INTRUSIVE ROCKS. 53 
granite, quartzites, hard dolomites, and brittle limestones any movement 
would produce a well-marked shearing zone or set of closely grouped 
fractures; in the more plastic shales, however, any slight movement would 
be taken up by the uniform yielding of the entire mass, until the disturb- 
ance was adjusted, and the motion would thus entirely die out near the 
bottom of these plastic beds. Cases of the disappearance of faults in soft 
and shaly beds have been noted by the writer in this district, it having 
sometimes been possible to observe in a single exposure the whole process 
of diminution and disappearance without noticeable deflection. 
AGE OF THE INTRUSIVE ROCKS. 
The relative age of the diorite-porphyry and the quartz-porphyry 
in Aspen can not be stated, for they were not found in any place in 
juxtaposition. The diorite-porphyry sheet ranges in horizon* from the 
top of the granite to the bottom of the Leadville dolomite; the quartz- 
porphyry sheet lies uniformly at the bottom of the Weber, and cross- 
cutting dikes are rare. Both these rocks have participated in all that the 
region has undergone in the way of folding and faulting. These disturb- 
ances began in late Cretaceous time, according to information furnished 
by neighboring districts, and Mr. Emmons’ has assigned to this general 
period both the eruptive activity of the Elk Mountains and that of the 
Mosquito Range. The rocks are therefore in a general way contempo- 
raneous, having been injected toward the beginning of the great mountain- 
making disturbance, and before the folding and faulting which followed. 
The movements in the rocks, however, must have begun very soon after 
these volcanic intrusions. It seems possible that the two processes may 
have begun simultaneously, and that the injection of the molten rock 
occupied only a comparatively brief period of time, while the strains 
folding and faulting. In the Aspen district there is abundant evidence that 
which were generated at the same time found relief very slowly in the 
the movement along most of the fault planes is still actively going on, and 
some important faults have originated in post-Glacial time. 
'Geologic Atlas U.S., folio 9, Anthracite-Crested Butte, Colorado; Geology of Leadville, p. 31. 
