Vol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOG. 243 



The best example of a spherulitic rock collected from this series 

 of rhyolites was taken from the road-section close to, and a little 

 north of, the junction of the rhyolites with the Pentre-newydd 

 ■diabase. 



(e) The Andesites on the Western Side 

 of the Dingle. 



The andesites on the western side of Coomb Dingle may be 

 divided into two groups, one consisting of augite-andesites, and 

 the other of hornblende-andesites. 



The group of augite-andesites, both as regards the flows and 

 the fragmental rocks, is petrographically identical with the upper 

 and lower andesitic series on the eastern side of the dingle, already 

 described (pp. 238 et seqq.). 



A rock exposed in the road, 320 yards west-south-west of Tre- 

 hyrn [E 4151], is seen to be a coarse tuff, made up largely of frag- 

 ments of vesicular hyalopilitic andesites and pumice : a rock identical 

 in character with those from the eastern side of the dingle. But an 

 interesting and important feature of this tuff is, that it contains, in 

 addition to the usual material, fragments of a devitrified rhyolite. 

 These rhyolitic fragments mostly present a patchy appearance, such 

 as is quite common in the rocks of Castell Cogan ; while some consist 

 solely of cryptocrystalline felspar and quartz, without any definite 

 arrangement. 



The hornblende-andesite is a pale-grey rock which weathers 

 almost white, and is thus hard to distinguish in the hand-specimen 

 from some of the rhyolites. It has a mean specific gravity of 2-66. 

 Sections cut from the least-weathered portions of the flows [E 4153, 

 4156], which form the crags above the stream, show that the rocks 

 are considerably decomposed. Under the microscope, they are seen 

 to consist of phenocrysts of felspar set in a fine-grained felspathic 

 matrix. 



The phenocrysts include well-formed plagioclase-felspars, belonging 

 to the oligoclase and oligoclase-andesine varieties, which are usually 

 twinned according to the Carlsbad and albite-laws. In contra- 

 distinction to the augite-andesites described on p. 239, interpene- 

 fcration-twins and stellate groups are uncommon. 



The presence of hornblende is inferred from strongly doubly- 

 refracting pseudomorphs, in an indeterminable micaceous mineral. 

 These pseudomorphs (*8 mm. in length) have the external form of 

 hornblende, and show a corrosion-border of pale-green chlorite and 

 dusky iron-ores. The groundmass consists of a felted mass of 

 felspar-microliths, which give almost straight extinction, and are 

 evidently of more acid composition than the phenocrysts. The 

 matrix is, however, considerably decomposed, and has given rise to 

 sericitic alteration-products and secondary quartz. 



The rock, as a whole, does not seem to have been vesicular ; but 



