May, 1892. 
A WEEK IN NORTH WALES. 
Ill 
side of the valley, in order to reach the path leading to the 
upper valley, Cwm-yr-Afongoch (the hollow of the red river). 
One of the special points which we had proposed to our¬ 
selves to be aimed at in our little expedition was the determin¬ 
ing whether certain blocks of a curious porphyritic rock 
which occur plentifully among the broken rocks at the foot 
of the falls are derived from the great mass of igneous rock 
over the edge of which the water comes, or are residues of 
the washing out of the “ marine drift,” of which the valleys 
of the north coast of Wales are full. 
The colour on the Ordnance map is stated to mean 
‘‘massive intrusive felspathic rocks;” it is the same in the 
case of the enstatite diabase of Penmaenmawr, and the little 
Dinas Hill behind Llanfairfechan, and the same again as the 
quartz felsite of Llanberis, and the hornblende porphyry of 
Mynydd Mawr, of which we speak later on. Various patches 
of crimson interspersed (the colour denoting “ Greenstone ”) 
make it evident that the surveyors found a variety of rocks, 
and were in doubt Ubout the real nature of some of them. 
It is quite plain, from the researches of more recent 
geologists, although the tract of country is by no means 
thoroughly explored, that within this red patch there is 
included a considerable complex of rocks, and the opinion 
seems pretty strong that we have here the remains of one of 
the great Ordovician volcanoes of North Wales. (See Harker’s 
“Bala Volcanic Series in Carnarvonshire,” and Sir A. Geikie’s 
“ Anniversary Address to the Geological Society, 1891.”) 
We were fortunate in obtaining specimens of the grano- 
phyre which at the falls forms the edge of the mass, and of 
the more granitic variety a little further in ; and also a curious 
specimen, which, on being examined microscopically, shows 
a crack filled with perfectly glassy clear felspar along with 
quartz, from which it can only be distinguished by exhibiting 
a biaxial figure in convergent polarised light. The crack has 
cut across an original felspar crystal of the rock, which has 
been cemented again by the secondary felspar, the twin 
structure being preserved in the new material. 
The porphyritic rock mentioned above was found in situ, 
overhanging the little river in the ridge running north from 
Bera Mawr, and is no doubt the greenstone of the Survey 
map. Harker describes it as apparently a bronzite bearing 
andesite. 
This upper valley of the Aber stream affords some grandly 
wild scenery ; spurs from the hills on either side seem at times 
to cross it almost completely from side to side, and indeed 
at one place, for possibly a quarter of a mile, the little river 
runs in a beautiful little rocky gorge in miniature, with little 
