148 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
close genetic connection with the eruptive rocks of the Mosquito Range, 
on the opposite side of the Sawatch. As already described, there have 
been found at Aspen dikes of porphyry which come directly across the 
sedimentary formations and connect with the overlying sheet. These 
dikes were found in the Bonnybel, Smuggler, and Park Regent mines. In 
the first named of these mines the dike sends out thin and limited sheets 
into the Carboniferous dolomite before reaching the horizon at which 
the main sheet spreads out. It has been concluded, therefore, that the 
porphyry at Aspen came up directly from below and was derived from 
some larger reservoir. Throughout the whole of this western mountain 
region igneous rocks are common as sheets, as lenticular bodies or 
laccoliths, and even as cylindrical pipes or ‘‘plutonic plugs,” as described 
by Russell. The same quartz-porphyry, or a rock very closely related, 
forms laccoliths on the Mosquito Range, and thick sheets, such as are seen 
at Leadville. A possible explanation of this Tourtelotte Park uplift would 
be that igneous rocks, probably derived from the same reservoir as the 
previously intruded porphyry, accumulated in a restricted area; that 
upward propulsion of this rock elevated the overlying rigid formations, 
and that this uplift caused the fracturing and faulting. We may conceive 
that if this upward tendency of the molten rock beneath had been actively 
continued, the rock would have forced its way to the surface, and what is 
now the limited, faulted, uplifted dome would have become the neck of a 
voleano. Actually, however, no rock was erupted, although, as previously 
noted, there is a late eruption of basalt along the (Casille Creek fault a few 
miles farther northwest, near Woody Station. 
Rate of faulting —One of the most striking things in the geology of Aspen — 
is the evidence of great activity in the deformation of the rocks, both in 
the past and in the present. This evidence offers, perhaps, an opportunity 
for estimating the rate of movement along these active faults. Mr. D. W. 
Brunton, in a letter written to the author, makes the following interesting 
observations concerning recent fault movements: 
That the movement along the fault planes is now going on is plainly proved in 
a great many ways. Survey monuments, which have been located by different 
engineers with exactitude, are now in some instances 4 and 5 feet from the position 
they occupied ten years ago. The upper portion of many shafts in the camp has 
been moved entirely across the lower portion, in some instances shutting off com- 
1 Jour. Geol., Vol. IV, pp. 25, 189. 
