454 S. ALLPORT ON PITCUSTONES AND PERLITES PROM 



the one to the west of the Wrekin, however, is the more important 

 for my purpose, as it affords the most interesting and typical 

 varieties, and also supplies the best specimens for examination. 



At " Lea Hock," near the south-western termination of the 

 ridge, there is a large quarry near the Shrewsbury road, in which 

 the rock is very well exposed. In one part it is intersected in all 

 directions by numerous joints and cross joints, which cause it to 

 break into small fragments ; so that fresh surfaces are difficult 

 to procure. The jointage-planes are generally smooth, and coated 

 with peroxide of iron, and frequently exhibit on their slightly wea- 

 thered faces numerous fine parallel lines, which are either straight 

 or tortuous, and even exhibit a complicated folding and crumpling, 

 like that seen in crystalline schists. Sometimes, however, they 

 widen out into distinct bands, and then produce a striped or lami- 

 nated appearance. In some cases the parallel stripes are so 

 distinctly marked that they closely resemble laminae of deposition, 

 or lines of foliation, and have in fact been regarded as evidence of 

 the original stratification of the rocks here described. An examina- 

 tion of their internal structure shows, however, that they invariably 

 indicate the presence of those remarkable streams of microliths pre- 

 viously described in the Hungarian perlites (p. 453, PI. XX. fig. 7)*. 

 These finely banded rocks also occur in the Wrekin. 



Among the most interesting examples collected in the quarry just 

 mentioned are several varieties of a remarkable spherulitic rock. 

 These exhibit the closest analogy with the comparatively rare though 

 well-known group of volcanic vitreous rocks already referred to, and 

 may. I think, be appropriately described as 



Ancient Spherulitic Pitchstones and Perlites. 



One beautiful variety of this rock consists of numerous bright- red 

 sphcrulites set in a grey or yellowish-green matrix. Sometimes 

 they occur singly, and are irregularly scattered throughout the 

 mass ; or, as frequently happens, several are crowded together so 

 as to interfere with the development of their regular apherical form ; 

 while in other specimens they are arranged in rows, like strings of 

 coral beads, and thus form parallel layers. This is a well-marked 

 feature even in hand specimens ; and when the spherulites are closely 

 pressed together, a thin slice exhibits a series of continuous red 

 bands, with undulating outlines formed by the mutual interference 

 of successive contiguous spheres (see fig. 8). This is an extremely 

 hard rock of a bright red colour, and closely resembles some varieties 

 of jasper. 



As seen in thin slices, the spherulites (fig. 9, PI. XX.) usually 

 consist of a circular central disk of bright red surrounded by a colour- 

 less ring (distinguished by two shades in the figure) ; the latter varies 

 greatly in width, and is perfectly continuous with the red portion, of 

 which it is merely the unstained border; and then there is an outer 

 zone of transparent glass (unshaded in the figure). 



* For an admirable description of laminated volcanic rocks, see Darwin's 

 ' Geological Observations on Volcanic Islands,' p. 74, second edition. 



