526 DR. C. A. MATLEY OX THE [Oct. I913,. 



known Cambrian rocks of North Wales it is clear that the Bardsey 

 and Lleyn rocks must be of pre-Cambrian date. The intrusive 

 diabase and granite-sills may be also of pre-Cambrian age. 



The sedimentary rocks of Bardsey can be correlated with the- 

 lower beds of the Llanbadrig Series of Northern Anglesey and with 

 their equivalents, the Llanfair-y'nghornwy Beds of North-Western 

 Anglesey. In the latter area there is also, as in Bardsey, an 

 intrusion of granite which has undergone much crushing, and 

 it may, perhaps, be correlated with the Bardsey granite. 



As regards the post-movement dolerite-dykes, similar dykes can 

 be seen in large numbers in the pre-Cambrian strip of the neigh- 

 bouring mainland, where I have mapped about 125 of them. They 

 are most probably of Tertiary age, as already suggested by Mr. E. 

 Greenly for dykes of the same composition and with similar trend 

 in Anglesey, 1 and Dr. Flett writes to me concerning a rock-slice 

 from one of the Bardsey dykes [9091], 2 that it is an 



' ophitic olivine-dolerite with fresh basic felspar, extraordinarily like some- 

 Tertiary dolerite-dykes of the West of Scotland.' 



VI. Glaciologt. 



In the 2nd edition of his North Wales Memoir Bamsay mentioned 

 the Glacial drift of Bardsey, and observed that the island 



' has been moulded by ice .... , but the rnammillated roches moutonnees- 

 have since been roughened by the weather.' {Op. cit. p. 212.) 



The only other reference to the glaciation is in Dr. Jehu's paper of 

 1909, where he remarks that 



' the island as a whole may be regarded as an example of the phenomenon 

 known as " crag and tail," the crag facing the north-east, from which direction 

 the ice-sheet came.' {Op. cit. p. 29.) 



Among the boulders and pebbles recorded by him from the 

 boulder-clay of the island may be mentioned Chalk-flints, Ailsa 

 Craig microgranite, Dalbeattie granite, picrite, Carboniferous Lime- 

 stone, and shell-fragments. 



The result of my own investigation of the drift-phenomena of the 

 Island is given below. Glacial drift, in the form of boulder-clay,, 

 covers most of the ground between Mynydd Enlli and the western 

 coast, the mountain itself being almost free from drift, though its 

 rocks are frequently moutonnees. The drift- covered area is 

 given over to agriculture. In the south of the island the drift is 

 patchy and very thin. Ice-worn surfaces are common, though few 

 of them have retained their scratches. I found striae, however, at 



1 ' On the Age of the Later Dykes of Anglesey ' Geol. Mag. dec. 4, vol. vii 

 (1900) pp. 160-64. 



2 The numbers in square brackets here and in the Appendix are the- 

 numbers of the rock-slices in the Museum of Practical Geology, Jermyn 

 Street, cut from material handed over to that Museum from my collection. 



