part 4] DISTRICT ABOUND COBEIS AND AEEELLEFEJOa. 525 



is their attenuation as they are traced from south-west to north- 

 east. In the south-west, on the margin of the area described, they 

 are about 2000 feet thick. At Corris itself they are probably 

 about 1800 feet, and immediately north of Garnedd-wen Station 

 about 1400 to 1500 feet thick. North-east of Aberllefenni the 

 diminution in thickness is very marked. At Cymerau they are 

 about 650 feet thick, while on the extreme eastern margin of the 

 area described they are not more than 500 feet thick. The 

 Garnedd-wen Beds, therefore, diminish in thickness from about 

 2000 to 500 feet in a distance of little more than 6 miles along 

 the strike. 



This diminution in thickness is accompanied by a loss of 

 arenaceous material. In the eastern part of the area, from Cymerau 

 to Ratgoed, the Garnedd-wen Beds are almost exclusively a mud- 

 stone group with no conspicuous grit-bands. A particularly good 

 example of this loss of arenaceous material is seen in the Ceiswyn 

 A r alley. From Aberllefenni over Foel Crochan, the Narrow Vein 

 is overlain by the conspicuous massive grit already mentioned. 

 This grit-band is maintained on the north side of Foel Crochan ; 

 but, almost immediately after crossing the Ceiswyn stream, it dies 

 out quite suddenly, and the Narrow Vein is overlain by a very 

 characteristic group of dark-blue mudstones, weathering to a pale 

 brown. These mudstones are traversed by thin irregular seams of 

 greyish siliceous material. These siliceous seams have a distinct 

 current-bedded appearance, and a characteristic contorted or ' curled' 

 bedding. Weathered fragments give concentric weathering tints. 

 These mudstones with siliceous seams overlying the Narrow Vein 

 are well known to the local quarrymen, by whom they are 

 appropriately called the ' ribbony rock '. 



The mudstones, gritty mudstones, grits, and conglomeratic bands 

 of the Garnedd-wen Beds in the Corris- Aberllefenni area are 

 precisely similar to the Ordovician rocks which crop out in the 

 crests of anticlines and occupy such considerable areas in the 

 northern part of Central Wales. 1 



No fossils have been found in these beds in the Corris- Aberllefenni 

 area ; but north-eastwards in the Dinas Mawddw} r country, I 

 have collected specimens of Pliacops mucronatus from the 

 lowest beds of the Garnedd-wen mudstones, immediately overlying 

 the Narrow Vein. 



Valentian. 

 Pont Erwyd Stage. 



Cwmere Group. — -As at Machynlleth, this group consists of 

 dark-blue shales, with subordinate siliceous bands and an occasional 

 band of grit. The group is especially characterized by its 

 pronounced rusty Aveathering, which serves to distinguish it from 



1 O. T. Jones & W. J. Pugh. Q. J. G. S. vol. lxxi (1915-16) p. 347; O. T. 

 Jones, ibid. vol. lxv (1909) p. 468 ; and id. Special Reports on the Mineral 

 Resources of Great Britain : vol. xx- Lead & Zinc, Mem. Geol. Surv. 1922. 



