INTRUSIVE ROCKS. 51 
spots. Under the microscope the frequent small feldspar phenocrysts are 
seen to be sometimes kaolinized, but mostly altered to fibrous muscovite. 
The groundmass is holocrystalline, being made up of quartz and musco- 
vite, which is derived from the alteration of the feldspars in the ground- 
mass of the fresh rock. Calcite in small grains is common. Actinolite in 
sheaflike clusters and spherulitic forms is also found, clustered in certain 
areas and intergrown with quartz, which appears secondary. 
Both megasecopically and microscopically this rock is almost exactly 
like the White Porphyry at Leadville. 
Source—Besides the main sheet, which has been described, there were 
found in several places in the vicinity of Aspen cross-cutting dikes, vertical 
or nearly so, which may connect the bedded sheet with some concealed 
body of porphyry below. One of these localities is on Aspen Mountain, at 
the Bonnybel mine; the others are on Smuggler Mountain, at the Bush- 
whacker and Park-Regent mines and in the Smuggler. In both these 
places the dikes cut the Leadville dolomite, and they are undoubtedly con- 
tinuous downward. It is also shown in the Bonnybel mine that a few small 
sheets were sent out from the dike, along the bedding, altering the dolomite 
to marble along their contact. In the dolomite these sheets are small and 
of limited extent, not extending outside of this mine; but on reaching the 
shales above the dolomite the dike merges into the main thick sheet. The 
actual junction of the dike with the sheet is not observable, having been 
removed by faulting. 
Along the fissures represented by these dikes much molten material 
must have ascended. It has been shown by various geologists that the 
ereat sheets, and even the laccolithic bodies, of intrusive rocks, which are 
so common in the Rocky Mountain region, have ascended along narrow 
fissures; and the whole body of porphyry at Aspen may well have come 
up through a few such dikes as have been observed. It therefore seems 
probable that most of the porphyry which is found in the immediate vicinity 
of Aspen ascended along vertical or steeply inclined fissures from some 
point nearly or directly below There was apparently little obstruction 
offered to the upward movement of the intrusive material until the horizon 
of the Weber shales was reached. At this point the resistance offered by 
the overlying rocks was so great that the accumulating material lifted the 
strata bodily, instead of forcing its way through, and spread out as a thick 
