138 PEOE. T. M'KENNY HUGHES ON THE 



western rib of syenitic porphyry " (p. 137) — pointing out that the 

 beds of its upper surface are of such remarkable structure, and offer 

 such apparent passages into the overlying slates, that he cannot in 

 all the quarries separate one formation from the other (p. 137). 

 Subsequently, in a paper read before the Geological Society, Feb. 25, 

 1852 (Quart. Journ. Geol. Soc. vol. viii. p. 146), he says that he 

 had not found a well-defined base for the Cambrian series of North 

 Wales, unless we take the metamorphic rocks as a kind of hypo- 

 thetical base, and gives the following descending series : — ■ 



Arenig slates and porphyry. 



Tremadoc slates. 



Lingula flags. 



Harlech grits. 



Llanberis slates (including slates, grits, and bands of porphyry). 



Metamorphic series. 



By this time the Geological Survey was at work in North Wales% 

 and Jukes was in correspondence with Sedgwick. So we find specu- 

 lations as to why the Lingula flags had not turned up where ho 

 expected them, and at last the announcement that the Survey had 

 found them. This settled the correctness of Sedgwick's correlation 

 of beds in that district. Subsequently, however, Prof. Ramsay ar- 

 rived at the conclusion that the altered rocks of Llyn Padarn, which 

 had been referred by Sedgwick to an ancient formation beneath his 

 Cambrian, were not a distinct underlying series, but the frizzled 

 ends of beds of Cambrian age, which were seen in an unaltered 

 state close by ; and in confirmation of this view it was pointed out 

 that the conglomerates taken as the base of the Cambrian dipped 

 towards the metamorphic axis. This opinion was published in 1866 

 in Memoirs Geol. Surv. vol. iii., and is illustrated by sheets 28 

 and 31 of the cross sections of the Geological Survey. 



In 1868 Mr. Maw described " a new Section of the Cambrian 

 Rocks in a cutting of the Llanberis and Carnarvon Railway " (Geol. 

 Mag. March 1868, vol. v. p. 121), about halfway along which he 

 considered that there was a visible unconformity in the lowest part 

 of the series of Cambrian rocks, where a " dark green rock," which 

 he had been following, " suddenly terminates, resting," as it seemed 

 to him, " on the upturned edges of an older slate." 



By this time the Laurentian, Labrador series, and Huronian of 

 America, and the fundamental gneiss of Scotland, had been assigned 

 to their true position below the Cambrian ; and some of the meta- 

 morphic rocks of Norway, and of various isolated areas in the 

 British Isles, were beginning to be suspected to be of earlier date than 

 Cambrian. So geologists were prepared to accept the views of Mr. 

 Salter and Mr. Hicks as to a Pre-Cambrian island at St. David's. 

 It was, however, pointed out that the coarse conglomerates, which 

 seemed to form a more natural base for the Cambrian, did not rest 

 immediately on what was at first defined as the Pre-Cambrian Island, 

 but on a higher series of bedded rocks, fragments of which could be 

 identified in the Cambrian conglomerates. 



It still remained for Mr. Hicks to work out this district; and the 



