ROCKS OF PEMBROKESHIRE. 515 
am inclined to put it, of a due proportion of the basic silicates from 
the Dimetian at St. David’s is the main cause why it is not a true 
gneiss instead of a massive granitoid rock. The various changes 
to which the Dimetian rocks have been subjected would undoubtedly 
have produced a schistose structure, were more of the basic silicates 
present. All, however, may even have had an igneous origin ; but if 
so, the evidence that such was the case is now entirely wanting. 
One thing, however, is certain beyond doubt, that is that the Dimetian 
at St. David’s is not, as asserted by the Director-General at p. 313, 
—“a central boss of eruptive granite with associated peripheral 
' dykes, elyans, or amorphous intrusions of quartz-porphyry,” which 
“has been protruded through the Cambrian strata,’ and this is the 
main point which we have to contest. 
VI. Tur Arnvontan Rocks. 
a. Evidence of Age.—That these rocks, like the Dimetian already 
referred to, are older than the Cambrian conglomerates is a fact 
which cannot now be disputed, as the Cambrian conglomerates contain 
rolled fragments in abundance which cannot be differentiated from 
these rocks. Moreover in some places, notably in Ramsey Island (and 
in areas in North Wales, as described in a former paper *), the con- 
glomerates repose unconformably upon them, and are almost entirely 
built up of materials which could only have come from such rocks. 
As in the case of the Dimetian, so with these, it has been maintained 
by the Director-General that they, or at least the majority of them, are 
properly indicated on the Survey maps as eruptive masses in Cambrian 
and Silurian strata. 
I have referred above to the fact that fragments of these so-called 
Post-Cambrian eruptive masses are to be foundin abundance in the 
basal Cambrian conglomerates, but we have still more important 
evidence which shows that the rocks I claim under the name 
Aryonian are not only Pre-Cambrian, but more than probably also 
of Pre-Pebidian date. In the lowest Pebidian breccias, more pro- 
perly in some places to be called agglomerates, fragments of the 
peculiar quartz-felsites (old rhyolites), of the halleflintas, and of the 
indurated argillites and breccias which together mainly make up the 
Aryonian group in Pembrokeshire, occur in such abundance, that 
no doubt can arise that they must have been derived from pre- 
existing rocks identical in their grouping and character with those 
we now class as Arvonian. The Pebidian agglomerates at Clegyr Hill, 
St. David’s, are very largely made up of fragments of old rhyolites 
(spherulitic) like those found zn situ in the ridge at St. David’s and 
to the north-east; and curiously some of the so-called “adinole 
nodules or concretions varying from the size of a pin’s head to 
that of a man’s head or larger,” described by the Director-General 
in his paper at p. 320 as occurring in the Porcellanite series near 
Nun’s Chapel, are nothing more than fragments of these old rocks 
included in the associated breccias. These are described by Mr. 
* Loc. cit. 
