AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 87 
had a square cross-section, and gave the comparatively high susceptibility 21 x 10“^ 
After being reduced to a circular cross-section and its density re-determined, the value 
of the susceptibility was found to be 23 x 10“^ 
In other cases the specimens were re-tested “ after demagnetisation,” for the 
following reason. It is well known that if a piece of iron be subjected to a consider¬ 
able magnetising force, and afterwards tested for permeability at lower forces, the 
permeability then found is smaller than would have been the case had the specimen 
been first demagnetised. The complete previous magnetic history of rocks is unknown, 
but some of them, owing to their retentivity, show that they must have acquired a 
high degree of magnetisation at some past time. By subjecting such a specimen to 
an alternating magnetic force, gradually diminishing to vanishing point, the magnetic 
effects, due to this retentivity, which have perhaps existed for centuries, can be 
removed in a few minutes, and it is then found that the specimen (for example the 
Dolerite, E. 6519*) may have increased susceptibility. 
Experiments have been made which prove that the method is applicable to powders, 
thus making it possible to deal with magnetic permeability of fair average samples 
obtained, if necessary, from relatively large areas. The powders are placed in a glass 
tube of known internal volume, and by taking into account the specific gravity and 
total weight of the powder in the tube, the measure of concentration is obtained, and 
results comparable with those of the same material in the solid state can be readily 
secured. 
Emphasis should be laid upon the fact that samples cut from the same hand-speci¬ 
men frequently show widely different susceptibilities, as illustrated by the Mount 
Sorrel granite (L. 15). The only satisfactory way in which to obtain an average 
result is to test a large number of specimens of the same rock. 
It may be observed that when a rock is spoken of as having a high susceptibility, 
as for example in the dolerites, the susceptibility-value is in reality exceedingly small 
as compared with the susceptibility of such a material as iron in which the suscepti¬ 
bility may have a maximum value of 40,000,000 x 10“^ in C.G.S. units. 
My best thanks are due to E. F. Herroun, Esq., F.I.C., for the use of his magneto¬ 
meter and for his valuable assistance. 
PART III.-THE GEOLOGICAL STRUCTURE AND THE MAGNETIC 
DISTURBANCES. 
(A). NORTHAMPTONSHIRE: IRTHLINGBOROUGH DISTRICT. 
The investigations in this district had for their special object the testing of the 
changes in the character of the disturbances caused by a flat sheet of iron-ore 
according as the observations were made in positions above the sheet, along its edges, 
and outside its edges. 
*'See footnote, p. 83. 
