

CRETACEOUS AND TERTIARY—GENERAL ARRANGEMENT. 155 
from that of its eastern development, that were sections from the two 
localities directly compared, it would be difficult to prove their equival- 
ency. The beds best exposed in the Bad Lands, it is true, are at the very 
base of the formation, and lower than any seen east of this place; still 
their difference, when compared with those of the Souris, is not very 
striking, and chiefly obtains with the lowest beds of division 8., which 
are more massive, and homogeneous, while the upper part of this division, 
with its lignites, corresponds closely enough with the lignite-bearing 
beds near Wood End. In some of the lower beds of 8., a pale greenish 
grey tint, not previously observed, appears; and the introduction of 
reptilian bones is a new feature, none having been found east of this 
place. No molluscous fossils were found in the beds of this locality. 
371. On the Milk River, where the next great series of exposures of 
this formation occurs, its character has undergone a further change. The 
pale greenish shade is now more pronounced, and characteristic of a 
greater thickness of the beds; and while well marked fresh-water mol- 
luses abound, they are mixed for the first time with the remains of Ostrea, 
a marine genus. Lignites still occur, though not abundantly, and 
reptilian bones are found in certain layers. Midway in the section, 
is an important band of sandstones, which may be placed, conjecturally, 
on the horizon of those of the Roche Percée and division a. of the Bad 
Lands. The change from salt to fresh water conditions, which, in the 
eastern regions seems synchronous with the change from Cretaceous to 
Tertiary, here begins to fail us as a test. 
372. In the vicinity of the Buttes, or Sweet Grass Hills, while lignites 
and carbonaceous clays still occur, the beds are characterized by the pre- 
sence of Ostrea shells in great numbers, with one or more species of mol- 
luses identical with those of the Bitter Creek coal series of Wyoming, 
and a Dostia (Neritina) apparently indistinguishable from those of Coal- 
ville in Utah. Near the first and second branches of the Milk River, 
where the formation is again well exposed, there is a remarkable rever- 
sion to fresh water conditions, quite at-variance with its general tendency 
to become more strictly marine westward. Dinosaurian and other repti- 
lian remains, however, still occur in connection with Unio, near the First 
Branch, and on the Second Branch the bone-bearing beds appear to overlie 
a considerable thickness of strata holding Limnea, Paludina, Planorbis, 
and Spherium—some of which are identical with those of the well- 
defined eastern Fort Union beds. Greenish-grey, arenaceous clays, now 
constitute a large part of the sections, coarse sandstones occur fre- 
