G. L. EUes — Qraptolite Zones in Arenig Bodes. 



20^ 



(b) Western District. 



In the western or Aberdaron tract the masses of shale are for 

 the most part faulted patches, and the only idea of their place in 

 the succession is obtained from the fauna they contain. 



The shales seen at Parwyd, since they contain Diclymograptus 

 liirundo and Jizygograptus suecicus, seem to represent the zone of 

 D. liirundo, while the lower zone of B. extensiis appears to be 

 represented by the beds at Forth Mendwy, though the graptolites 

 here are rather fragmentary, and it is certainly exposed in the 

 quarry opposite Dvvyrhos Farm, where numerous, though badly 

 reserved, specimens of the zone fossil may be obtained. 



Arenig. (Figs. 2 and 3.) 



In the Arenig district there are two graptolite-bearing shale bands 

 associated with the volcanic group. One occurs in the middle of 



^* Volcanic 

 f * ^ Group. 





Fig. 2.— Sketfh-niiip of Xaut Elios Ddu. Scale, 6 indies = .J mile. 



the volcanic series, the other underlies it, and both have hitherto 

 been regarded as being of Arenig age. The upper shale band, 

 known to some as the Daear Fawr shale, contains Diplograptidaj 

 and a few Dicranograptidte : the Biplograpti, which are chiefly 



