ON THE ARCHAEAN R0CK8 OF GREAT BRITAIN. .339 



There is no break at the base of the conglomerate, and further there is 

 evidence of volcanic action in the admitted Cambrian, so that there is no 

 reason for separating the Pebidian from it. The so-called Dimetian is 

 not an ancient granitoid gneiss, but a true gi'anite, and is intrusive in 

 the Cambrian series ; the Arvonian consisting partly of apophyses from 

 this granite mass, partly of a volcanic series, inseparable from the so-called 

 Pebidian, and thus from the Cambrian. 



To this attack Dr. Hicks has replied, admitting (as indeed had been 

 done previously) that some of the indications upon which he had relied 

 to prove the metamorphic character of the Dimetian — viz., the chloritic 

 schists and dolomitic beds — were fallacious, the former having been found 

 on microscopic examination to be diabase dykes, rendered schistose 

 by subsequent compression ; the latter, to be probably bands of infiltra- 

 tion along lines of fracture or crushing — but asserting unhesitatingly (and 

 here his view is supported by independent microscopic study on the part 

 of Mr. T. Davies) that the Dimetian rock, whatever be its true nature, 

 has contributed recognisable fragments to the basement conglomerate of 

 the Cambrian, as have many of the supposed apophyseal felsites ; that 

 the latter frequently cut the granitoid (Dimetian) rock ; that the sup- 

 posed cases of intrusion of the last-named into the superjacent beds 

 are non-existent ; that there is no evidence (but the contrary) of a 

 great fold asserted by the Director- General to exist in the volcanic 

 series ; that there is a marked break below the conglomerate at the base 

 of the Cambrian, and that the assez'ted proofs of volcanic action in the 

 latter are of a very slight and uncertain character. 



Between the publication of the two papers one had been written by 

 Professor Blake, 1 who confirmed in strong terms the existence of a 

 marked line of separation between the Pebidian and the Cambrian, and 

 the non-intrusive character of the Dimetian, but regarded it as a true 

 granite — the core of the volcano which had ejected the rhyolitic lavas and 

 tuff's (Arvonian of Hicks) which formed the lower part of the volcanic 

 series. He thus maintained the existence of a pre-Cambrian series, but 

 regarded it as one approximately continuous group. 



Perhaps, as I have examined a portion of the district rather carefully, 

 and have studied a very considerable collection of microscopic slides, 

 including all those submitted to Mr. T. Davies, I may venture to express my 

 own opinion, which is this — that I fully agree with him in recognising 

 ' Dimetian fragments,' as well as those of Archaaan schists (not now visible 

 in situ), in the Cambrian conglomerate : that while in the present state of 

 our knowledge I will not venture to say whether the Dimetian be a granite 

 or not, I feel certain that there is no valid evidence of its being connected 

 with any of the felstones, and none whatever (but much to the contrary) 

 of its being intrusive in the stratified series ; and that the Cambrian and 

 Pebidian appear to be at least as widely separated as the Ordovician and 

 the Silurian, the break at the base of the Cambrian, physically and litho- 

 logically, being far more marked than any one that occurs between it and 

 the top of the Ordovician. Of the advisability of separating the Arvonian 

 from the Pebidian I have always been more than dubious ; but, with this 

 exception, I think that Dr. Hicks's main position as regards the geology of 

 St. David's remains unshaken. He also notices in his last paper r 



' Q. J. G. S., vol. xl. p. 294. 

 1 (j. ./. (r. 8., vol. xl. p. 007. 



