522 DR. C. A. HATLEY ON THE [Oct. I913, 



to north-west. There is a small quarry in these slates, about 

 50 yards south-east of the school, exposing a purple igneous rock 

 which microscopic examination shows to be a fine-grained de- 

 composed variolite. It underlies a tufaceous shale containing 

 pebbles of the same rock with chert and some sediment, and so 

 there is no doubt that the igneous rock in this case is a lava and 

 not a sill. 



On the east the beds below the thrust-plane dip at higher angles, 

 usually from 60° to 90°, and the dip is generally a reversed one. 

 A conspicuous outcrop of quartzite about 8 feet thick helps us to 

 elucidate the structure here. On the crest, south of the summit, 

 at about 300 feet O.D., the quartzite stands up as a prominent 

 rock, and can be followed along the hill-slope as a sigmoidal fold, as 

 shown in fig. 4. Though broken by a fault, this bed can be traced 



Fig. 4. — Sigmoidal fold on the ridge of Mynydd Enlli, 

 | at the 300-foot contour, looking north. 



[Q — Quartzite. L = Limestone. th.= thrust-plane.] 



northwards for a distance of about 400 yards as a group of thick 

 lenticles which lie between the crest and the sole of the thrust. 

 Still farther north several more large lenticles or ' quartz-knobs ' 

 are to be seen, one of which is represented in PI. XLIX, fig. 2. 

 They may be portions of the same bed. Two other fine examples of 

 large isolated lenticular masses are perched, one near the other, on 

 the steep slope above Ogof Morlas, and dip east-north-eastwards 

 at 40° to 50°. One is of quartzite, the other of limestone. 



On the bare northern slopes of the hill cataclastic gritty slates 

 dip steeply, though with many minor contortions and small over- 

 folds, below the two great lenticles just mentioned. They contain 

 the usual broken bands of grit and quartzite; while a zone of thin, 

 flaggy, quartzose grits and lenticles of a fine clastic breccia (or 

 angular grit) make their appearance in places. At the northern 



