234 



MESSES. T. C. CANTEILL AND H. H. THOMAS ON THE [May I906, 





I 1 



I 



be 



(Section continued.) Thickness in feet. 



Tuffs 20 



Compact andesite and tuffs 20 



Fine-grained vesicular andesite 6 



Tuffs 35 



Thin vesicular andesite 3 



Tuffs 10 



Thin vesicular andesite and tuffs 173 



Bhyolitic breccia. 



In the above series six flows have 

 been identified ; bnt, owing to some of 

 the ground being obscure, it is impos- 

 sible to say whether any more are 

 present in the section. 



(b) The Western Side of the 

 Dingle. 



(i) The Ehyolites of Castell 

 Cog an.— The rhyolites of this area are 

 faulted off from those on the eastern 

 side of Coomb Dingle, and form the 

 , high ground to the south of Tre-hyrn. 

 The country rises rapidly from the 

 stream-level (100 feet above Ordnance- 

 datum) to the rampart of the ancient 

 camp of Cogan, at a height of 426 feet. 

 It then gradually falls again towards 

 Pentre-newydd, to an altitude of 200 

 feet. The rhyolites are pale-yellow and 

 grey rocks weathering white, which 

 locally show good spherulitic and 

 fl uxion- structures. 



Although the area occupied by these 

 rocks is considerable and debris of them 

 is plentiful, outcrops of solid rock are 

 not numerous ; exposures may be seen 

 to the best advantage, midway between 

 the camp and the dingle, at a point 

 situated at the top of the wood ; and also 

 about 200 yards west of the road near 

 the junction with the andesites. At 

 the top of the wood, the rocks dip south- 

 eastward at 60° to 80°, while at the 

 last-named exposure they are vertical 

 and strike west 30° south. 



The exposures and dips of these beds, 

 provided that there has been no strike- 

 faulting, would indicate a thickness of rock of in all probability not 

 less than 1150 feet. 



(ii) The Andesitic Series. — North of the rhyolites of Castell 

 Cogan, and probably faulted from them, is a series of some 



