530 THE GEOLOGY AND [Oct. 1913,. 



of the rocks might* be described as calc-chlorite schists. In 

 places a broken crystal of albite is left in these sheared rocks, and 

 indicates the original igneous structure. It would be difficult, in 

 fact, from the microscopic slides alone to recognize many of these 

 rocks as pillow-lavas ; and it is impossible to discuss adequately 

 what was their original composition and structure. 



It is clear, however, that there were porphyritic and non-por- 

 phyritic types, the former containing felspar phenocrysts and tho 

 latter being often very vesicular. The pillow-lavas are extremely 

 liable to alteration, and, in fact, are nearly always much decom- 

 posed ; and this, in addition to their highly vesicular character, 

 makes them weak rocks, which are very readily crushed by folding 

 movements. From the northern summit of Mynydd Enlli some of 

 the specimens [9303] are exceedingly like the more completely 

 altered vesicular schalsteins of Cornwall and Devon, 1 such as occur 

 around Brent Tor. 



A specimen obtained 50 yards south-east of the school is inter- 

 esting, as exhibiting traces of the variolitic structure which is so 

 characteristic of the pillow-lavas of the Lleyn 2 and Anglesey. 3 

 Another specimen from the same locality contains fragments of 

 glassy variolite ; this resembles the ' palagonitic tuffs ' that accom- 

 pany pillow-lavas in the Mevagissey 4 and Porthallow 5 districts of 

 Cornwall. 



(3) The Diabases. 



Most of the basic volcanic rocks of Bardsey Island are so fine- 

 grained and so highly vesicular that they are more probably lavas 

 than intrusions. None of the specimens sliced is a typical albite- 

 diabase, 6 such as those that occur so commonly among spilitic rocks 

 elsewhere. There is, however, one remarkable rock from Mynydd 

 Enlli, near the 505*5 bench-mark [9302], which is rather coarsely 

 crystalline, and shows one of the characteristics of the 'spilitic suite' 

 in that it is exceedingly rich in albite. It contains also chlorite,, 

 iron-ores, and a fair amount of quartz ; and it has been so much 

 crushed that it resembles a breccia rather than an igneous rock. 

 This rock is abnormal among albite-diabases in containing much 

 primary quartz, and perhaps micropegmatite ; but no doubt can be 

 entertained that it is intrusive, and closely connected in origin with 

 the pillow-lavas. 



A chert-like rock, associated with a green diabase north of 



1 'The Geology of Dartmoor ' Mem. Geol. Surv. 1912, p. 20. 



2 Miss C. A. Raisin, ' Variolite of the Lleyn & Associated Volcanic Rocks * 

 Q. J. G. S. vol. xlix (1893) p. 152. 



3 E. Greenly, ' The Origin & Associations of the Jaspers of South-Eastern 

 Anglesey ' Q. J. G. S. vol. lviii (1902) pp. 429-30. 



4 'The Geology of the Country around Mevagissey' Mem. Geol. Surv. 1907 ^ 

 p. 54. 



5 'The Geology of the Lizard & Meneage' Mem. Geol. Surv. 1912, p. 184. 



6 H. Dewey & J. S. Flett, ' On some British Pillow-Lavas & the Rocks- 

 Associated with them' Geol. Mag. dec. 5, vol. viii (1911) p. 206. 



