NATURAL SCIENCES Or PHILADELPHIA. 
147 
were so closely related to forms occurring in the Great Lignite Basin, which we 
knew to be Tertiary beyond a doubt, that there was a question whether 
they were not specifically identical ; also that a species of Ostrea found in the 
upper Judith beds could not be distinguished from a species observed in 
the lowest bed of the Lignite Basin, and a species of Trionyx was con- 
sidered by Dr. Leidy common to the two deposits, led both Mr. Meek and myself 
to express the opinion that the Judith beds were probably Tertiary, and were on 
a parallel with the lowest beds of the great Lignite Basin. Other facts have 
now been brought to light which lead us rather to suspect that the suggestion 
first made by Dr. Leidy, that Judith River beds, or at any rate a portion of those 
containing the estuary fossils, as well as those occurring in the Black Hills, may 
be the American representatives of the Wealden of England. On the map I have 
not distinguished the Wealden deposits by a separate color, but included them 
with Lower Cretaceous formation No. 1. 
From the evidence thus far obtained in regard to the Laramie Mountains 
and the Black Hills, I have inferred that there have been the following 
oscillations of the surface of the country in this region. In the first place, 
after the deposition of the azoic strata, there was an upheaval of the granitic 
rocks, which threw into a highly inclined position these older strata. If this 
upheaval elevated the old azoic strata above the ocean level, there must have 
been a subsequent subsidence, after which the Potsdam Sandstone was depos- 
ited. Then there was a later elevation that raised the Potsdam Sandstone 
above the surface of the ancient sea, which was followed by a long period of 
repose, sufficient for the deposition of all the formations from the Potsdam to 
the Upper Carboniferous, when there was another subsidence followed by the 
deposition of the formations of the Upper Carboniferous, Permian, (possibly the 
Triassic,) Jurassic and Cretaceous Periods. After all these disturbances, and 
probably at the same time that most of our continent was raised above the 
ocean level, the whole of these strata were again elevated to nearly their 
present position previous to the dawn of the Tertiary period. 
What changes may have occurred in the physical features of the country 
during the long period which must have elapsed between the deposition of 
the Lower Silurian and the Carboniferous rock, it is impo»sible now to deter- 
mine. We know that in our present seas there is a constant deposition of 
sediment going on, forming sandstones, &c., or a calcareous precipitation form- 
ing limestones. In order to account for the hiatus between the Potsdam 
sandstone and the Carboniferous rocks in this region, we must suppose that 
i^ither the surface occupied by the former was above the waters during this 
long interval, or that the intervening formations were deposited and subse- 
quently removed by erosion and denudation, prior to the Carboniferous era. It 
seems quite improbable that so great thickness of strata could have been re- 
moved, so as to leave no trace of their former existence over so large an areao.. 
We are, therefore, inclined to the opinion that they were never deposited 
this region. 
VIII. Tertiary Basins of the Upper Missouri. 
Our present knowledge of the geological formations of the far west, leads me 
to modify, to some extent, the divisions of the Tertiary Basins given in my 
notes explanatory of the first edition of the geological map of Nebraska, pub- 
lished in May, 1857. Considering the fresh water and estuary deposits at the 
mouth of the Judith as probably the equivalents of the Wealden of Europe, , 
we have the following subdivisions : 
1st. Grreat Lignite Tertiary Basin. 
3nd. White River Tertiary Basin. 
IsL Great Lignite Tertiary Basin, 
As this formation, in its extensive development on the Upper Missouri, has 
been described quite fully in several preceding papers, I will here simply 
