Vol. 62.] IGNEOUS AND SEDIMENTARY ROCJKS OF LIANG YNOG. 241 



There is little doubt that, in these rocks, there is a considerable 

 proportion of clastic material, and it is uncertain whether they should 

 be classed as true ashes rather than as ashy sediments. 



The pumiceous tuffs are not of any great thickness, when 

 compared with the banded rocks described above, but they occur, 

 and present exactly similar characters, in both the lower and the 

 upper andesitic series. They consist of lapilli of pumiceous and 

 hyalopilitic andesite, of which the vesicles are filled with a pale- 

 green chlorite (PL XXYI, fig. 1). The fragments are set in a 

 fine-grained matrix, made up almost entirely of broken felspar-laths 

 and microliths, with a certain amount of chlorite and a few clastic 

 quartz-grains. The felspars in the lapilli are usually all oligoclase- 

 microliths ; but occasionally some of the larger fragments may 

 contain small lath-shaped crystals of a slightly more basic variety. 



(b) The Ehyolites on the Eastern Side 

 of the Dingle. 



The rhyolites on the eastern side of the dingle are pale, highly- 

 silicified rocks. They present both perlitic and spherulitic struc- 

 tures. A specimen collected from a small quarry above the 

 main road [E 4143] proved to be a beautifully-spherulitic rhyolite 

 (PI. XXIV, fig. 1). The few phenocrysts that occur are felspars of 

 the albite-anorthite series ; they are twinned according to the 

 Carlsbad and albite-laws, and give low symmetrical extinctions 

 indicative of a fairly-acid oligoclase. Subordinate orthoclase also 

 occurs. These phenocrysts range up to 1*3 mm. in length, and are 

 set in a groundmass which was once glassy but is now completely 

 de vitrified. The matrix consists of cryptocrystalline quartz and 

 felspar, but in parts of the slide traces of the original flow-lines and 

 a perlitic structure may still be made out. 



The spherules, which are abundant, and are often observed to 

 coalesce, average 0*4 mm. in diameter ; they are well-bounded, and 

 in ordinary light appear more transparent than the rest of the rock. 

 They are composed of radiating and branching felspar-fibres, of 

 which the zone of elongation has a negative sign, and giving 

 practically-straight extinction. Between crossed nicols the spheru- 

 lites show a well-defined cross at the centre, which becomes blurred 

 and indistinct outwards towards the margin, owing to the branching 

 and change of direction of the felspar-fibres. 



It appears that these spherulites are a direct result of the 

 devitrification of the rock, as they occur as the nuclei of the perlitic 

 structure. A very few minute patches of fibrous brightly-polarizing 

 chlorite (p. 239) occur in the groundmass, and may perhaps 

 represent vesicles ; while others, of a chlorite with lower bi- 

 refringence, may possibty represent some ferromagnesian mineral. 



(c) The Ehyolitic Breccia. 



This rock seems to be a true fluxion-breccia, and consists of 

 fragments of perlitic and spherulitic rhyolites set in a matrix of 



