Vol. 69.] 



GEOLOGY OF BABDSEY ISLAND. 



523 



summit, in the core of an overfold, a highly-vesicular, rotten, 

 basic igneous rock, presumbly a pillow-lava, occurs in association 

 with limestone and qnartzite. Not far away, near the 505'5-foot 

 bench-mark, is a dyke-like mass of diabase, some 25 to 30 feet wide, 

 referred to by Dr. Plett in the Appendix (p. 530). The adjacent 

 slates show contact-alteration. 



The steep eastern face of Mynydd Enlli has not been examined 

 by me, but the rocks are seen from the sea to be mainly cataclastic 

 slates frequently containing masses of quartzite and limestone. 

 Towards sea-level the structural planes dip westwards at moderate 

 angles. On the shore at Bau Pelen occur some ferruginous 

 ochreous rocks which may be basalts, but I was unable to examine 

 them. Farther south the coast is occupied by great crush-con- 

 glomerates of quartzite and limestone in a slaty matrix, which can 

 be conveniently studied on the headland of Pen Cristin. 



(e) The Crush-Conglomerates of Pen Cristin. 



Between Henllwyn and Pen Cristin some stages in the process 

 of disruption which has given rise to the formation of these crush- 

 conglomerates can be 

 Pig. 5. — Overfolding in quartzite-bands traced. The shore-sec- 

 interbedded with slates ; cliff west of tion here is remarkably 

 Pen Cristin. like, and is as instruc- 



tive as, that exposed on 

 the shores of Cemaes 

 Bay (Anglesey). 1 North- 

 east of the harbour of 

 Henllwyn the quartzite 

 and limestone group is 

 well exposed, as thick 

 beds contorted, folded, 

 sheared, and broken into 

 lenticles, lying among 

 slates with irregular 

 and contorted cleavage. 

 Parther east come slates 

 with numerous broken 

 bands and long lenti- 

 cles of quartzite and 

 occasional lenticles of 

 limestone, which ex- 

 hibit overfolding (fig. 5). 

 At Pen Cristin they 

 give place to more 

 of a slaty matrix full 



[The longest band shown is some 20 feet in length.] 

 thoroughly cataclastic strata, consisting 



1 Described by me in Q.J.G.S. vol. lv (1899) pp. 661-65. As the lime- 

 stones and quartzites of Bardsey are precisely like those of Northern Anglesey, 

 no lithological description is given of them in the present paper. 



