Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/442

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Dr. Mac Culloch on the Geology of

of curved penicilliform groups of acicular crystals frequently an inch in length, assuming an aspect of great singularity. In this direction the schist is visible, and appears to form the largest part of the stone, while in the cross fracture the lamellæ of hornblende alone being seen, the whole rock seems to consist of this mineral. Occasionally the hornblende displays crystals disposed in so many different ways, that the schist is discernible even in the cross fracture, but this variety is the least common.


The veins of granite found at Portsoy are well known, not only for the peculiar character of their crystallization, called graphic, but also because they contain crystals of schorl (tourmaline) of great magnitude. The singular disposition, and mutual relations of the crystallized substances which form this compound rock, are known to have afforded Dr. Hutton an argument for its igneous origin, and its peculiar character has been supposed to arise from a simultaneous, or nearly simultaneous crystallization of the several substances contained in it. The specimens which I have to enumerate, are such as not only throw considerable doubt on this explanation, but are in fact, sufficient to prove a sequence of epochas even in this limited space, and to show that the compound rock in question has been formed by successive operations, the nature of which however I fear we shall not easily determine. I would entreat the pardon of the Society for occupying so much of its time on the fracture of a schorl crystal, were I not persuaded that much light must at some period inevitably be thrown on the greater geological phenomena, by considering the chemical and mechanical relations existing among the smaller portions which