356 THE PRE-C AM BRIAN AGE OP ROCKS IN N.W. PEMBROKESHIRE. 



cannot be newer than the Lingula-flags ; and, indeed, it seems almost 

 certain that it must be of Lower Cambrian age, and that it happened 

 to be left here lying on the Pre-Cambrian eroded surface when 

 the beds between it and the Liugula-ilags were cut out by the fault. 

 That the volcanic series at Trefgarn and Plumstone mountains are 

 older than the Lowest Cambrian rocks in those areas, there cannot 

 be the shadow of a doubt ; therefore there is every reason to believe 

 that the rocks of the same nature in this quarry are also of that 

 age. Id any case the evidence here is perfectly conclusive as to 

 their being older than the Lingula-flags (Upper Cambrian). There- 

 fore the Survey maps and sections are misleading in indicating any 

 of these as intrusive masses in Cambrian and Silurian sediments. 



Conclusions. 



The new facts now laid before the Society, combined with those 

 given on previous occasions, show that there is exposed in N.W. 

 Pembrokeshire a great group of Pre-Cambrian rocks. There are 

 also material differences to be made out in these Pre-Cambrian rocks 

 at various points, and there are indications pointing to their being 

 of different ages ; the Dimetian rocks are evidently the oldest, and 

 some of the indurated argillites and the more basic portions of the 

 Pebidian series the newest. The position of the great felsitic series 

 (Arvonian) is certainly between the Dimetian and Pebidian, but it 

 is difficult to state decidedly whether it should be classed as of Pre- 

 Pebidian age. Prom the facts to be obtained at Eamsey Island, 

 also to the north of St. David's, and elsewhere, I am still inclined to 

 look upon that series as marking a definite period. Still it must be 

 admitted that at present the evidence for this is not conclusive. 

 That the newest of the Pebidian rocks must have been folded, 

 cleaved, and have undergone great structural changes before the 

 Lower Cambrian Conglomerate was deposited is absolutely certain, 

 as the conglomerate rests quite unconformably on the Pebidian s, 

 and the rolled fragments of the latter found so abundantly in 

 it show clearly that the Pebidians had undergone all their im- 

 portant changes before the fragments were broken off. That the 

 underlying felsitic and Dimetian rocks had also assumed their 

 peculiar characters in Pre-Cambrian times, is equally certain, as 

 may be seen by examining the rolled fragments so plentiful in 

 some of the Cambrian grits and conglomerates. 



The presence of these fragments in the Conglomerate is also con- 

 clusive proof that the Dimetian and the felsitic rocks are not in- 

 trusive in the Cambrian and Silurian sediments, but are, as previously 

 claimed by me, undoubtedly of Pre-Cambrian age. 



I therefore maintai-i that lliere is the most ample evidence to 

 show that there is a great r^roup of Pre-Cambrian rocks exposed in 

 N.W. Pembrokeshire, nr.d hence that I have proved conclusively 

 that Dr. Geikie's views in regard to these rocks, as given in his 

 paper and more recently in his text-book, are entirely erroneous. 



(Por the Discussion oil this paper, see p. 363.) 



