490 MR. W. B. E. KING ON THE [vol. lxxix, 



III. The Geneea.Ii Succession. 



In the area under discussion the rocks fall into the following- 

 natural lithological and paheontological divisions : — 



Salopian. Mudstones with grits. 



f(a) pale-grey mudstones, with sandy beds. 

 | (b) wavy-bedded sandy shales, with thin dark shaly bands, 

 Valentian. -j passing down into 



I (c) 15 feet of massive bedded fossiliferous sandstone. 

 [_ [Meristina-crassa Sandstone.] 



Break. 

 f(a) leaden-blue blocky fossiliferous mudstone [ Trimicleus- 

 ' bucklandi Mudstone], up to 50 feet exposed. 



I (b) grey-blue to grey-brown sandy mudstones, fossiliferous 

 .,.,.. ! throughout, about 1000 feet thick. [Calymene-quad- 



° ' j rata 1 Mudstones and higher Phillipsinella. Beds.] 



(c) dark blue-black blocky mudstones, somewhat calcareous, 

 with many fossils about 15 feet. [Lower Phillips- 

 { inella Beds.] 



f(a) jet-black graptolitic shales and black limestones, with 



a phosphate-band 18 inches thick. Total thickness == 



| about 50 feet. [Pen-y-garnedd graptolite- shale.] 



p 1 • J (k) calcareous ashes, with mudstones and irregular lime- 



" | stone-bands, many fossils. [Ortlris-actoniie calcareous 



beds.] 



j (c) sandstones and fine sandy mudstones, with an ash-band 



^ in the south and west. [Orthis- alternata Sandstones.] 



IV. Local Details. 



A detailed description of the localities where the various heels 

 are well developed ma}' now he given, beginning with the earlier 

 formations. 



Caradocian. 



Ortli is-alternata Sandstones. — Rocks of undoubted 

 Caradocian age form all the high ground from Brvn Cownwy in 

 the south to Bwlch Greolen and the Das Eithen ridge, and the 

 ground immediately north of Pen-y-garnedd, in the north. 



The most conspicuous rock-type is a tough, blue-hearted, fine, 

 someAvhat muddy sandstone, which occurs in beds measuring up to 

 2 or 3 feet in thickness, and separated one from the other by beds 

 of varying thickness of sandy blue mudstone. Some of the bedding- 

 surfaces of the sandstones and shales are covered with fossils, the 

 commonest forms being : — 



Orthis (HeterortMs) alternata Sowerby. 

 O. (H.) alternata var. retrorsistria Davidson. 

 Plectambonites sericea (J. de C. Sowerby) ; Souldey type. 

 Strophomena (RafinesquinaJ expansa (Sowerby). 



This is, in fact, the fauna which characterizes the Glyn Gower 

 Sandstones and Allt-dclu Mudstones of the Bala country. 3 



1 See p. 504. 



2 G. L. Elles, Q. J. G. S. vol. lxxviii (1922) p. 171. 



