298 H, HICKS ON THE PRE-CAMBRIAN ROCKS IN 
are succeeded by quartz-felsites of the true Aryonian type (with 
probably some bands of hiilleflintas), which may be traced con- 
tinuously to near Bangor, where the Pebidian beds, I believe, rest 
upon them unconformably. 
According to this view the three formations made ont in Pem- 
brokeshire are found closely associated also in Caernarvonshire; and 
the evidence, as there, seems to point to their being unconformable 
here also to one another. 
How far these unconformities may be considered as evidence of 
important changes or simply as local accidents, it is of course im- 
possible to say ; but it seems as if a considerable lapse of time was 
indicated, and that other formations may possibly have even inter- 
vened, from the different state of metamorphism in which they are 
found, and the different direction usually cf the strike in the beds 
in each formation, which, if constant, would certainly mean dif- 
ferent periods of upheaval. 
Lleyn Promontory. 
In the Lleyn Promontory, or that part of Caernarvonshire which 
gradually tapers towards the $.W., with an imaginary base-line 
drawn from Clynnog-fawr on the one side to Criccieth on the other, 
numerous masses of so-called intrusive rocks are marked on the 
Geological Survey maps. That some of these are, as supposed, intrusive 
masses, which have altered to some extent the surrounding rocks, 
there can be no doubt; but that by far the larger proportion 
are not so, and do not in any way alter the beds in contact with 
them is equally certain. That many of these also are of Pre- 
Cambrian age I think I shall be able to prove, though, in the ab- 
sence generally of true Lower Cambrian rocks in this area, the evi- 
dence has to be based frequently on the general character of the 
rocks themselves and the behaviour of the beds in contact with 
them. I may say also that as no contemporaneous lavas of Lower 
Cambrian or even Upper Cambrian age have been recognized in 
Caernarvonshire, this fact will be considered as of importance in 
attempting the correlation of the rocks, though not as absolute proof 
alone. In the term Upper Cambrian I include the rocks now gene- 
rally recognized as Lingula-flags and Tremadoce rocks only. 
The rock of Mynydd-y-Cennin is described by Professor Ramsay 
as “a quartz-porphyry similar to that of Llyn Padarn near Llan- 
beris, and probably having a like origin ;” this is therefore un- 
doubtedly an exposed portion in continuation of the Llyn-Padarn 
and Moel-Tryfaen ridge, and, like it, of Pre-Cambrian age. The 
Lower Cambrian rocks have here been faulted down ; but they are still 
surrounded, partially at least, by beds of Upper Cambrian age, and 
which are not in the slightest degree altered at the junction. The 
mass to the north of Clynnog-fawr is of the same character, and also 
surrounded by Upper Cambrian rocks, and on the one side by even 
Lower Cambrian beds also unaltered in contact. The masses 
further south, and forming the mountains of Bwlch-mawr, Gryn-goch, 
Pen-llechog, and the beautiful and very conspicuous Eifl (or Rivals) 
