34 DE. STAMP AND MB. WOOLDKIDGE ON THE [vol. lxxix, 



Devitrification has everywhere affected the fragments, but the 

 original character is often indicated by well-developed perlitic 

 cracking. In other fragments, commonly of a slightly greenish 

 hue, minute felspar-crystals may be distinguished ; but their nature 

 is much obscured by the greenish decomposition-products of the 

 glass. A flow-structure is generally apparent, and the rock from 

 which the fragments were derived was probably a fine-grained 

 spilite. A point of some interest is the transition in one fragment 

 from a fine-grained spilite (as described above) to a true perlitic 

 rock. We appear here to have evidence of a spilite-glass, which in 

 places carried felspar-microlites and in others developed perlitic 

 cracking (PI. I, fig. 1). The fragments commonly show a smooth 

 or rounded outline. They are accompanied by a few greatly- 

 altered felspar-crystals. 



The appearance of these rocks in the field justifies the use of 

 the appellation ' rhyolitic ' or 'felsitic,' while the perlitic texture is 

 further suggestive of such a nature. We have shown that this 

 same material passes into a rock of spilitic appearance; but it must 

 be admitted that the composition of the felspar is indeterminable. 

 Further, it is unlikely that analysis would afford any conclusive 

 evidence of the affinities of the rock, since the original composition 

 has been radically changed. 



Whether the rock is a flow-breccia or not, it is difficult to 

 determine. The contrast between rock-fragment and matrix 

 seems too pronounced to reconcile with autobrecciation ; yet an 

 absence of that mixture of rock-types which would probably result 

 from explosion is significant. The original glassy nature of the 

 fragments points to rapid cooling or to high pressure, while the 

 absence of marked vesicularity is a point of some interest as 

 possibly indicating high pressure. Vesicular glass is certainly a 

 characteristic explosion-product, but it appears more likely that 

 a non-vesicular glass would be formed during submarine eruptions, 

 where the pressure of the superincumbent water-column helped 

 to imprison the fluxes. Now, Mr. J. F. N. Green l considers auto- 

 brecciation to be a submarine feature ; hence we may perhaps 

 take the characters of the glass as indicative of submarine flow- 

 brecciation. 



It may be pointed out, however, that no definite conclusion as 

 to the origin of the rock _ is possible in the absence of wider 

 exposures. The horizontal extent of the breccias would clearly 

 have a most important bearing on the question ; but this cannot be 

 determined, since the rocks are exposed only in the core of the 

 anticline. 



(3) The Spilite-Breccias and Spilites. 



The spilite-breccias occurring below the flows seem to be of 

 somewhat local distribution, and, so far as the rather poor exposures 

 show, are better developed on the south of the Nant-Cerdin Valley 

 than on the north. 



1 ' The Vulcanicity of the Lake District ' Proc. Geol. Assoo. vol. xxx 

 (1919) p. 157. 



