CHAPTER X 
THE CAMBKIAN YOLCAKOES OF SOUTH WALES 
In tlie southern part of the Principality of Wales a remarkably varied 
display of British Cambrian volcanic rocks has been preserved. The 
district around St. David’s has the distinction of being the first in which 
volcanic rocks of such high antiquity were recognized. As far back as the 
year 1842, Eamsay found that “ felspathio volcanic ash ” was associated with 
other proofs of igneous action, and this fact was recorded by him on the 
published Horizontal Sections of the Geological Survey. Unfortunately he 
afterwards came to regard the rocks as “ altered Cambrian,” thus following 
certain hypothetical views which, as will be further alluded to in the sequel, 
he had adopted in explanation of the phenomena in Caernarvonshire and in 
Angle.sey. The volcanic nature of these ancient materials was subsequently 
rediscovered by Dr. Hicks, who has devoted much time and labour to their 
study. Distinguishing the volcanic series of St. David’s by the name 
“ Pebidian,” he has contended that it forms a pre-Camhrian system separated 
Iiy an unconformability from the base of the Cambrian formations. He 
likewise endeavoured to show that an older system of rhyolitic lavas, felsitic 
breccias and hiilMiutas could be distinguished, which he termed “ Arvonian ” ; 
and more ancient still, a core of granitoid or gneissic rocks, which he 
separated under the name of “ Dimetian.” My own investigation of the 
ground thoroughly convinced me that there are no pre-Camhrian rocks at 
St. David’s; that the “Arvonian” and “Dimetian” series are merely 
intrusive rocks (quartz -porphyry, granite, etc.) which have invaded the 
volcanic series ; and that the “ Pebidian,” instead of being a pre-Cambrian 
formation on which the Cambrian base rests unconformably, is a group of 
volcanic rocks into which the Cambrian strata pass down conformably, and 
which in the St. David’s district constitutes the lowest group of the Cambrian 
system.! 
! For Dr. Hicks’ views, .see especially liis papers in tlie Quart. Joum. Geol. Soc. vols. xxxi. 
^xxiii. xxxiv. xl. My criticism of them will be found in op. cit. vol. xxxix. (1883), subse- 
TWeutly in the main confirmed by Prof. Lloyd Morgan, op. cit. xlvi. p. 241. See also Prof. Blake, 
‘’P- cit. xl. (1884). Dr. Hicks in his more recent papers has merely reiterated his previously 
published opinions. 
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