On the Quartz-felsite and Associated Rocks at the base of the Cambrian Series in North-western Caernarvonshire

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Abstract
Conclusions: We see, then, that the rock in the quartz-felsite massif exhibits every characteristic of an igneous origin; we see also that fragments closely corresponding with it occur again and again in the overlying rocks, and that these rocks, as a rule, are comparatively little changed— the evidence of alteration being illusory, and quite ordinary slates or grits in some cases interposing between the quartz-felsite and the supposed highly metamorphosed rocks. We must then refuse to these Caernarvonshire “porphyries” an origin different from that of other igneous rocks of similar composition, and cease to quote them as examples of what extreme metamorphism can effect. I may repeat again that, allowing for slight mineral changes brought about by the agencies to which all rocks have been exposed in the long lapse of ages (such as devitrification, the formation of viridite, &c.), there is no difference of any importance, so far as I can see, between these quartz-felsites and comparatively modern rhyolites; and if I could prove that a base still remained undevitrified, I would give them the latter name. That they were rhyolites in pre-Cambrian times I have no doubt.

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