286 Mr W. Galbraitb on the Magnetic Properties of 



Doubtless similar rocks will exert the same influence; and as 

 there is some analogy between this rock and Arthur's Seat, it 

 should have occurred to geologists that the same consequences 

 might have been expected, as have lately fallen under my own 

 obs<.'rvation, though previously, so far as is known to me, few 

 accurate observations of the kind have been recorded. 



On the evening of the 10th of August 1831, I took a walk 



but broader. Both rocks were stratified, with a dip of twelve degrees, and 

 parallel to the basaltic range on which they reposed. On presenting a mag- 

 netic needle to them, it was subject to sudden and violent changes. The cir- 

 cumference of one of them attracted the north pole through half its extent, 

 but repelled it for the remainder. The manner in which the needle was af- 

 fected by the other rock, may be understood by drawing a line lengthwise 

 through the centre of the upper plane of the rock, and another crosswise 

 through the same plane, so that the point of contact shall occupy the centre 

 of the plane ; the north pole of the needle was attracted at the extremities 

 of the longer line, while the opposite pole was attracted at the extremities of 

 the shorter one. 



M. Reuss of Bilin, obsei-ved the same property in a mountain of dark 

 greyish-black basalt, free from magnetic ironstone, in the JNIittelgebirge. 

 The mountain, 1800 feet high, is covered with wood to its summit, and pre- 

 cipitous on all sides. Its polarity is so great, that the needle at its eastern 

 foot was moved 40°, and at the summit itself 90° W. At the western foot of 

 the rock, the contrary was the fact ; but the polarity is shown not only in the 

 whole mass of the rock, but likewise in the larger detached pieces, and even 

 in the smallest fragments; the north point of the needle being at one end 

 distinctly attracted, and at the opposite end as distinctly repelled. 



Many years ago, I noticed this property in the trap-rocks of the Island of 

 Canna, and in other trap districts in Scotland. A late writer remarks, that this 

 magnetic influence is occasionally limited to a space of three or four feet ; 

 but it is sometimes also extended to distances much more considerable, so as 

 to produce a decided effect on the variation of the needle. There is no doubt 

 that it has, where unobserved, been a frequent cause of error in maritime 

 surveying, as well as in surveys on shore, where the compass is used for the 

 observations, or when the position of the theodolite is regulated by the needle. 

 Independently of the local disturbances produced in the Western Islands by 

 the vicinity of masses of trap, there is a general irregularity of the magnetic 

 variation prevalent throughout the western coast, produced, doubtless, by 

 the combined influence of the larger tracts, whether of trap or of other rocks. 

 It is sufficiently sensible at sea to diminish materially the use of the compass 

 in navigating these islands ; fortunately that instrument is not often wanted, 

 as it rarely happens that some land cannot be seen. At a period when the 

 general variation was stated at 26° west, it was found to be 19° in Loch Ryan, 

 36° on the east shore of Skye, and 21° near the Craig of Ailsa. The trials on 

 land were made with a needle elevated as high as possible above the surface. 

 —Editoiu 



