Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/540

456 as distinct and unquestionable as in any of the more recent glassy- rocks. Fig. 6 represents a portion of a thin slice of one of these ancient perlites; and beside it I have placed an equally careful drawing of a typical perlite from Schemnitz (sec fig. 5). The older rock has undergone a considerable amount of alteration ; and that constitutes the only perceptible difference between them. In some of the Meissen pitchstones the perlitic structure is nearly or quite absent, while in others it is well developed; and it fortunately happens that specimens occur in various stages of alteration.

In one of the red varieties the colour of the mass is due to the in- filtration of bright red ferric oxide, which has followed the lines of fissure, and has also stained the glass for a short distance on each side, as represented by the light shade in fig. 1. In the same specimen, however, a yellowish-brown substance here and there takes the place of the red oxide, and shows a marked tendency to aggregation in little spherical nodules. In a brown variety from the same locality, a pale brown flocculent substance is alone present, and has invaded the glass in the same manner but to a far greater extent than in the preceding examples; the parts permeated by it have a distinct action on polarized light; and it is quite evident that a further extension of the process of alteration would impart to the mass a pseudo-felsitic aspect.

The kind of alteration here described has clearly been produced by chemical action; and it has followed precisely the same course as that which has so frequently converted fractured crystals of olivine into serpentinous pseudomorphs.

In the ancient perlite a similar process has been in operation, and has produced the appearance represented in fig. 6. The shaded parts indicate the presence of a yellowish-green substance, which accompanies the lines of fissure and has invaded the glass on each side.

Devitrification of the glassy magma.—In addition to the chemical alteration just described, the original glassy base of these old rocks has also undergone certain molecular changes which it is highly important to notice. A slight examination of the two thin slices represented in figs. 5 & 6 suffices to show the identity of their general character as seen in ordinary light; but when placed under the polarizing microscope between crossed prisms, it is at once seen that the matrix of the Schemnitz rock remains dark, while that of the older one transmits light in many places, and the field of view exhibits an irregular mosaic of light and dark grains.

On rotating the object, some of the dark grains may then be seen to transmit light in certain positions; but the greater number always remain dark; and it becomes evident that the mass consists of a homogeneous glass with numerous doubly refracting patches disseminated through it. The extent to which the two substances prevail varies considerably in different parts of the rock: in some of the highly altered spherulitic varieties there may be seen a perfect mosaic of varying pale shades of colour, while in others the glassy substance predominates. The peculiar character of the doubly refracting portion of the base is extremely well shown when the axes of the Nicols are inclined to each other; by a slight rotation of