92 PEOCEEDINGS OF THE GEOLOGICAL SOCIETT. 



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 porj^hyry"*. 

 In conformity with these ideas, the remarkable felspathic strata which 

 lie nearest the porphyry were regarded as metamorphosed Cambrian 

 rocks, nnd where similar rocks reappear over a large area near Ban- 

 gor they were coloured on the map with 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 convictions to which he gave expression in the passages I 

 have just quoted. It is easj^ 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 offer confirmation of the ideas concerning meta- 

 morphism which were then in vogue. There cannot, however, 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 he should be now 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, felsite, or rhyolite of Llyn Padarn, as well 

 as that of Llandeiniolen, 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 must have been already in 

 existence before these Cambrian strata were deposited. These con- 

 clusions must be regarded as wholly indisputable. But most of the 

 critics of the work of the Survey have proceeded to certain further 

 deductions. They have maintained that the presence of fragments 

 of the porphyry in the overlying conglomerate marks an unconforma- 

 bility between the two rocks, that the conglomerate shows the base 

 of the Cambrian system, and that the porphyry is therefore pre- 

 Cambrian. These assertions and inferences do not seem to me to be 

 warranted. They have already, in my judgment, been disproved by 

 Mr. Blake in an excellent memoir read before the Society in 1888, 

 with the main conclusions of which I agree. Mr. Blake shows, in 

 my opinion conclusively, that there is no break in the Cambrian 



^ Ibid. p. 174. 



