Vol. 60.] IGNEOUS ROCKS OF PONTESFORD HILL. 463 



once solid spherulites (as contended by Prof. Cole and Mr. Harker) l 

 have been destroyed, and replaced by secondary mineral matter, 

 generally quartz, which now forms the so-called amygdaloid. 

 If the mass is coarsely vesicular or lithophysal, there will be a 

 strong tendency for the fibres to surround these cavities, extending 

 outward into the matrix and inward towards the centre of the 

 vesicle. Similarly, fibrous matter may develop radially outward 

 from phenocr} T sts, or, as in ordinary spherulites, from central 

 points or lines, where the conditions have been such as to induce 

 crystallization. 



It would seem, then, that many of the nodules are spherulitic 

 growths, where the spherulitic fibres develop in general, not from 

 a central point outward, as in the small, true spherulites, but 

 locally from vesicles or other cavities, crystals, etc., coalescing finally 

 to form in some cases larger and larger growths. Those nodules 

 which have roughl}--concentric or crescent-shaped cavities, now filled 

 with quartz, may be due in some cases to a progressive or ' spas- 

 modic ' crystallization of a ' hydrous patch ' during the solidification 

 of the rhyolite-magma (see p. 461). But in other cases, they may 

 have arisen as compound vesicles, due to the local distension of the 

 magma, and the subsequent development of the brown, fibrous and 

 spherulitic material. The spherulitic type of devitrification is not all 

 of the same age, for fibrous growths undoubtedly traverse small 

 and earlier-formed spherulites, which have been dissolved out and 

 replaced by quartz. 



In a specimen of the South-Eastern Khyolite, a spherulitic growth 

 has taken place around an undoubted vesicle, now filled with 

 quartz, for the flow-lines can be seen distinctly curving round it. 



(3) The Andesite-Group. 



(a) The more Acid Grits and Tuffs. — The actual junction 

 of the Northern Rhyolite and the succeeding tuffs is not seen, but 

 the felsitic-looking grits and tuffs follow on immediately, the line of 

 junction being marked by a hollow in the ground with springs. No 

 reliable dip in these basement-tuffs can be made out, but when a 

 good dip is seen higher up in the andesite-series, the beds are 

 dipping at about 80°. These acid-looking tuffs crop out along the 

 road and lower skirts of the hill (537, 566, 556, 555, 554, 553, 552, 

 551). They are pink and green, fine-grained, gritty tuffs, with a 

 distinctly-acid look, though containing very few quartz-grains. 



No. 566 is a fine-grained grit, the grains being pink, set in a 

 greenish matrix. Under the microscope the grains, measuring up 

 to 0*04 inch across, are seen to consist of lapilli of vesicular, 

 devitrified glass with well-marked fluxion-banding, together with 

 broken crystals of felspar with lamellar twinning. One fragment, 

 0*05 inch across, contains skeleton-crystals of orthoclase in a de- 

 composed greenish glassy matrix, a few subangular quartz-grains, 



1 G, A. J. Cole, Geol. Mag. 1877, p. 299; A. Harker, 'The Bala Volcanic 

 Series of Caernarvonshire ' [Sedgwick Prize Essay for 1888] 1889, pp. 28-40. 



