310 T. G. BONNEY ON THE BASE OF THE 
Bangor. If I rightly understand the Map, these are coloured as 
intrusive ; but in the Memoir (pp. 140, 141) we read :—‘ Below [the 
purple slates | are other thinner bands of slate, grit, and conglomerate, 
the lowest of which, by Llyn Padarn and elsewhere, passes into the 
quartz-porphyry that for thirteen miles stretches along the midst of 
the Cambrian strata..... So closely does the matrix of the altered 
rock resemble the adjoining typical porphyry, in colour, texture, and 
even in porphyritic character, and by such insensible gradations do 
they melt into each other, that the suspicion, or rather the convic- 
tion, constantly occurs to the mind that the porphyry itself is 
nothing but the result of the alteration of the stratified masses. . 
This conclusion is further aided by the capricious variation of the 
strata adjoining the porphyry... . This I can only account for by 
the supposition that these beds have, as it were, been partly eaten 
into by heat and themselves converted into porphyry.” 
The points which I hope to establish are:— — 
(1) That the former and a part of the latter massifs are neither 
intrusive nor metamorphic, in the ordinary sense of the word, but 
are parts of ancient lava-flows, which, were they of modern date, 
we should probably not hesitate to call rhyolites. 
(2) That the southern portion of the second massif is quite dis- 
connected from the northern, and is a group of metamorphic rocks 
of earlier date. 
The former of these propositions is the subject of the present 
paper. The latter is treated in a separate communication. The 
proofs can, I think, be given most conveniently by describing the 
sections which I was able to examine. 
Moel-Tryfaen District. 
The section at the adit in Moel Tryfaen has been described by 
Dr. Hicks*. As our party was no better provided with lights than 
his, I did not think it worth while to examine the adit, but collected 
specimens carefully from the spoilbank outside. The following 
(besides the usual Cambrian slates) are the principal varieties :— 
(1) rather gritty greenish slate banded with rounded grains of a 
pinkish or purplish felsite and light-coloured felspar; (2) a rock of 
mottled and streaky aspect containing larger fragments of the same 
felsite; (3) a conglomerate of the same; (4) a number of greenish 
slates and grits. The first three closely resemble members of the 
Cambrian conglomerate; and the last group (4) resembles some of 
the rocks which, in the Bangor sections, lie at no great distance 
beneath the Cambrian conglomerate. 
Dr. Hicks says+, “we did not succeed in touching the conglo- 
merate in the tunnel.” I have, however, no doubt that the “‘ more 
porphyritic-looking rock, mostly dark-coloured with spots of highly 
vitreous quartz in a base of felsitic matter,’ &c. was really the con- 
glomerate, which, owing to the imperfect light and other causes, to 
be mentioned hereafter, he failed to recognize. The outcropping 
* Quart. Journ. Geol. Soc. vol. xxxiy. p. 147. 
t Loe. cit. p. 148. 
