My own examination so far confirms the conclusions arrived at by these observers. Like my predecessors in the Geological Survey, however, I have been unable to detect anywhere in Caernarvonshire or Merionethshire a base to the Cambrian system, and I am compelled to agree with them in regarding as Cambrian (partly even as Lower Silurian) all the rocks from Bangor to Llanllyfni, which have more recently been classed as pre-Cambrian. But though thus supporting their general stratigraphy, I am bound to acknowledge that they failed to recognize the existence of a great volcanic series below the Arenig horizon. The existence of this series, noticed by Sedgwick, was first definitely stated by Professor Hughes,[84] and his statements have been confirmed and extended by subsequent observers, notably by Professor Bonney and Mr. Blake. The Cambrian period is thus proved to have been perhaps even more continuously volcanic than the Lower Silurian period was in Wales.
[84] Proc. Camb. Phil. Soc. vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales are represented in [Map II.]
The following table shows the subdivisions of the Cambrian system now recognized in Britain:—
| Wales. (Ranging up to 12,000 feet or more.) | Western England. (About 3000 feet.) | N.W. Scotland. (About 2000 feet.) | ||
Upper or Olenus Zones. |  | Tremadoc Slates Lingula Flags (Lingulella, Olenus, etc. | Shineton Shales (Dictyograptus or Dictyonema, Olenus, etc.). | Limestones, about 1500 feet thick, divisible into seven groups (Archæocyathus, Maclurea, Ophileta, Murchisonia, Orthoceras, and vast quantities of annelid castings). |
Middle or Paradoxides Zones. |  | Menevian group (Paradoxides). | Conglomerates and limestones (Comley), with Paradoxides, etc. | |
Lower or Olenellus Zones. |  | Harlech and Llanberis group with basement volcanic rocks; bottom not seen. | Thin quartzite passing up into green flags, grits, shales and sandstone (Comley Sandstone), containing Olenellus. | Shales ("fucoid beds"), with Olenellus, Salterella, etc. Quartzites with annelid burrows. The base of the series lies unconformably on 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]
[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.
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Walker & Boutall sc.