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



Magnetic Disturbances in the Isle of Skye. By Mr Alfred 

 Harker. 



[With Plates XI, XII.] 



[Received 23 January 1900.] 



It has long been known that in the Isle of Skye and other 

 parts of the Inner Hebrides there are local magnetic disturbances, 

 sometimes of such magnitude that the compass becomes useless 

 as a guide. This fact may plausibly be connected with the 

 geological constitution of the islands in question, the prevalent 

 rocks being basalt, gabbro, and other igneous rocks rich in iron- 

 compounds, largely in the form of the magnetic oxide. But 

 although casual allusions are often met with, there seems to be 

 little or no information available as regards the amount and 

 distribution of these magnetic irregularities, and the probable 

 explanation of the phenomena. This must be my excuse for 

 offering some results of observations made during the last few 

 years in the central part of Skye. The observations were taken 

 incidentally in the course of geological work, and were of a very 

 rough kind, the only instrument used being the azimuth-compass 

 or often the ordinary pocket-compass. 



In their ' Magnetic Survey of the British Isles 1 ,' Professors 

 Riicker and Thorpe give the magnetic elements for about a score 

 of stations in the Western Isles and on the adjacent coast of 

 Scotland, with their differences from the calculated normal values. 

 The disturbances there indicated are, however, of a totally 

 different order from those with which I proceed to deal, rarely 

 amounting to one degree in declination and less in inclination, 

 with a variation up to about three per cent, in horizontal force. 

 The much larger, and at the same time much more local, disturb- 

 ances, which can be observed with a compass, were outside the 

 scope of that survey, though a special examination was made of 

 part of the Isle of Carina, which has long had a reputation for the 

 abnormal behaviour of the compass. Here the authors record 

 disturbances up to as much as 25°"3 in declination, the highest 

 figures being for points just E. and W. of the Compass Hill. The 

 influence diminished rapidly with distance, and was trifling at 

 200 yards. This is almost the only record from the Western Isles 

 which I can find to compare with the phenomena to be described 

 in Skye. 



These phenomena are exhibited in the Cuillin Hills, composed 

 mainly of gabbro, which occupy the central part of the island, and 

 on the moorland plateaux, consisting of basalt, which extend 

 thence northward and westward, covering about half the area of 



1 Phil. Trans, (a), vol. 181, pp. 53—328, plates 1-14; 1890. 



