136 



IIARDWICKE'S SCIENCE-GOSSIP. 



expressed by Professor Hughes and himself at the 

 last meeting, and it proved also that the supposed 

 facts, relied upon by the author to support his views, 

 were clearly mistakes made by Professor Geikie from 

 an imperfect acquaintance with the district and the 

 rocks. By the admission of the author, the 

 appearances are abnormal in the St. David's area. 

 With respect to the intrusive character of the so- 

 called granite, he asked what had become of the 

 materials displaced by the intrusion. He regarded 

 the so-called tuffs alternating with the Cambrian 

 conglomerates as derivative rocks, full of quartz- 

 grains, &c. The junction of the sedimentary rocks 

 with the granitoid rock was a faulted and not an 

 intrusive junction. The fault was marked by 

 slicken-sides, but not by any contact metamorphism. 

 He demurred to the author's views as to the double 

 series of foliations. He showed that while a dyke of 

 greenstone 50 yards wide had produced enormous 

 alteration in the surrounding rocks, the great 

 granitoid mass had produced no alteration. At 

 Ogof-llesugn, a place specially [referred to by the 

 author, it was possible to get between the Dimetian 

 and the quite unaltered Cambrian conglomerate. 

 Another mass of conglomerate was jammed in 

 through the action of a fault. The amount of 

 faulting and crushing in this area was enormous, a 

 fact which did not seem to have been recognised at 

 all by Professor Geikie. The supposed porphyries in 

 the Pebidian were really, for the most part, indurated 

 ash. The author now admitted that unconformity 

 existed between the Pebidian and the Cambrians. 

 Examined with more care than appeared to have 

 been given to it by the author, the conglomerate was 

 found to consist in very large part of the Pebidian 

 rocks, and of derivative materials from the still older 

 Arvonian and Dimetian series. This fact was 

 remarkably confirmed in Ramsey Island. He 

 maintained the existence of a great unconformity 

 between the Pebidian and the Cambrian con- 

 glomerates, the materials of the former having been 

 metamorphosed before the deposition of the latter. 

 He pointed out the existence of great masses of 

 agglomerate in the midst of the supposed intrusive 

 masses. 



Mr. Peach, from the sections of Dr. Hicks, argued 

 that the Cambrian conglomerate rests always on the 

 same member of the underlying beds, and there could 

 be no unconformity. He regarded the so-called 

 Pebidian pebbles as segregations, and not pebbles. 



Mr. Hudleston said that no one could suppose the 

 Pebidians to be of sufficient importance to constitute 

 a system by themselves ; and the great question was 

 whether they should be grouped with the Cambrian 

 or the Archean. He had difficulty in recognising 

 the supposed unconformity between the Cambrian 

 and the Pebidian, and he thought that the volcanic 

 series was the natural base of the Cambrian system. 



Mr. Topley stated that the faults invoked by Dr. 



Hicks would account for the non-passage of a dyke 

 from the granite into the Cambrians. Dr. Hicks 

 had not distinguished between local and regional 

 metamorphism. The species from the conglomerate 

 exhibited by Dr. Hicks were certainly exceptional ; 

 but the great mass of the conglomerates are of quart- 

 zose character. Local and small unconformities 

 between the Cambrian conglomerates and the main 

 volcanic group (Pebidian) had been admitted both at 

 this and at the last meeting ; but he differed from 

 Dr. Hicks as to the great significance to be attached 

 to them. 



Dr. Callaway objected to Professor Geikie's views 

 as to bleaching and induration being proofs of local 

 metamorphism ; he regarded them on the contrary as 

 evidence of faulted junction, the result of pressure 

 and the infiltration of water. He remarked that the 

 key found by Dr. Hicks at St David's had supplied 

 us with an explanation of most of the similar 

 Archean series in England and Wales, which was a 

 great confirmation of the truth of the theory. 



Mr. Rutley said that some of the felsites of the 

 district resembled certain spherulitic rhyolites. He 

 thought that they represented a transition between 

 granitic rocks and ordinary rhyohtic lavas. Most of 

 the Welsh lavas of the same kin i with which he was 

 acquainted were of Lower Silurian age. 



Mr. T. Davies did not agree with Professor Geikie 

 in regarding the so-called Dimetian as a granite. It 

 contained no mica, nor had it contained any ; for he 

 could not regard the green mineral as the result of 

 the alteration of mica in situ, but rather as derived 

 from the interbedded basic rocks. Among 500 

 specimens of granite from about 400 localities he 

 could find nothing resembling the St. David's rock, 

 and he could not regard the latter as a granite at all. 

 A rock in the very heart of this supposed intrusive 

 mass was found to be a breccia with fragments, some 

 of them water-worn, of the stratified rock of the 

 district. 



Professor Renard said that he had had a collection 

 of specimens and of microscopic slides from the rocks 

 of St. David's submitted to him by the author, and 

 had examined them in concert with Professor Zirkel, 

 of Leipzig, and Professor Wichmann, of Utrecht. 

 The conclusions arrived at regarding them were as 

 follows : — 1. The so-called "Dimetian" rock of St. 

 David's is unquestionably a true granite. 2. The 

 quartz-porphyries are just such rocks as might be ex- 

 pected to occur as apophyses of the granite, and the 

 specimens from Bryn-y-Garn, Rock House, and St. 

 David's left no doubt on our minds that such is really 

 their origin. They cannot be confounded with 

 rhyolitic lavas. 3. The conglomerate from the 

 granite-contact shows secondary quartz between its 

 pebbles. 4. The bands of fine tuff found intercalated 

 with, and on various horizons above, the conglome- 

 rate, consist of true tuff, and cannot have been 

 derived from the mere superficial waste of older 



