CAERNARVONSHIRE AND ANGLESEY. 301 
tempting to unravel so difficult a region as the Lleyn Promontory 
has proved to be. 
Nearly the whole of the area to the west of the Rhos-Hirwain 
ridge is occupied by so-called altered Cambrian rocks. These, it will 
be seen, are all of Pre-Cambrian age, chiefly belonging to the Pebi- 
dian formation. 
They are described generally by Mr. Selwyn, in Professor Ramsay’s 
memoir, as ‘a series of green, grey, and purple schists, often very 
hard and siliceous, sometimes arenaceous and gritty, with large and 
small veins, beds and masses of quartz rock, also bands and patches 
of calcareous rocks, and grey and pink siliceous crystalline limestone 
occasionally burnt for lime,” and to be pierced “by numerous 
greenstone dykes, and contain patches of serpentine, and veins and 
nodules of red jasper. These beds are much contorted on a small 
scale, but their general dip is north-westerly from 25° to 50°.” In 
this concise and clear description by Mr. Selwyn we see nothing to 
indicate that these are in any way like the Cambrian rocks in other 
areas in Caernarvonshire; but his description at once calls to mind 
the Pebidian rocks of St. David’s, Bangor, and other places. Our 
personal examination of these rocks was still more conclusive; and 
I have not the slightest hesitation, after the evidence obtained, in 
associating these rocks with those of Pre-Cambrian age, and more 
immediately with those in the Pebidian formation. 
They occupy an extensive area and ‘“‘form a band (exclusive of 
Bardsey Island) 14 miles long from Bardsey Sound to Nevin, and 
generally from a mile to 2 miles wide. A long spur also proceeds 
southward from the main mass to the east coast of Aberdaron Bay,” 
where they are described as “green chloritic siliceous schists.” 
Bardsey Island also consists of similar rocks. Though frequently 
much contorted, and hence repeated in folds, it is perfectly clear 
that they, on the whole, dip away from and follow the outline of 
the axis to the east formed of Dimetian and Arvonian rocks. We 
haye therefore a succession here almost identical with the one in 
the next promontory to the south, viz. the St. David’s one. On the 
whole also the rocks representing the three formations in both 
promontories are exceedingly alike; but it is probable that in Caer- 
naryonshire a greater thickness is shown, and hence that some bands 
occur which are but imperfectly represented at St. David’s, as, for 
instance, the calcareous and serpentinous ones. ‘The unravelling of 
this district, occurring, as it does, intermediate between St. David’s 
on the one hand and the great metamorphic areas in Anglesey on 
the other, seemed to me of the utmost importance, as any interpreta- 
tion which it might offer could not fail to have an important bearing 
on several other districts in North Wales, and especially in Anglesey. 
Anglesey. 
Some previous excursions which I had made into Anglesey had 
not enabled me to arrive at any thing like a satisfactory conclusion 
with regard to some of its rocks. Fortified, therefore, with this new 
evidence, we this year again carried our investigations there ; and 
