THE CAMBRIAN VOLCANOES 
BOOK in 
1 66 
and more mingled with ordinary argillaceous and sandj^ sediment as they 
are followed in upward succession. Among them occvir hands of grit and 
fine conglomerate containing pebbles of porphyry and pieces of slate. Some 
of these grits are mainly composed of white felspar, felsite and clear grains 
of cpiartz, evidently derived from the disintegration of a rock like the 
poi'phyry of the main ridge. ' As the ordinary sediment of the Llanberis 
group sets in, the tuffs are restricted to thinner and more widely-separated 
bands. Some thin layers of felspathic breccia, seen among the slates close 
to the Crlyn Peris Hotel, piohahly mark the last discliarges of the slowly- 
expiring vents of tliis region. Here, as at St. David’s, from the most 
ancient of our volcanic records, striking evidence is furnished of the gradual 
extinction of volcanic action. Through many hundreds of feet of strata 
which now supervene, representing the closing ages of the Cambrian and 
the earlier ages of the Silurian period, no trace of volcanic material has 
been found in tliis district until we reach the Bala lavas and agslomer- 
ates of Snowdon and the Pass of Llanberis. 
In the neighbourhood of Bangor another area of similar rocks wraps 
round the northern end of tlie western porphyry ridge. The Geological 
Survey map, in conformity with the ideas that governed its repre- 
sentation of the older rocks of Anglesey and Caernarvon, colours 
these as altered Cambrian. That this error should have been made, 
or, when made, should not have been speedily corrected, is all the more 
surprising when we consider the thorough. mastery which the surveyors had 
acquired of the aspects and the interpretation of ancient volcanic rocks in 
Wales, and when, moreover, we remember that as far back as 1843, long 
before the Survey of Caernarvonshire was published, Sedgwick had pointed 
out the true volcanic nature of the rocks. That great pioneer recognized 
the presence of “ trappean conglomerates ” and “ trappean shales (Schaal- 
stein) ” among these deposits at Bangor ; but he could not separate them 
from the CaTnbrian series of the rest of Wales.^ And in his section he 
represents them as undulating towards the east and passing under the 
great mass of the Caernarvonshire, slates and porphyries. 
This interpretation, which I believe to be essentially accurate, was 
modified by Professor Hughes, who, fixing on a conglomerate as the base of 
the Cambrian system, regarded all the rocks below it, or what he termed 
his “ Bangor group,” as pre-Camlirian.** He has been followed in this view 
by sub.sequent writers;® hut Mr. Blake has argued that here, as in the 
Llanberis district, there is no evidence to separate the volcanic detrital 
deposits above the porpliyry from the Cambrian system.'^ 
A little southward from Bangor the quartz-porphyry is overlain by a 
most interesting group of fragmental rocks, the “ Bangor group ” of Pro- 
fessor Hughes. Tm’gely of volcanic origin, they must be some hundreds of 
' Froc. Geol. Soc. vol. iv. p. 212; Quart. Journ. Geul. Soc. vol. iii. (1847), p. 136. 
^ Quart. Journ. Gcol. Soc. vol. xxxiv. (1878), p. 137. 
® Prof. Boimey, op. cit. vol. xxxv. (1879), p. 316; Pr. Hicks, ibid. ji. 296. 
^ Op. cit. vol. xliv. (1888), p. 278. 
