NATURAL SCIENCES OF PHILADELPHIA. 
49 
more nearly lower than upper Carboniferous forms, though some of them cer- 
tainly are like coal measure species. 
The color, texture, and composition of the beds are such as to carry the mind 
at once to the lower Carboniferous series, and quite unlike those of any rock 
known in the western coal measures. 
Lower Silurian. 
Potsdam Sandstone, 
At several places in the Black Hills, the oldest Carboniferous bed of that re- 
gion (H) was seen reposing, to all appearances conformably, upon a bed thirty 
to fifty feet in thickness of reddish and grayish sandstone, composed of angular 
grains of quartz, cemented by silicious, and sometimes small portions of cal- 
careous matter. The fossils obtained from this rock were Lingula antigua^ 
(Hall), and great numbers of a small shell very similar to L. "prima (Conrad), 
but from its thickness and structure probably an Oholus; also a shell nearly 
related to 0. appolinus^ as figured by Murchison and De Vernueil, in#heir work 
on the geology of Russia, but perhaps nev/, and fragments of a Trilobite belong- 
ing apparently to one of the forms figured by Dr. Owen from the lower sand- 
stones of Minnesota. 
The above mentioned fossils, as will be at once understood by the Paleon- 
tologist, clearly prove this lower sandstone (I) to belong to the oldest portion of 
the Silurian system, or in other words, to the Potsdam sandstone of the New 
York series. The identification of this rock, at this remote point in the far 
west, we regard as a matter of peculiar interest, proving, as it does, the exten- 
sion of that formation several hundred miles further westward than it has 
hitherto been known to occur in this country.* 
Metamorphic and Igneous Rocks. 
At almost every place where the base of the Potsdam sandstone was seen in 
the Black Hills, it was found to repose upon what appears to be upturned edges 
of an ancient series of sedimentary rocks. These older rocks present the ap- 
pearance of having their strata thrown into a vertical or highly inclined condi- 
tion, while the Potsdam, although much disturbed, rests unconformably upon 
them. 
Further in towards the interior of the mountain, and beneath the metamor- 
phic rocks, as already stated, the main body of the Black Hills is composed 
chiefly of a coarse feldspathic granite and other igneous rocks. 
DESCRIPTIONS OF NEW FOSSILS.-^ 
Pentacrinus asteriscus. 
Our knowledge of this crinoid is entirely derived from detached pieces of its 
column, and other parts, as seen imbedded in a sandy matrix cemented by cal- 
careous matter. These segments or joints of the column may be characterized 
as rather thin, small, and very symmetrical pentagonal, star-shaped bodies, the 
rays of which are usually longer than wide, and rather acutely angular at their 
extremities. Through the centre of each joint there is a very small circular per- 
foration, from which five regular lance-oval, petaloid areas radiate, one to the ex- 
tremity of each of the angles ; the areas being bounded by^rather narrow, slightly 
elevated transversely crenulate margins. 
The above description applies more particularly to the largest sized speci- 
mens, measuring about -18 inch across from point to point of the opposite angles. 
Associated with these there are other much smaller joints, varying from -05 to 
•10 inch in diameter. These have proportionably shorter and broader rays, or 
* Figures and more extended descriptions to appear in Lieut. Warren's final report. 
1858.] 4 
