i 62 
THE CAMBRIAN VOLCANOES 
BOOK III 
That the material of each of the two main ridges is the result of more 
than one eruption has been inferred from the supposed intercalation of 
bands of slate and of breccia in the rockd Considerable lithological 
differences may be detected in each mass, but they are not greater than may 
be observed in single sills and bosses. In some parts of the Llyn Padarn 
porphyry a distinct nodular structure appears which shades off into bands 
and lenticular streaks, reminding one of the characters of some of the Bala 
rhyolites. Other portions are markedly brecciated, the separated fragments 
being surrounded in a matrix of the rock, which shows flow-structure 
sweeping past them. On Moel Gronw angular fragments of a dark pinkish 
tint are scattered through the general mass. Again, some parts are 
crowded with quartz-grains, while others are comparatively free of these, 
and occasionally a spherulitic structure has been observed.'^ 
The microscopic structure of this ancient eruptive rock has been studied 
by Professor Bonney, who found that the general type was a compact dull 
grey felsite, with porphyritic crystals of felspar and grains of quartz, closely 
resembling some modern rhyolites. Though unable to detect any actual glass 
in the base, he had no doubt that the rock was originally vitreous, and he 
found abundant and fresh examples of the most perfect flow-structure.® 
Eeference may be made here to the remarkable influence of the intense 
cleavage of the district upon this rock.'* Along its southern margin, where 
it has been exposed to pressure from the south-east, the (|uartz-porph)U’y 
has been so crushed that it passes here 
and there into a fine unctuous slate or 
almost a schist. Nowhere can this change 
be more clearly seen than on the slopes 
of Myiiydd y Cilgwyn. The cleavage 
jilanes strike about N. 40° E., with an 
inclination to dip towards the N.Ah 
Within a space of a few yards a series 
of specimens may be collected showing at 
one end an ordinary or only slightly- 
sheared quartz -porphyry with abundant 
quartz -blebs, and at the other a fine 
greenish sericitic slate or phyllite, where- 
Fuj. 42. —Basic ilyke traver.sing quartz- in the quartz has been almost entirely 
poriiliyry and converted iiito a kind of i _ i -r • pi* i 
slate by cleavage. West side of Llyn ClUSheCi dO^\ll. LlUeS of sll6arillg may be 
Paiiani. detected across the breadth of the por- 
pp, quartz-porpliyry; ilyke and connected phyry ridge, each of them coinciding with 
the prevalent trend of the cleavage. Some- 
times also certain basic dykes, which traverse the porphyry in some numbers, 
^ See for exanijilc. Prof. Bonney, Quart. Jonm. (reol. Hoc. vol. xxxv. (1879), p. 312 ; Mr. Blake, 
op. oil. vol. xliv. (1888), pp. 277, 287. But some at least of the supposed “slates,” as stated 
in a previous footnote, have been since shown to be dykes. 
Mr. Blake, ihid. ji. 277. ® Op. cit. vol. xxxv. p. 312. 
The secondary planes due to cleavage must not be confounded witli tlie original flow- 
structure. 
