<^HAP. XI 
NORTH WALES 
165 
Still more striking are the sections on the south-west side of the lake.^ 
Starting from the porphyry of the ridge, we cross a zone of conglomerate 
and grit largely composed of the dehris of the poi’phjTy, until we reach a 
hand of felsite or quartz-porphyry, which at its eastern end is about ten 
feet thick, while it seems to increase in dimensions westwards.^ In the 
centre the rock is dark purplish -red, exceedingly compact or flinty, 
sprinkled with a rariable proportion of quartz-hlehs and felspar crystals. 
Towards its southern or upper edge (for the rocks, though nearly ■\'ertical, 
dip southwards) it has been cleaved into a variety of purple slate, and 
Would there at once he classed among the ordinary slates of the neigh- 
bourhood. But the fissile character is merely a marginal structure which 
the rock shares with the highly-cleaved tuffs that follow it. Traced west- 
wards, this bed is found to enclose a core of qirartziferous porphyry, which, 
though it has escaped from the most complete results of crushing, is never- 
theless cleaved along its margin as well as partially in its interior. It 
Would not he possible to distiugiiish parts of this intercalated less crushed 
core from portions of the porphyry of the main ridge. The difference of 
colour does not count for much, for even in this core the purple tint gives 
place to greenish grey, and what in the centre at the east end is a solid 
dark purplish-red felsite passes westward into a greenish slate, like that 
already noticed on Mynydd y Cilgwyn. 
The microscopical examination of this rock shows it to be a true felsite 
cf the rhyolitic type, which in the central uncleaved part exhibits a wavy 
flow-structure like that found in the quartz-porphyry of the ridge. So 
intense has been the cleavage in its upper part that the original structure 
of the rock is there effaced. The immediately overlying tuffs, which are 
likewise so thoroughly cleaved that it is not easy to draw a sharp and 
continuous line between them and the intercalated lava, precisely resemble 
those found below the conglomerate on the opposite side of the lake. 
They include bands of coarse volcanic breccia as well as fine compact 
material, showing the varying intensity of the volcanic discharges. Their 
'ncluded stones consist of vainous felsites, andesites and slates.® 
The thin sheet of interstratified quartz-porphyry Iiere described is not 
the only one to be found in the section. Others thinner and more intensely 
cleaved lie among the tuffs higher up. They have been sheared into mere 
pale uuetous slates, Imt the remains of their quartz-blebs may still be 
detected in them. 
The tuffs, with their interstratified bands of porphyry, liecome more 
Jor various ruadings of these sections, compare Mr. Blake {QuctA. Lout. GgoI. Soc. vol. xlix. 
(1893), p. 450) with Professor Boiiney and Miss Raisin {op. cit. vol. 1. (1894), p. 581). 
See Professor Bonuey and Miss Raisin, op. cit. p. 593 ct seq. 
I see no reason to doubt that the less acid igneous fragments were ejected during the 
rtosing ]ihases of volcanic action, even though no such rocks have been found at tbe surface in 
tVe must remember how frequently mi.xtures of acid and basic materials are to be found 
ni the same continuous series of volcanic ejections and even in the same vent, of which illu.stra- 
tion will be given in subsequent pages. Xor should we forget what a mere fragment oi a 
' olcanie group is expo.sed at the surface in the Llanberis district. See Professor Bonney and 
jlliss Raisin, op. cit. p. 596, footnote. 
