lOO PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



volcanic episode, but the comparative infrequence of distinct inter- 

 stratifications of shale or sandstone may be taken to imply that as 

 a rule the pauses between the eruptions were not long enough to 

 allow any considerable accumulation of sand or mud to take place. 



No satisfactory proof has yet been obtained of any interstratified 

 lavas among the tuffs of the Bangor district. Some rocks, indeed 

 can be seen on the road between the George Hotel and Hendre- 

 wen which, if there were better exposures, might possibly furnish 

 the required proof ; but at present little can be made of them, for 

 their relations to the surrounding rocks are everywhere concealed. 



From what I have now adduced, it is obvious that while both 

 felsitic and andesitic lavas existed within the volcanic foci and 

 were ejected in fragments to form the tuffs and breccias, the lavas 

 poured out at the surface during the Cambrian period in Caernarvon- 

 shire were mainly, if not entirely, felsites (rhyolites) in which the 

 chief porphyritic constituent was quartz. These lavas thus stand 

 entirely by themselves in the volcanic history of Wales. Thouo-h 

 felsites of various types were afterwards poured out, nothing of the 

 same quartziferous kind, so far as we yet know, ever again appeared. 



I have now to consider the relation of this volcanic group of Eano-or 

 to the strata which overlie it. The geological horizon of these strata 

 is not, perhaps, very definitely fixed. It may be Arenig, possibly even 

 older. But for my present purpose it will be sufficient to consider 

 the strata in question as lying at the bottom of the Lower-Silurian 

 series. Professors Hughes and Bonney have taken as their base a 

 marked but impersistent band of conglomerate. Mr. Blake, however, 

 has more recently shown that, as this band is succeeded by tuffs like 

 those below it, it cannot be claimed as marking the upper limit of 

 the volcanic group. He therefore classes it in that group and traces 

 what he thinks is an overlap or unconformability at the bottom of 

 the Lower-Silurian strata to the east. Mr. B. N. Peach, who ac- 

 eompanied me in an examination of this ground, agrees with me in 

 confirming Mr. Blake's observation as to the position of the con- 

 glomerate, which is undoubtedly overlain by the same flinty felsitic 

 tuffs as are found below it. But we were unable to trace any un- 

 conformability. According to the numerous observations which we 

 made, there does not seem to be any discordance in strike or dip 

 between the flinty tuffs and the overlying shales and grits. The 

 two groups of rock appeared to us to be conformable and to pass 

 into each other as at JAyn Padarn. 



An unconformable junction here would, in some respects, have 



