Yol. 60. IGNEOUS ROCKS OF POXTESFORD HILL. 459 



the whole mass, though often very imperfectly. In the brown 

 fibres are many circular clearer spaces, which were originally 

 spherulites (in some cases possibly vesicles) with the brown fibres 

 crossing them, but now filled with a mosaic of irregularly-outlined 

 quartz-crystals. The brown fibrous growth is by no means confined 

 to the nodules : it occurs sporadically in small and often quite 

 irregular patches anywhere in the perlitic. and what I have termed 

 the microlithophysal, matrix. But usually it seems to have started 

 to develop along a definite line, such as a crack, 1 or the edge of a 

 vesicle or crystal, and then gradually spread, fungus-like, through 

 the surrounding material. 



The following descriptions are from slides kindly lent to me by 

 Mr. Parkinson: — 



(a) Pyromeridal nodule from the north-west of the hill, about 

 1*3 inch across, with a roughly-oval cavity filled with quartz and 

 pale-brown angular chips, and with a double fractured border of 

 yellowish and reddish-brown fibrous material (PL XLI, fig. 1). 

 Under the microscope, the matrix, in which the nodule is em- 

 bedded, is greenish and yellow-brown, and much stained with iron- 

 oxide, and it shows in polarized light a microcrystalline aggregate 

 largely made up of secondary quartz-grains. The wall or border 

 of the nodule is much fractured and veined with secondary silica, 

 while angular, broken portions of the wall appear towards the 

 centre of the amygdaloid. This border is made up of the usual 

 brown microfelsitic fibrous matter, often in radiating tufts, and 

 spreading out into mushroom-like growths, where it has had a free 

 space in which to develop. Groups of felspar-phenocrysts or 

 isolated individuals, showing simple or albite-twinning, occur in 

 the fibrous border ; and the fibres are usually deflected round the 

 crystals, and not infrequently radiate outward from their walls. 

 The cavity is now filled with a mosaic of clear quartz-grains, 

 enclosing small brown spherules, generally with a well-marked 

 radial structure, and showing the black cross in polarized light. 

 Usually, these spherulitic bodies are surrounded by a border of 

 perfectly-clear quartz, the smaller ones by perfect little hexagons 

 of quartz. Often chalcedonic silica is arranged in agate-like bands. 



(6) Another nodule from the same locality, about 1*5 inch long, 

 contains an irregular quartz-amygdaloid, and shows a much- 

 fractured border, looking, indeed, as if the fracturing occurred when 

 the nodule was hollow (PI. XL, fig. 2). Under the microscope, 

 many felspar-crystals, some 0*1 inch long, are seen in the fibrous 

 border, and the material of the latter is often arranged in radial 

 bunches, like that of the spherulitic bodies of the Lea Rock. The 

 central cavity is now filled with a brightly-polarizing mosaic of quartz, 

 in which are crowds of small brown spherules, with a pronounced 

 radial structure and showing the usual black cross with crossed 

 nicols. Lining the inside of the surrounding fibrous border is a 

 thin band of clear silica, and then a layer of the small, brown, 



There is, of course, the possibility that, in some cases, such cracks are the 

 result of contraction due to the crystallization of the fibrous material. 



