622 MR. E. GREENLY ON THE GEOLOGY OE THE [Nov. 1 896, 



the unconformity noted by Mr. Selwyn (Kamsay, ' Geology of North 

 Wales,' Geol. Surv. Mem. vol. iii. ed. 2, 1881, p. 223). 



As seen from the beach (fig. 6, p. 621), the slates form a bard and 

 well-marked feature (in which the sharp anticline of scbist is in- 

 cluded), over which pass the softer black shales, their bedding-planes 

 dipping at very nearly the same angle as the top of the feature. 1 The 

 discordance between their dip and the cleavage of the slates is most 

 marked, the latter being nearly vertical. The strikes also are in 

 different directions, the slates running E.N.E., and the shales at 

 this point N.W. Moreover, though it is difficult to trace bedding 

 for any distance in the slaty group, yet none of the beds, even when 

 at high angles, pass up into the group above, or over the crest of 

 the anticline, the schist rising nearly, if not quite, to the base of the 

 dark shales. Most important of all, the cleavage of the green 

 slates stops short at the base of the shales, which are quite un- 

 cleaved, even where seen, as at one place, actually in contact with 

 the lower group. 2 



Now these shales are not hard or coarse beds which would 

 resist a cleavage, but, it must be noted, are even finer and more 

 susceptible of such a structure than the lower rocks which possess 

 it in a high degree. Had the compression, therefore, operated at a 

 post-Ordovician date, these dark shales could not have escaped, but 

 would have received a cleavage readily, as even much coarser beds 

 of that age have in the Caernarvonshire mountains. 



The unconformity would seem, then, to be not merely local, 

 but of systematic value. But if we have here a fragment of 

 an older system, that system could not be later than Cambrian ; 

 and indeed, as no break so pronounced has been observed in North 

 Wales between Ordovician and Cambrian, it would almost certainly 

 be older still. The occurrence, therefore, of undoubted organic 

 remains in these rocks is of great interest, as they would appear to 

 be of pre-Cambrian age. Moreover, it would follow that the 

 group of crystalline schists here seen must be assigned, both as 

 regards their first formation and metamorphism, to a yet earlier 

 period. 



Experience has shown, however, that it is unwise to build too 

 much upon the evidence of single sections, however decisive it may 

 appear, especially when the rocks have been subjected to powerful 

 earth-movements. There has been powerful movement here at 

 two, if not three, distinct periods. Confirmation from other 

 sections, or possibly from palseontological evidence, must therefore 

 be looked for. 



But unless such evidence contradict that which is seen here, it 

 would seem that the crystalline schists of at least this region of 

 Anglesey must be of extreme antiquity. 



1 The surface of the feature is marked by slickensides, which occur also 

 in the schists ; but the discordance cannot be due to movement of this kind. 

 A false appearance of such movement has been produced in fig. 1 (p. 619) 

 W my exaggerating somewhat the width of the anticline in the shales. 



2 Mr. G-. J. Williams, F.G.S., kindly permits me to add that he concurs 

 with me in this reading of the Oareg Onen section. 



