358 MR. p. LAKE AND PROF. S. H. EETNOLDS ON [Sept. I9I2, 



fracture whicli becomes more conchoidal as the flintiness increases. 

 The colour of the weathered surface in the field is variable, some- 

 times dark, often very pale. On a freshly-broken surface the 

 colour is generally grey or greenish grey, becoming in some cases 

 nearly black, and in others, especially when somewhat weathered, 

 pale pink or white (272). 



The texture is sometimes extremely uniform ; sometimes the 

 development of patches and strings of a chloritic mineral imparts to 

 the rock a variegated appearance. Flow-structure is occasionally 

 well seen in a hand-specimen (179), but the rocks are never vesicular. 

 Little phenocrysts of quartz and felspar are nearly always apparent 

 in the freshly- broken surface. Small grains and crystals of pyrite 

 are often conspicuous in a hand-specimen. 



Microscopic characters. — The rhyolites are, in general, 

 remarkable for their uniformity of character and the small number 

 of minerals represented. The correspondence with those described 

 by Mr. Harker from Carnarvonshire is, in most cases, very close. 



The ground-mass is, as a rule, cryptocrystalline, frequently with 

 strings and patches of microcrystalline material, which are not 

 sharply marked off from the cryptocrystalline. Elow-structure is 

 frequently seen when the section is viewed in ordinary light, but 

 ceases to be apparent in polarized light. Tt is often brought out 

 by the wisps or bands of chloritic material which has collected in 

 the loops of the bends. 



No examples of perlitic structure have been met with. Occa- 

 sionally the ground-mass breaks up under polarized light into a 

 mosaic of irregularly-interlocking blurred patches (raicropoikilitic) 

 (189). This structure sometimes extends uniformly over the whole 

 section, sometimes occurs locally in bands. One section (176) 

 shows curious little micropegmatitic groups, exactly similar to 

 those described by Mr. E. E. Cowper Eeed ^ from Fishguard. They 

 clearly represent felspar crystals, and in one case the felspar of the 

 micropegmatite shows Carlsbad twinning. 



In almost every slide there are numerous crystals or grains of 

 felspar and quartz, the latter being often corroded by the ground- 

 mass. The felspars nearly always occur as small crystals, having 

 various lengths up to about 5 mm. Although the crystals occa- 

 sionally show only Carlsbad twinning, they are more often twinned 

 on the albite type, and, from their low extinction-angles, belong 

 to the albite-oligoclase series, as in the case of the Carnarvonshire 

 rhyolites.^ ISo ferromagnesian minerals have been met with. 

 Most of the slides show small irregular patches of iron-ore, which, 

 from the leucoxenic alteration, are no doubt ilmenite. A chloritic 

 decomposition-product, in the form of patches and strings of a pale- 

 green feebly polarizing substance, is very common. In some cases 

 (179) much finely-disseminated pyrite occurs. 



1 Q. J. G. S. vol. li (1895) p. 162. 



^ The * Bala Volcanic Series of Caernarvonshire ' 1889, p. 20. 



