THE CAMBRIAN VOLCANOES 
BOOK III 
164 
end of Llyn Padarn. They are intercalated among fine tuffs, grits, 
volcanic breccias and purple slates, sometimes full of fine ashy material. 
On the south-east side of the ridge, where the rocks have suffered intense 
cleavage, they assmne a fissile unctuous character, and then resemble parts 
of the cleaved Cambrian tuffs at St. David’s. But on the north-west side, 
wlrere they have in large measure escaped the effects of the cleavage-move- 
ments, their original structures are well preserved. 
One of the first featirres of these detrital deposits to arrest attention is 
the amount and variety of the fragments of igneous rocks in them. Some 
of the conglomerates, though enclosing pebbles of quartz, ipiartzite, granite 
and other rocks not found in situ in the immediate district, are mainly com- 
posed of the debris of the cjuartz-porphyry of the ridge. Indeed, this 
latter material appears to have contributed a large proportion of the 
detritus of which the general body of strata here is made up. But there 
are to be noticed among the contents of the conglomerates and breccias 
pieces of many volcanic rocks not to be found on the porphyry ridge. 
Among the.se, besides felsites showing sometimes beautiful flow-structure 
(rhyolites) and various quartz-porphyries, there occur abundant fragments 
of less acid lavas (andesites) and pieces of older tuffs. Some of the frag- 
mental rocks are green in colour, probably from the abundance of fine 
basic volcanic dust in them. Certain bands are full of large angular pieces 
of shale, similar in character to the Cambrian slates, and doubtless due to 
the disruption of pre-existing Cambrian strata by volcanic explosions. It 
is clear that from vents in this neighbourhood there continued to be an 
abundant discharge of dust and various andesitic and other lapilli, which, 
falling on the sea-floor, mingled there with the ordinary mechanical sedi- 
ment that was being deposited at the time.^ 
But we have evidence that, during the period when these showers of 
volcanic detritus were thrown out, streams of lava, though on a greatly 
Fki. 44. — Section of Clegyr on the north-east side of Llyn Pailaru, near the lower end. 
diminished scale, continued to be poured forth. The hill of Cleg-yr (Fig. 
44), near the lower end of Llyn Padarn, on the north-east side, consists 
mainly of cleaved tuffs {t) and slates with conglomerates (c), overlying the 
quartz-porphyry (p). Near the summit a band of felsite is intercalated in 
these rocks. 
’ On the composition of the conglomerates or breccias, see Professor Bonney and Miss Raisin, 
Quart. Jour. Geol. Soc. vol. 1. (1894), p. 698. 
