Mr Harher, Magnetic Disturbances in the Isle of Skye. 273 



the prominent features among the moorland hills, it is formed, 

 not by the basaltic lavas, but by a more durable rock, also of basic 

 composition, intruded among them in the form of a sheet or sill. 

 The long axis of the ridge bears about N. 18^-° W., and is there- 

 fore nearly in the normal magnetic meridian for the place and 

 epoch (N. 21° W.). Violent local disturbances connected with 

 intense magnetisation of the rocks occur at several spots, notably 

 at the two summits, G and E, and at other salient points such as 

 F and H; and these disturbances are of the character already 

 described as distinguishing the ' first order.' Fig. 2 is a ground- 

 plan of part of the rock-surface at C, the point which exhibits 

 the strongest disturbance of this type. It may be remarked in 

 passing that this summit, though not quite so high as E, is a 

 more marked prominence : hence perhaps its more intense mag- 

 netisation. 



Observations of declination were taken at intervals of a few 

 yards, or in critical places one yard, along the crest of the ridge 

 and also along a line at right angles to the long axis, passing 

 through the summit E: the results of these are indicated in 

 diagram (PI. XII.). It is clear from the figures there given that 

 the field of disturbing force is of a very complex nature, but on 

 examination some indication of law is evolved from the apparent 

 irregularities. Most striking are the manner in which the de- 

 flections increase towards each of the centres of strong local dis- 

 turbance, and the abrupt change in sign of the deflection as we 

 pass such a centre. The relation of all the deflections along the 

 northern half of the ridge to the centre of disturbance at the 

 summit G is very manifest. This summit produces negative 

 deflections, diminishing with distance, at points to the north of it ; 

 and positive deflections, also diminishing with distance, at points 

 to the south. To the north, where no other important centre of 

 disturbance complicates the result, the effect is felt to at least a 

 hundred yards, and probably with more delicate instruments 

 might be proved much farther. The centres E and F produce 

 somewhat similar effects, except that the deflections are in the 

 opposite sense. It is interesting too to note the abrupt change of 

 7° at the point B. This may be interpreted as the superposition 

 on the effect of G of an effect of the same kind but much smaller 

 magnitude, and probably points to the existence at B of a centre 

 of disturbance too feeble to be detected in a rough and hasty 

 examination. Other irregularities may be conjectured to have a 

 like origin. 



The observations at this locality seem then to indicate that, 

 besides the powerful local forces which cause the needle to spin 

 rapidly about as the compass is moved over the surface, a summit 

 magnetised in this apparently fortuitous manner exerts neverthe- 



