THE CAMBRIAN VOLCANOES 
BOOK III 
1 68 
on the road between the George Hotel and Hendrewen, 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 wdiile 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 
surtace during the Cambrian period in Caernarvonshire w'ere mainly, if not 
entiiel) , felsites (rhyolites) in w’hich the chief j)or])hyritie constituent was 
quartz. These laA as thus stand entirely by themselves in the volcanic 
histoiy ot Wales. Though felsites of various types were afterwards poured 
ouC nothing of the same quartziferous kind, so far as we yet know, ever 
again appeared. Further south, in Merionethshire, as will be shown in 
Chapter xii., the Cambrian volcanic eruptions appear to have been on 
the whole less acid, and to ha^-e begun with the outpourino- of andesitic 
lavas. 
I have now to consider the relation of the volcanic group of Eangor 
to the stratti which overlie it. The geological horizon of these strata is 
not, perhaps, very definitely fixed. It may be Arenig, possil)ly 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 w'hat he thinks is an overlap or 
unconformability at the bottom ot the Lower Silurian strata to the east. 
Mr. B. H. Peach, who accompanied me in an examination of this ground, 
agreed with me in confirming Mr. Blake’s observation as to the position 
of the conglomerate, which is undoubtedly overlain by the same flinty 
felsitic tuffs as are found below it. But we were unable to trace any 
unconformability. According to the numerous oljservations 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 
Llyn Padarn.^ 
An unconformable junction here would, in s(mre respects, have been 
welcome, for it would at once have accounted for the superposition of 
Lower Silurian strata directly np)oir the Canrbriarr volcarric series, arrd for 
the disappeararrce of the Llanberis slates and grits which form so conspicu- 
ous a feature above the tuffs and conglomerates at Llyn Padarn. In the 
abserrce ot srrcli a structure wo nrust accept the order of succession as 
apparently urrbroken, arrd rely on some such explanation us was proposed by 
Sir Andrew Puirnsay to accourrt for the overlap of the Arenig rocks on 
' See Mr. Blake ou this point, Quart. Joimi. OeoJ. Soc. vol. xlviii. (1892), p. 252, note. I 
retain the opinion e.xpre.ssed above. 
