270 Mr Harker, Magnetic Disturbances in the Isle of Skye. 



These phenomena are not confined to high mountain-summits, 

 but are exhibited equally by insignificant knolls on the moorland 

 plateaux, provided they form the highest ground in their imme- 

 diate neighbourhood. The localisation of these intense magnetic 

 disturbances in such situations seems to point very decidedly to 

 atmospheric electricity as the cause of the magnetisation of these 

 salient points of rock. It is not necessary to suppose that the 

 exposed points have been struck by lightning. Indeed, the re- 

 markable rarity of thunderstorms in Skye rather suggests that 

 the summits act to some extent as lightning-conductors. It is 

 easy to show too that no great lapse of time is required for rocks 

 so exposed to become magnetised. The stones composing the 

 cairns erected by the Triangulation Survey are invariably highly 

 magnetic, while the loose stones lying about the base of the cairn 

 are much less so. This is true even of small cairns erected by 

 climbers within a few years past. (See fig. 1.) 



The very violent, and at the same time very local, disturbances 

 which have been briefly described may be styled for the sake of 

 clearness disturbances of the first order. As stated, they are 

 found generally in connection with salient points of rock all over 

 the gabbro mountains and, so far as my examination has extended, 

 over the basaltic plateaux of Skye. It may be taken as established 

 that they are directly due to permanent magnetisation of the 

 rocks in these exposed places. The enquiry naturally suggests 

 itself, whether a like explanation is applicable to any disturbances 

 of a lower order. My observations on this point are rather sug- 

 gestive than conclusive, but some of the results may be worth 

 putting on record. They apply to what may be called disturb- 

 ances of the second order, affecting a larger area than the pre- 

 ceding, but having a smaller magnitude. Observations of de- 

 clination (the only kind taken) still show large deviations from 

 the normal magnetic meridian, but not such as involve com- 

 plete rotation of the needle, except where centres of disturbance 

 of the first order occur within the area affected. Two examples 

 will suffice to illustrate the nature of the phenomena. 



I take first the summit of Glamaig, a hill of more than 

 2500 feet altitude in the neighbourhood of Sligachan. The summit 

 consists of basalt, metamorphosed by the proximity of the granite 

 on which it rests and in which it has been involved. The hill 

 has a round top, which falls away in smooth slopes on the south 

 and west sides, but the north-east face is at first steep and rocky. 

 It forms a cliff, the edge of which runs in a curved line N.W. and 

 N.N.W., passing very near the summit. Centres of intense local 

 disturbance due to permanent magnetisation (disturbances of the 

 first order) occur at several places where crags project prominently 

 on the face of the steep slope, and the summit-cairn, standing 



