SHARPE ON NORTH WALES. 291 



it passes through Bryn-y-ddinas, and, at the distance of a mile and a 

 half, is lost beneath the Wenlock beds at their junction with the 

 Lower Silurian rocks*. To the S.S.W. it runs along the high hills 

 on the western side of the Penmachno valley, crosses that valley 

 at Moel Machyria, and may be traced in the same direction about 

 two miles further. Throughout the whole of this course, in length 

 about nine miles, it forms a line of abrupt separation between two 

 systems of beds, the system to the east of the dike striking east, 

 the other to the west of it striking north-east. 



On the exact line of the direction of the dike, to the further di- 

 stance of several miles, the beds are in great confusion. The road 

 from Ffestiniog to Bala crosses this line of disturbance at Rhaiadr 

 Cwm, where, at the bottom of the deep and inaccessible chasm 

 formed by the waterfall, the greenstone may perhaps be exposed. 

 On the further prolongation of the same line the greenstone re-ap- 

 pears north of Traws fynydd, and there covers a considerable area. 

 In this, as in the former part of its course, the igneous rock forms 

 the boundary between two systems of beds differing in strike. 



The greenstone then bends to the south-west, but its course in 

 that direction cannot be distinctly traced through the intricate 

 number of greenstone rocks belonging to the chain which extends 

 from Barmouth northwards. To the west of this chain the dike is 

 not recovered, but a fault appears, running south-west; and this 

 fault separates the prolongation of the Ffestiniog slates hereafter 

 noticed from the grey slaty flagstones of Harlech. The author en- 

 deavoured to connect the bedded rocks of Carnarvonshire with those 

 of Merionethshire, by tracing a section across the line of disturbed 

 beds near Rhaiadr Cwm, where the Bryn-y-ddinas dike does not 

 reach the surface. But he was foiled in this endeavour by a great 

 eruptive mass of felspathic i-ock, which, a little further to the east, 

 entirely cuts off the beds. 



As it appears therefore that Carnarvonshire is completely severed 

 from the rest of Wales, it was from the evidence afforded by the 

 Carnarvonshire rocks themselves, in respect of their position, order 

 of succession, organic remains and internal character, that the author 

 sought to determine their age. 



To obtain in the first instance a general section of the position of 

 the beds in Carnarvonshire, he ascertained their dip along two prin- 

 cipal lines of section ; the first line extending from Bryn-y-ddinas 

 along the course of the Holyhead road to the Menai ; the second 

 line passing through Rhaiadr Cwm, Snowdon and Llanberris, to 

 the Menai. 



Holyhead Road Section (Section 2). — On approaching the dike of 

 Bryn-y-ddinas from the south-east, the beds of Bala grit are affected 

 by it to the distance of nearly a mile. The dip varies from N.N.E. 

 15° to N.N. W. 25°, and the cleavage dips N.W. 50°. On the western 

 side of the dike, the beds for 3^ miles dip, at moderate angles rarely 

 exceeding 35°, to the N.W. or N.N.W. : to the north of the Holy- 



* The author names it tlie Brvn-v ddiiias dike. 



