Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/538

454 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 weathered 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 laminated appearance. In some cases the parallel stripes are so distinctly marked that they closely resemble laminæ of deposition, or lines of foliation, and have in fact been regarded as evidence of the original stratification of the rocks here described. An examination of their internal structure shows, however, that they invariably indicate the presence of those remarkable streams of microliths previously described in the Hungarian perlites (p. 453, Pl. 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

One beautiful variety of this rock consists of numerous bright-red spherulites 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 spherical 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, Pl. XX.) usually consist of a circular central disk of bright red surrounded by a colourless 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).