4() GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
occasional interstratified limy beds. The formation is broken up by pro- 
fuse, irregular jointing in many places, so that the outcrop is very friable. 
In these places the red rock becomes mottled with gray, although there is 
no apparent difference between the gray and the red portions except the 
color. Often these gray spots mottle the rock thickly; often they are 
arranged in bands parallel to the stratification; and often they unite so as 
to form a continuous seam. At one locality there was observed a system 
of fractures at right angles to the stratification, and the gray bands follow 
these fractures to the exclusion of other parts of the rock; the gray color 
penetrates for varying but always slight distances from the crevice. By a 
combination of these phenomena an irregular patch of gray 4 or 5 feet in 
diameter is formed in places in the red rock. These occurrences show that 
the gray is derived from the red by the removal of iron; the probable agent 
is carbonated surface waters, which penetrate along fractures and porous 
areas, dissolve the iron, and carry it away. 
Under the microscope this red shaly sandstone is seen to be very fine 
erained and to be made up mostly of very small grains of detrital quartz, 
with altered ferruginous materials, apparently detrital, which can not be 
exactly determined. These minerals are inclosed in a plentiful cement of 
calcite, which is in places finely granular and in other places without recog- 
nizable crystallization. There are also flakes of gypsum. The red rock is 
colored by earthy iron oxide disseminated throughout the rock; and the 
gray part differs from the red only in the absence of most of this oxide, so 
that only slight yellow stains are left. The transition from red to gray is 
gradual, as seen under the microscope, but takes place within a short 
distance. 
This formation, consisting of the basal sandstone and the overlying 
shales or shaly sandstones described, and having an aggregate thickness of 
approximately 400 feet, is the stratigraphical and lithological equivalent 
of the Gunnison formation of Eldridge,’ as described in the Crested Butte 
area. Mr. Eldridge assigns this formation to late Juratrias age, basing his 
correlation on its stratigraphical and lithological equivalence to the Atlanto- 
saurus beds of the eastern side of the Rockies and the similarity of the 
molluscan fauna found in the two localities. The fossils described by Mr. 
Eldridge in the Crested Butte district were fresh-water forms, showing that 
1Geologic Atlas U. 8., folio 9, Anthracite-Crested Butte, Colorado, 1894. 
