88 On the Silurian System. 



pears at Cincinnati, Columbus, and other places in Ohio, and ter- 

 minates on the south in 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 sufficient 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, 

 two very distinct formations, in the same division, whence I infer 

 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 Caradoc sandstone. Traces 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 Roches- 

 ter, (calciferous slate, Eaton.) It contains in considerable abun- 

 dance, the Asaphus hmulurus. Green, (A. longicaudatus, Murch.) 

 Trimerus delphinoceplialus, so common in this shale, is said to 

 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- 

 stroyed in another, like the Calymene Blumenbachii. 



Wenlock Limestone. 



This is represented by a series of limestones admirably devel- 

 oped in the Helderberg mountain, of which I have noticed six, 



