Mr Harher, Magnetic Disturbances in the Isle of Skye. 269 



Skye. I have found no remarkable magnetic disturbances in the 

 Red Hills, composed of granite (a rock very poor in iron-oxides), 

 nor do they occur, so far as my information goes, in the south- 

 eastern part of the island, which is built mainly of stratified 

 sedimentary rocks. 



The first fact to note is that on any salient point, either of 

 the gabbro mountains or the basalt plateaux, the rocks are per- 

 manently magnetised, and very often to such an extent that, by 

 moving the compass about, the needle may be made to point in 

 any direction. The disturbance affects dip as well as declination, 

 so that, with the dial held horizontally, the needle will often jam 

 against the card. When a rock-face forming part of such an 

 eminence is tested more closely with the compass, it is found 

 that certain points of the rock behave as north poles and others 

 as south poles, such poles being irregularly distributed at intervals 

 varying from a few inches to a few feet. A vertical and a hori- 

 zontal rock-face give similar results in this respect. Attempts to 

 measure roughly the strength of a pole (assumed to be on the 

 surface of the rock) gave various and not very satisfactory results. 

 One method was to select a fairly isolated north pole on a rock 

 facing south and present the compass to it, adjusting until the 

 needle lay in the normal magnetic meridian, but in reverse 

 position. The compass was then withdrawn in the same direction 

 until the needle became unsteady and swung round. In one 

 case this did not occur until the needle was 2 feet from the 

 rock. 



Fig. 1. Rough sketch of cairn on Sgurr nan Gobhar, looking down on it from 

 above. The small arrows show the positions assnmed by the compass needle. 

 When one end of the needle dips very strongly, this is indicated by a small circle. 

 The highest point of the cairn is on the edge of the stone on the extreme left of the 

 figure, and this behaves as a north pole. Another strong north pole is indicated 

 towards the right, of the figure. 



