285 



no vestige of an unconformability, but the volcanic groups which 

 Dr. Hicks has included in his " Pebidian " can be seen in many 

 places graduating insensibly into the altered shales which form a 

 great part of his so-called " Arvonian," In fact the latter group, as 

 above stated, consists of portions of the volcanic breccias and tuffs 

 (that is, the " Pebidian " strata) where these are invaded by quartz 

 porphyry. This is well seen in the series of quarries north from 

 the Church Schools, where, as already remarked, a perfect gradua- 

 tion can be traced from highly altered shales and tuffs, next the 

 intrusive quartz porphyries, northward into the normal condition of 

 these strata in the district. The asserts n that the Pebidian strata 

 are made up chiefly of" Dimetian" fragments must, from the evidence 

 already adduced as to the late date and intrusive nature of the 

 " Dimetian " mass, be founded on error of observation. I need hardly 

 say that, after the most patient search, neither Mr. Peach nor my- 

 self could detect anywhere in these tuffs and breccias the smallest 

 fragment which, by the utmost stretch of fancy, could be referred to 

 the granite of the " Dimetian " ridge. There occur indeed, abundant 

 lapilli of felsite, as I shall more fully describe in the sequel ; but 

 these fragments can readily be discriminated from the material form- 

 ing the eruptive porphyry dykes and bosses. 



But the most extraordinary statement in the passage just cited is 

 that in which the writer asserts that the great agglomerates of 

 Clegyr Hill rest unconformably upon his " Arvonian " group. 

 The agglomerates in question are well seen on the road-side east of 

 Clegyr Bridge, where they dip towards S.S.E. at 65°, and are inter- 

 banded with siliceous layers (halleflintas). These siliceous bands are 

 precisely the same as those seen near the quartz porphyries north of 

 the Board-Schools, on the shore at Nun's Chapel, and elsewhere. They 

 are characteristic of the volcanic group where it. is traversed by 

 intrusive siliceous eruptive rocks, and particularly of a zone that 

 lies not far below the conglomerate to be referred to in a later 

 part of this paper. The presence of these bands in what Dr. Hicks 

 himself cites as typical " Pebidian " agglomerates is an important 

 fact, in still further proving that, at St. David's itself, " Pebidian " 

 and "Arvonian" are only different names for the same series of 

 rocks. 



On Clegyr Hill the same agglomerate, interbedded with fine tuff 

 and bands of siliceous schist, and traversed by a dyke or boss of 

 spherulitic quartz porphyry, appears to dip towards the JST.W. at 

 40°-50° — that is, actually towards the rocks which it is said to 

 overlie. Nothing is seen at the surface for a distance of nearly a 

 quarter of a mile, when the observer finds that a few square yards 

 of quartz porphyry have been laid bare in a quarry to the south of 

 Trepewit, while, about thirty yards distant, fine tuff, nearly vertical, 

 but preserving the normal E.N.E. strike, is seen on the road-side. 

 Yet Dr. Hicks boldly asserts not only that the rocks of Clegyr Hill 

 overlie the porphyry, but that they do so unconformably ! 



