354 The Rev. Prof. Blake — Base of the Sedimentary Series. 



V. — On the Base of the Sedimentary Series in England and 



Wales. 

 By the Eev. Prof. J. F. Blake, M.A., F.G.S. 



{Concluded from the July Number, p. 315.) 



3. North-west Carnarvonshire. — There are here two or three separate 

 areas which, at all events historically, must be considered more or 

 less independently, viz. the district between Bangor and Carnarvon, 

 the Llyn Padarn and Moel Tryfaen range, and the Lleyn Peninsula. 

 Except in reference to the last of these, it is certain that the 

 belief in Precambrian rocks in Carnarvonshire began with Prof. 

 Hughes. This author, indeed, quotes Prof. Sedgwick as recognizing 

 such rocks here, but the passages he quotes have a very different 

 meaning in the original. The slates near Bangor and Carnarvon, 

 which Sedgwick says are amongst the oldest of North Wales, are those 

 which occur " along the shores of the Menai Straits from Bangor to 

 Carnarvon," which he describes as "dark earthy-coloured slates which, 

 were we to judge only by mineral structure, might easily be con- 

 founded with Upper Silurian rocks," and these are "cut through by 

 a great intrusive rib of syenitic porphyry of a different epoch, which 

 ranges nearly with the beds." In the section he draws, from the 

 Menai Straits to Glyder Fawr on the Carnarvon Chain, in an E.S.E. 

 direction, which therefore must pass near Carnarvon, he indicates by 

 the same letter a the rocks on both sides of the porphyry, and inserts 

 no fault. From all this it is plain that what Sedgwick regarded as the 

 very oldest rocks in North Wales are those which have since been 

 recognized as Carboniferous shales on one side, and Arenig slates on 

 the other ! Like Prof. Sedgwick, Prof. Eamsay also taught the in- 

 trusive character of the crystalline rock, about which, at Llyn Padarn, 

 he held a very peculiar theory. It was in 1878 that Prof. Hughes 

 for the first time brought forward evidence in favour of the 

 Precambrian age of the "porphyries," and of the rocks near Bangor. 

 The principal point of his paper is that there are always con- 

 glomerates on the eastern boundary of this group, which proves that 

 the latter are older than the conglomerates. But what is the age 

 of these conglomerates ? On this point he brings forward no 

 evidence whatever, but merely asserts that the conglomerates " form 

 everywhere the basement bed of the Cambrian." Now there are 

 three conglomerates in question : (1) that at Twt Hill; (2) that at 

 Llandeiniolen ; (3) that on the east of Bryniau Bangor. All these 

 three Prof. Hughes takes to be the same, and Prof. Bonney now 

 identifies Nos. 1 and 3. My own examination of the district, how- 

 ever, yielded satisfactory evidence that they are all of different ages. 

 The Twt Hill conglomerate is quite of a different character to either 

 of the others, being far more quartzose ; it is followed rapidly at 

 that spot first by grits and then by slates, which are perfectly con- 

 tinuous with those carrying Arenig fossils, and the same succession 

 may be traced on a curved line as far as Llandeiniolen, and thence 

 along a line which runs east of and is transverse to the Bryniau 

 Bangor conglomerate. This then is an Arenig conglomerate forming 



