190 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
intervening blue limestone. There are occasional blocks of porphyry in 
the breccia along this fault throughout the workings, and in the southern 
part of the mine a continuous band comes in at or near the fault. There 
are also bowlders of blue limestone in the breccia in the southern part of 
the workings, showing that the drag has been from the south. 
Faults—The Della S. fault has an east-west trend, and dips to the south 
about 30 degrees. It has an offset to the west on the north or lower side of 
about 200 feet. According to Mr. D. W. Brunton, striation on the fault 
plane shows that there has also been a southerly movement, so that the top 
block has moved southeast over the bottom. This is the only large fault 
in the Della S. mine, but there are many small ones, which are generally 
parallel to it and belong to the same system. None of them, however, 
appear to have over 50 feet displacement. 
Occurrence of ore—he ore occurs in the neighborhood of the Silver fault 
in certain localities. Throughout the mine there are recognized two streaks 
or veins—the foot-wall streak and the hanging-wall streak. The former is 
in dolomite on both sides, and is generally 5 or 10 feet from the contact 
with the shale. In some of the larger stopes below the tunnel, where the 
ore occurs in solid dolomite, the exact locality of the Silver fault is not 
known. The hanging-wall streak is in the Silver fault itself, or even in 
the shale above. At one place in the fourth level ore has been taken out 
40 feet from the fault plane m the solid shale. 
In the bottom of the Della 8. incline were observed two bands of 
barite, which are conformable to the Silver fault. Hach of these bands 
was about a foot thick and constituted rich ore. Between them was 5 or 6 
feet of dolomite, which was mineralized so as to be profitably mined. The 
Silver fault was not seen here, for upon the highest barite vein there still 
lay barren dolomite. These veins, which constitute the so-called foot-wall 
streaks, as well as those forming the hanging-wall streaks, are evidently 
formed on slips auxiliary to the main Silver fault, on both sides of which 
they-lie. In the dolomite these slips are especially well marked and seem 
to be indefinite in number. They are nonpersistent and are generally 
more or less mineralized. The slip nearest the main fault is nearly always 
the richest, beg generally richer than the ore on the fault plane itself. 
It is not at all points, however, that the Silver fault with its auxiliary 
slips in the dolomite and the shale has been made the locus of very 
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