| rocks; bottom not | and sandstone | burrows. The base of | | seen. | (Comley Sandstone), | the series lies | | | containing | unconformably on | | | _Olenellus_. | pre-Cambrian rocks. | +----------------------+----------------------+-------------------------+ CHAPTER X THE CAMBRIAN VOLCANOES OF SOUTH WALES In the 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, Ramsay found that "felspathic 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 Anglesey. 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-Cambrian system separated by an unconformability from the base of the Cambrian formations. He likewise endeavoured to show that an older system of rhyolitic lavas, felsitic breccias and hälleflintas 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-Cambrian 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.[85] [Footnote 85: For Dr. Hicks' views, see especially his papers in the _Quart. Journ. Geol. Soc._ vols. xxxi. xxxiii. xxxiv. xl. My criticism of them will be found in _op. cit._ vol. xxxix. (1883), subsequently in the main confirmed by Prof. Lloyd Morgan, _op. cit._ xlvi. p. 241. See also Prof. Blake, _op. cit._ xl. (1884). Dr. Hicks in his more recent papers has merely reiterated his previously published opinions.] [Illustration: _Walker & Boutall sc._ Fig. 40.--Map of the volcanic district of St. David's.]