Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/334

 conspicuous as to have become an object of popular curiosity, notwithstanding which, it has rarely attracted the attention of geologists, unless, as in cases like those here alluded to, it has been exerted in an uncommon degree. I may therefore add that it is very generally diffused among the rocks of this family, although as we might reasonably expect, in very different degrees, those variations of energy depending at times on the peculiar composition of the rock, at times on its state of decomposition, and at others on circumstances which we have not the means of ascertaining, little acquainted as we are with the laws which regulate the movements, the accumulation, or the permanence, of this mysterious power. In a paper on Canna, which I presented to the Society some years ago, I pointed out the great action exerted by the trap in several parts of this island, as well as in that spot so well known to the inhabitants and to mariners by the name of Compass-hill, and took occasion at the same time to recommend to surveyors who are in the practice of using the magnetic needle in their art, the necessity of attending to these disturbances hitherto held of no moment, or rather, I might say, unknown and unattended to by them. I also pointed out with similar views the same effects occurring in several parts of the island of Sky, where they are frequent and powerful. Among the places in that island where the needle undergoes violent disturbances the hill of Glamich is the most remarkable, but as in that paper I have entered into the details which respect this spot, I need not repeat them here. I must however add that this hill consists partly of clinkstone, together with the porphyries which are usually associated with it,