392 
Mesozoic Rocks of Colorado. — Stevenson. 
mountains southward from Denver. The group is shown in 
isolated areas only, having been removed by erosion from the 
greater part of the region. The rocks are gray to bluish-gray 
and yellow sandstones with shales and numerous coal beds, 
which are of great value. The areas best known are the small 
coal field near Canon City, Col., the extensive coal field tribu¬ 
tary to Trinidad, Col., and tying on both sides of the line be¬ 
tween Col. and N. Mex.; and the Galisteo area, very small, at 
say 25 miles south from Santa Fe, N. Mex. Leaf-beds are not 
rare and at many localities they yield abundance of excellent 
specimens. The extreme thickness is not far from 2,000 feet. 
The Fox Hills is composed mostly of sandstones, blue to 
gray to yellowish, with irregular harder layers which resist 
weathering. Coal beds of some importance are found in the 
section. Northward from Denver on the Platte, where the thick¬ 
ness is more than 1,000 feet many of the subordinate beds are 
rich in characteristic marine -fossils, while others are crowded 
Avith the curious nodose fucoid, Halymenites major Lesq. 
Southward, the thickness decreases until at Canon City the 
vertical range of the Halymenites is about 350 feet; there im¬ 
portant coal beds occur in this interval. The thickness di¬ 
minishes until in the Trinidad field the vertical range of 
the Halymenites is but 80 feet. No traces of the fucoid Avere 
observed in the Galisteo area and there the Fox Hills is sup¬ 
posed to be absent. 1 
The Fort Pierre is a great mass of shales forming the slopes 
of many mesas on the plains of the Arkansas Canadian and 
Purgatory rivers and Avell shown for long distances in the im¬ 
mediate foothills as well as around the Laramie plateau of the 
Trinidad coal field. Beyond the Mora river in New Mexico 
they can be followed Avithout difficulty to the Pecos river at 
the south; they are distinct at Galisteo, N. M., Avhere they 
underlie the Laramie and rest on the Niobrara. The shales 
are black, gray and yellow, both sandy and argillaceous. The 
upper portions carry layers of calcareous and ferruginous con- 
1 Dr. C. A. White, in a letter, advises against placing much reliance on 
this Halymenites , as he has found it in the Fort Pierre of Colorado, and 
in the Laramie of the same state, as well as in the marine Eocene near 
Laredo, Texas, where it is associated with Carditaplanicosta, &c. The 
writer, however, uses it only as marking within this region along the 
mountain front, the disappearance of the Fox Hills conditions and not 
for positive identification of horizons. 
