SILURIAN OF THK DEE VALLEY. 281 



the Silurian beds, and nevertheless are troughed in by large faults 

 on all sides ; hence they must be very high up in the Bala series ; 

 and they are quite dijfferent lithologically from any other Bala beds 

 in the neighbourhood. Orthis hirnanteiisis is recorded by Salter from 

 Cerrigydrudion ; and it most probably comes from these beds. 



It may be remarked that ashy beds occur in the Ashgill Shales 

 of the Lake district, at Spengill. 



5. The May-Hill Group. 



In a paper by Prof. Hughes (Q. J. G. S. vol. xxxiii. p. 207) a 

 series of gritty beds is described under the name of the Corwen 

 Grits, and these beds are taken to represent the base of the Silurian 

 (Sedgw.), and correlated with the Austwick Conglomerates &c. of the 

 Lake district. At the rectory, Cerrigydrudion (see Section, p. 280), 

 an undulating series of calcareous grey grits, false-bedded and 

 ripple-marked, with numerous clay-galls, is seen resting uncon- 

 formably on the Bala beds ; and these grits are intermediate in 

 lithological character between the Corwen Grits and the Plasuchaf 

 beds near Cyrnybrain, described also by Prof. Hughes in the above- 

 cited paper. They were apparently unfossiliferous where examined, 

 though Pentamerus ohlongus, quoted from Cerrigydrudion, was pro- 

 bably found in these beds. 



The beds at the rectory form a strong feature ; and the base is 

 seen to rest in an undulating manner upon the underlying even- 

 bedded Bala beds. These gritty beds, and the immediately over- 

 Ij^ing ones, which I shall presently describe, are included by the 

 Geological Surveyor in the Bala Series. 



In lithological character, the gritty series of Cerrigydrudion 

 resembles the calcareous and gritty bands at the base of the Silurian 

 of the Lake district proper, e. g. at Skelgill and Pullbeck near Am- 

 bleside, and Appletreeworth Beck near Coniston. Towards the top 

 it contains one or two finer blackish grey bands, which show 

 cleavage. 



6. The Graptolitic Mudstones. 



In the Cambridge Museum, " Graptolites sp. with very narrow 

 ceUs " is placed among the middle Bala fossils and recorded (Cat. 

 Cambr. & Sil. Foss. Woodw. Mus. p. 39) as from Bala. The matrix 

 in which the fossil occurs is excessively like that of the Graptolitic 

 Mudstones of the Lake district ; and the species itself is Monograptus 

 argenteus, Nich., which characterizes a single zone of the Graptolitic 

 Mudstones in the neighbourhood of "Windermere &c. I was there- 

 fore led to infer the occurrence in this district of the equivalents 

 of the Graptolitic Mudstones, and, although unable to detect them 

 in the country immediately around Bala, obtained the following 

 section near Cerrigydrudion (see fig. p. 280). 



In this section the gritty beds on the horizon of the Corwen Grits 

 lie, as already described, unconformably upon the Bala beds of the 

 valley, and they pass up into a series of well-cleaved blackish 

 pyritous mudstones, exactly like those seen in Church Beck, near 



Q. J. G. S. No. 142. u 



