HISTORICAL PALAEONTOLOGY. 



GENERALISED SECTION OF THE CAMBRIAN ROCKS IN WALES. 



Fig. 27. 



Arenig Group (Base of the 

 Lower Silurian). 



Tremacloc Slates. 



Upper Lingula Flags. 



Middle Lingula Flags. 



Lower Lingula Flags. 

 Menevian Group. 



..... LongmyndorllarlechGroup. 



Pre-Cambrian Rocks. 



The above may be regarded as giving a typical series of the 

 Cambrian Rocks in a typical locality ; but strata of Cambrian 

 age are known in many other regions, of which it is only 

 possible here to allude to a few of the most important. In 

 Scandinavia occurs a well-developed series of Cambrian de- 

 posits, representing both the lower and upper parts of the 



