42 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
Maroon Creek with Roaring Fork, and another half a mile or so up the 
creek from the butte. This limestone is from 50 to 75 feet thick, and is 
overlain by thin-bedded shaly limestones, which pass upward into the soft 
eray or black shales of the Montana formation. On Red Butte the dense 
limestone itself is somewhat fissile, which doubtless arises from the squeez- 
ing to which it has been subjected in the formation of the overturned told 
that is exhibited here; but in the outcrop farther up Maroon Creek it is 
more massive. This limestone is a close lithological as well as stratigraph- 
ical equivalent of the Niobrara limestone of the Crested Butte area, and 
its correlation is based chiefly on this equivalency. In the Crested Butte 
area Mr. Eldridge found fossils which indicated its Niobrara age, and the 
occurrence at this horizon of a limestone similar to the one described is 
widespread in this region. 
The average thickness of the Niobrara in the Aspen district may be taken 
at about 100 feet. The line of division between the upper shaly beds of 
the Niobrara and the overlying Montana shales is not distinct. 
MONTANA FORMATION. 
Above the Niobrara comes a very great thickness of gray or black shales, 
generally carrying thin bands or lenticular masses of impure black lime- 
stone. Some of the limestone beds and lenticular bodies are partly silici- 
fied. The main outcrops extend down Roaring Fork from Red Butte to the 
border of the area mapped, a practically continuous exposure being afforded 
along the banks of the stream and in the cut which has been made for the 
railroad, and also up Maroon Creek from Red Butte for half a mile, passing 
across the overturned syncline to the underlying Niobrara limestone. The 
upper part of the formation, which occupies the greater part of the space 
between the Roaring Fork below Red Butte and Woody Creek, is mostly 
concealed by glacial detritus, so that no close examination could be made. 
From the nature of the drift, however, and from occasional doubtful out- 
crops, it seems probable that the upper part of the series becomes slightly 
more arenaceous. The two subdivisions which the term Montana covers— 
namely, the Fort Pierre and the Fox Hills—ean not, however, be well dis- 
tinguished in this area In thin limestone layers ranging from below the 
middle to near the top of the formation were found great numbers of fossils, 
identified by Mr. T. W. Stanton as Inoceramus barabim. 
