154 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
steepening the beds recover and assume a flatter westerly dip, which sud- 
denly changes to a very gentle dip in the bottom of the syncline. Near 
the point where the dip changes there is developed the Aspen fault, vertical, 
and with only a slight throw, which is down on the eastern side. The 
workings of the Durant mine show this structure well, the whole of it 
having been accurately demonstrated from the surface to the bottom of 
the syncline. Most of the workings lie in a comparatively narrow zone 
between the vertical Aspen fault and the Contact fault, which is here 
turned up with the beds so as also to be vertical and for a long distance 
parallel with the Aspen fault. West of the Aspen fault there is developed 
in the workings another fault belonging to the same system, namely, the 
Schiller. There are many smaller faults parallel to these greater ones, and 
also many complicated cross slips and intermittent fractures. Many of these 
occur between the Aspen and the Contact faults. They have been admira- 
bly and carefully worked out by Mr. Rohlfing, the superintendent of the 
mine, but are not of sufficient importance to be dwelt upon in this general 
summary. The Silver fault is also cut in the southern part of the workings, 
but has nowhere been very greatly explored. 
The ore has been found chiefly in the narrow vertical zone between the 
Aspen and Contact faults. On account of the northerly pitch of the Aspen 
Mountain syneline, the point where the steep eastern limb of the syncline 
flattens out to form the bottom of the fold becomes gradually deeper toward 
the north; and for this reason the narrow vertical zone which is highly 
productive in the Durant mine extends still farther downward in the Aspen, 
where it has been followed nearly 1,500 feet from the surface before the 
bottom of the syncline was reached. In the Durant ore has formed in 
immense bodies, especially along the various faults and fractures. Of 
these there are many, and «those which have a slight throw or no throw 
at all are apparently as important in forming ore as are the greater ones, 
showing that the only effect which a fault has in the formation of ore is to 
furnish a channel through which the ore-bearing solutions may circulate. 
The ore is nearly always found in the dolomite at the intersection of one 
or more of these faults with the Contact fault. In such cases ore may 
occur above the Contact fault in the blue limestone, when it is generally 
surrounded by an envelope of dolomite, the dolomite in turn being sur- 
rounded by limestone. In one case lead ore was found directly in the 
