Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/543

Rh fragments of pitchstone, in some of which small crystals of orthoclase and plagioclase are surrounded by streams of microliths, while others have a felsitic or crypto- crystalline texture and are crowded with minute crystals of felspar. A few fragments of spherulites have been detected; and there are also great numbers of broken felspar- crystals scattered through the mass; some of the thin layers are, in fact, almost wholly composed of abraded felspar crystals, with small fragments of microcrystalline trap disseminated among them.

At one time some of these ashes must have been slightly vesicular; for they now contain many cavities whose walls are coated with chlorite, and the interior filled with crystalline quartz. In a few cases calcite and quartz are associated together. There are also some interesting examples of microscopic cavities filled with calcite and epidote; bright yellow prisms of the latter project from the walls and cross each other in various directions, the intervening spaces being filled in with the calcite. Pale lemon-coloured epidote is by no means uncommon in the coarser ash-beds; it usually occurs in fan-shaped groups of flat prisms, which exhibit delicate bright tints in polarized light. In all the descriptions of the optical characters of this mineral which have come under my notice, it is said to be strongly dichroic; this, however, is certainly not the case with any of the numerous examples of pale yellow epidote which I have observed in these rocks, and in the altered syenites of Leicestershire and other localities.

The masses of rock here described represent a portion only of a series of similar products which have been erupted along an old line of volcanic action; porphyrinic and other varieties occur at Charlton Hill and Caer Caradoc near Church Stretton: while in the hilly district to the west there are still traces of old volcanic vents, accompanied by a most interesting variety of basic and acid lavas, which I hope to describe on a future occasion.

The results arrived at in the present memoir may be briefly summed up as follows:—

1. The highly characteristic internal structure of some of the rocks affords the clearest proof of their original vitreous condition; for the peculiar perlitic and spherulitic formations, with their associated microliths, have never been observed except in connexion with the obsidian or pitchstone varieties of volcanic glass.

2. It appears also that, in the older as in the younger series, there is the same gradation between the vitreous and stony varieties; and as the perlitic and other glassy rocks are well known to be subaerial volcanic products, the rocks here described afford strong evidence that during the earlier geological periods volcanic action was of the same kind and produced the same results as in more recent times. I will only add, in conclusion, that probably no one who exa- mines a good series of the Schemnitz rocks, or the beautiful rhyolites of the Euganean Hills, will fail to recognize among