•516 DR. C. A. MATLEY ON THE- [Oct. T913, 



apparently the lowest strata exposed, in view of the fairly steady 

 dip of the beds over most of the island. 



Dr. Jehu's work in Bardsey dealt exclusively with the drift and 

 the glacial phenomena, and will be referred to later. 



III. Stratigraphy of the Pre-Cambeian Bocks. 



The rocks of the island consist principally of sediments, usually 

 •green, gritty, schistose slates or phyllites, in and among which 

 occur irregular beds, lenticles, and masses, large and small, of 

 quartzite, grit, and limestone. Igneous rocks are also present. 

 Granite, diabase, and spilite, nearly always much crushed by 

 earth-movement, have been observed, and there are also dykes of 

 ■olivine-dolerite, quite uncrushed and therefore of post-movement 

 age. The sediments have, it is true, the prevalent westerly to 

 northerly dip mentioned by earlier writers, but no general infer- 

 ence as to the succession can be drawn from this fact, for the field- 

 work reveals frequent inversions and repetitions of the strata. The 

 structure is often masked by slaty cleavage, and the rocks in many 

 places are in a thoroughly cataclastic condition. The rocks have 

 been subjected to intense earth-pressure acting mainly from the 

 north-west, with the result that they have been disrupted, over- 

 folded, and overthrust. All the pre-movement rocks are, for reasons 

 given below, considered to be of pre-Cambrian age ; but the age of 

 ■the movements themselves will not be here discussed- 

 There is a close lithological resemblance, as earlier writers have 

 pointed out, between these rocks and those of the northern district 

 ■of Anglesey ; and, from my knowledge of both regions, I am able to 

 state that this .resemblance extends also to the manner in which 

 both areas have been affected by intense earth-movements. Just 

 as in the Cemaes area of Northern Anglesey, rocks once regarded 

 as volcanic agglomerates have proved to be cataclastic sedi- 

 mentary strata, 1 so also in Bardsey, although volcanic rocks do 

 occur in the island, I have failed, to recognize any true ag- 

 glomerates, and I find that those which have been described as 

 •such are bedded rocks which have been torn to pieces by earth - 

 movement, and are now in the condition of crush-breccias and 

 crush-conglomerates. Similarly, the isolated ' quartz-knobs ' of 

 J. F. Blake, of which several good examples occur in Bardsey, 

 prove to be, as in Northern Anglesey and elsewhere, portions of 

 massive beds of quartzite that have been broken up into lenticles 

 and separated from their fellows by differential movements between 

 these hard masses and their less rigid associates. 



The rocks are excellently exposed on the shore and in the sea- 

 cliffs, but inland, except on Mynydd Enlli, they are largely covered 

 by boulder-clay. The accompanying coast-sections (figs. 1, 2, & 3, 

 pp. 517, 520) are given, in order to illustrate the geological structure; 



1 See Sir Archibald Geikie, Geol. Mag. ser. 4, vol. iii (1896) p. 481 ; and 

 ■C. A. Matley & W. W. Watts, Q. J. G. S. vol. lv (1899) pp. 657-66 & 677-78. 



