242 MESSES. T. C. CANTRILL AND H. H. THOMAS ON THE [May 1906, 



similar material, which occasionally shows a well-defined flow- 

 structure. 



A section cut from one of the larger fragments showed it to be a 

 devitrified patchy rhyolite [E 4144], with a pseudospherulitic 

 structure. The phenocrysts consist of oligoclase and oligoclase- 

 andesine felspars, both considerably decomposed, set in a felsitic 

 groundmass which has suffered more or less complete recrystal- 

 lization both of the felspar and the silica. The secondary felspar is 

 usually clear and granular, but the slide also shows one fairly- 

 large spherule. 



Another section [E 4146] shows a devitrified perlitic rhyolite, 

 with included fragments of a hyalopilitic andesite caught up in it 

 and seemingly in part absorbed. The rhyolite has a well-marked 

 flow-structure in places, and locally becomes perlitic, or less 

 frequently spherulitic. The fragments of andesite consist of masses 

 of felspar-microliths with some interstitial material, while the 

 rhyolite often exhibits good flow-structure around these xenoliths. 



Two sections [E 4438 & 4443] cut from the rhyolitic breccia at 

 the point where it crosses the road, show patches of rhyolite with 

 perlitic structure. This structure occurs chiefly in those patches 

 which have either escaped devitrification, or in which the recon- 

 struction has proceeded to no great extent. The still glassy part of 

 these rhyolite -fragments is usually stained a pale-yellow colour, and 

 may be distinguished by this character in the hand-specimen. A 

 good example of a perlitic fragment from this breccia [E 4443, 

 specific gravity 2*69] is figured in PI. XXIV, fig. 2. 



(d) The Rhyolites of Castell Cogan. 



These rhyolites are similar in many respects to those on the 

 eastern side of the dingle, both in the hand-specimen and under the 

 microscope. They have a mean specific gravity of 2*54. 



Slices cut from several localities [E 4140, 4141, 4159] showed 

 almost entirely-devitrified rocks, in which little or no trace of 

 original structures could be identified. The phenocrysts, which are 

 by no means common, consist of fairly-large felspars, measuring as 

 much as 3 mm. in length, all but completely converted into sericitic 

 decomposition-products. They show traces of Carlsbad twinning, 

 and the crystals apparently have the sanidine-habit. 



Well-formed quartz-crystals (*3 mm. in length) occur : these are 

 in all probability of secondary origin, although they are surrounded 

 by a zone of cryptocrystalline material, which seems to pass 

 gradually into their peripheries. 



The groundmass is completely devitrified in all cases, and usually 

 has a cryptocrystalline character ; occasionally, however, it becomes 

 somewhat coarse, and presents a patchy appearance, due to the 

 secondary crystallization of both the felspar and the quartz. Generally 

 speaking, no other structures are present; but occasionally it is 

 possible to make out the original flow-lines, and in places the rocks 

 become spherulitic. 



