part 4] GEOLOGY OF COERtS AND ABERLLEFENNI. 539 



the lithological characters of the groups in both areas being 

 practically identical. 



When the thicknesses of these groups are compared, however, 

 there are man}- striking differences. Prof. O. T. Jones informs me 

 that he considers the Brynglas and Drosgol Groups to be approxi- 

 mately 900 and 1150 feet thick respectively : that is, a total 

 thickness of 2050 feet. The Garnedd-wen Beds in the south- 

 western part of the Corris area approximate to that total thick- 

 ness ; but there is a marked diminution in thickness in the eastern 

 part of the area, where the Garnedd-wen Beds are not more than 

 500 feet thick, probably rather less. 



The Nant-y-Moch Group is about 2000 feet thick, and even 

 then the base of the group is not exposed. The Red Vein and 

 Narrow Yein are only about 400 feet thick throughout the Corris- 

 Aberllefenni area. 



It is, of course, possible that the Narrow Vein may be a 

 development of the lower part of the Drosgol Group ; but that 

 does not materially affect the values given, for the Narrow Vein 

 rarely, if ever, exceeds 60 feet in thickness. 



The Plynlimon Stage is about 4000 feet thick, as exposed at 

 Plynlimon. At Corris the total thickness of the corresponding 

 Abercorris Group varies from 2400 feet in the west to 900 feet in 

 the east. The point where the Abercorris Group diminishes in 

 thickness to about 900 feet is approximately 16 miles due north 

 of Plynlimon (see fig. 6, p. 538). 



Further, it is interesting to observe that at Plynlimon, Dicello- 

 graptus anceps occurs in a band about 600 feet above the shales 

 which }deld Orthograptus truncatus. At Corris, D. anceps is 

 found in a band which is considered to be a short distance, probably 

 only a few feet, above the shales that yield O. fruncaius var. abbre- 

 viate in such abundance. 



It will, therefore, be seen that the correspondence between the 

 two areas is very close, despite the great difference in thickness of 

 the rock-groups. 



The Bala area. — The lower part of the Ceiswyn Beds recalls 

 in many ways the lithological characters of the Nant Hir Shales 

 which overlie the Ordovician volcanic rocks west of Bala. 1 At 

 Corris, however, there is no trace of the Derfel Limestone recorded 

 at Bala. The rocks immediately above the volcanics at Corris are 

 well exposed, and so it would appear that that highly characteristic 

 limestone-band dies out before this area is reached. 



At present, it is difficult definitely to correlate any of the groups 

 established at Corris with those described at Bala, although the 

 two areas are only about 20 miles apart. The Foel-y-Dinas 

 Mudstones yielding the Phacops-mucronatus fauna are probably 

 equivalent in general to the Garnedd-wen Beds, since the latter, 

 north-east of the area at present described, yield Ph. mucronatus. 



1 G. L. Biles, Q. J. G. S. vol. lxxviii (1922) p. 132. 



