144 
GEOLOGY OF THE FOURTH DISTRICT. 
14. PENTAMERUS LIMESTONE. 
(Part of No. 6, Pennsylvania Survey.) 
According to the Report of the Third District, this rock, being one of the subordinate 
divisions of the Helderberg series, disappears from thinning out, in the vicinity of Oneida 
creek. It is not recognized in the Fourth District as a distinct rock, though the charac¬ 
teristic Pentamerus, or one very similar to it, has been found near Buffalo. This circumstance 
with others, induces the belief that some of the lower rocks of this series appear farther west 
but that they have not yet been distinctly recognized. The constant association of the Pen¬ 
tamerus galeatus (Atrypa galeata, Dalman, Hisinger ; Murchison, Silurean Researches , 
p. 623, pi. 12, f. 4), with this rock gives the name by which it is known. It will not be con¬ 
founded with the Pentamerus limestone, which is a subordinate member of the Clinton group, 
the typical fossil of which is the Pentamerus Ohlongus (see page 70). Another singular 
fossil of this rock is the Lepocrinites Gebhardii. * 
15. DELTHYRIS SHALY LIMESTONE. 
Catskill Slialy Limestone. Geol. Report of the Third District. 
This rock is enumerated in the order of succession, though it does not occur in the Fourth 
District. 
As its name implies, it is a shaly mass, or consists of shaly with alternating beds of com¬ 
pact limestone. Its present and former name is derived from the abundance of the Delthyris 
macropleura of Conrad which everywhere marks the rock. The name of Catskill Shaly 
Limestone, which has been proposed on account of its great development on the Catskill Creek, 
is found to be objectionable, as it at once carries the mind to the Catskill Mountains, a very 
different group of rocks, thus tending to propagate a false impression. Since the name now 
used has been previously adopted, there can be no objection to continuing it. 
It is an exceedingly interesting rock from the great number of species, abundance and 
perfection of its fossils, many of which are very similar to those of the Niagara shale, and 
of the Wenlock shale of England, some indeed appear to be identical with those of the latter 
rock. Fossils appear in this rock very analogous to the following forms, which are figured in 
* Sefe Report of the Third District, page 117. Also the. forthcoming Report of the Second District, under the head of 
Pentamerus limestone. 
