Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/103



I have concluded, perhaps without sufficient evidence, that the pitchstone both of Ben-na-Caillich and of Glamich, has been detached from veins. This deduction is made from the small quantity of fragments which are to be found, and from the circumstance that all the pitchstones of Scotland hitherto observed actually occur in veins.

A large portion of the summit of Glamich has the power of affecting the magnetic needle at even a considerable distance, a property extremely common in the rocks of this family. I was desirous of ascertaining if any regularity existed in the position of the magnetic poles, similar to that which Humboldt has observed in a rock of serpentine which he has described: for which purpose I observed the affections of the needle over many parts of the space as far as that was accessible, and I have represented them in the accompanying diagram, Pl. 3, fig. 1.



A consideration of those positions in the diagram will explain immediately that which would require much circumlocution to describe in words. In five places, extending from the eastern to the south—western side of the ground included in the circle, it will be seen that the needle remains uninfluenced. At the northern limit of the hill its position is equally true, but from the strength of its polar tendency, I conceive it was there affected by the coincidence of its position with the meridian of some neighbouring magnet, although by some oversight I neglected to take any other bearings near the same spot which might have verified this supposition. On the north east rhumb in five several places taken at the distance of five yards from each other, the positions are such as could not result from the influence of any one magnet, however