RT Journal Article
SR Electronic
T1 The Pebidian Volcanic Series of St. Davids
JF Quarterly Journal of the Geological Society
JO Quarterly Journal of the Geological Society
FD Geological Society of London
SP 241
OP 269
DO 10.1144/GSL.JGS.1890.046.01-04.16
VO 46
IS 1-4
A1 Morgan, C. Lloyd
YR 1890
UL http://jgs.lyellcollection.org/content/46/1-4/241.abstract
AB 1. Introduction.Two strongly contrasted interpretations of the structure and geological genesis of the St. David's promontory are before geologists. The first of these is that advocated for many years by Dr. Hicks and finally summed up by him in 1884 (Quart. Journ. Geol. Soc. vol. xl. p. 507). On this view there are at St. Davids, besides the strata of Cambrian age, three distinct Pre-Cambrian systems the Dimetian, Arvonian, and Pebidian. Each is unconformable to the others. The Dimetian was very much in its present state before the Arvonian was laid down. The Arvonian hälleflintas, again, were brought to their present mineral condition before the Pebidian series was deposited. The Pebidian volcanic beds, once more, were nearly as highly metamorphosed as they are at present before the Cambrian Conglomerate was formed. Each is therefore separated from the succeeding group by a systematic unconformity, and we have, on this view, to quote the words of Mr. Tawney, ”a vista opened up to us in the rocks of St. Davids of age behind age most pleasing to the imagination of the geologist.”The second interpretation is that advocated by Dr. A. Geikie (Quart. Journ. Geol. Soc. vol. xxxix, p. 261). According to this view the Pebidian constitutes a volcanic series at the base of and forming an integral part of the Cambrian system; the Dimetian consists of granite intrusive into the Cambrian and of much more recent age; while the Arvonian group comprises portions of the Pebidian volcanic breccias and tufts, where these