OF THE UPPER MISSOURI. 97 
coides, Physa longiuscula, P. rhomboidea, P. Nebrascensis, Planorbis subumbilicatus, Ancy- 
lus minuta, Valvata parvula, Bulimus? teres, B. vermiculus. The last two species will 
doubtless prove to belong to the genus Clausilia, or to an intermediate new genus 
between Bulimus and Clausilia. ‘There were also many seeds of plants in this ligneous 
material. ‘The upper portion of this bed is a dark gray silicious grit, sometimes of a drab 
color, from the numerous particles of coaly matter which are disseminated through it. 
It contains numerous impressions of dicotyledonous leaves, of the same species with those 
found in the same bed ten miles below this point, and holding nearly the same position. 
But one of the most remarkable features of this bed at this locality, is the presence of 
myriads of spherical concretions, covering an area of several miles in extent. ‘They vary 
in size from half an inch to several feet in diameter, and are formed of thin layers of sand- 
stone, concentrically arranged about a nucleus. They contain much ferruginous matter, 
of a grayish color internally, but becoming of a reddish iron-rust color on exposure. 
About thirty miles below the mouth of Milk river the beds of the lignite basin begin 
sensibly to rise above the water-level of the Missouri, by the reversed dip of the strata, and 
on reaching Round Butte they cap the hills, though still maintaining considerable thick- 
ness. Near the mouth of Mussel-shell river, the Cretaceous formation No. 4 occupies the 
country, the lignite beds having entirely disappeared. At Round Butte the lignite de- 
posit is mostly seen in outliers in the form of denuded conical hills, scattered over the 
broad upland plateau, the highest about one hundred feet, resting upon No. 4 or a blend- 
ing of Nos. 5 and 4, which have a thickness of two or three hundred feet. The northern 
outliers of the lignite basin pass off on the Cretaceous hills, presenting very similar litho- 
logical characters to those along the southern border. 
The same deposit underlies the country bordering upon the Yellowstone, at least to the 
mouth of the Big Horn, which is the highest point to which my explorations have ex- 
tended. It presents nearly the same lithological characters as on the Missouri, and most 
of its organic remains are specifically identical. 
Near the mouth of the Yellowstone river are a series of hills, composed of variegated 
sands and clays, with some impure lignite and large numbers of animal and vegetable 
remains. 
Near O’Fallon’s creek, about one hundred miles by land abeve the mouth of the Yellow- 
stone, is a very high rugged portion of country, called Bad Lands, which is cut through 
by the river, so as to expose a fine vertical section of the strata. This section will repre- 
sent most of the Tertiary beds seen on the Yellowstone, but they vary much in thickness 
and appearance at different localities. 
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VOL. XI1.—13 
