104 Cincinnati Society of Natural History. 
quite precipitous sides. Numerous joints occur in many of the strata, 
particularly in the more compact kinds, and fine examples of concre- 
tionary structure or weathering are not rare. The tendency of the 
thick beds of marly sandstone on the banks of Green river, at the 
crossing, to weather spheroidally, is very noticeable, and this is repeat- 
ed in various degrees in the argillaceous and calcareous rocks as well. 
The Bridger Group, though succeeding the Green River Group, is 
closely related to it, for the transition from one to the other is not 
abrupt, either in the structure of the beds or their contents. The 
Group is exposed at the surface over a considerable extent of country, 
northward and eastward from Fort Bridger as far as Little Sandy 
river and beyond, forming the top layers of numerous isolated buttes, 
During this epoch it is probable that the land was covered with fresh 
water in a lake as large as in the previous era, if not more extensive. 
The beds are mainly composed of dull-colored, indurated clays, and 
arenaceous layers of considerable thickness, the latter usually brown- 
ish, or dull yellow or gray, often.with more or less of a concretionary 
structure. The clays are generally compacted, but they become dis- 
integrated upon exposure to the atmosphere, and readily yield to the 
eroding forces. Some thinner layers of more calcareous material, with 
silicious seams, often affording interesting concretions, are interspersed, 
but they are rather exceptional than otherwise. The Green river and 
Bridger Groups are readily distinguished by the effects produced by 
erosion. The former presenting nearly vertical cliffs, so that the i1m- 
pression in crossing the country where it forms the surface rocks is 
that of traveling over an ordinary plain with occasional descents, by a 
succession of terraces, to the narrow valleys of the streams. On the 
contrary, where it is concealed, or only occasionally capped by the © 
Bridger Group, the country is very irregular, often simulating the 
‘“ Bad Lands,” the beds of the latter being eroded without complete de- 
nudation, so that they stand out in buttes, or rude architectural forms. 
The deposits in the Yellowstone Lake basin, and in the valley of the 
main river and its tributaries, which may be regarded as Pliocene, are 
mainly the sediments of an ancient lake, of which the present body of 
water is the representative on a much reduced scale. Beautiful and 
highly instructive sections of the old beach formations are exposed in 
the valleys of the streams, particularly in the lower valley of Pelican 
creek, and far down the Yellowstone river, where they become more 
complicated and more interesting. An examination of these shows 
that the lake formerly extended over a mych larger area, and that it 
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