CLASSIFICATION OF THE EOZOIC ROCKS OF THE BRITISH ISLES. 293 
(Dimetian ?)’ I have since tlien carefully re-examined these 
rocks, and have come to the conclusion that they should be 
included rather with the Arvonian. Whether they occupy a 
higher or a lower, or an equivalent position to those of Bo- 
dafon Mountain, it is difficult at present to decide. Still the 
evidence seems to point to the quartz group at Holyhead being 
the highest. At St. David’s, rocks like those of Holyhead 
exposed near Llanhowel are, I believe, higher in the succession 
than the more typical ‘ halleflintas ’ of Treffgarn Hock. The 
strike is usually somewhere near north and south. This for- 
mation may be divided into two groups under the names 
of Treffgarn and Holyhead. 
4. Pebidian. 
This is a formation undoubtedly of enormous thickness, as may 
be seen from its partial exposure at St. David’s (Sect. 1, p. 308), and 
contains several groups of rocks, in some respects considerably 
unlike each other. At present, however, it may be sufficient to 
divide the formation into two main groups. The lower one con- 
sists of the thin-bedded gneissic, micaceous, and talcose rocks ; 
and the higher, of the brecciated rocks with their associated 
schists and slates. At St. David’s the lower group is either 
but partially exposed or cut off by a fault. Some of the beds 
may be seen at Porthlisky Harbour resting unconformably 
upon the Dimetian axis. These are rather highly felspathic ; 
others are the so-called talcose rocks, found frequently 
at this horizon in Canada, and described by Dr. Sterry Hunt as 
the 4 commonly called talcose or nacreous schists, owing their 
peculiar characters to a soft, hydrous mica, which is not unfre- 
quently disseminated in very quartzose beds and gives to such 
a schistose character.’ Exactly similar rocks to these at Porth- 
lisky occur at Dulas Bay, on the north coast of Anglesey; and I 
believe it will be found that they occupy a low position in relation 
to the chlorite- schists so well developed in that island. That the 
chlorite- schists in Anglesey, with a general north-east and south- 
west strike, belong to a period in the history of the globe not far 
removed from that indicated by the Lower Pebidian rocks of St. 
David’s, I feel convinced. The deposits are sufficiently alike in 
material and in the changes they have undergone to be correlated. 
That they should now appear to be rather too much altered for this 
period, is probably due to the readiness of the chief materials 
entering into their composition to undergo change as compared 
with those in many of the other rocks. The view put forward 
by Sterry Hunt and Delesse, that 4 talcose and chloritic rocks 
have been directly formed from the molecular re-arrangement 
or diagenesis of aqueous magnesian sediments,’ seems to be the 
correct one, and to be capable of explaining many of the ap- 
