OF THE UPPER MISSOURI. 125 
were seen. It is now well known that O. congesta and I. problematicus range down into 
No. 2, so that No. 3 in the West and Southwest may give place to an increased develop- 
ment of No. 2. Nos. 4 and 5 are largely developed everywhere, when not concealed by 
the overlying Tertiary deposits, especially along the Laramie mountains and in the valley 
of Cache la Poudre. In the valley of Wind river all the Cretaceous rocks down to No. 2 
appear to have been removed by erosion prior to the deposition of the Tertiary beds, and 
the characteristic fossils of No. 2 are quite abundant. As we pass over mountains, we 
have inclining against the western slope six to eight hundred feet of alternations of black 
plastic clays, arenaceous marls, and beds of sandstones and limestone, with a few seams of 
Carbonaceous matter passing up into calcareous and arenaceous compact rocks. In some 
arenaceous limestones near the middle of the series and extending upward, quite abundant 
fossils were observed, among them a large Mnoceramus, two species of Ostrea, a large Pinna, 
four inches in length, a Cardiwm, and a number of undetermined species with fragments of 
silicified wood. ‘The general dip of these rocks is about 20°. These well-marked Creta- 
ceous beds pass up quite imperceptibly into an enormous thickness of Lignite Tertiary. 
Passing over the dividing crest to the head waters of the Missouri, we did not observe any 
indications of Cretaceous rocks until we had descended below the three forks, where we 
find traces left after erosion. They do not reveal themselves conspicuously-until we arrive 
within twenty or thirty miles of Fort Benton, where the black plastic clay begins to over- 
lap the Jurassic rocks with its characteristic fossils, and on reaching Fort Benton the 
plastic clay is quite homogeneous, and is developed to a thickness of eight hundred feet. 
As we proceed toward the mouth of the Judith river and near the Judith mountains we 
find quite thick beds of concretionary sandstone, which form the “Stone Walls,” “Citadel,” 
&c. It is from these beds that we have obtained a group of fossils which we have referred 
provisionally to No. 1, but which seem to be specifically distinct from all others in the 
West. It may be that when this group of beds, now referred to Nos. 1 and 2, comprising 
a thickness of fifteen hundred to two thousand feet in this region, are more carefully 
studied, that several subdivisions will be made, having equal importance with the others. 
During the past season our route led us along the “divide” between the Missouri and 
Yellowstone rivers south of the Judith mountains, so that we passed outside of any good 
exposures of No. 1, as well as beyond the limits of the estuary beds at the mouth of the 
Judith. We must await a more thorough and detailed exploration of this region before 
we can state with entire confidence the succession of the beds. 
In describing the Tertiary deposits in the Northwest, I separated them into two divisions, 
but more recent investigation has added many new facts in regard to them, and rendered 
the following divisions necessary. Ist, Estuary deposits; 2d, True Lignite beds; 3d, 
Wind River valley deposits; 4th, White River Tertiary deposits. 
The estuary deposits, of which the Judith basin may be regarded as the type, are quite 
