296 H. HICKS ON THE PRE-CAMBRIAN ROCKS IN 
altered beds were undoubtedly Pre-Cambrian. ‘The majority of 
the rocks in this area are breccias and felspathic schistose rocks ; 
but there are also some porcellanitic and compact and schistose 
chloritic rocks. I look upon them as belonging to the Pebidian 
formation and probably unconformable to the central ridge previously 
described. In my description of that ridge * I associated the rocks 
composing it with the Pebidians also, as we were then unaware of a 
third group as distinct from the others. It seems likely now, 
however, that the more central or highly altered portion will 
have to be associated with the Arvonian, and that only the least- 
altered beds along the flanks can be grouped with the Pebidians. 
On the accompanying map (p. 297) the three Pre-Cambrian groups 
are indicated ; and places where each may be examined will be men- 
tioned in the description given of the separate areas. 
Caernarvon to Bangor. 
I propose to refer very briefly to this area, as it was partially 
described by Professor Hughes in his papery read before the Society 
in December 1877; and it has subsequently been examined by Pro- 
fessor Bonney, in whose paper it will be fully referred to. As the 
results, however, obtained by me when examining the rocks in this 
area in August last seem to bear out the interpretation which I had 
previously given to it, I feel it necessary to lay a few of the facts 
before the Society. In the former paper I stated that ‘‘ represen- 
tatives of the two great unconformable series, Dimetian and Pebidian,” 
occurred in Caernarvonshire. “ With the former of these I would 
associate the so-called syenitic ridge (granitoid rocks) of Caer- 
naryon; and with the latter the altered beds which rest directly on 
the syenite ridge towards Bangor.” These expressions were used 
after a careful examination of portions of the area in 1877; but I 
felt it necessary to devote more time to the district before venturing 
to express myself more strongly on these points. The results 
obtained in our explorations last year have fully borne out my 
previous views; and it is also a great source of satisfaction to 
find that the careful labours of Professor Bonney, both in the 
field and with the microscope, tend also in every way to confirm 
those views. 
The new facts which I need to refer to chiefly bear upon the vary- 
ing character of the rocks as we proceed from Caernaryon to Bangor. 
At Caernarvon the true Dimetian type is found. About four miles 
to the north of this, on the road to Menai Bridge, a sudden change 
takes place, and a greenish-grey compact rock is seen haying more 
of the character of some of the Aryonian or hilleflinta rocks of 
St. David’s than of the Dimetian. The microscopical examination 
of this rock is given by Professor Bonney in the Appendix, No. I. 
I believe it will be found that a stratigraphical break on a fault 
occurs near here, and that rocks belonging to two distinct formations, 
Dimetian and Arvonian, are thereby brought into contact. These 
* Quart. Journ. Geol. Soc. vol. xxxiy. p. 147. t Ibid. p. 137. 
