252 THE EOCKS MAPPED AS CAMBRIAN IN CAEBNAPvVONSHIEE. 



of the hill overlooking the town on its south-eastern side, we find 

 the Purple Slate in characteristic form. The boundary between it 

 and the Halleflinta runs near the top of the Recreation Ground and 

 cuts off the north-eastern corner, descending? to the road-level just 

 south of the Mountain Square Slate works. These are of course the 

 Lower Purple Slates, and show the usual character of that portion in 

 being more massive and bedded, more sandy and less cleaved. They 

 have a high dip of about 70° to the east, and are overlain by the 

 Silurian grits and micaceous slates, with a dip of about 30° to S.S.E. 

 along an irregular line, which end by overlapping them and coming 

 into contact with the ' Halleflinta.'^ 



§ 5. The Phiw-wen Geit. 



That there is a grit at the base of all the Purple Slates, and thereby 

 distinct from the St. Ann's Grit, is very evident on Moel Rhiw-wen 

 (fig. 6). The summit of this enclosed hill is crossed in a N.N.E. 

 direction by a band of green grit with quartz-pebbles, which occa- 

 sionally weathers brown here. On the western side it is seen lying 

 on Pale Banded Slate, particularly at a quarry at Rhiw-wen, while 

 on the east — after passing a conglomerate, which is believed to be 

 unconformable — we come to old quarries of Purple Slate on the rise 

 to Drysgol : and the same relations of strata are seen at the northern 

 end. By the aid of this observation we are able to recognize the 

 true position of the grit on Moel-y-Ci (fig. 6). It is seen rising 

 from beneath the Purple Slate on the eastern side by Bron-y-foel, 

 and is succeeded on the upper slopes by Pale Banded Slate. This 

 last occupies most of the hill, but capping the western slopes we 

 find crags of grit again. Here ib dips slightly into the hill, but 

 soon turns up again and forms the summit. The grit on this hill 

 also weathers brown and is rather coarse, and I take all the masses 

 to be the same grit, forming part of a partially denuded semi- 

 anticlinal, and corresponding with that on Moel Rhiw-wen. 



We cannot, however, always recognize a grit as a distinct bed in 

 this position. The base of the Purple Slates gets gritty, and so does 

 the top of the Pale Slate series, but often no line can be drawn. 

 In the broken ground south of Tregarth we can, by selecting the 

 proper direction, pass over in downward succession purple slate, 

 grit (St. Ann's), purple slate, grit (Rhiw-wen), and pale slate. The 



^ I have previously shown (Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 271) 

 that rocks of this age lie uneonformably on all the members of the Cambrian 

 or older series from Bangor to Caernarvon, and this disposition of them I 

 have again confirmed by a careful examination of (I believe) all the exposures 

 near the former tovrn. Sir A. Geikie, in his Presidential Address for 1891, 

 states that neither he nor Mr. Peach, after 'numerous' observations, could find 

 ' any discordance in strike or dip between the flinty tuffs and the overlying 

 sliales and grits,' and accepts what to me appears a reduciio ad absurdum in 

 consequence. The fact, however, of the unconformity throughout is. to my 

 mind, so abundantly clear that I am sure a further examination would con- 

 vince Sir Archibald ; and I will not therefore crowd this paper by adducing 

 further proofs. 



