AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 75 
If the magnetic disturbances were, as was reasonable to suppose, due to the local 
distribution of iron, it became important to ascertain in what form and quantity this 
iron was present. 
It is well known that although iron is the only element capable of showing 
magnetic phenomena on a large scale, yet the magnetic influence exerted by a given 
weight of it is dependent on the particular state of combination of the iron. In the 
metallic state iron is, or can be rendered, very strongly magnetic, but its susceptibility 
is rapidly diminished by the presence of small amounts of certain substances, notably 
manganese.* In the combined state as magnetite (Fe 304 ) it is still strongly magnetic, 
although not nearly so strongly as would be the corresponding amount of iron in the 
free state. In the case of magnetic pyrites (FegS^) the susceptibility sinks considerably 
below the value for magnetite itself, and in the case of other compounds of iron the 
susceptibility is still further diminished, so that haematite, for example, is ordinarily 
spoken of as non-magnetic. Definite examples are given later (p. 84 et seq., and 
p. 105). In general, it may he said that iron in the ferrous condition has a 
much greater magnetic susceptibility them the corresponding amount of iron in the 
ferric state. 
It follows that although magnetic disturbances are in all probability connected 
with the distribution of iron in some form or other, yet the amount of disturbance 
tvill not he a direct measure of the amount of iron present. A rock containing a 
small percentage of iron in the form of magnetite (Fe 304 ) will exert a much greater 
effect than a rock containing a large percentage of iron in the form of haematite 
(FeaOg). Further, magnetite will continue to exert a disturbing influence even when 
disseminated through a large amount of material of low susceptibility, as in the case 
of the magnetite of igneous rocks. In general, a number of magnetic observations 
at different points round the disturbing centre are necessary before the effects 
due to- deep-seated rocks of high susceptibility can be distinguished from those due 
to rocks of low susceptibility situated nearer the surface. A magnetic disturbance, 
therefore, does not necessarily imply the presence of iron in a form economically 
workable. 
Melton Mowbray is only one of a number of centres of magnetic disturbance, but 
it is of importance as being partly surrounded by an iron-ore producing country. 
The ore-bearing strata occur in the form of a flat sheet capping a ridge of high 
ground which makes a crescent round the town, the inner diameter of the crescent 
being about six miles. Within the crescent the sheet has disappeared as the result of 
denudation. The observation-stations in the magnetic surveys by Rucker and 
Thorpe, and by Mr. Walker, were situated about four miles from the outcrop 
of the iron-ores, and in rocks below their horizon. The importance of determining 
* Sir R. Hadfield, C. Cheneveau, and Ch. Geneau, ‘Roy. Soc. Proc.,’ Ser. A, vol. 94 (1917), 
p. 65. 
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