88 1 On the Silurian System. 
pears at | Cincinnati, Columbus, and other places in Ohio, and ter- 
minates on the southin Tennessee. It occurs at Bedford Springs 
and other localities in Pennsylvania. It is a singular fact, and 
may be the means of lessening the faith of some geologists in the 
value of organic remains, that the well known trilobite, Calymene 
Blumenbachii is not known to occur in the Caradoc sandstone, 
but is here characteristic of the Trenton limestone. But we re- 
gard groups, not a single species, in comparing rocks of distant. : 
localities, and it will be found that such discrepancies occur in 
other-formations without ever being in amount suflicient to create 
confusion and prevent comparison. ‘There is one fossil figured 
by Murchison as a Caradoc species, which here lies immediately 
below the Wenlock shale. This arrangement would embrace 
Salmon river sandstones and shales and the Niagara sandstones, 
that the Salmon river rocks'are wanting in Wales, and the Niag- 
ara sandstone very rarely present, which brings the Pentamerus — 
rock in contact with the Caradoe sandstone. Traces of of the Niag- 
ara sandstone in Wales may be recognized by the occurrence of 
Agnostus latus, (nob.) and Planorbis trilobatus, (Bellerophron, 
Sow.) which belong exclusively to this formation. It is remark-_ 
able that the Caradoc rocks should consist of sandstone in Wales, © 
whilst the Trenton limestone and slate form so prominent a fea 
ture of the series in this country. Had it been otherwise, a more 
extended correspondence would probably have occurred between 
the fossil groups on each side of the Atlantic. 
Wenlock Shale. 
This formation is clearly identified with the shale of Rocha 
ter, (calciferous slate, Eaton.) It contains in considerable abun- 
dance, the Asaphus limulurus, Green, (A. longicaudatus, Murch.) 
two very distinct formations, 1 in the same Mivision, whence I infer i 
_ Trimerus delphinocephalus, so common in this shale, is said t0 
occur in the Wenlock limestone, but to characterize the Ludlow 
formation in Wales, whilst here it has never been found above 
the Rochester shale ; it is, therefore, a curious instance of a spe- 
cies having been preserved in one region after it had been de- 
so he in another, like the Calymene Blumenbachii. 
Wenlock Limestone. 
This is represented by a series of limestones admirably devel 
— in the Helderberg mountain, of which I have noticed si%; 
5p 
