Rocks and Regional Magnetic Disturbances. 531 



its horizontal attraction strengthens that due to the plateau as a 

 whole, the resultant force would be about 450, a value which is sur- 

 passed only by such stations as Canna and Soa, which are islands in 

 the West of Scotland. 



While, then, in order to account for the high vertical forces, we 

 have been obliged to make favourable assumptions as to the shape of 

 the masses, as to the position of the smaller on the larger mass, and 

 as to the magnetic properties of the rocks, we now find that these 

 lead to possible values of the horizontal disturbing force considerably 

 greater than any which have been measured. 



The most obvious explanation of the discrepancy is the assumed 

 vertically of the sides of the magnetic masses. If the sides of the 

 valley slope for half the height at an inclination of one in ten, as 

 described above (p. 527), the largest horizontal force due to the 

 plateau at any of the points for which the calculation has been made 

 is 123, as against 227 when the sides are vertical. It must, however, 

 be remembered that in this case the mean range of vertical forces is 

 reduced from 159 to 107. It is not, therefore, convenient to assume 

 that the slopes are very gradual. 



The supposition that the main mass is surrounded by magnetic 

 matter of less permeability than itself reduces the horizontal forces, 

 but also reduces the vertical forces below the observed values. 



An irregular outline, on the other hand, might tend to increase the 

 range of the vertical force at points near the edge, while it would 

 diminish the horizontal forces. 



It appears on the whole, however, that the gentle slopes or gradual 

 changes of permeability which would reduce the horizontal forces to 

 the observed values, would give vertical forces about one-third too 

 small. 



I do not think that this can be considered an unsatisfactory result, 

 but I will defer comment upon it in order to turn to another point. 

 So far, we have been discussing districts of widespread disturbance. 

 In Scotland the forces are more localised, but more intense. The 

 most rapid change of vertical force disturbance which has been 

 measured is in the Southern Hebrides, where it varies from 736 at 

 Loch Boisdale to +369 at Bernera, which is only 20 miles distant. 



Dr. Thorpe and I have proved (' Roy. Soc. Proc.,' vol. 47) that a 

 very intense centre of disturbance exists in this neighbourhood, and it 

 is remarkable as being near the highly magnetic rocks of Mull, but, as 

 its effects appear to be more far reaching than those of that island, 

 it is legitimate to assume that it is produced by rocks of exceptional 

 magnetic power. An equal range of force could, however, be pro- 

 duced by the mean permeability of basalt in the west of Scotland, 

 t.e., 0-00271. 



If, as before, we take the attracting mass to be the frustum of a 



