OF THE UPPER MISSOURI. 43 
itself worthy of a separate position in the series. It is composed of a dark leaden gray 
plastic clay, containing few fossils, but great quantities of sulphate of lime in crystals, 
which assume a variety of beautiful forms. Its greatest thickness is seen about five miles 
below the mouth of James river. 
Below the mouth of Vermilion river we have a perpendicular exposure showing Nos. 1, 
2 and 3 in their order of superposition. 
a. Gray and lightish yellow calcareous marl, containing in great numbers Jnoceramus problematicus and commi- 
nuted fish remains. 40 feet. No. 3 of general section. 
b. Dark plastic clay, with abundant fish remains in a fragmentary condition, also Ammonites percarinatus, Serpula 
tenuicarinata, and a species of Ostrea like O. congesta. 30 to 40 feet. No. 2 of general section. 
c. Ferruginous sand-bed just above water’s edge. At low water are seen large quantities of arenaceous concretions, 
with vegetable impressions and a species of Pharella. No. 1 of general section. 
At this point ¢ represents No. 1 as it dips beneath the water-level of the river; a No. 3 
when it is seen for the first time largely developed and forming an independent bed. 
About five miles above the mouth of Vermilion river, on the right side of the Missouri, No. 
2is finely exposed. It here contains several layers of a very hard, compact, dark gray, con- 
cretionary limestone. The fossils observed at this locality were an Ammonite, Cytherea, 
and quite numerous well-preserved teeth and other remains of fishes. At Dixon’s bluffs 
I found Serpula tenuicarinata, an Ostrea, perhaps QO. congesta, and large masses of the 
sulphuret of iron. Twelve miles above the mouth of James river, No. 2 is only about ten 
feet above the water’s edge. At this locality overlying No. 2 is seen, quite well developed, 
formation No. 3, with Ostrea congesta, and above it, capping the hills, the first appearance 
of formation No. 4, in a thin outlier. No. 2 is exposed over a small area by the upheaval 
of the older rocks in the valley of Old Woman’s creek, a tributary of the south fork of the 
Shyenne. It is also a very conspicuous bed around the Black hills, presenting the same 
lithological characters as on the Missouri, and containing a great abundance of fossils, 
Ammonites, Scaphites, Cytherea, Ostrea, &c., with large quantities of fish remains. It here 
attains a thickness of about 200 feet. 
ForMATION No. 3 or GENERAL SECTION. 
The geographical extension of this formation and its influence on the scenery render it 
one of the most interesting on the Upper Missouri. It is first seen in thin outliers at and 
a short distance below the mouth of the Big Sioux, and becomes quite conspicuous on the 
summits of the bluffs ten miles above Iowa creek. At Dorion’s hills there is a fine section 
of this bed, about eighty feet exposed above the water's edge, containing its most abundant 
VOL; XIt.—19 
