Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/542

458 I., II. Ancient Pitchstone; analyzed by J. A. Phillips.

III. Perlite, from Hlinyik, near Schemnitz; analyzed by Von Sommaruga, quoted by J. Roth.

IV. Pearlstone (Hungary); analyzed by Rammelsberg.

An inspection of a list of analyses shows that there is quite as close an agreement between these examples of ancient and Tertiary perlites as can be found in a series of the latter only; and as this similarity of chemical composition exists notwithstanding the alteration to which the older rocks have been subjected, it may be inferred that in this instance, as in others, the changes have been almost entirely molecular, little or nothing having been taken from or added to the mass.

Intimately associated with the characteristic devitrified pitchstone there occurs a hard compact variety of a dark red colour and semi-vitreous aspect, with a subconchoidal fracture and sharp cutting-edges. Some specimens show a banded structure, even to the naked eye; while others, apparently amorphous, exhibit under the microscope most interesting examples of fluidal structure, but with no indications of perlitic or spherulitic formations. These also appear to consist of devitrified glass, and are found to pass into masses of ordinary "hornstone" or "compact felspar," which were probably never in a vitreous condition.

A close examination of very thin slices shows that the red colour of these rocks is entirely due to the diffusion of hydrous ferric oxide through the mass; a magnifying power of 800 enables one to perceive that the matrix consists of a colourless glassy substance crowded in parts with minute yellowish-red specks, which have the appearance of a fine dust even under the highest powers. In many cases the colouring-matter clearly has its origin in minute ochreous patches presenting crystalline forms with ragged outlines, from which the red stain has spread in all directions.

An examination of the stratified fragmental rocks of Lawrence Hill and the Wrekin leaves no room for doubt as to their real character.

The various beds consist for the most part of a breccia composed of small angular and slightly rounded fragments of compact red felsite and altered pitchstone, quite similar to those forming the masses with which they are associated; these, together with other materials, have been cemented together, and now form an extremely hard rock, which frequently exhibits manifest signs of subsequent alteration. Fragments of larger size, however, are not uncommon; and in one of the coarser ash-beds a block of beautiful spherulitic pitchstone, 8 inches in diameter, was found imbedded in the mass. A vertical slice of one of the finer ash-beds exhibits under the microscope a series of thin layers composed of angular