File:The Quarterly journal of the Geological Society of London (13204715765).jpg

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,