124 ON THE GEOLOGY AND NATURAL HISTORY 
the Missouri we see occasional indications of their existence, but not so conspicuous as to 
be readily identified. The age of this group of rocks may be now considered as thoroughly 
established, so great a number of fossils which appear to be of undoubted Jurassic types 
have been obtained. 
I have remarked that the older fossiliferous beds doubtless pass beneath the more recent 
Cretaceous and Tertiary deposits, and occupy a greater or less area underneath the prairie 
country east of the “divide” of the Rocky mountains. I have made this inference from 
the fact that where any elevations occur the complete series of fossiliferous beds are ex- 
posed around the axis of upheaval. For the benefit of those geologists who may be con- 
structing geological maps of this portion of the West, I would say, that I have never seen 
any of the older fossiliferous rocks, from the Potsdam to the Jurassic inclusive, exposed, 
except in narrow outcropping belts around the margins of the mountain elevations. The 
Carboniferous rocks occupy a belt from one to two miles wide, and the red arenaceous de- 
posits are exposed over about the same area, while the Jurassic form a zone never more 
than one-fourth of a mile to three miles in width. 
The following additional facts in regard to the Cretaceous rocks of the Upper Missouri 
were obtained during the last expedition to that region. I have preferred to allude briefly 
to them here, inasmuch as they will appear in detail in a forthcoming report. 
The various subdivisions of the Cretaceous group in the West were observed at numerous 
localities. The strata in many places occupy large geographical areas, holding a horizontal 
position, in others forming a belt or zone of greater or less width around the mountain 
elevations. No. 1 is a well-marked and distinct division along the Missouri river from 
De Soto to a point above the mouth of the Big Sioux river in the eastern portions of Kan- 
sas and Nebraska and in the South and Southwest. But when we come into the vicinity of 
the mountain ranges in the Northwest its typical lithological characters are wanting, and 
apparently an increased development of No. 2 only is seen.* Along the Big Horm moun- 
tains, No. 2 is eight hundred to one thousand feet in thickness, composed of black plastic 
clay, with several layers of gray and yellowish calcareous sandstones, ten to fifty feet in 
thickness. Along the Laramie and Wind River mountains the same characters are shown. 
After leaving the Missouri near the mouth of the Niobrara river, No. 3 is never seen pre- 
senting its typical marly character. In the vicinity of the Black hills we saw a series of beds 
composed of alternate thin layers of arenaceous and argillaceous sediments, with Ostrea 
congesta and Inoceramus problematicus, which may possibly represent No. 3. Along the 
Big Horn mountains and from Red buttes to Cache la Poudre creek the same fossils were 
often found, and some other indications of its existence, but no well-marked typical beds 
* We mean that the distinctive lithological characters of No. 1, as seen on the Missouri, do not appear along 
the mountain elevations. It is probable that Nos. 1 and 2 are merged into each other, and lithologically form one 
division. Further evidence is needed before we can speak positively in regard to this point. 
