Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/440

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Dr. Mac Culloch on the Geology of

arguments drawn from theoretical principles. We have, however a perfect certainty (as far as we have any certainty in geological induction) that it is not stratified, because veins are found arising from it, and entering the mass of incumbent schistus in the well known junction at Loch Ranza.

I have not thought it necessary to take notice of the probable cause of the prismatic form which occurs so frequently on the summits of the high ridges in this granite, having before spoken of them in my remarks on that of Cornwall and Devonshire.

I cannot quit this subject without mentioning that many of the masses of granite on the summit of Goatlield, are magnetical, affecting the poles of the needle in situ, and influencing it also even in detached pieces. This occurrence may probably be more common in granite than we are aware of, but as yet, I believe it has been observed only in the Hartz mountains.[1]


The sappare (disthène) said to have been originally discovered in this place, is now known to be inherent in many different rocks. Thus, it has been found in granite, in mica slate, and in talc. The specimens of it seen here, occur in a vein of quartz, which traverses a talcy clay slate, a slate accompanied by a singular rock, which I will also describe. It only offers the common aspect of this mineral, which hitherto has exhibited no remarkable varieties, and is of a very decided character. The crystals of disthène occur in the quartz vein, as well as in the clay slate which bounds it, and pass indifferently through both, without

  1. I have since observed it in the mountain Cruachan.