THE LOWER SILURIAN PERIOD. 



95 



GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS 

 OF WALES. 



Fig. 34- 





May Hill Sandstone 

 (base of Upper Silu- 

 rian). 



Llandovery Group. 



Upper Bala. 



Lower Bala. 



Upper Llandeilo. 

 Middle Llandeilo. 

 Lower Llandeilo. 



Upper Arenig. 



Middle Arenig. 



f Lower Arenig (Up- 

 I per Tremadoc 

 [ Group). 



f Tremadoc Slates 

 j (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 

 regarded as constituting an exceedingly full and typical series 



