52 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
sheet, becoming thinner in regions remote from the vent or vents. The 
corresponding White Porphyry of Leadville is also very largely developed 
as sheets at this same horizon the main sheet in Leadville overlying the 
Leadville limestone, as in Aspen. This general rock type is characteristic 
of the Mosquito Range, and is foreign to the Elk Mountain type, of which 
the diorite-porphyry is a representative. It is quite probable, therefore, 
that the White Porphyry of Leadville, and perhaps the other porphyries, 
ascended from the same source as the porphyry of Aspen; and when the 
intervening Sawatch Range is carefully mapped it is likely that dikes of 
porphyry will be found between the two districts. The intrusive rock, as 
already noted, would be represented in the granite or other very rigid rock 
only in the form of narrow dikes, although the amount of material forced 
up may have been as great as in the sedimentary rocks. These latter, on 
account of their plasticity, offer resistance to upward movement, and invite 
lateral movement along the channels naturally afforded by their stratifica- 
tion planes. Thus the intrusive rock accumulated in subterranean reser- 
voirs, which deformation and erosion have now brought to the surface; but 
where the rocks have been planed down to the underlying granite only 
narrow and oceasional dikes appear. The situation of Aspen is, therefore, 
intermediate between two great axes of eruptive activity, each of which 
has certain distinguishing features, one lying in the Elk Mountains and 
the other in the Mosquito Range or the Arkansas Valley; and the district 
evidently is, in spite of its geographical position, most closely associated 
with the latter axis. 
Mr. Cross! has come to the conclusion that in the case of the huge 
laccolithic: bodies of intrusive rock which are common in the Rocky Moun- 
tains, the narrow fissures along which the molten rock ascended, now 
occupied by dikes of the same material, must have ceased to exist as 
channels at the horizon of the laccolith, and suggests that the formation of 
such nonpersistent channels was due to some gradually exerted force, and 
not to any sudden violent rupture, such as might arise from earthquake 
action. A gradually exerted force would more easily be deflected by 
various causes, and the fissure might easily pass into some marked bedding 
plane. In the case at Aspen, however, any slight vertical fissure, however 
produced, would tend to disappear on reaching the shales. Through the 
1Pourteenth Ann. Rept. U. S. Geol. Survey, Part IT, 1894, p. 240. 
