Vol. 69.] THE GEOLOGY OF BARDSE? ISLAND. 521 



(c) The South-Eastern Coast. (Part of fig. 2, p. 520.) 



This shore consists mainly of the siliceous slates with their usual 

 quota of grit- and quartzite-bands, already described as occurring 

 on the south-western coast ; but diabases also occur, in two long 

 strips separated by a slaty crush-conglomerate that contains abun- 

 dant quartzite and rare lenticles of limestone. These diabases 

 undoubtedly belong to the same group as those seen about Maen Du. 

 They have often a sheared aspect, and pass into ' schalsteins ' ; parts 

 of them look pillowy and may be lavas, but some of the masses are 

 intrusive. They contain bands and lenticles of chert-like sediment, 

 apparently folded intensely, and these seem to be contact-altered 

 rocks or adinoles. Some of the igneous rocks distinctly cut across 

 the bedding, and form lenticular sills. 



Farther north-west, in the bay of Henllwyn, a large mass of 

 pillowy diabase or spilite, showing remains of vesicular and 

 variolitic structure, occurs below high-water mark, and there may 

 be more lenticles at that locality ; but the rocks are obscured 

 by thick groAvths of Fucus. Grit, slate, and some calcareous sand- 

 stone are also in evidence, the whole being broken up into a 

 crush-breccia. 



From the shore-sections just described, it will be apparent that, 

 despite a fairly constant direction of dip, there is much repetition 

 of the strata, evidently the result of overfolding and overthrusting. 

 The following rock-groups are recognizable, and seem to occur in 

 the order shown below : — 



(vii) Cataclastic grits and slates, with occasional limestone, 

 (vi) Variolite and diabase, partly as intrusive sills and partly as 



pillow-lava (spilite). 

 (v) Gritty slates and fine-grained sandstones, usually showing regular 



cleavage and containing thin flaggy quartzite and grit-bands, 

 (iv) Limestone-and-quartzite group : tbat is, slates with included thick 



beds and lenticles of quartzite and limestone, 

 (iii) Cataclastic grits and slates. 



[Granite-sill intruded usually at about this horizon.] 

 (ii) Sandy slates, showing regular cleavage. 

 \i) Sandy slates, with quartzite-bands. 



(d) Mynydd Enlli. 



A thrust-plane hading westwards can be followed, as already 

 stated (p. 518), from Bau y Kant in a southerly direction. It can 

 be traced along a bracken-clad hollow on the western slope, and 

 divides the hill structurally into two portions. It reaches the 

 shore near the harbour of Henllwyn, and may be continued across 

 the bay as one of the thrusts that traverse the south of the island 

 with a west-south-westerly course. 



The western, or overthrusting, limb consists of cataclastic green 

 slates, with one or more purple zones^ enclosing many small phacoids 

 of grit and occasional larger lenticles of flaggy grit, quartzite, and 

 limestone. The cleavage, usually phacoidal, dips at 30° to 40° west 



2 m 2 



