Mr Barker, Magnetic Disturbances in the Isle of Skye. 275 



Again, there are cases in which the deflections are small or 

 vanishing near the summit itself (the small area of magnetised 

 rock being excluded), and reach a maximum value at some little 

 distance from it. This may perhaps be a complex result due to 

 the joint effect of distinct centres of local disturbance, as illustrated 

 by the , summit portion of the ridge of Sgurr nan Banachdich 

 (fig. 3). Here the summit-cairn (B) is highly magnetised, and so 

 also are the points of rock A, 14 yards north, and C, 21 yards 

 south. Going north from the summit, the negative deflection 

 increases from zero to — 17° (at four and five yards), then decreases 

 towards A, and there changes sign. Going south from the summit, 

 the positive deflection increases from 1° to 34° (at fourteen yards), 

 then diminishes, and at C changes both magnitude and sign 

 abruptly. I have not observed such phenomena in connection 

 with summits which exert a sensible effect to long distances. 



The next enquiry in natural sequence seems to be whether a 

 mountain, as distinguished from a mountain summit, or (to go 

 further) a group of mountains, composed of such rocks as gabbro 

 and basalt, exerts an appreciable effect of the nature of magnetic 

 disturbance over the adjacent country. Rough compass observa- 

 tions cannot be expected to contribute much to the solution of 

 this question, for the disturbances contemplated will be of what 

 may be regarded as the third order of magnitude, usually not 

 exceeding a degree or two in declination and affecting a corre- 

 spondingly extensive area. A large number of observations were 

 taken on the belt of low moorland country bordering the Cuillins 

 to the north and west, but, as might be anticipated, the results are 

 found to be quite inconclusive. Not only are the errors of obser- 

 vation : too nearly comparable in magnitude with the actual 

 deflections, but on this ground, composed of basaltic rocks, it is 

 impossible to eliminate satisfactorily the disturbing effect due to 

 slight permanent magnetisation of small knolls and ridges. Prob- 

 ably better results might be obtained in places where the gabbro 

 mountains are bordered by a broad track of granite, a rock not 

 itself susceptible to magnetisation. 



In this connection we may notice the case of Garbh-bheinn, 

 which may perhaps be regarded as supplying a connecting link 

 between magnetic disturbances of the second and of the third 

 order. Garbh-bheinn, the northern termination of the Blaven 

 range, is a mountain of 2650 feet, consisting in the main of gabbro, 

 but this rock is obliquely underlain by granite, with a small inter- 

 vening wedge of volcanic agglomerate and basalt. The granite is 

 continuous with a large tract of the same rock extending northward 



1 The method adopted in every case was to take the magnetic bearings of one or 

 more distant objects, and to compare with the true bearings as plotted with the 

 protractor on the Ordnance Survey map. 



VOL. X. PT. V. 20 



