516 HARTFELL-VALENTIAN SUCCESSION AROUND PLTXEOION. [Nov. 1909. 



Supplementary exposjurc. 



Half-a-mile south of Craig-y-Ceffyl is a good section in the 

 blue-black mudstones and overlying shales, in the small stream of 

 Nant Hylles. About 50 yards up stream from an old sheepfold, 

 the mudstones contain a thin sandy shale-band which weathers to 

 a buff colour (F. 58) ; it yielded many graptolites, mostly in a 

 fragmentary condition, among which the following species were 

 identified : — Monographis exiguns (c), M. galaensis, M. holmi (v.c), 

 M. cf. holmi approaching priodon, M.proteus, and M. turriculatus 

 (v.c). 



The result of the traverses across the southern part of the 

 district has been to demonstrate the existence within the Ystwyth 

 Stage of three distinct rock-groups, which always follow one another 

 in the same order from west to east, namely : — (i) The group of 

 mudstones with regular alternations of thin grits, to which the name 

 of Devil's Bridge Group is given; (ii) a group of mudstones 

 almost devoid of grits, but containing numerous shale bands, espe- 

 cially in the upper part: for this the name Myherin Group is 

 suggested ; and (iii) a shale group, in which occur bands of coarse 

 speckly grit and evenly-bedded, dark, gritty flags : the name of 

 Bhuddnant Group seems appropriate for this, as it is not so 

 well displayed anywhere else within the limits of the district. 

 Again, within the Myherin Group two subgroups can be readily 

 distinguished : — (a) a lower, characterized by the pale-blue or grey 

 colour of the mudstones and the comparative rarity of shaly bands : 

 from their fine development on Banc Dolwen, south of the Afon 

 Myherin, they may be distinguished as the Dolwen Mud- 

 stones; (b) the upper is easily recognized by the dark-blue colour 

 and almost glossy character of the rocks, and by their peculiar 

 property of weathering in concentric rings of a bright orange-red 

 or vermilion : shaly bands-are relatively more frequent and thicker 

 than in the lower subgroup, and I can suggest no better appellation 

 for them than that of Blaen Myherin Mudstones, after the 

 name of the farm at the head of the valley. 



That the order of these groups is one of ascending sequence from 

 west to east is shown by the magnificent sections along the Myherin 

 and Bhuddnant Valleys, where, despite numerous undulations, the 

 predominant effect of the easterly dip is very clear. This is also 

 only what might be expected from the disposition of the lower 

 strata of the district; for it has been shown, when dealing with 

 those groups, that, in any traverse from west to east or from north- 

 west to south-east, we pass (on the whole) from older to younger 

 strata : that is, the sequence is an ascending one in that direction. 



Fig. 13 (p. 517) illustrates the general sequence within the stage, 

 and the graptolites which characterize certain horizons. 



