Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/434

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. Mac Culloch on the Geology of

may perceive that it is of a large lamellar form, or such that one of the lateral dimensions is always much greater than the other, the leading lines being nearly vertical. The cross fracture by which it is separated into small masses and becomes detached, is comparatively so rare as not to give any general feature. If it is possible to transfer a term from a hand specimen to a rock, I might say that it was formed of distinct lamellar concretions, and a perfect idea may be acquired of it by conceiving the lamellar pitchstone of Arran magnified to the requisite size. It contains some caverns, formed in a vertical direction, but whether by a falling out of some parts, or a subsidence and consequent separation of contiguous ones I could not determine. I have already said that its texture and aspect are very various, its varieties occurring as far as I could perceive without any certain order or regularity. Its basis appears to be a felspar, occasionally of the compact sort, but more generally crystallized in a confused manner. It differs much in colour, and offers the following remarkable varieties, brown red, umber brown, iron grey, purple brown, ash colour, yellow green, olive green, pale grey green, and grass green. The crystals of felspar which are imbedded in the mass are of a small size, rarely exceeding the tenth of an inch, and are either grass green, brown, or white. Nodules of green earth are also found imbedded in it, and the green varieties appear to derive their colour from this substance.

I have contented myself according to the established language with calling this rock by the simple name of porphyry, a name so vague, and so carelessly applied to rocks of widely different characters, as to be a cause of unspeakable confusion. Since the term porphyry is now applied to all rocks, whatever they may be, which in a given base contain imbedded crystals of felspar, or of any other substance, it is evident that it can determine nothing with regard to