38 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
favorable to the preservation of animal remains; and as the same conditions 
prevailed throughout a great part of the Rocky Mountain region, the entire 
fossil evidence at this horizon is extremely scanty. It has been hitherto 
believed, however, that these beds belong to the Triassic. The Triassic 
was recognized in the vicinity of Aspen by the geologists of the Hayden 
Survey, although the sandstones on Woody Creek, mentioned above, were 
included in their maps with the Carboniferous. In the Crested Butte dis- 
trict the Triassic appears to be wanting, though it may be so altered as to 
be unrecognizable. 
Microscopically these rocks are essentially fine-gramed, impure sand- 
stones with much ferruginous material. The thickness of the Maroon and 
the Triassic together can not easily be ascertained, on account of the dis- 
turbances which have taken place, and especially on account of the Castle 
Creek fault, which prevents the obtainment of any continuous section. It 
is therefore difficult to say how much the thickness varies in different parts 
of the limited area mapped; but from various sections a mean thickness 
for both formations of about 6,600 feet was obtained, of which the Maroon 
beds take up 4,000 and the Triassic 2,600 feet. These are the thicknesses 
which have been shown on all the sections. 
Conditions of deposition of the Maroon and Triassic beds —The lithological characters of 
the entire Maroon series show that it is derived almost wholly from the 
disintegration and rapid erosion of a granitic land mass. The rocks are 
made up in large part of this granitic material—quartz, feldspar, and mica 
chiefly, with some material derived from the limestones and dolomites 
which had previously been deposited. Mingled with this detrital material 
is some that is probably organic; of this class is some of the cryptocrys- 
talline calcite. A mineral which is also of organic origin, and which is 
significant of the conditions of deposition, is glauconite. This mineral is 
found rather in the finer-grained and more calcareous beds than in the 
purely granitic strata, showing that its formation was in water slightly 
deeper than that in which the rest of the beds were deposited. The zone 
at which this mineral is ususally formed is that of the outer edge of the 
land-derived sediments. On the other hand, some of the grits are of nearly 
pure granitic material, very little worked over by water action; these were 
evidently deposited close to the shore. 
In the Crested Butte area the corresponding Maroon beds are typically 
