304 H. HICKS ON THE PRE-CAMBRIAN ROCKS IN 
most of those in the neighbourhood of Holyhead and along the north- 
west coast, seem to me closely allied to rocks of the Pebidian forma- 
tion in other areas; and I propose, therefore, to associate them with 
that group, though possibly some, such as the rocks of Holyhead 
mountain, may prove to be of different age and probably older 
(Dimetian?). The remaining area is described as “a belt of meta- 
morphic rocks, from two to four miles wide, that strikes north-east 
from Newborough Sands across the island to Beaumaris and Llan- 
dona.” The prevailing rocks in this area, as seen, at least, on the 
road from Menai Bridge to Beaumaris, may be described as green or 
greenish grey, with a schistose foliation. Interstratified with these 
are also some hard green bands frequently containing veins of epi- 
dote. Some micaceous schists and purplish rocks are also described 
as occurring in this area. 
The rocks collectively, as seen here, are peculiarly like those in the 
Pebidian group at St. David’s, and there can, I think, be little doubt 
of their position. Vide Appendix, No. XII. 
Ffestiniog and Dolgelly. 
In addition to the Pre-Cambrian rocks now described, there are 
in North Wales at least two other areas in which they are found ; 
but as these are in the county of Merioneth, and not intimately 
associated with the districts under consideration, I hope to be able 
to describe them more fully in a future communication. One of 
these is in the neighbourhood of Ffestiniog and partly shown in 
Map, the so-called Moel-tan-y-Grisiau syenite, and was examined 
by me some years since, but has been more recently explored by 
Mr. Tawney, who has, I think, most satisfactorily proved that it 
is mainly of Dimetian age. ‘The other is in the neighbourhood of 
Dolgelly, and was recently mentioned, in my paper to the British 
Association, as being chiefly of Pebidian age. 
Conclusions. 
Collectively, these additions to the Pre-Cambrian areas hitherto 
known are large and important, and many of them were not in the 
slightest degree suspected until we commenced our researches 
amongst them a few years since. Of others, especially some of the 
rocks in Anglesey, Professors Sedgwick and Phillips many years ago 
expressed the opinion that they were older than the Cambrian rocks. 
These views, however, did not receive much support from geologists, 
but were for the time crushed by the combined weight of authority 
vested in the Geological Survey as represented by its chiefs, who all 
declared that these must be altered Cambrian and Silurian rocks 
only. Consequently they were so marked on the geological maps, 
nd still remain so, ‘ 
