ON TJrE ARCH/EAN KOCKS OF i.KKAT HIIITAIN. 



539 



icaceous 



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

 ovideneo of volcanic action in tlio admitted Cambrian, so that there is no 

 roaHou for soparatinfj the Pobidian from it. Tlio so-called Diraetian in 

 n )t an ancient granitoid gnois.t, but a true gfi-anito, and ^'s intrusive in 

 the Cambrian series; the Arvonian consistin;^ partly of apophyses from 

 tliis granite mass, partly of a volcanic scries, inseparable from the so-called 

 J'ebi(lian, 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 ho had relied 

 to prove the metamorphic cbaract(>r of the Dimetian — ^viz,, the chloritic 

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

 on microscopic examination to be diabase dykes, rendoi-ed schistose 

 1)y subsequent compression ; the latter, to bo pr(jbably bands of infiltra- 

 tion along lines of tVactnre or ci'iisliiug — but asserting unhesitatingly (and 

 here his view is sup]iorted by independent microscopic study on the part 

 of J\rr. T. Davies) that the IJitaetiau rock, whatever be its true nature, 

 has contributed recognisable fri'ginents to the basement conglomerate of 

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

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

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

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

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

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

 of the Cambrian, and that the asserted proofs of volcanic action in the 

 latter arc of a very slight and uncertain character. 



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

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

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

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

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

 tuffs (Arvonian of Hicks) which formed the lower part of the volcanic 

 series. He thus maintained the existence of a px'e-Cambrian series, but 

 regarded it as one approximately continuous gi'oup. 



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 

 * Diraetian fragments,' as well as thoso of Archajan schists (not now visible 

 //( ."•■//«), 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 

 ^t David's remains unsliaken. He also notices in his last paper* one or 



^11 



' Q.J. G. S., vol. xl. p. 204. 

 -■ (^'. ./. (,'. >'.. v.il. >;'.. [t. .'.o:. 



