96 ON THE GEOLOGY AND NATURAL HISTORY 
sides of the Missouri river are great quantities of the shells of mollusca in a fine state of 
preservation, looking very much like those strewn upon the shores of our present rivers 
or lakes. Silicified wood occurs everywhere in the greatest abundance, in the same perfect 
state of preservation before mentioned. 
A vertical section of the different beds in descending order, as exposed within twenty 
or thirty miles of Fort Union, would be as follows: 
Feet. 
1. Ferruginous marl, with arenaceous concretions, caps the hills, and is covered with angular blocks of 
granite; sometimes the upper part of this bed for several feet in thickness is composed of concretionary 
sandstone, forming ledges. Most common fossil, Padudina trochiformis, . . : . 20 to 30 
Drab indurated arenaceous clay, . . : : 5 : 0 6 : . 2 - > Al) 
Impure lignite with numerous crystals of selenite, 12 inches. 
eH g2 bo 
Gray and drab indurated clay, contains at various localities very abundant impressions of leaves of dicoty- 
ledonous trees with a species of fern, . : c : : 0 0 ‘ : c . 50 to 70 
5. Impure lignite with much silicified wood. One mass lay in the bed eighteen inches in diameter, and 
thirty feet in length, 18 inches. 
6. Gray indurated sand, with a slight mixture of clay, contains numerous freshwater mollusca, as Paludina 
trochiformis, P. retusa, P. Leai, P. Leidyi, Melania Nebrascensis, also many fragments and entire 
stumps of silicified trees, among the debris of which I noticed that the shells were most abundant, . 30 
7. Impure lignite, 4 inches. 
8. Dark gray and drab indurated sand, .. 6 F 6 0 : 0 : 9 0 c 20 to 80 
About six miles northeast of Fort Union a local bed occurs, containing a somewhat 
peculiar class of fossils, of freshwater and land species, a section of which I give to show 
the position of the fossils. 
Feet. 
1. Indurated silicious grit, variable in color and structure, sometimes light gray, drab or ferruginous, with 
layers of clay and concretionary sandstone, near the lower portion of the bed. ‘The terrestrial and fluvia- 
tile shells are inclosed in a hard reddish carbonaceous matrix, thrust in between layers of the 
sandstone, . : : 0 : ‘ j : : : 5 : : b F 80 to 100 
2. Impure lignite, 2 
e . . ° ° ° . . . . . . a 
3. Yellowish gray indurated clay. 
The lower portion of bed 1 of the above section contains at this locality a profusion of 
land and freshwater shells, inclosed in a compact matrix, well calculated to preserve them. 
The bed occupies an area of only a few yards square, exposed in the channel of a little 
stream, and was observed nowhere else on the Missouri. 
The mollusca that occur at this locality, though belonging to extinct species, are closely 
allied to forms now living on the land and in the little streams of the country. The species 
as yet described from this place are Cyclas fragilis, C. subellipticus, C. formosa, Pwpa helt- 
