326 Dr. Mac Culloch on the Geology of Glen Tilt. 



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 paperf 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, 



* This paper docs not appear, having been retained for the purpose of connecting it 

 with the liistory of the neighbouring and similar islands. 



t Piiated in this volume. 



