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). 



( 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 



