"Vol. 69.] GEOLOGY OE'bAEDSEY ISLAND. 519 



4o be a crushed pillow-lava (spilite) brought up by a thrust from 

 the uorth. This rock may be correlated with the variolite already 

 seen in Bau y Nant, as it is similarly overlain by siliceous slates 

 which are practically cleaved sandstones. The latter are contorted. 



South of these is to be seen on the shore a conspicuous group of 

 thick lenticles of quartzite dipping westwards at about 40° to 50°. 

 They represent a broken-up bed about 7 feet thick. A limestone- 

 band, also now represented by isolated lenticles, lies in the slates a 

 few feet below the quartzite. Still farther south, in Forth Solfach, 

 granite is exposed again, as shown by Miss Raisin, at and near 

 high-water mark ; I found it also at low-water mark at Trwyn 

 Dihirid, and at the next point to the north-west of it, dipping 

 north-westwards at 65° to 75°. At all these localities it is 

 crushed and cataclastic, as also are its sedimentary associates. 



From the south side of Porth Solfach to the lighthouse, a distance 

 of 800 yards, irregular beds and lenticles of quartzite and limestone 

 occur among slates which, as a rule, are more irregularly cleaved 

 and contorted in the immediate neighbourhood of the larger 

 lenticles. Close to the lighthouse is a massive quartzite 20 feet thick, 

 dipping north 15° west at an angle of 65°. Separated from this 

 bed by a narrow interval of highly-contorted slates with thin flaggy 

 quartzite-bands, is another quartzite of irregular thickness up to 

 15 feet, dipping in the same direction at 45° to 50°. I consider 

 the two quartzites to be probably a single bed repeated by a broken 

 isoclinal fold, especially as limestone occurs under the southern and 

 above the northern of these two outcrops. Lenticles among the 

 slates and limestones in the rocks on the north can be identified 

 with the same bed, indicating additional broken isoclines. 



The coast south of the outcrop of the 20-foot quartzite-bed is 

 eroded in white-weathering siliceous slates, like those seen in the 

 north of the island at and near Trwyn y Gorlech. They contain 

 one or more zones of quartzose flags, the rapid isoclinal folding of 

 which brings out the structure of the slates ; broken bands and 

 lenticles of quartzite, and a zone of flaggy grits that forms several 

 irregular outcrops, are also present. At the southernmost extremity 

 of the island (Maen Du and neighbourhood) there has been much 

 movement, thrust-planes abound, and the rocks are considerably 

 shattered, sheared, and cut up into lenticles, the dynamic forces 

 -acting from the north-west and west. Here and at Ogof Lladron, 

 green diabase is found associated with a green phacoidal crush- 

 breccia of greenish-grey grit in a slaty matrix. A portion of the 

 igneous rocks here present a phacoidal aspect, and it is just possible 

 that this may be crushed pillow-lava ; but much of the igneous 

 material in this area is almost certainly intrusive, and the sedi- 

 ments in contact with some of the diabase have been converted into 

 chert-like adinoles. The diabase occurs in a manner suggesting 

 that it forms broken synclines lying among overfolded slates. 



Q. J. G. S. No. 275. 2 m 



