SHARPE ON NORTH WALES. 297 



District between the eastern side of the Carnarvonshire Trough and 

 Arenig, — It has been before stated that the author endeavoured to 

 connect the beds on the east side of the Carnarvonshire trough with 

 those in Merionethshire, by prolonging his line of section across the 

 pass of Rhaidr-cwm in a south-eastern direction ; and that he was 

 foiled in this endeavour. The following are the rocks that he met 

 with in that line of section. 



The schist that is the lowest rock on the west side of the fault of 

 Rhaiadr-cwm, is found also to the east of the pass, dipping first east 

 25°, then north-east 30° ; and further east it forms a poor slate, 

 dipping north 20°, and resting on the eruptive mass of Moel-llech- 

 wydd-gwyn, which wholly cuts off the stratified series. This igneous 

 mass consists of quartzose felspar- rock and porphyry, and belongs 

 in point of date to the porphyry of Cader Idris. Between this mass 

 and the porphyritic chain of Arenig, schistose rocks occur of vari- 

 able character ; and towards the south-east these rocks are found 

 dipping north-east from 80° to 45°: but on the road to Bala two 

 lines of felspathic rock, proceeding to the N.N.W. from Arenig-fawr, 

 intervene to complicate the section. Between these two lines is a 

 hard blue schist dipping north-east 30°. At Tai-hirim, at the south- 

 west foot of Arenig-bach, is a quarry of poor slate, which dips north- 

 east 30°*. Arenig-bach forms the northern extremity of the por- 

 phyritic chain of the Arenigs, which is fourteen miles long, and 

 reaches southward nearly to Cader Idris. On the line of section 

 No. 3, the porphyritic mass of this chain is nearly two miles wide. 

 In the centre it consists of grey and black porphyry ; on the flanks 

 the porphyry is usually flesh-coloured, passing into an amorphous 

 quartzose felspathic rock. The beds which lie to the west of Arenig 

 were described by the author in a former paper (see Proc. Geol. 

 Soc. vol. iv. p. 10). 



Western side of the Carnarvonshire Trough. — The position of the 

 beds which lie between the synclinal axis and the valley extending 

 from Capel Cerrig to Beddgelert, leads to the inference that these 

 beds are the equivalents of the rotten schists of Merionethshire, 

 which lie above the Bala grits. The beds in question are hard, 

 grey, slaty grits, alternating with slate, grey hornstone or quartz- 

 rock ; rocks very different in their mineral character from the schists 

 which they are supposed to represent ; but this difference may be 

 attributed to the numerous dikes and interstratified beds of green- 

 stone by which this Carnarvonshire district is traversed; whereas 

 that of Merionethshire is quite free from igneous action. The ag- 

 gregate thickness of these beds is much greater than that of the 

 rotten schists of Merionethshire. 



The valley which reaches from Capel Cerrig to Beddgelert is the 

 line of a considerable fault. To the west of this fault, as far as the 

 eastern sides of Glyder-fawr and Snowdon, are hard grey schists and 

 schistose grits. These the author regards as the equivalents of the 



* The bed which is next to the porphyritic rock is so often found to be a true 

 slate, that this character, when it occurs under such circumstances, must be attri- 

 buted to metamorphic action. 



