9 6 



HISTORICAL PALEONTOLOGY. 



stones, and limestones (the " Cincinnati" rocks proper). The 

 exact parallelism of the Trenton and Cincinnati groups with 

 the subdivisions of the Welsh Silurian series can hardly be 

 stated positively. Probably no precise equivalency exists; 

 but there can be no doubt but that the Trenton and Cincin- 

 nati groups correspond, as a whole, with the Llandeilo and 

 Caradoc groups of Britain. The subjoined diagrammatic 

 section (fig. 35) gives a general idea of the succession of the 

 Lower Silurian rocks of North America : 



GENERALISED SECTION OF THE LOWER SILURIAN ROCKS 

 OF NORTH AMERICA. 



Fig. 35- 



Medina Sandstone (base of 

 Upper Silurian). 



Cincinnati Group proper. 



Ulica Slates. 

 -Trenton Limestone. 



Black River Limestone. 



Bird's-eye Limestone. 



Chazy Limestone. 



Quebec Shales (Levis Beds). 



Calcifcrous Sand-rock. 

 Potsdam Sandstone. 



