94 



HISTORICAL PALAEONTOLOGY. 



GENERALISED SECTION OF THE LOWER SILURIAN ROCKS 

 OF WALES. 



Fig. 34- 



( 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). 



j 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 



