CRETACEOUS SERIES. : 41 
the beds were deposited in fresh-water lakes; and this conclusion probably 
holds good of the formation in the Aspen district. 
CRETACEOUS SERIES. 
DAKOTA FORMATION. 
Lying above the Gunnison formation, as exposed on Red Butte and on 
Maroon Creek, is a massive white sandstone which has been recognized in 
this same stratigraphical position in many parts of the Rocky Mountains, 
and which has been found from its fossil remains to belong to the Creta- 
ceous. To this has been given the name Dakota formation. The sandstone 
varies in color from white to grayish and pinkish; often it becomes fine 
grained, and in bands is gritty and conglomeratic, and not only quartz, but 
feldspar and other granitic detritus can be observed in the coarser parts. 
In the upper part of the formation the rock is finer grained and contains 
abundant plant remains, which, however, on account of the porous nature 
of the rock, are not well preserved. Locally the rock becomes a quartzite, 
the secondary cementing silica being often distributed in irregular bands 
and lenticular areas in the sandstone; frequently it is found only in 
irrecular bunches, so that there are nodules of quartzite in the sandstone. 
These nodules become conspicuous on weathering 
2, since they resist erosion 
better than the sandstone. 
The average thickness of this sandstone, taken from various measure- 
ments, is about 250 feet. 
COLORADO FORMATION. 
The two divisions of the Colorado formation—the Fort Benton shales 
and the Niobrara limestone 
are both recognizable in the Aspen district. 
Benton shales —A hove the Dakota sandstone comes an estimated thickness 
of 350 feet of black calcareous shales, with some thin-bedded and shaly 
limestones. These shales are best exposed on the west side of Red Butte, 
where they are inverted. From some thin-bedded limestones in the upper 
part of the formation fossils were collected which were identified by Mr. 
T. W. Stanton as Gryphea newberryi and Ostrea lugubris. 
Niobrara limestone — A hove the Benton shales comes a bed of dense gray or 
blue limestone with a close texture and conchoidal fracture, which is per- 
sistent throughout the district. This formation is well exposed in the bed 
of Maroon Creek at two points, one at Red Butte, near the junction of 
