94 



HISTORICAL PALEONTOLOGY. 



Generalised Section of the Lower Silurian Rocks 

 OF Wales. 



Fig- 34- 



r/- / / / jUit 



///// / / it-t-r-H- 

 / / / / / 



May Hill Sandstone (base 

 of Upper Silurian). 



Llandovery Group. 



Upper Bala. 



Lower Bala. 



Upper Llandeilo. 



Middle Llandeilo. 



Lower Llandeilo. 



Upper Arenig. 



Middle Arenig. 



Lower Arenig (Upper 

 Tremadoc Group). 



Tremadoc Slates (Lower 

 Tremadoc Group). 



In North America, both in the United States and in Can- 

 ada, the Silurian rocks are very largely developed, and may be 



