PRE-CAMBRIAN ROCKS IN PEMBROKESHIRE. 289 
The discovery of new areas, however, has now made it clear that 
these quartz-felsites are more characteristic of the Aryonian group, 
and usually in association with the halleflintas; and that at St. 
Dayid’s, as elsewhere, they are for the most part old rhyolitic lavas 
interstratifying rocks of the true hiilleflinta type. 
Much of the ground to the N.E. of St. David’s is obscured by 
drift, bogs, and mossy ground, and hence some difficulty has been 
experienced in unravelling it satisfactorily. I have, however, by 
carefully tracing all lines of streams, the materials derived from deep 
wells, as well as all quarries that could be found, now proved con- 
clusively that at least two thirds of the central axis will have to be 
associated with the Arvonian—or,, in other words, that nearly all 
of the red patch, coloured as syenite and felstone in the Geological 
Survey Map, to the N.E. of St. David’s will have to be assigned 
to that formation, the lower portion only, that of which I have 
already given sections, being Dimetian. 
The discovery of the new areas already referred to, viz. of Roch 
and Treffgarn, and of a new type or group of rocks there, has cleared 
up much of the difficulty which we had to deal with in the rocks of 
St. David’s and in North Wales; and I venture to hope that it has 
given us the clue by which many other areas hitherto doubtful may 
be unrayelled. 
The junction of the Arvonian with the Dimetian may be seen 
at St. David’s, about a quarter of a mile to the south of the Cathedral 
and near Rock House. ‘The Dimetian is well exposed at Bryn-y- 
garn, where it stands out as a prominent ridge. A little to the 
north of this there is a slight depression in the surface of the ground ; 
and this I consider to mark a line of fault, for immediately beyond 
this the Arvonian rocks are seen striking up towards the ridge. The 
lowest beds, as seen on the road-side near the Deanery, show some- 
what of a brecciated appearance ; but when the rock is submitted to 
microscopical examination this character is less evident, and it may 
possibly be a form of brecciation in situ. Both this rock and the 
immediately succeeding one, according to Mr. Davies’s descriptions 
(Nos. 4 and 5), come sufficiently near the halleflinta type to associate 
them together. ‘The difference in any case between them and the 
Dimetian is most marked; and the abrupt junction between the one 
and the other at this point has led me to place here the line of 
demarcation. Succeeding these, as we trace the line to the N.E., 
are the quartz-felsites and porphyries mentioned in my former 
paper, and which are well exposed near the church and _board- 
schools. No. 6 in Appendix also comes from this point. Nearly 
opposite this, on the east side of the ridge, between Trepewet and 
Clegyr, true hiilleflintas are found, in some cases showing a brec- 
ciated appearance, but in others the compact variety found at Roch 
and Treffgarn. These show, under the microscope, the characteristic 
nests of quartz, and one of the brecciated ones is described by Mr. 
Davies in No. 7. In association with these are found the quartz- 
porphyries of the Church-school quarry type; and resting uncon- 
formably upon the whole are the great agglomerates of Clegyr 
