Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/102

From Wikisource
Jump to navigation Jump to search
This page has been proofread, but needs to be validated.

Among the rocks, for which I could not find a place in the geological description without disturbing its order, pitchstone requires to be noticed. Although not found in situ it offers as a mineral specimen some appearances which are interesting, and which I shall therefore describe. It was on the hill of Glamich that I found the specimens in question, and it is probable that they had been detached from some veins which I was unable to trace. There are two varieties, a black one very little differing from that of Rum, except that it contains a few dispersed crystals of glassy felspar; and an olive green one, which as it offers some apparently important peculiarities hitherto unobserved, I shall describe more fully. It is often of a granular combined with a small conchoidal fracture, and is generally disposed in distinct concretions which are either of the flat or curved lamellar form. It is remarkable for containing irregular rounded cavities similar to those of the amygdaloids, filled with compact grains of a grayish hue. The structure of these is so singular as to be deserving of notice. On breaking the smaller ones, they are discovered to consist of a grayish white enamel similar to that which is formed by the fusion of felspar. But if we break the larger grains we can distinctly see that the center is composed of glassy felspar, the crystalline transparency and platy fracture of which are perfect, while the surface to a certain depth is converted into the white enamel I have described. I have not observed this very peculiar and striking appearance in any other pitchstone which has come under my notice, although there are appearances not much unlike it in some of the varieties found in Arran.

Those who conceive pitchstone, like basalt, to be of igneous origin, will have little difficulty in explaining this phenomenon, and will even find in it strong evidence to support that theory. It is unnecessary to enter on a reasoning so obvious.