152 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
some little distance. <A little farther on, however, on the south side of the 
downfaulted block, the quartzite resumes its original position. Farther up 
the hill this block is characterized by the Weber shale which lies in it, 
between the blue limestone on the north and that on the south. The 
block between the two main faults is not simple, but is cut by very many 
slips, which are in a general way parallel to those that bound the block, 
so that actually the block is a sort of downthrust shear-zone in which the 
rock is cut into thin slices. Besides these parallel faults there are also 
minor slips running in all directions. A peculiar feature in the structure is 
the absence of the blue limestone in the Bonnybel mine, for here porphyry 
and Weber shale directly overlie the Leadville dolomite. That this dolo- 
mite probably belongs to the permanent and original bed which forms the 
lower portion of the Leadville horizon is indicated by its estimated thick-. 
ness, which is that usually found in the Leadville dolomite. There is 
evidently, therefore, a fault which here locally cuts out the blue limestone 
and brings the Weber formation down upon the dolomite below. This is 
the condition of affairs in the northern district, from the bottom of the 
Roaring Fork Valley to Lenado, and it results from the action of the 
Silver fault. Throughout Aspen Mountain and Tourtelotte Park, however, 
there is usually blue limestone between the dolomite and the shale. In 
the outcropping cliff on either side of the downfaulted block in which the 
Bonnybel mine is situated there is continually shown the usual Contact 
fault overlain by the blue limestone, and thus it is in this block alone that 
the limestone is missing. The fault, therefore, can be only a local one. 
Another unusual and especially interesting feature of the mine is the 
occurrence of porphyry in the dolomite in the form of a large crosscutting 
dike and of small interbedded sheets. The dike seems to be nearly 
vertical as exposed at many points in the workings, being present in the 
rock as far down as explorations have gone, or nearly to the horizon of the 
Parting Quartzite. From this dike there run off one or two interbedded 
sheets, perhaps more. ‘These sheets are cut in the main workings, and have 
always altered the dolomite along their contact for several feet to a coarsely 
crystalline, nearly white marble. This is the only place in the district 
where porphyry sheets have been found at a horizon lower than the Weber. 
In the dolomite, however, these sheets are small and thin, and no important 
thickness is found till the horizon of the Weber shales is reached, at which 
