46 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
feet at the northern limit of the Tourtelotte Park special area, and this is 
reduced to 15 or 20 feet at the northern end of Aspen Mountain. This 
thinning ends in the ultimate disappearance of the rock, for it is not found 
or. the other side of the valley, north of Aspen. 
There is also a noticeable thinning of the sheet from the east toward 
the west. While the outcrop of the bed seems to be practically continuous 
on the east side of the mountain spur which lies between Roarmg Fork 
and Castle Creek, so far as can be determined in view of the subsequent 
complicating faults, there are on the west side places where it not only 
thins out but entirely disappears for a short distance, only to reappear farther 
along on the same horizon. It has thus on this western slope the character 
of an intermittent sheet. 
Description —Very little of the internal structure of the rock can be made 
out with the naked eye. It is always fine grained, dark green in color, 
and uniform in appearance. Occasionally phenocrysts of darker green 
than the groundmass, and having the habit of hornblende, are present. 
There are also occasional feldspar phenocrysts, small and profoundly 
altered around the edges, so as to present an irregular shape; and crystals 
of pyrite are sometimes present. In texture the rock is granular and 
slightly porous. 
Microscopic structure—Under the microscope it is seen that the rock has 
always a porphyritic structure, although the phenocrysts are of small size, 
and the structure might easily change to granular. The phenocrysts are 
always much altered, often so much so that none of the original mineral 
remains. The most common form is a collection of alteration products 
which form pseudomorphs after hornblende. These pseudomorphs are 
primarily of chlorite; further alteration has brought about the formation 
of secondary quartz, epidote, carbonates, and limonite. No unaltered 
hornblende was found in any of the sections examined. Biotite is also 
common among the phenocrysts, often completely altered like the horn- 
blende, chiefly to chlorite, but often having residual areas of green or 
brown mica, which frays out along the edges to chlorite. Feldspar erys- 
tals are common, usually more or less completely altered to muscovite and 
calcite; these seem to be mainly orthoclase, although some show the 
moultiple twinning of plagioclase. Ina peculiar phase of the rock found 
in contact with the granite on Aspen Mountain there occurs a feldspar 
