XXX INTRODUCTORY REMARKS. 



Fort Steele, and at other localities in that region, some of the fossils charac- 

 teristic of this formation, such as Inoceramus problematicus, 'Scaphites War- 

 renanus, &c, occur in arenaceous beds. Farther westward, at Aspen station 

 on the railroad, at an elevation of about eight thousand feet above tide, 

 there is also an extensive series of hard, whitish and bluish, thinly-lami- 

 nated, argillaceous rock, three hundred to four hundred feet in thickness, 

 containing scales and detached bones of fishes, fragments of Ammonites, 

 etc., that probably belongs to this division of the Upper Missouri section. 

 The Cretaceous sandstone connected with a workable bed of coal, and contain- 

 ing numerous casts of Inocrramus problematicus, at the old Bear River mine, 

 Wyoming, likewise appears to belong to the horizon of this division; as is 

 also probably the case with the dark clay or shale above the lower main 

 coal-bed mined at Coalville, Utah. Professor Marsh also brought a few fos- 

 sils from Brush Creek, Utah, indicating the presence of this or the succeed- 

 ing division, or perhaps both, at that locality, overlying or including coal. 

 The species from this group are figured on plates 3 to 8, inclusive. 



So far as yet known, the facts-seem to indicate that this rock, like the 

 others, both in the north and in the south, thins out in an easterly direction, 

 its greatest thickness being in the region of Fort Benton, about eight hundred 

 feet; while along the Missouri in Nebraska, below the Great Bend, it 

 probably does not attanin a thickness of more than one hundred feet. In 

 New Mexico, it is probably at least as thick as at any place in the Upper 

 Missouri; yet, in Texas, Dr. Shumard estimated its thickness at not more 

 than fifty feet. 



Niobeaea geoup. — This third member of the section in the ascending 

 order is well developed along the Missouri for some distance below the Great 

 Bend, near which it is first seen in coming down the river to rise from beneath 

 the Fort Pierre group. Farther down, it continues to rise with the other 

 members of the series higher and higher, until, in the region of Niobrara 

 River, it forms perpendicular escarpments, sometimes from ninety to one 

 hundred feet in height. Below this, it is seen resting directly on the Fort 

 Benton group ; and on the higher country both of these divisions continue on 

 to near the mouth of Big Sioux River, and on the west side of the river far 

 beyond, the Fort Benton group being here seen to lap upon the Dakota 

 group. The dip of all the rocks here being in a west-northwesterly direction, 

 each member of the section can be clearly seen to pass beneath the next sue- 



