Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/333

 three years ago, the same circumstance is described, and I may here repeat that the granite in question is not decomposed, and contains a very large proportion of flesh coloured felspar, the quartz and mica being in small quantities. As granite is a rare rock in Scotland, I have not been able to extend my observations on this substance far, but I have also found the same magnetic power in some of the granites which form the Ross of Mull. The porphyries also occasionally possess the same power. I have observed it in different places, but most remarkably in those of Cruachan, and expect that it will be found most conspicuous in those, which from their black colour contain much hornblende and approach to trap in their composition: it is not always that circumstances allow of the repetition of these observations. I have not often discovered it among the primary schists, but I must add at the same time that my trials on those rocks have been but few. Yet it occurs among the schists in the hills which border Glen Tilt in more situations than one, although seldom in an energetic degree. It is not limited to the hornblende slate, but is found in those specimens of argillaceous schist into which that mineral does not enter. I have also observed that some of the beds of dark blue limestone exert a disturbing force on the needle, and imagine that I found it greatest in a stratum which crosses the upper part of Glen Fernat at the foot of Cairn Ree. I have however examined limestones of the same apparent nature in other parts of Scotland without having discovered a similar property in them, so that I conclude it will be found but rarely either among the rocks of this family, or the schistose ones.

It has so long been known and so often remarked as a quality common to the rocks of the trap family, that instances of it must have occurred to every geologist. In these indeed it is often so