90 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
the beds of the west side of the Castle Creek fault is nearly parallel with 
the dip of the fault itself, it follows that these slips are nearly parallel 
with the beds which they traverse. Their action is to bring up wedges of 
the different formations against the Castle Creek fault sooner than these 
formations would come up with the normal structure. The dip of the 
faults, however, is not exactly coincident with the dip of the beds; the 
blocks brought up, therefore, are wedge shaped, and in cutting across such a 
series of slips the various formations are encountered in their normal order, 
but usually very much reduced from their normal thickness, and are locally 
sometimes entirely absent. This structure is shown in the Dubuque tunnel 
in Queens Gulch, which cuts across the steeply dipping beds on the west side 
of the fault and finally crosses the fault itself into the granite. . In passing 
through this tunnel, which is over 800 feet long, the first solid rock encoun- 
tered is the gray micaceous limestone, which is recognizable throughout the 
district as lying at the base of the Maroon formation. Beyond this is a 
considerable thickness of black Weber shales; then comes a large mass of 
porphyry, which in turn gives way to another body of shale. Near the 
end of the tunnel, just before reaching the granite, there is a highly altered 
and mineralized zone, which has apparently all the characteristics of 
altered blue limestone of the Leadville formation. This succession is that 
normally found going downward from the base of the Maroon to the top of 
the Leadville formation, although in this place the beds are dipping steeply 
to the east and are therefore locally overturned and in the reverse of their 
usual position. Although the succession is normal, the thickness of the 
various formations is much less in this section than usual, as is shown by 
comparison with the beds on Red Mountain and on the ridge south of 
Queens Gulch. The Weber shale, for example, which lies between the 
porphyry and the Maroon gray limestone, and which normally has a thick- 
ness of not far from 1,000 feet, is here only about 200 feet thick. The 
porphyry, of which there should be normally 300 or 400 feet, is here only 
about 200 feet thick, while the shale underlying the porphyry, which is 
normally 250 feet thick, is here 100 feet or less. The mineralized zone 
which has been taken as altered blue limestone is a narrow strip along the — 
fault, averaging 50 or 60 feet, while the normal thickness of the Leadville 
blue limestone is about 150 feet. The contacts of these various formations, 
as exposed in tunnels, are always greatly brecciated, showing that they are 
