CHAP. XI 
NORTH WALES 
i6i 
eruptions have heen traced by him, as well as by Professor Hughes aud Mr. 
Plake, in the stratified rocks which stretch north-eastwards to Pangor. 
The extent and persistence of these ancient volcanic phenomena, and their 
probable connection with the remarkable northward attenuation of the Cam- 
brian sedimentary rocks, deserve special attention. 
It is generally agreed that the rocks variously termed quartz-porphyries, 
telsites or rhyolites form the oldest members of this volcanic series.’^ 
Ihey 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 Naut 
irancon (Map. II.). Wliether the materials of these two ridges are jiarts of 
one originally continuous sheet or group of sheets, or, if different protrusions, 
w'hether 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 connected 
underground, as a continuous phittorm beneath the overlying pyroclastic 
materials. 
These acid rocks have been regarded by some observers as intrusive 
sheets, by others as lava-streams that were poured out at the surface. If 
account be taken simply of their petrographical characters, 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 superficial streams. But this structure may be 
found in dykes and intrusive sheets as iierfectly as in lava-flows, so that it 
cannot liy 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 
'T- superficial lava-stream ; ’ while, on the other hand, the rock generally 
presents the tolerably uniform flinty texture so familiar in intrusive sheets 
of similar material.^ My own impi’ession is that these igneous masses were 
probably erupted to the surface as long banks wliich rose above the waves ; 
that they were thus exposed to prolonged subaerial and marine denudation ; 
that by this means any iqqier more cellular portions of the lava which may 
have existed were broken up and pounded down into detritus, and thus that 
what is now visible is a part of the eruptive rock which originally lay 
at some depth within its body. This view is confirmed by a study of other 
lavas which are found on different platforms in the detrital deposits that 
overlie the Llyn Padarn quartz-porphyry. 
^ Whether the granitic rock of Twt Hill, Caernarvon, is connected with the porphyry or 
elongs to an older eruption is immaterial for my present purpose. 
But the Llyn Padarn rock, like many acid lavas, may never liave possessed a vesicular 
structure m any portion of its mass. The sheets of felsite which occur among the overlying tuffs 
are not cellular, but present the closest resemblance to the main mass below. 
■ Mr. Blake brought forward the evidence of a section on the north or under side of the 
, lidge to show that the I'ock has thei'e been intruded into the Cambrian strata 
War . uiirn. Qcol. vol. xliv. (1888), p. 288). But tlie rock supposed by liiiu to be altered 
greenstone ” (Miss Raisin, op. eit. vol. .\lvii. (1891), p. 336). 
M 
slate has been shown to be intrusive 
VOL. I 
