50 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
area. Here, where shafts have gone deep enough to obtain portions of the 
rock removed from immediate surface alteration, the color is light gray, with 
a greenish tinge. Small phenocrysts of quartz and dark mica are rather 
sparingly disseminated, with bunches of pyrite, in a groundmass of fine and 
compact texture. Feldspar phenocrysts are very small and scarcely notice- 
able. Under the microscope the outlines of the quartz phenocrysts are 
somewhat rounded, and there are little bays which are occupied by the 
groundmass, showing corrosion by the magma previous to the consolidation 
of the groundmass. The mica is nearly colorless, with brilliant polariza- 
tion colors; therefore it is probably muscovite, instead of biotite, as it seems 
in the hand specimen. Small, stout crystals of orthoclase are frequent, often 
showing Carlsbad twinning; these are often replaced, sometimes to a large 
extent, by crystalline calcite, which penetrates the feldspar along the edges 
and the cleavage cracks. This calcite is evidently an infiltration; it is 
coarsely erystalline and effervesces freely in the hand specimen. Pyrite 
occurs in small grains; almost invariably it is found embedded in calcite, 
when the latter is present, thus showing its secondary origin. Rarely there 
are small, slender crystals of plagioclase feldspar. The groundmass is 
finely microcrystalline, sometimes showing a tendency to micrographic 
structure. Zircon is found in small grains, as well as apatite; these are 
ordinarily inclusions in the mica. 
In most of the district where active mining is carried on it is not possi- 
ble to find any rock which is nearly so fresh as that described. ‘The same 
tendency to alteration and change which has brought about the deposition 
of the ores is shown in the decomposition of the associated rock. On Aspen 
and Smuggler mountains and in Tourtelotte Park the porphyry is very light 
in color, with a gray or green tinge, which locally becomes brown from 
staining with iron oxide from the surface and along joints. It is porous and 
usually contains an abundance of pyrite. ‘The circumstance that this pyrite 
is more abundant in the altered than in the fresh rock shows that its forma- 
tion was comparatively late and probably a feature of the alteration itself. 
No mica phenocrysts remain in the altered rock, and the small altered 
phenoerysts of orthoclase, and rarely quartz, are barely distinguishable to 
the naked eye. Surface oxidation produces a phase speckled with brown, 
the spots being of iron oxide derived from the alteration of the sulphide; 
microscopic examination usually shows a kernel of residual pyrite in these 
