234 



MR. C. A. MATLEY ON THE 



[a) The Llandeilo Strata. 



[May 1900, 



A glance at the map (PI. XIII) will show that these rocks occupy 

 four strips of ground in the western part and three in the eastern 

 part of the area. These latter three and the northernmost one of 

 the former lie on the same line of strike. The full succession does 

 not occur in any one of these outcrops, but can be demonstrated by 

 piecing together the sections seen in several localities : it is as 

 follows : — 





Probable 

 thickness. 



Fossils. 



Cd. Black argillaceous shales , 



Co. Ironstone, in part oolitic 



Cb. G-rey quartzose shales or 1 

 slates striped by thin black I 

 lamina?. J 



Ca 2. Pale conglomerates and 

 grits. 



Ca 1. Red-purple conglomerate. 



40 feet 

 (top not seen) 



20 feet 



150 feet 



500 feet 



180 feet 



See (a) & (j8) Quart. 



Journ. Geol. Soc. vol. 



lv (1899) p. 639. 

 See (7) ibid. p. 640. 



Crinoid-stems. 



See {$) to ( V ) p. 640 of 

 the above-quoted paper. 



(1) Between Hell's Mouth and Forth Wen Bay.— The 

 basal beds are best exposed here. The lowest bed is a great purple 

 conglomerate which dips north-north-eastward at 65° to 70°, but the 

 outcrop is ven r variable in width. Its relation to the underlying 

 rock will be described in discussing the quartzites of the Llanbadrig 

 Series (p. 244-). The matrix is very sandy, reddish purple in colour, 

 and the included pebbles, of all sizes up to more than 2 feet in length, 

 are quartzite and quartzose grit, mainly reddish, some white. 



Pebbly grits, interbedded with finer slaty beds and containing 

 some bands of conglomerate, succeed the purple conglomerate to the 

 north. The junction may be natural near Hell's Mouth, but it 

 is a fault between Craig Wen and Porth Wen Bay. These beds are 

 usually pale, or even white, sometimes of a sage-green tint. About 

 12 feet of soft blue-black shale is seen at Porth Wen Works: this, 

 however, may be faulted in. 



The series is repeated in the headland of Torllwyn. The purple 

 conglomerate, with part of its thickness cut out, forms a band from 

 Porth Adfan to Porth Wen Bay. It has been brought up and 

 thrust over the grits, and in Porth Adfan a few feet of a still higher 

 zone, namely the banded sandy shales, cleaved, crushed, and full of 

 small thrusts, appear below the thrust. The pale gritty and pebbly 

 beds follow, and continue to the northern extremity of the headland, 

 where their dip is about 50° north-north-eastward. 



All these beds have been strongly compressed by movement from 

 the north, the effect of compression being especially marked in the 

 case of the pebbles of the purple conglomerate. The principal 

 cleavage agrees in amount with the dip of the conglomerate, but the 



