80 ON THE GEOLOGY AND NATURAL HISTORY 
the series, not only from its extent, thickness, and difference of composition, but also from 
the more favorable influence that it exerts upon the country underlaid by it. In ascend- 
ing the Missouri river it first makes its appearance near the mouth of Grand river, about 
one hundred and fifty miles above Fort Pierre. Near Butte aux Gres it becomes quite 
conspicuous, acquiring a thickness of eighty or one hundred feet, and containing great 
quantities of organic remains. Here it forms an extension of what is called Fox ridge, a 
series of high hills, having a northeast and southwest course, crossing the Missouri river 
into Minnesota at this point. Its northeastern limits I have not ascertained. In its 
southwestern extension it continues for a considerable distance nearly parallel with the 
Missouri, crosses the Moreau river about thirty miles above its mouth, then forms a high 
dividing ridge between the Moreau and Shyenne rivers, at which locality it first took its 
name. Continuing thence its southwesterly course, it crosses the Shyenne, and is seen 
again in its full thickness at the heads of Opening creek and Teton river, forming a high 
ridge, from which tributaries of the Shyenne and Teton take their rise. The little streams 
flowing into the Shyenne have a northwesterly course, while those emptying into the Teton 
take a southeasterly direction. We thus find that this bed underlies an area of about two 
hundred miles in length and fifty miles in breadth, or about ten thousand square miles. 
The general character of formation No. 5 is a yellow arenaceous and argillaceous grit, 
containing a great amount of ferruginous matter and in localities a profusion of organic 
remains. It forms a much more fertile soil, sustains a more healthy and luxuriant vegetation 
than formation No. 4, and abounds in springs of good water. 
Like No. 4 this formation yields in the greatest abundance quite perfect and well-pre- 
served organic remains. Many of the species approximate so closely to Tertiary forms 
that, did we not everywhere find them associated with Ammonites, Scaphites, Buculites, and 
other genera which are not known to have existed later than the Cretaceous epoch, we 
should at once pronounce the formation in which they occur to belong to the Tertiary 
period. Fossils are found throughout this formation to a greater or less extent, and the 
species are too numerous to mention any but the most characteristic and abundant ones. 
‘The greatest proportion of the species are restricted to this bed; and those which are 
common to it and formation No. 4 are chiefly Cephalepoda, which everywhere have an 
extensive vertical as well as geographical range. At Butte aux Gres on the Missouri we 
find great quantities of fossils inclosed in tough ferruginous silicious concretions, as 
Scaphites nodosus, S. Conradi, Nautilus Dekayi, a most abundant bivalve, Mactra Warrenana, 
&c. Along the Moreau river and on Fox hills, Busycon Bairdi, Cuculloea Nebrascensis, 
0. Shumardi, Fusus Haydeni, occur in great numbers. At the head of Teton river, where 
this formation attains a great thickness and presents its usual lithological characters, very 
few fossils are found, a single fish-tooth, a small undescribed mollusc, and a few impressions 
