Mesozoic Rocks of Colorado. — Stevenson. 393 
cretions, which very often are rich in fossils. The extremes of 
thickness observed are 900 and 1,700 feet. 
The Niobrara consists of limestones, gray to blue, more or 
less magnesian, separated by black to grayish blue shales. 
The lithological features are very striking, even more so than 
are those of the overlying Foi't Pierre. These rocks are well 
shown below Canon City, Col., and at many other localities 
along the immediate foot-hills until the Spanish peaks over- 
flow is reached. Thence exposures are few though character- 
istic until the synclinal runs out in the Moreno valley 35 miles 
south from the Colorado line ; but along Cimarron creek, 
around Ocate mesa, the Canadian hills, the plains south from 
the Mora river, and at Galisteo the exposures are excellent. 
The interruptions in continuity are due only to erosion by 
the Canadian, Mora and Pecos rivers and their tributaries. 
The greatest thickness observed is approximately 700 feet. 
The Fort Benton, is made up of dark shales and some 
irregular sandstones ; it is thin, seldom more than 150 feet 
thick, and its material is yielding, so that one rarely finds 
any but very fragmentary exposures. These, such as they are, 
are numerous enough along the foot of the mountains as well 
as near the Canadian hills in New Mexico and near Galisteo. 
The Dakota everywhere underlies the Fort Benton but 
the contact between the two is frequently shown only on the 
waters of Mora river. The Dakota can be traced almost un- 
interruptedly from central Colorado southward to Galisteo 
creek in New Mexico. In the writer's opinion, its variations 
southward are more notable than those of the Fox Hills 
northward. But before any detailed statement can be made 
respecting these changes it is necessary to say something 
about the underlying rocks. 
The Jurassic south from Denver, Colorado, is repre- 
sented by shales with thin limestones ; near Caiion City it is 
composed of shales. The Triassic is represented in the same 
region by variegated sandstones, more or less conglomerate 
in many places, with beds of gypsum, the whole making a 
mass of great thickness. But southward from Canon City, 
these groups decrease in importance, so that before the waters 
of the Purgatory river are reached, they are so thin as not to 
be recognized or they are altogether wanting. They are cer- 
tainly wanting in the Moreno valley of New Mexico, 35 miles 
