168 



SCIENCE. 



[Vol. II., No. 27. 



feels it his duty to ' defend the Tiews of his prede- 

 cessors ' as there indicated. There is a still larger 

 area of the Dimetian rocks about ten miles to the 

 east of St. David's; and there, as at St. David's, tliese 

 granitoid rocks underlie the lowest Cambrian beds 

 ■without producing the slightest alteration in the 

 latter. Indeed, I have now found no less tliau six 

 areas in Wales where typical Dimetian granitoid rocks 

 occur under the Cambrian or pre-Cambrian rocks; 

 and in neither of tliese areas, tliough several excellent 

 observers have, in addition to myself, searched the 

 boundaries carefully, have we found the slightest 

 indications of their being intrusive in those rocks, 

 though they are all colored as intrusive roclcs in the 

 survey-maps. In several of these areas the fact that 

 they must be pre-Cambrian rocks is rendered perfectly 

 certain, as large fragments of the granitoid rocks in 

 exactly the same condition in which they are now 

 found occur in the basal conglomerates of the Cam- 

 brian. In one area only, in Wales, have I found 

 Dimetian rocks entirely surrounded by roclcs newer 

 than the Cambrian; and here the Llandovery con- 

 glomerates and sandstones repose upon them, and 

 are largely made up of materials derived from the 

 Dimetian. In the other areas newer rocks than the 

 Cambrian are found occasiimally in contact with 

 limited portions of the Dimetian exposures ; but these 

 effects are clearly seen to have been produced by 

 faults. 



In his paper to the Geological society, referred to 

 in Science, Professor Geikie maintained "that the 

 'Dimetian group' is an eruptive granite which has 

 disrupted and altered the Cambrian strata, even above 

 the horizon of the supposed basal conglomerate." 

 The evidence adduced to support this view was from a 

 section at Ogof-Llesugn, where, as he supposed, " the 

 conglomerate had been torn off and enclosed in the 

 granite, and has been intensely indurated so as to be- 

 come a sort of pebbly quartzite." I'rofessor Hughes 

 and myself, along with a number of other competent 

 observers, have since examined this spot; and we 

 found that the conglomerate lies quite loosely upon 

 the Dimetian, that at almost every point we could 

 pass our hand between the conglomerate and the 

 granitoid rock, and that the Cambrian conglomerate 

 had no change whatever induced in it beyond that 

 common to it in all parts of the district. Two other 

 sections were mentioned, and drawings exhibited to 

 show the ' Dimetian ' intrusive in the Cambrian, and 

 as having eaten deeply into the series at Porthclais. 

 These sections I knew perfectly well, at the time, to 

 be in the lines of faults; but for greater satisfaction 

 I asked Professor Hughes and party to re-examine 

 these with me. The result proved that I was entirely 

 right, and that Professor Geikie and his assistants 

 had mistaken a junction produced by well-marked 

 faulting for an intrusion, and the beds which he sup- 

 posed had been eaten away had simply been dropped 

 by the fault. He could not produce a single speci- 

 men showing contact alteration between the grani- 

 toid (Dimetian series) and overlying rocks. His 

 evidence, therefore, fails utterly, on examination; 

 and the pre-Cambrian age of the granitoid rocks of 

 St. David's is rendered, if possible, more than ever 

 certain. An attempt was made to show that the 

 quartz-porphyries which I had pointed out as being 

 intrusive in the Pebidian rocks, which alter the rocks 

 in their immediate vicinity, were just such rocks as 

 might be apophyses of the 'granite,' but, with a curi- 

 ous want of knowledge of the fact that these quartz- 

 porphyries are common to many other parts of the 

 area far distant from the granitoid series, that they 

 also actually in some places cut across the latter. 



As Professor Geikie did not spend the time neces- 

 sary to examine the area wbere the Arvoni.tn rock* 

 are chiefly exposed, but hastily arrived at the conclu- 

 sion, without seeing tbeni, that the lialleflintas, brec- 

 cias, and porcellanites must be inlru;-ive felstones, I 

 need scarcely refer to Professor Geikie's views on this 

 point. I shall refer fully to this question in my paper, 

 in reply, to the Geological society. I may, however, 

 mention, that I exhibited a series of magnificent brec- 

 cias from this group, and showed large masses of the 

 Cambrian basement conglomerates from liamsey Is- 

 land, consisting almost entirely of the rocks of the 

 Arvonian group upon which they repose. The latter 

 are colored in the survey-map as intrusive in beds high 

 up in the Silurian (fossiliferous Arenig). 



The Pebidian, Professor Geikie says, 'forms an in- 

 tegral part of the Cambrian system.' He acknowl- 

 edges that it underlies the Cambrian conglomerate, 

 but says the latter rests quite conformably upon the 

 former. In the survey-map these Pebidian beds are 

 supposed to be Cambrian beds higlier than the con- 

 glomerate, but altered by the so-called intrusions. 

 Here, therefore, some modification of the map is 

 acknowledged to be necessary. Had Professor Geikie 

 and his assistants used ordinary care in examining 

 these conglomerates, they would have seen also that 

 they are constantly in contact with different mem- 

 bers of the underlying rocks, that they lie unconform- 

 ably on tlie edges of those beds, and also that they 

 are very largely made up of the rocks below. 



Professor Geikie did not refer to North Wales in 

 his paper; but as the facts are, if possible, clearer 

 there than in South Wales, I may be allowed to call 

 the attention of the readers of Science to some 

 sections just published by the Geologists' association 

 of London, preparatory to the visit to be paid by the 

 members to Carnarvonshire and Anglesey, July 23- 

 28. These sections show in a very clear manner how 

 the Cambrian conglomerates creep over the Dime- 

 tian, Arvonian, and Pebidian rocks in that area. The 

 rocks of the first two and lowest groups are in that 

 area, as at St. David's, colored as intrusive rocks in 

 the survey-maps, and the last as altered Cambrian 

 and Silurian rocks. 



The sections have been prepared by Prof. T. McK. 

 Hughes (Woodwardian professor of geology at Cam- 

 bridge, and formerly of the Geological survey), who 

 has carefully worked out the geology of this district. 

 He and I were the first to point out, in the year 1877, 

 the similarity of the conditions exhibited here to 

 those at St. David's; and since then he has devoted 

 much time to the elucidation of the facts bearing 

 upon the questions in that area. 



In a diagram (no. 1) he shows how tlie basement 

 conglomerate of the Cambrian, between Bangor and 

 Carnarvon, creeps over no less than four sub-groups 

 of the archean rocks: viz., at Bangor, over the Bry- 

 niau beds (Pebidian) ; at Brithdir, the Dinorwig beds 

 (Arvonian ?) ; at another part farther south, the Crug 

 beds (upper Dimetian); and at Tut Hill, tlie Carnar- 

 von beds (lower Dimetian). In section 2, tlie uncon- 

 formable overlap of the Cambrian over the Pebidian 

 near Bangor is clearly sliown ; and in no. 3, a diagram 

 section showing the sequence of tlie rocks from Car- 

 narvon to Snowdon, the basement beds of the Cam- 

 brian are shown rolling over the Carnarvon and Din- 

 orwig groups at different points.- 



Altogether, the evidence afforded by these sections 

 is of the most conclusive kind; and it seems impos- 

 sible to believe that the surveyors, when they have 

 seen and examined these sections, and have had more 

 experience with the Welsh rocks, can still cling to the 

 antiquated faith that all these pre-Carabrian rocks 



