Mr Barker, Magnetic Disturbances in the Isle of Skye. 277 



the most wide-spread of those which I have attempted to establish 

 within my limited area. 



In these latitudes a peak magnetised by induction should 

 attract the north pole of the needle, and this is a simple test to 

 apply to disturbances of what have been called the second and 

 third orders. Some peaks, such as Glamaig noticed above, con- 

 form to this rule ; but it clearly affords no general explanation of 

 the phenomena. At the same time certain considerations, and 

 especially a comparison of N. — S. with E. — W. ridges, suggest 

 that induction may be one element contributing to the general 

 result. Its influence might be sought in disturbances of low order 

 among the granite hills bordering the gabbro tract or on the low 

 ground built of Jurassic strata near the basaltic plateaux. 



Prof. Riicker 1 has endeavoured to show by calculation that 

 induction is capable of producing regional magnetic disturbances 

 comparable in magnitude with those detected by his survey ; but 

 the argument is open to serious criticism from the geological side. 

 The author calculates the effect due to a large rectangular slab of 

 rock of given magnetic susceptibility (this figure being obtained 

 by experiments on basalts from Mull) magnetised by the earth's 

 induction. The thickness of the slab is taken at 12 miles, it being 

 supposed that beyond that depth, where the temperature would be 

 about 700° C, magnetic matter does not exist. It is thus assumed 

 that basalt, or rock of similar magnetic susceptibility, extends 

 downward to a depth of twelve miles, an assumption not warranted 

 by the known geological structure of any part of the globe. In 

 Skye the basalt and gabbro are in the form of an irregular sheet, 

 the base of which is seen in many places, the underlying rocks 

 being sandstones, etc., for which no magnetic properties can be 

 postulated. The base of the sheet is in some places above sea- 

 level, in others a little below, and the average thickness of the 

 sheet must be roughly about the same as the average elevation of 

 the land, or probably not more than the fiftieth part of twelve 

 miles. 



It may be remarked in conclusion that, although little in- 

 formation has been published concerning magnetic polarity among 

 British rocks, there are numerous records and some detailed 

 accounts from the Harz, the Fichtelgebirge, the Eifel, and other 

 districts on the continent of Europe. The facts are very fairly 

 comparable with those described above. Any considerable mag- 

 netic properties are confined in general to basalts, diabases, 

 serpentines, and other rocks rich in iron- compounds and usually 

 in the mineral magnetite. The phenomena are such as would be 

 produced by a large number of small magnets distributed through 



1 Proc. Roy. Soc, vol. xlviii., pp. 505—535; 1890. 



20—2 



