PEE-CAMBRIAN EOCKS OF ST. DAVID S. 



287 



Fig. 7. Contemporaneous Erosion accompanying Cambrian 

 Conglomerates. Caer-fai, St. David's*. 



the coast-section, or looking at the occasional exposures inland, 

 and finding that all the tuffs, breccias, and diabase sheets dip 

 steadily in a north-westerly direction, would infer that he is 

 crossing a continuous succession of beds, the highest being at the 

 north-west end and the lowest at the south-east end of the section. 

 This natural inference has been drawn by Dr. Hicks, and may 

 partly account for some of the errors into which he has fallen. 

 Further comparison, however, would have shown him that the strata 

 are here isoclinally folded ; that is, they have been thrown into an 

 anticline, which has been bent over to the south-east, so that the 

 strata in the south-eastern half of the fold are inverted (figs. 1 & 2, 

 p. 268). That this is the case, was proved by Mr. Peach and myself 

 in the identification of the same beds on the two sides of the arch. In 

 particular, the peculiar group of shales or schists immediately below 

 the conglomerate on Ramsey Sound reappears at Porth-lisky. The 

 conglomerate accompanies them ; but at the latter locality it has been 

 cut out by the granite. It appears, however, a short way inland in 

 the Allan valley, and on the east side of the granite at Ogof-llesugn. 

 The reversed dip continues along the coast-line ; but the beds are 

 eventually seen to right themselves, and they appear in normal 

 order to the east of Caer-fai. I shall return to this interesting 

 structure in the second part of this paper (p. 309) f. 



* Dr. Hicks figures this junction as an unconformability of the Cambrian 

 conglomerates on the Pebidian Beds. But he reverses the visible dip, making 

 tbe rocks inclined towards the sea instead of towards the land (Quart. Journ. 

 Geol. Soc. vol. xxxiii. p. 236). This subject is again referred to in the text. 



t It may be proper to notice here that the structure above described proves 

 that Dr. Hicks's estimate of the visible thickness of his " Pebidian " group is 

 greatly exaggerated. He makes the thickness at least 8000 feet (Quart. Journ. 

 Geol. Soc. vol. xxxiv. p. 159). Were the beds absolutely vertical all the way, 

 they could not be more than 4000 feet ; for they extend across a belt which, to 



