122 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
striking changes in the rocks lying on either side of it; that along this fault 
there appear and die out gradually, owing to its character as above described, 
comparatively slight discordances, so that at one point the Weber shales 
rest against the blue Leadville limestone and at other points against the 
Leadville dolomite; and near Hunter Creek the shales rest against the 
Parting Quartzite and the Silurian dolomite. This fault has been more 
important than any other one thing in determining the deposition of min- 
erals throughout the whole Aspen district. 
The Silver fault originated earlier than any of the other main structural 
disturbances in the district, with the exception of the folding. It is sup- 
posed that it took place at about the same time with the folding, and that it 
represents the slipping of one formation over the other in their endeavor to 
accommodate themselves to the new conditions. 
East-west faults —T'here are shown on the map two faults having a general 
east-west trend. These evidently belong to the same system, and there 
seems to be more of them in the district just north of the northern end of 
the map, running in the same direction. The chief of these two faults is 
called the Lenado fault. It has, in general, an east-west trend; and its dip 
is always steep, often approaching the vertical. In the Bimetallic tunnel it 
dips steeply north, and since the downthrow is on the north side, the fault 
is normal. This northerly dip causes the outcrop to recede in a southerly- 
direction toward the west, so that it runs from the Aspen Contact mine 
in a southwesterly direction. In the Aspen Contact and the Leadville 
mines, however, the dip is nearly vertical, and even slightly overturned, so 
that it is steeply toward the south; in this case the fault is reversed, and, 
from its course east of the Aspen Contact mine, it may be judged that its 
southerly dip persists to the eastern edge of the area mapped. 
This fault may be traced throughout most of its distance across the 
area. It was first observed in the bend in the road above the Bimetallic 
tunnel, where the Leadville dolomite comes into contact with the hard 
black Weber limestones. This contact is one which normally appears 
throughout this district as the Silver fault, which has led to a great deal of 
confusion among the miners. But in this case the fault cuts across the for- 
mations diagonally, while the Silver fault is always nearly or quite parallel 
to them. The true Silver fault runs into the Lenado fault very near the 
point mentioned, and is cut off, bemg displaced so that its continuation on 
