ROCKS OF PEMBROKESHIRE. 5k 
3. Porth-clais and Allan Valley. 
In Porth-clais farmyard, between the area last referred to and the 
Allan Valley, the Dimetian rocks, in their usual condition, are 
exposed, and they are seen here to be penetrated by dykes of porphy- 
ritic felsites. In the lane leading from the farmyard to the moor to 
the east I obtained specimens from the actual junction of one of the 
dykes* with typical Dimetian, containing parts of each. One of these 
is described in Note 8. The felsite is exposed also in the field to 
the north of thislane. This evidence, again, is conclusive in proving 
that the porphyries are not, as stated by Dr. Geikie, “‘ grouped round 
the central boss of granite,” our Dimetian, or, “intimately con- 
nected with it, like the elvans of granite districts” (p. 3138). 
In Porth-clais harbour, and in the Allan Valley immediately to the 
north, some of the most important sections referred to by the 
Director-General to prove that the Dimetian is intrusive occur. Of 
these he says it ‘is difficult to believe that they can have been 
actually seen by any one who could afterwards maintain the rock to 
be Pre-Cambrian in age” (p. 274). It is also stated on the same 
page that they ‘“‘show the granite to be unmistakably eruptive ; 
for the strata adjacent to it present examples of the induration and 
silicification so commonly, though not universally observable, along 
the borders of a granite boss;” also that he ‘‘searched in vain 
among the published papers for any account of these localities.” 
In reference to these statements I need only say that all those who 
took part in the discussion, who had visited St. David’s at any pre- 
vious time, gave ample evidence to show that these sections were — 
well known to, and had been examined by them, and that fortunately 
they had interpreted them in their true light. My papers also 
contain ample evidence to show that they had been carefully studied 
by me; and I need only say further that when the former Director- 
General, Sir A. Ramsay, accompanied by Mr. Etheridge, visited St. 
David’s, their special attention was called by me to these sections. 
Fearing that further difficulties may arise as to the true interpre- 
tation of these sections, I have had some sketches prepared to 
show the exact spots where the facts are to be observed, so that 
they may be readily examined by those who visit the area in 
the future. The ‘“tongue-like projection across the river at the 
ford” is indicated in fig. 6, and in the plan, fig. 5. Instead of 
being an intrusive tongue penetrating the Cambrian strata, as 
* In Professor Blake’s map (Quart. Journ. Geol. Soc. vol. xl. p. 295), these 
are indicated as fringing the Dimetian. The evidence is perfectly clear that they 
are intrusive in the Dimetian ; hence conclusions based on this view are valueless. 
The want of knowing that these felsitic rocks here and about Nun’s Chapel are in- 
trusive dykes, and not old rhyolites like those I have included in the Arvonian, 
has caused him to suppose that the latter everywhere fringe the Dimetian, and 
therefore that the Pre-Cambrian Groups at St. David’s belong all to one vol- 
eanic series. ‘The evidence is clear, however, that this supposition, as stated 
above, is incorrect. The actual junctions here between the Dimetian.rocks and 
the felsites are most sharply defined, and there is not a particle of evidence to 
show a gradual passage of the one into the other. 
