NATURAL SCIENCES OF PHILADELPHIA. 
45 
thus far favors the conclusion that this lower series (No. 1) is of the age of the 
Lower Green sand, or Neocomian of the old word, we yet want positive evidence 
that portions of it may noi be older than any part of the Cretaceous system." 
Although we have little direct additional evidence at this time in regard to 
the age of this series, as we have always understood it, we now know that from 
beneath its lower beds, around the base of the Black Hills, there rises a series 
of very similar strata, as may be seen by the foregoing section, separated from 
its base by no well-marked line of demarkation, and containing many fossils 
closely similar to those considered characteristic of the Jurassic system of the 
old world. At the same time we have failed to recognize amongst these fossils 
any forms peculiar to the Cretaceous epoch, or even very nearly analogous to 
species common in rocks of that age. 
The formations above alluded to as containing Jurassic types of fossils, 
are marked in the foregoing sections A, B, and C inclusive. But as before 
stated, these beds often pass so gradually, in their lithological characters, 
into No. 1 above, and so few fossils have been found near the junction, that 
we confess we have but a vague and indefinite idea in regard to the particular 
horizon at which the line should be drawn between them. Indeed, the gene- 
ral aspect of No. 1, and that of the formations below, are so very similar, and 
they are all so unlike the beds above, that if we were to classify them by 
their lithological characters alone, we should be inclined to view No. 1, and 
formations A, B, and C as forming one natural group, or at least to think 
that portions of No. 1, as we now understand it, should be classed with the 
series below. This view also appears to be the more reasonable when we 
take into consideration the great thickness of No. 1 in the vicinity of the 
Black Hills, and the fact that the beds A, B, and C contain a group of fossils 
apparently more nearly related to lower than upper Jurassic forms. 
Inasmuch, however, as numerous leaves beyond a doubt belonging to dicoty- 
ledonous trees, closely analogous to the oaks, willows, and other existing forest 
trees, are known to occur in No. 1 along the Missouri, near the Big Sioux, and 
in northeastern Kansas,""" and we have a BacuUte from similar beds, apparently 
of the same age, near the mouth of Judith River, on the upper Missouri, — while 
we also learn from the letters and notes of our deceased friend, Mr. Henry Prat- 
ten, that he saw a species of Baculite in formations presenting the same charac- 
ters, and seeming to occupy the same position, along the Platte above Fort La- 
ramie, we think we hazard little in viewing at least a considerable portion of 
No. 1 as belonging to the Cretaceous system. 
Another fact favoring the opinion that No, 1, even down as low as we have 
provisionally carried it in the Black Hills section, probably belongs to the lower 
Cretaceous, is the occurrence at its base of a bed conl&mmg Amm.onites and Os- 
trea^ along with Unio^ Planorbis, and Paludina ; an association of fossils which, 
in that position, carries the mind rather to the Wealden than to older forma- 
tions. 
The occurrence of these forms at this horizon, also leads us to suspect that a 
considerable portion of the estuary beds at the mouth of Judith River, above 
Fort Union, in regard to the age of which we have been so much puzzled, may 
be, as first suggested by Dr. Leidy, a representative of the Wealden, and as we 
were then inclined to suppose, belong to our No. 1. 
The close similarity between the lithological characters of these deposits, and 
those of some of the Tertiary formations of the north-west, and the estuary 
character of their fossils, together with the analogy of many of the species of 
mollusca found in one of the upper beds, (which may be an outlier of Tertiary 
resting on older formations), taken in connection with the fact that amongst the 
fossils collected from one of the middle beds (see section, page 124, vol. viii. Pro- 
ceedings, 1857,) there were some fragments of a Trionyx^ regarded by Dr. Leidy as 
identical with a species occurring in well-marked Tertiary deposits near Long 
* We are indebted to Maj, Hawn for our knowledge of the occurrence of these leaves in 
No. 1, as seen in Kansas. 
1858.] 
