126 DE. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAxMPTONSHIRE 
magnitude observed. Hence the granites would require to come very close to the 
surface, and to fill up a large part of the area between Rempstone and Lough¬ 
borough, and again between Melton, Asfordby and Waltham. The known geology 
is, I gather, against this. Dr. Cox informs me that it is most unlikely that any 
granite could come within 1000 feet of the surface at Melton Mowbray, or even 
within 3000 feet of the surface if, as seems probable, the Coal Measures occur under 
the cover of the newer rocks. Thus we appear to be forced to an explanation by 
rocks of still higher susceptibility, such as the dolerites, involving logically a greater 
depth or a smaller bulk of such rocks.”—G.W.W. 
With regard to the influence elsewhere of granites and similar rocks, Ruckee and 
Thoepe* state that : “In general, masses of non-basaltic rock, igneous rock, produce 
little or no efiect on the needle ” . . , . but they note exceptions to this rule m the 
case of the granites of the Cheviots and of Galway. 
PART IV.—SUMMARY AND CONCLUSIONS. 
After considering the geology of certain areas in which anomalous magnetic 
disturbance had been brought to the notice of the Conjoint Board of Scientific 
Societies, as stated on p. 75, the Iron-ore Committee of the Board formed the 
conclusion “ that it is not possible at present to see any connexion between the 
observed magnetic disturbances and known occurrences of iron-ore in this country.” 
The committee resolved that “ the causes of the disturbances are deserving of further 
investigation and they recommend that attention should be concentrated upon two 
dissimilar areas, such as the neighbourhood of Melton Mowbray and that of Strachur 
and Locligoilhead, that a detailed geological and petrographical survey of the rocks 
in each region investigated should be carried out, and that the magnetic permeability 
of these rocks should also be investigated.” 
In pursuance of this recommendation the magnetic and geological surveys were 
carried out in the summer and autumn of 1917, and the materials collected 
subsequently submitted to magnetic, chemical, and petrographical examination. 
The district chosen was that of Melton Mowbray, where the nearest superficial 
iron-ores are four miles distant from the point at which the magnetic disturbances 
were originally observed, and as a control, a parallel examination was made in an area 
of undisturbed Northampton iron-ore near Irthlingborough, the boundaries of which 
have been surveyed and are accurately known. 
The weak magnetic disturbances of the Irthlingborough district can be correlated 
with the distribution of the Jurassic iron-ores, which are in the form of a horizontal 
sheet of weakly susceptible ferrous carbonate when protected from weathering by 
overlying impervious strata, passing into less susceptible hydrated ferric oxide at the 
weathered outcrop. 
* ‘ Phil. Trans.,’ Ser. A, vol. 188 (1896), p. 653. 
