ANNIVERSARY ADDRESS OF THE PRESIDENT. 93 



series, that the various porphyries and their acconipariiments are 

 parts of tliat series, and that there is no certain proof of the exist- 

 ence 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 eruptions have been traced by him as well 

 as by Professor Hughes and Mr. Elake in the stratified rocks which 

 stretch north-eastwards to Bangor. But the extent and persistence 

 of these ancient volcanic phenomena, and their probable connexion 

 with the remarkable northward attenuation of the Cambrian sedi- 

 mentary rocks, have hardly received adequate attention. 



It is generally agreed that the rocks variously termed quartz- 

 porphyries, felsites, or rhyolites, form the oldest members of this 

 volcanic series f. They come to the surface in two long ridges, one 

 running from Caernarvon to near Bangor, the other from Llanllyfni 

 to Ann's Chapel, at the mouth of Nant Prancou. AVhether the 

 materials of these two ridges are pares of one originally continuous 

 sheet or group of sheets, or, if different protrusions, whether they 

 belong to the same geological horizon, or whether, as Mr. Blake 

 believes, they are distinct masses, separated by a considerable thick- 

 ness of detrital material, cannot in the present state of our knowledge 

 be positively decided. It seems to me probable that they are con- 

 nected underground, as a continuous platform beneath the overlying 

 pyroclastic materials. They have been regarded as intrusive sheets, 

 more recently as lava-streams that were poured out at the surface. If 

 we take account simply of their petrographical characters, we must 

 admit that they find their nearest analogies among the intrusive 

 quartz-porphyries of older geological periods. The presence of flow- 

 structure in them has been thought to indicate that they were super- 

 ficial streams. But this structure may be found in dykes and intru- 

 sive sheets as perfectly as in lava-flows, so that it cannot by itself be 

 taken as proof of a surface-discharge of lava. It must be confessed 

 that, both in the main mass of quartz-porphyry and in the abundant 

 fragments of it in the overlying conglomerates and breccias, there 

 is an absence of such scoriform portions as one would naturally look 

 for in a superficial lava-stream ; while, on the other hand, the rock 

 generally presents the tolerably uniform flinty texture so familiar 



* Quart. Journ. Geol. Soc. vol. xliv. p. 271. 



t Whether the granitic rock of Twt Hill, Caernarvon, is connected with the 

 porphyry or belongs to an older eruption is immaterial for my present purpose. 



