SIJRVEY OF THE BRITISH ISLES FOR THE EPOdH JANUARY I, l89I. <351 
comparable with their linear dimensions, yet if the sides slope gradually away the 
Forces may be appreciable. 
It must also he remembered that although the highest susceptibility we have taken 
as a basis of calculation is the mean of a number of specimens from Scotland, much 
larger mean values were obtained in groups of specimens from Skye. All the largest 
values of the Forces above calculated might be doubled without the susceptibility 
exceeding that of rocks which occur on the surface, and, even if in any given locality 
the surface rocks are only feebly susceptible, it is possible that they may overlie 
more strongly magnetic masses. 
On the whole then, it apjDears that all the observed facts, the smallness of the 
Disturbances in some cases, and their magnitude in others, are consistent with the 
theory that they are caused—in part, at all events—by basic rocks magnetized in 
the earth’s field. 
We now proceed to enquire how far the main centres in which basic rocks occur 
are also centres of magnetic attraction. 
The easiest way of conducting the enquiry is to study stations near to the 
main masses of igneous rocks, but yet sufficiently distant to make local Disturbances 
unimportant as compared with the regional attraction. 
If at all such stations the Horizontal Disturbing Forces tend towards the igneous 
mass, it is obvious that it is on the whole a centre of attraction. 
Of course doubtful points arise, in deciding which there is sometimes room for 
difference of opinion. 
Thus if a comparatively insignificant group of magnetic rocks appears on the 
surface, near to, but completely detached from, a much larger intrusive mass, it may 
be doubtfid whether it should or should not be included within the girdle of 
external stations. 
If it is included, it may divert the line so far from the central region, that the 
cogency of the proof is diminished. If it is not included, there is the risk that, 
though of trilling superficial area, it may represent an important underground 
extension, and that the Disturbing Forces in the intermediate s^mce may be subject 
to local variations. 
On the whole we have thought it best in each case to keep as near to the principal 
mass of rock as possible!, but if the district between it and an apparent outlier appear 
to be locally disturbed, we have carried the boundary outside the latter. In all 
cases, where any doubt can arise, the reasons foi' the selection of the stations 
are given. 
Another difficulty arises from the fact that the Disturbances due to igneous rocks, 
which appear on the surface, are interfered with by important lidge lines, wdiich are 
either connected with another group of rocks or traverse a neighbouring district 
where the superficial soil is non-magnetic. 
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