ROCKS IN N.W. CAERNARVONSHIRE. 283 



Prof. Ramsaj and, after him, Dr. Hieks draw the section, though 

 they introduce a fault as well. 



But is this so ? On the north-west side of the felsite there is a low 

 alluvial valley, about 200 yards wide. On the opposite side, quite 

 close to the valley, there are exposures of rock ; but it is not a con- 

 glomerate, but ordinary fine slates, with vertical cleavage, but dip- 

 ping towards the felsite at an angle of about 20°. This is suggestive, 

 but affords no certain datum ; our only chance is on the edge of the 

 felsite itself. It is ill founding a determination on one section, but 

 in this case a single proof could scarcely be more decisive. On the 

 west side of the road from the bridge to Eryn Efail, and exactly on 

 the letter " E" of that word in the map, there is a quarry in the 

 felsite, which here shows a precipitous face towards the valley. 

 The plan of this quarry is shown in fig. 6. The edge of the felsite 



Pig. 6. — Plan of Quarry at Bryn Efail. 



N. S. 



4 



1. Porphyry. 2. Slate. 3. Grit. 4. Grreenstone. 



is ragged and irregular, but perfectly clean. In the centre of the 

 quarry it runs against a boss of greenstone, and cuts it into pro- 

 montories and twisting tongues. At the sides of this the junction 

 is, for the most part, with a vertical bed of slate, which is followed 

 on the west by a bed of grit, with which also the felsite occasionally 

 comes in contact. The strike of these beds is nearly parallel to that 

 of the Cambrians just noticed. The felsite is so welded with the 

 slate that it is easy to obtain a junction specimen, and two such 

 have been examined. In one the line of junction is perfectly 

 straight. The felsite shows the quartz-crystals somewhat broken and 

 exhibiting signs of pressure, and the ground-mass is crypto-crystal- 

 line and of varying coarseness ; the whole is singularly like the 

 band at the edge of the Twt-Hill mass in the Field-quarry, and I 

 have no doubt they were formed under similar circumstances. 

 Towards the junction both felsite and slate are infused with ferric 

 oxide, which produces a coloured band in the slate of about J inch 

 in width. In the slate are developed abundant crystals of chias- 

 tolite, to judge from their long shape, their rhombic cross-section, 

 and their dark line down the centre, and their being broken up into 

 an infinitude of small low-polarizing crystals, which refuse to extin- 

 guish as a whole. There are also a number of small high-polarizing 

 crystals which I cannot identify. In other words, the slate is 

 altered by contact with the felsite. The same phenomena are 



u2 



