Vol. 60.] IGNEOUS KOCKS OF POXTESFORD HILL. 471 



Under the microscope, the andesite-lava (516 c, 57 a) is found to be 

 made up of a felted mass of felspar-laths about 0*02 inch in length, 

 milky-white by reflected light, but still showing both simple and 

 lamellar twinning, generally extinguishing parallel to their length, 

 or nearly so, indicating a felspar of the oligoclase-series. A good 

 deal of very pale-green, nearly-colourless, highly-refractive and 

 much-cracked augite (malacolite) occurs in short prisms, usually with 

 octagonal sections, and exhibiting a well-marked prismatic cleavage. 

 It is frequently twinned, and occasionally encloses felspar-prisms. 

 These minerals are embedded in a dull, greenish-brown, glassy 

 matrix, largely converted into yellow-and-green palagonite, which 

 in its turn has been replaced in part by zeolites. Small magnetite- 

 or ilmenite-granules are plentiful. The ilmenite, which is evidently 

 secondary, occurs in minute rhombs and hexagonal plates, with the 

 ordinary white leucoxene-products, some of the skeleton-crystals 

 showing very good examples of the characteristic mesh of white 

 rods. The altered glass has the same general character as that of 

 the palagonite-tuffs ; it occurs in roundish patches, portions of 

 which are milk-white in incident light, and with weak chromatic 

 polarization, and sometimes exhibits a fibrous or spherulitic structure. 

 Minute green granules, milk-white in reflected light, are common 

 in these palagonite-areas, especially along their borders, representing 

 a further change in the alteration of the glass. It is possible that 

 some of the larger circular areas represent vesicles. The rock is an 

 augite-andesite with a hyalopilitic groundmass, in which much 

 of the residual glass is converted into palagonite, and a good deal 

 of secondary ilmenite occurs (PI. XLIII, fig. 5). 



In some cases (516 a, 528) phenocrysts of felspar measuring up 

 to 0*05 inch, as a rule simply twinned, and often arranged in radial 

 groups, are embedded in a mesh of much smaller crystals : while in 

 (528) many elongated vesicles are seen, filled with a pale-green, 

 spherulitic, brightly-polarizing substance, often with a bordering 

 zone of colourless zeolite, which, between crossed nicols, shows 

 a fibro-radiate or minutely-spherulitic structure. 



Xo. 525, just below the dolerite, is much darker in colour than 

 the typical andesite, very fine-grained, with pale-green flecks, and 

 in the hand-specimen it is almost impossible to distinguish it from 

 the fine-grained compact dolerite. Under the microscope, the 

 matrix is nearly black, and, with a high power, appears dusted all 

 over with very minute grains of secondary magnetite and ilmenite, 

 which appear not only in the matrix, but covering largely the 

 phenocrysts. Much pale augite is present, together with squarish 

 felspar-phenocrysts of low extinction-angles, as well as felspar- 

 microlites in the glassy matrix, and many vesicles filled with a 

 pale-green, faintly-polarizing substance (? delessite), often showing 

 zonary banding, and a fibrous or spherulitic structure. In spite of 

 the close microscopic resemblance of this rock to some of the finer 

 dolerites or basalts, there can be no doubt that it belongs to the 

 Andesite-Group. 



At the top of the gullev. a little way down the north-eastern 



2k 2 



