122 B. N. A. BOUNDARY COMMISSION. 
river, the same beds were again observed, and beyond this point small 
exposures very frequently occur, and show yellowish nodular s:ndstones, 
and arenaceous clays, precisely resembling those of the Milk River. 
300. Some miles further west, near the 620 mile point on the Line, 
a very interesting section was observed on the eastern side of a small 
coulée, where the flood-water had cut away the bank. (Plate IV., 
Fig. 4.) The upper layer is of massive grey sandstone, about eight 
feet in thickness, and immediately below it a softer bed filled with Ostrea 
shells occurs. Below this, about six feet of arenaceous clay, with a few 
shells of Ostrea, and then an irregular and impure lignite about a foot 
thick, which is underlaid by about seven feet of arenaceous clay similar 
to that above it, but more perfectly stratified. Next, a carbonaceous 
shale, overlying a grey arenaceous clay filled with the remains of small 
molluses, the most abundant of which appears to be identical with 
Corbula undifera of Meek; a second species, probably referrable to C. 
mactriformis, is also sparingly represented. Below the shell-bed is 
another seam of impure lignite, which is slightly thicker than the 
upper, and holds in some places rather large drops of amber or fossil 
resin. Below this lignite, is a bed of grey arenaceous clay, underlying 
which is a foot or two of carbonaceous shale, which rests on a grey, soft 
sandstone, constituting the base of the section as here exposed. 
301. The beds in this section are exceptionally rich in the remains 
of Mollusca, and the form referred to Corbula undifera, constitutes about 
half the substance of the bed which it characterizes, occurring in all 
stages of growth. The proportion of carbonaceous beds and lignites, is 
also greater in this section than in the corresponding beds exposed in the 
valley of Milk River. The conditions of deposit implied,. are those of 
shallow water, perhaps estuarine, but of distinctly saline character. The 
section is situated about fourteen miles north-eastward from the most 
eastern of the Three Buttes, and the beds are horizontal. In a second 
locality, nor far removed, is a fossiliferous bed containing a few speci- 
mens of Corbula undifera, mingled with Goniobasis insculpta, Meek, 
fragments of Ostrea, and a species of Dostia or Neritina, not yet 
specifically determined, but probably either NV. patelliformis or WN. 
Bellatula ; forms described by Prof. Meek from Colville in Utah. 
302. On approaching the Kast Butte to within ten or twelve miles, - 
the hitherto horizontal beds are found to assume a gentle dip away from 
it, or on this side, to the northward. In the vallies of the streams which 
seam the flanks of the hills, and furrow the surface of the prairie around 

