AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 127 
The disturbances arising from material only feebly magnetic are quite capable of 
detection and may be of use in determining the boundaries of the sheets in areas 
not afiected by larger disturbances of deep-seated origin. 
In the Melton district the disturbances cannot be correlated with Jurassic iron- 
ores, which have been removed by denudation from all but a margin of the area, or 
with the Coal Measure clay-ironstones (weakly susceptible) which may possibly occur 
in depth. 
On the other hand the magnetic disturbances, which are inferred from their 
distribution to originate at a considerable depth, exhibit a remarkable correspondence 
with certain easterly to westerly faults which cut the Mesozoic rocks, and in some 
cases are also seen to cut the Palaeozoic rocks to the west of the area. It is 
practically certain that all these faults must extend downwards into the deeper 
seated rocks, which may be of Coal Measure age or older. 
As none of the sedimentary rocks known in the district have susceptibilities high 
enough to account for magnetic disturbances of the magnitude observed, attention 
was given to the igneous rocks which are known to occur in parts of the district and 
might possibly underly the areas of disturbance. 
Measurement of the susceptibility of a mimber of igneous rocks shows that this 
property is largely independent of the acidity or basicity of the rock but is chiefly 
related to the amount of magnetite present, even if in a widely scattered form. Of 
the rocks examined, all except certain dolerites have susceptibilities not high enough 
to account for the magnitude of the disturbances when allowance is made for the 
depths at which they are likely, if present, to occur. 
Dolerites intrusive into the Coal Measures, however, prove to possess the highest 
susceptibilities of any rocks found in the district, sufficiently high to produce the 
results observed if occurring at depths about equal to the probable thickness of the 
Mesozoic and Permian rocks there. Basic rocks elsewhere are known to give rise to 
magnetic disturbances in several districts where they are largely developed, as in 
Skye and Antrim, and Rucker and Thorpe have suggested that the magnetic pull 
towards the coalfield of the Midland Valley of Scotland is due to the abundance ot 
basic rocks there. 
Such rocks, though absent from the exposed coalfield, have been found in several 
borings and shafts in the concealed coalfield, and probably may be associated, directly 
or through faulting, with the easterly bending of the north—south strike of the 
rocks of the coalfield. They may be a feature of the southern rim of the coal basin. 
A consideration of the evidence renders it likely that the intrusions have been 
injected into faults, though the latter may possibly have undergone secondary 
movement at a later date. 
The outcrop of the Mesozoic rocks gives evidence of an anticlinal axis parallel with 
the Rempstone—Melton Fault, which may well be founded on an anticline in the 
deep-seated Palseozoic rocks, the higher beds of which, if the above explanation of 
