ROCKS OF PEMBROKESHIRE. p41 
that portion of the cliff at Porth-lisky harbour, where the Pebidian 
beds are in contact with the Dimetian. ‘The line of demarcation is 
a perfectly clear one, but the beds are much crushed at the junction, 
and it seems almost incredible that any one could have supposed 
this boundary to indicate the outline of a mass of intrusive granite, 
The Pebidian beds here have a curiously arkose-like appearance ; 
indeed some specimens, taken from the section a few yards higher 
in the succession than the position of the diabase dyke indicated, 
~ seemed by the aid of an ordinary lens to consist almost entirely of 
quartz grains and decomposed felspar. The microscopical evidence, 
however, does not quite bear out this view, though it shows un- 
mistakably the same facts that have been so frequently referred to 
in reference to the Lower Pebidian beds on the other side of the 
Dimetian—that is, that rocks of the Dimetian type must have 
yielded a considerable amount of the detrital material found in the 
Pebidian beds, especially in the lower ones. Note 20 refers to the 
arkose-like rock mentioned above, 21 to a specimen from beds still 
higher up in the section, and 22 to one irom the east side of the 
west harbour. These together give a fair representation of the rocks 
in the section between the stream and the Dimetian. West of the 
stream and forming the cliffs on the west side of the main harbour, 
the beds consist of acid breccias of various colours, with occasionally 
some bands of argillites and other detrital materials ; Note 31 refers 
to a slide from a fragment tolerably typical of this series. At Ogof- 
henllys and behind the cottage on the top of the cliff near that place, 
some diabase sheets or dykes are found in the series ; and they occur 
again further south, at Pen-y-foel, as mentioned by the Director- 
General (Note 34). The next series, ¢ on the map, consists of the 
quartz-felsite breccias and schists of various shades of colour, which 
are so easily recognizable throughout the district, and have a well- 
marked horizon. They reach to the coast of Ramsey Sound at 
Ogfeydd-duon, and they are overlain unconformably by the Cambrian 
conglomerates as in fig. 14, in the first little creek to the north. 
These beds have had a schistose structure developed in them by 
crushing, and they appear to be, for the most part, old rhyolites, 
but with marked differences from those characteristic of the Arvonian 
group. Some of these beds are correlated by Dr. Geikie with the 
Porth-lisky beds, though they differ entirely from those. Moreover, 
he has failed to recognize the true direction of the strike in these 
beds, which is not in a line immediately below the Cambrian con- 
glomerates, but inland and under, instead of over, the Trefaiddan 
and Rhosson series of basic ashes and sheets. Had the position of 
these beds and their true nature been recognized by the Director- 
General, he would have seen how fatal the facts are to his theory 
of an isocline in this area. The Note 33 refers to one of these schis- 
tose quartz-felsites from Carn-ar-wig to the south of Ogfeydd-duon, 
and Note 32 to another, from the same series inland, in the valley 
of St. David’s, from a quarry in a field south of Emlych. The latter 
will be again referred to in describing that area. On the south 
shore of Carn-ar-wig and at Pen-maen-melyn, there are some beds 
Q.J.G.8. No. 159. on 
