110 B. N. A. BOUNDARY COMMISSION. 
269. Below these are sombre Cretaceous clays of division §., and 
they extend downward to the water level of the river; showing a thick- 
ness of 273 feet, the base not being seen. The portion of these clay-slates 
most nearly resembling those last described, and those of the Pembina 
Mountain series, lies immediately below the yellow sands; below this, to 
the bottom of the valley, they show rather the crumbling earthy 
character and more sombre colour of the Bad Lands and Wood 
Mountain Astronomical Station exposures. This would tend to prove 
that rocks like those of the upper part of the typical Pembina Mountain 
series, are not confined to any particular horizon in the western represen- 
tatives of that group. About 100 feet below the base of the yellow 
sands, a bed characterized by the great abundance of the remains of a 
fine species of Ostrea, occurs. It is referable to Ostrea patina of Meek 
and Hayden; and fragments of a thick Jnoceramus appear in the same ~ 
stratum. The Ostreas, for the most part, are quite perfect, and have 
been intombed where they grew, the valves being still attached. They 
are frequently roughened externally, and crusted with selenite crystals, 
produced apparently by the action of acidulous waters on the shell 
itself. 
270. A short distance below this Ostrea bed, is a zone containing 
many large septarian ironstone nodules. In some places, a horizontal 
surface of this bed has been exposed, forming an arid wind-blown 
expanse of crumbled fragments of the shale, which here and there 
supports an Artemisia, and from which the nodular masses stand up at 
intervals, as they have been exposed by weathering. The concretions 
_are often as much as twelve or fifteen feet in diameter, and lenticular in 
form, but are generally broken into fragments by the action of the frost. 
They hold remains of Ammonites and Baculites, the former at times two 
feet in diameter, and referable to A. placenta, a form, like Ostrea patina, 
characteristic of the 4th group of the Missouri River section. The 
fossils are unfortunately intersected by the cracks which traverse the 
mass of the nodules, in such a way as to render their preservation very 
difficult. Some of them retain their nacreous lustre in all its original 
perfection. Bleached bands like those already described, occur in many 
parts of these clays. 
271. The beds here appear to be perfectly horizontal, and the 
increased elevation of the general surface of the country will more than 
suffice to account for the reappearance of the yellow sandy deposits last 
seen in the Bad Lands—~without supposing the existence of any gentle 
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