200 G. L. Elles — Graptolite Zones in Arenig Rocks. 



While the occurrence of one or other, or both, of these zones has 

 been indicated by Matley ^ and Fearnsides - in North Wales, and 

 by Marr & Eoberts,' Crosfield & Skeat,* and the officers of 

 the Geological Survey * in South Wales, so far as I am aware, 

 it has not been so generally recognised that this belt of rocks 

 characterised by * tuning-fork ' graptolites gives place downward 

 to a belt of rocks characterised by graptolites of the ' extensiform ' 

 type (horizontal series) wherever the graptolitic facies of the rocks 

 is developed. This belt characterised by 'extensiform' graptolites 

 may also be divided into two zones — 



1. A.n\x^^er zone oi Bidymograptus hirimdo. 



2. A lower zone of Bidymograptus extensiis. 



The succession of these zones has been determined in widely 

 separated ax'eas, and other localities will no doubt be found in 

 the future, so the object of the following notes is to indicate 

 where they are at present known to occur, and the characteristic 

 association of the graptolites which constitute the sub-faunas. 



There are then, three well-defined graptolite zones in the Arenig 

 rocks of Wales — 



1 . The zone of Bidymograptus bijidns. 



2. The zone of Bidymograptus hirnndo. 



3. The zone of Bidymograptus extensus. 



These probably represent the upper part of the series only. 



The following are the principal localities where they may be 

 studied : — 



EiVER Seiont. (Fig. 1.) 



The occurrence of fossils in the rocks in the neighbourhood of 

 Pont Seiont was indicated long ago by Eamsay " and later by Marr.- 

 The rocks present a certain amount of variation in lithological 

 character, but have a practically identical fauna; they are for the 

 most part sandy or slightly micaceous blue-black shales with a few 

 calcareous bands. They are well exposed on the left of the road 

 before reaching the white gates into the Public Park, and again 

 just beyond the gates, though at this point they have been 

 somewhat altered by the intrusion of a dyke of diabase. The fossils 

 collected from these beds include — 



Bidymograptus hijidus. Hall (very Climacograptus confcrtus, Lapw. 



common). CI. Scharenhergi, Lapw. 



Biplograptiis dentatns, Brong. Caryocaris, sp. 



The beds clearly, therefore, must be regarded as belonging to 

 the zone of D. hijidus. 



1 Geol. Mag., 1902, p. 118. 



'- Brit. Assoc, 1903. 



3 Quart. Journ. Geol. Soc, 1885, p. 476. 



^ Quart. Journ. Geol. Soc, 1896, p. 523. 



* Summary of Progress, 1902. 



^ Mem. Geol. Survey, vol. iii, p. 161. 



' Quart. Journ. Geol. Soc, 1876, p. 134. 



