378 MR. H. H. THOMAS AND PEOF. 0, T. JONES ON THE [Sept. 1912,, 



The Pebidian of this region is apparently not so well developed 

 as in the district of St. Davids, nor is it separable into so many sub- 

 divisions. It is, however, possible to detect several different stages- 

 which may be correlated, in a general way, with the upper portion 

 of the St. David's Pebidian as described by Mr. Green under the- 

 iiames of the Treginnis, Caerbwdy, and possibly Eamsey Sound 

 Series. 



Broadly speaking, the Pebidian of the Hayscastle region may be 

 divided into two groups of rocks : one best-exposed in the neigh- 

 bourhood of Pont-yr-hafod, and the other around Rhindaston and' 

 Gignog. 



(i) The Pont-yr-hafod Group. (P^— P^ of the Map, PI. XL.> 



Between Hayscastle and Tre-rhos there seems to be a definite 

 sequence of rock-types appearing in the same order on both sides- 

 of the stream which crosses the road at Pont-yr-hafod ; the distri- 

 bution of beds suggests either a synclinal, or an anticlinal, axis- 

 ranging nearly along the valley. The beds on the north side of 

 the valley are exposed on the same line of strike at Hayscastle, 

 where they have a dip of 45° to the north-north-west ; this renders- 

 it probable, apart from other considerations, that the beds lie in the 

 form of an anticline. 



Assuming that this inference is correct, the oldest strata exposed 

 in the district are highly felspathic tuffs, some beds of which are 

 coarse and even conglomeratic. Other bands are fine-grained, 

 frequently laminated, and present a striped appearance on a fractured 

 surface. The rocks of coarsest texture occur in the lowest exposed 

 part of the group. 



The lower and coarser tuffs are prevailingly pink and green ; 

 and, as the differently coloured fragments in the finer varieties 

 are fairly evenly distributed throughout the rock, the result is a 

 mottled or speckled appearance. 



In the coarser tuffs, such as those cropping out along the northern 

 side of the valley below Pfynnon-gron, the fragments are rounded 

 and reach an inch in diameter. These fragments are variously 

 coloured, and consist of quartzite, rhyolite, quartz-bearing fine- 

 grained tuffs, and large fragments of keratophyres and rocks of 

 intermediate composition : an assemblage which points to the 

 existence of an older group of bedded deposits unexposed within 

 the district under description. Most of the rock-types, however, 

 are represented in the later Pebidian deposits. With these coarse- 

 pink and green tuffs are associated some apparently thin bands of 

 purple or maroon-coloured fine-grained tuff which furnish highly 

 characteristic debris. 



The coarser rocks are succeeded by a thin series of blue and 

 variously-coloured pale speckled tuffs of acid composition, which are 

 in turn followed by buff- weathering rhyolitic tuffs containing bands- 

 of purple, less-acid, speckled tuffs that link them with the older 

 rocks of the group. 



