part 2] STRUCTUEE, ETC. OE THE BALA COUNTRY. 



137 











Graptolite 









Shelly Faunas. 



Zones. 



r Zone of 



Monograptus 



< 



C\vm-yr-Aethnen Shales. 







•{ crispus. 



M 









Zone of 











^ M.sedgiviclci. 



Hiniant Grits and Mudstoiies, 





Orthis-hirnantensis 







including local Hirnant 





fauna. 







Limestone. 









Foel-y-Ddinas Mudstones. 





FJiacops-mucronatus 





!?5 







fauna. 





-«) 











3 



Hi 



M 



Moel-fiyn Sandstones. 





Unfossiliferous. 





w 



< 



Rliiwlas Limestone and Mud- 





FMllipsinella para- 



DiceUograptus- 





stones. 





bola fauna. 



anceps Zone. 





Gelli-Gi-in Calcareous Asli, with 



ct 









Gelli-Grin (Bala) Limestone, 



^-1 



Chasmops & Nicnl- 







Moel-ftyn Limestone, 



e 



eUa-actonice fauna. 





)k 



Bryn-Pig Limestone, 



« 







•< 



H 



o 



Caerhafott.y Limestone. 



S 









Pont-y-Ceunant Ash. 











Allt-Ddu Mudstones. 





AsapTms-poioisi & 



1 ^ g 







&, 



Seterortli is-a Iter- 



1-^ 





Frondderw Ash. 



S 



nata fauna. 







Glyn-Gower Sandstones. 















•l'^ 



1^5 









J^ 











<4 













Nant-hir Shales and Derfel 







Climacograptus- 



Iz; 



Limestone. 







peltifer Zone. 













Dicranogra'ptus Shales and Derfel Limestone. 



Resting upon the well-known Volcanic Series of the Arenig 

 Mountains come the Dicranogj^aptus Shales, in this area con- 

 spicuously devoid of* fossils except at the base and summit, and of 

 somewhat different lithological character from those developed in 

 other Welsh areas. In the Bala district they consist, for the 

 greater part, of hard, black, gritty shales with sandy wisps, 

 showing in many sections a structure strongly suggestive of current- 

 bedding. Close to their base they contain the Derfel Limestone 

 recorded by Prof. W. Gr. Pearnsides as the Ortliis or Derfel Lime- 

 stone'^; the shales with which this limestone is interbedded are 



1 Q. J. G. S. voL Ixi (1905) p. 627. 



