i6o 
THE CAMBRIAN VOLCANOES 
BOOK III 
in the formation of granites, syenites and other rocks, commonly called 
intrusive.” ^ Certain structural lines in the quartz-porphyry he looked 
upon as indicating “ traces of stratification in a rock, the original felspathic 
and quartzose material of which has been metamorphosed into true 
porphyry.” “ In conformity with these ideas, the remarkable felspathic 
strata which lie nearest the porphyry were regarded as metamorphosed 
Cambrian rocks, and wliere similar rocks reappear over a large area near 
Bangor they were coloured on the map witli the same tint and lettering as 
were used for the so-called “ altered Cambrian ” of Anglesey. 
No one who has examined this Caernarvonshire ground can have failed 
to find the sections which doubtless led my predecessor to form the con- 
victions to which he gave expression in the passages I have just quoted. 
It is easy to see how these sections, wherein it is certainly difficult to draw 
a sharp line between the igneous rock and the clastic materials derived 
from it, would be welcomed as. appearing to otter confirmation of the ideas 
concerning metamorphism which were then in vogue. There cannot, how- 
ever, be any doubt that my friend was mistaken in his interpretation of the 
structure of that part of the country. It is to me a subject of keen regret 
that in his later years, when the subject was revived, he was no longer able 
to re-examine this ground himself, for no one would have confessed more 
frankly his error, and done more ample justice to those who, coming after 
him, have been able in some parts to correct his work. 
The quartz-porphyry, lelsite or rhyolite of Llyn Padarn, as well as that 
of Llaiideiniolen, is not a metamorphic but an eruptive rock, as has been 
demonstrated by Professors Hughes and Bonney. There is no true passage 
of the sedimentary rocks into it ; on the contrary, the conglomerates which 
abut against it are in great part made out of its fragments, so that it was 
already in existence before these Candmian strata were deposited upon it. 
These conclusions must be regarded as wholly indisputable. But most of 
the critics of the work of the Geological Survey have proceeded to certain 
fm'ther deductions. They have maintained that the presence of fragments 
of the porphyry in the overlying conglomerate marks an unconformability 
between the two rocks, that the conglomerate shows the base of the Cam- 
brian system, and that the porphyry is therefore pre-Cambrian. 
These assertions and inferences do not seem to me to be warranted. 
They have, in my judgment, been disproved by Mr. Blake,® who shows that 
there is no break in the Cambrian series, that the various porphyries and 
their accompaniments are parts of that series, and tliat tliere is no certain 
proof of the existence of any pre-Cambrian rocks in the whole district.^ 
That the igneous rocks of the Llyn Padarn area mark a volcanic period 
has been recognized by most writers since Professor Bonney pointed out the 
flow-structure of the quartz-porphyry, and other proofs of active volcanic 
' 31em. Geol. Sun. vol. iii. 2iid edit. p. 173. ^ Ibid. p. 174. 
^ In an excellent memoir read before the Geological Society in 1888, with the main con- 
clusions of which I agree. 
■* Quart. Joum. Geol. Soc. vol. xliv. p. 271. For subsequent paper-s by Mr. Blake, see op. cit. 
vols. xlviii. (1892) p. 243, xlix. (1893) p. 441. 
