14^8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



(6.) The section next cuts across the ridges which range on the 

 south-east side of Tal-y-llyn nearly parallel to the strike of Cader 

 Idris. Dark soft slate prevails, containing however some harder 

 quartzose bands, and (on the line of strike towards the south-v^^est) 

 some masses of coarse greywacke, also containing beds in which 

 roofing-slate has been extensively worked. Farther to the south- 

 west, on the strike of the beds, are some bands of contemporaneous 

 porphyry. The beds in this division of the section have not a great 

 inclination ; still their aggregate thickness must be very considerable. 

 I would include in this great group the slate-quarries of Aberllefeni, 

 and the great quarries opened on the side of the road from Tal-y-llyn 

 to Machynlledd. 



(7.) Farther still on this ascending section the rocks gradually 

 change their structure. They become more mechanical, — more 

 regularly thin-bedded, — alternate with gritty bands, — contain few 

 large thick beds of slate fit for economical use, and are violently 

 contorted. The dips and strikes continually change, but the cleavage- 

 planes remain nearly constant. Shells do not (so far as I know) 

 appear among these rocks ; but in a few places where the cleavage- 

 planes for a short space become tangents to the curved beds, fucoids 

 are very abundant. This series exhibits a mineral structure of no 

 common interest, — rolls in countless undulations from Machynlledd 

 to Aber Dyfi, and in like manner (with the same undulations and 

 the same structure) is carried to the south side of the Dyfi, and so 

 mingles itself with the great physical system of South Wales. 



When I found, during the summer of 1843, that some of the high 

 ridges, extending from the neighbourhood of Mallwyd towards Plyn- 

 liramon, were Upper Silurian^ I supposed it possible that the whole 

 of them might be Upper Silurian ; and by a geographical error, I 

 called them the higher mountains of South Wales ; whereas the 

 greater number of them are within the limits of xMontgomery shire, 

 and therefore in North Wales. I thought it probable also (before 

 I began the labours of the last summer), that some of the beds on 

 the line of the Dyfi might prove to be Llandeilo flag. These notions 

 were purely hypothetical, and I now abandon them for reasons to 

 be given in the sequel. 



By way of general conclusion to all the previous details, I may 

 now state, that all the higher mountains of Carnarvonshire and 

 Merionethshire, &c. form one vast but connected physical system, 

 which is fossiliferous almost to its base ; though many parts of it 

 (which admit of no physical separation from the remaining portions) 

 are, apparently from the effect of local conditions, entirely without 

 fossils. 



Sections in the great South-western Promontory of Carnarvonshire. 



The rocks of this region are very important in the physical history 

 of North Wales, but my limits permit me to add only a few sen- 

 tences respecting them. They form the following groups : — 



(1.) The zone of crystalline hypozoic slates (above mentioned). 



(2.) Great bosses of syenitic rock, which have risen in numerous 



