HISTORICAL PALEONTOLOGY. 



which are known as the " Lingula Flags, " from the great 

 abundance in them of a shell referable to the genus Lingula. 

 These are followed by 1000 feet of dark shales and flaggy 

 sandstones, which are known as the " Tremadoc slates, " from 

 their occurrence near Tremadoc in North Wales; and these 

 in turn are surmounted, apparently quite conformably, by the 

 basement beds of the Lower Silurian. 



GENERALIZED SECTION OF THE CAMBRIAN ROCKS IN WALES. 

 Fig. 27. 



Arenig Group (Base of 

 the Lower Silurian). 



Tremadoc Slates. 



Upper Lingula Flags. 

 Middle Lingula Flags. 



Lower Lingula Flags. 

 Menevian Group. 



Longmynd or Harlech 

 Group. 



Pre-Catnbrian Rocks. 



