110 DR. E. L. SHERLOCK ON THE [vol. lxxiv, 



ash forms a surface-cap over much of the hill-slope ; but, near 

 the bottom of the hill, the intrusive rock emerges from under- 

 neath the ash. Upwards, from Adit No. 3, the thrust trends in 

 a north-westerly direction, resting on the shales until it passes 

 beneath the greenstone plateau at the top of the hill. 



Proceeding northwards along the hillside, we note that crags of 

 ash become prominent as the debris-fan gives place to wooded 

 slopes, and we then find ash covering the whole hillside from the 

 greenstone plateau to the alluvium at the bottom. Soon, however, 

 as we proceed, the ash is cut off completely hj a fault having an 

 approximately east-and-west direction, and bringing in rhyolite. 

 It is possible that the ash may be capable of subdivision into 

 different types, as it varies considerably in appearance in different 

 -crags, and, in fact, near the fault the rock is doubtfully pyroclastir 

 and may be a lava. Here it is greatly sheared and is a serpen tinized 

 and highly altered diabase showing phenocrysts of albite-oligoelase. 



(C) The Rhyolite. 



This rock is the cause of the precipices of Clogwyn Mawr. 

 It is a very hard rock, dark grey, of fine texture, resembling a 

 halleflinta. It extends from the greenstone plateau to the alluvial 

 flat below and is well seen in a road-metal quarry situated by the 

 roadside, 600 }^ards north of the 4th milestone from Llanrwst. 

 Towards the south it is bounded by the fault referred to above, 

 and northwards it passes beyond the area surveyed. Petro- 

 graphically it is a silicified soda-rhyolite, containing albite and 

 perthite phenocrysts in a fine-grained felsitic ground-mass, with 

 some indications of banding. Some granular sphene, probably 

 pseudomorphous after ilmenite, is present. 



(D) The ' Greenstone/ 



Not only does this rock form the plateau at the top of the hill 

 from Rhibo, past Blaen-y-wern and Pen'rallt Inco, but it is 

 exposed in the lower slopes of the hill, near the high road, where 

 it is intrusive in the rhyolite, from 340 to 580 yards north of the 

 4th milestone. The rock is clearly the newest in the area, cover- 

 ing as it does all the others, which in turn disappear beneath it as 

 they are followed up the hill. 



The microscope shows it to be a partly albitized and epidotized, 

 ophitic dolerite, with fresh augites and large plates of decomposed 

 ilmenite. 



Dr. A. Harker 1 gives reasons for considering these later green- 

 stones to be probably of post-Carboniferous age. Prof. C. Lapworth 

 <fe Prof. W. W. Watts 2 show that in the Shelve district the later 

 •dolerites alter JPentamerus Limestones, and must therefore be post- 

 Llandovery at least. 



1 ' The Bala Volcanic Series of Caernarvonshire A: Associated Books 

 Oatnbridge, 1889, p. 107. 



2 ' The Geology of South Shropshire ' Proc. Geol. Assoc, vol. xiii (1894 1*5) 

 pp. 339-40. 



