270 PROF. A. H. COX AXD MR. A. K. WELLS 0>~ THE [vol. lxxvi, 



in the near neighbourhood of diabase-intrusions. A very small degree of 

 contact- metamorphism evidently suffices to emphasize the slight differences 

 in composition between the various bands. With the incoming of coarse 

 material these fine-grained ashes pass over into the massive type already 

 described. On the other hand, they may pass by a rapid transition into 

 almost normal slates. 



In places the more roughly-weathering, rather felspathic ashes 

 include peculiar lumps and strings of fine-grained ' china-stone " 

 ash, which may be seen, for example, in the thin middle ash-hand 

 exposed in the fault-gully east-north-east of Bryn Brith. This 

 phenomenon is important, as it represents the first incoming of 

 the peculiar ' agglomeratic ' type of rocks that attains a much 

 greater development in the Bhyolitic Series east of the Gwynant. 

 and at a somewhat higher level in the Bryn Brith Beds. (See 

 postea, p. 275.) 



Another type consists of a blue rock resembling the ' China-stone ' Ashes in 

 possessing a splintery conchoidal fracture like that of flint, but differing from 

 them in that no lamination is visible in the hand-specimen. Also the non- 

 laminated rocks, unlike the 'China-stone' Ashes, show no tendency to pass over 

 into slates ; they usually occur in very thick beds, and, although themselves 

 massive, they are frequently separated one from the other by thin films of soft 

 well-cleaved ashy material, after the manner often seen among rhyolitic flows. 

 The weathered surface is, as usual, white. Rocks of this type are best 

 developed low down in the Volcanic Series. It is possible that some of these 

 rocks may represent actual rhyolite-flows rather than ashes, and the same 

 hypothesis may even apply to certain of the cream-coloured rocks. No single 

 •example of either type has, however, shown any of the special characteristics, 

 such as spherulitic or perlitic structures, or the contorted flow-lines, that 

 would enable one to identify a rhyolite-lava with certainty. Since there exist 

 clear transitions from the cream-coloured rocks into rocks that are undoubtedly 

 ashes, while there also appear to be other transitions from the cream-coloured 

 rocks to the flinty types, it may well be that the whole of the rhyolitic rocks 

 consist of nothing but ashes, and that no true flows are present. 



We find, however, that elsewhere, as, for example, on Mynydd-y-Gader, true 

 xhyolites belonging to the Lower Acid Series often present an extraordinary 

 resemblance to tuffs. In one case a rock, that at first sight appears to be 

 ■a cleaved tuff passing over into ashy slates, suddenly develops a nodular 

 structure, and is then obviously a typical nodular rhyolite. Another well- 

 cleaved example, which in a small exposure exactly simulates a cleaved ash, 

 is seen, when well exposed along a large dip-joint face, to possess a broad 

 and rather irregular flow-banding cut across obliquely by the cleavage. 



Seeing that such cases occur on Mynydd-y-Gader, it is quite possible 

 that in the westerly extension of the same series, some of the rocks which 

 appear to be ashes, may really be lavas. The difficulty of distinguishing 

 between rhyolite-flows and rhyolite-tuff s is, of course, no new one, having been 

 met with frequently in other districts. 



The question of the existence of actual rhyolitic flows is, however, 

 of minor importance in this case. The essential feature of the 

 Lower Acid Series is that the volcanic emissions were most 

 markedly acidic in character, and do not contain any bands of 

 trachytic or andesitic composition. 1 In this respect they offer a 



1 In the area east of the Gwynant certain agglomerates in the Lower Acid 

 Series do contain blocks of highly-vesicular lava of almost basic composition 

 (see p. 273) ; but it still remains true that no definite bands entirely composed 

 of trachytic or andesitic material are to be found. 



