distance in, it enters Weber 
shales, much broken up, which soon change to a veritable breccia, in which 
shale and porphyry are commingled. On the east side of this comes solid 
porphyry, which is considerably softened and decomposed, and beyond 
this again is shale and porphyry-breccia nearly to the Castle Creek fault. 
On the west side of the fault the rock is hard black chert, evidently a 
silicified limestone. On passing through this granite is reached. The dip 
of the Maroon beds near the mouth of the tunnel is about 60 degrees 
to the northeast. The sequence of beds shown from west to east is the 
normal succession, consisting, after the sandstone and gray limestone at 
the base of the Maroon, of shale, then porphyry, then shale again, and 
finally the silicified limestone next the fault, which may be the blue Lead- 
ville limestone. The thicknesses of these formations, however, are much 
less than normal, and the nature of the contacts show that they are fault 
‘contacts, so that there is probably some displacement. These faults are 
nearly vertical, being mainly parallel with the Castle Creek fault, and so 
do not diverge much from the bedding, which is also very steep. On both 
sides of the porphyry, to the east and to the west, are especially well- 
marked zones of crushing. That to the east has been called the Dubuque 
fault, while that to the west, which is also found in the Little Annie mine, 
is called the Annie fault. There are, however, many smaller slips parallel 
to these. 
The chief ore in the Dubuque tunnel is the zone of silicified limestone 
which lies next the granite. This silicified rock contains pockets of 
copper pyrite and galena, which do not appear to have great continuity. 
There is often a good deal of barite. At the contact of Weber and Maroon 
there is another mineralized zone which dips steeply to the east with the 
formations, but it does not appear to be very rich. There is a similar zone 
farther west, in the Maroon limestones. 
