102 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
of the butte. This cliff is made up entirely of Silurian dolomite. Above 
it come the sandy and shaly beds of the Parting Quartzite, which have 
been eroded back so as to forma terrace. Next above is the Leadville 
dolomite, reaching nearly to the top of the butte, which is of the blue 
Leadville limestone. As may be seen in the plate, the Carboniferous 
dolomite shows some evidence of subaerial erosion, presenting a steeply 
sloping hill which carries some vegetation. The upper rocks are soft and 
crumbling, and are often weathered into tall pinnacles. This castellated 
structure is better shown in Pl. VI, which is a view of the butte taken from 
a point on the scarp a little east of the main development of the cliff. In 
the foreground the scarp is visible, although somewhat obscured by talus, 
while in the center of the picture are some of the bold pinnacles and cliffs 
which have been described. These structures are, of course, due to post- 
Glacial subaerial erosion. It must be remarked, however, that the effects 
of such subaerial erosion are much better marked at the top of the cliff 
than lower down, so that there is a progressive tendency to freshness in the 
outcropping rocks, and a diminution of the effects of erosion toward the 
bottom of the cliff. The Parting Quartzite, occurring a little more than 
halfway down, has been eroded back a few feet, forming a terrace on the 
top of the Silurian dolomite, but the dolomite itself shows no extensive 
weathering. The cliff which it forms is strictly perpendicular, having no 
tendency to crumble or to form a sloping surface. The rock is hard and 
fresh, well polished along its whole face, and heavily and unitormly 
striated. From the base of the cliff these striz can be distinctly seen to 
extend as high as 75 or 100 feet from the bottom; higher than that they 
appear to be obliterated. They are plainest and freshest at the base, and 
grow dimmer progressively toward the top. he line of this cliff is the 
outcropping of the Butte fault, as is shown by the relations of the strata 
on both sides. Below the cliff the Parting Quartzite outcrops a con- 
siderable distance farther down the hill, showing a downthrow to the south 
of about 400 feet, which, it may be remarked, is the total height of the 
escarpment. 
The strize on the polished surface of the dolomite cliff dip 70 degrees 
to the west on the vertical face of the cliff and indicate by their form an 
unmistakable downward throw on the south side. It seems clear that this 
polished surface is the actual plane of movement of the Butte fault, and 
