428 Geological Society. 



in the Coal Measures. These facts suggest the possibility of the 

 occurrence of dolerites intrusive into Coal Measures beneath the 

 Mesozoic rocks of the Melton Mowbray district. 



The distribution of the dolerites actually proved, and of those the 

 presence of which is suspected by reason of the magnetic dis- 

 turbances, appears to be controlled by the faulting. Moreover, 

 whereas the character of the magnetic disturbances is such that 

 it would not be explained by a sill or laccolite faulted down to the 

 north, in the manner demanded by the observed throw of the 

 principal fault, it would be explained by an intrusion that had 

 arisen along the fault-plane. The faulting itself is connected 

 with a change of strike in the concealed Coal Measures, and the 

 incoming of doleritic intrusions in the concealed coalfield, in con- 

 trast with their absence from the exposed coalfield, appears to 

 depend upon the changed tectonic features. The change of strike 

 is apparent, but to a less degree, in the Mesozoic rocks which, in 

 the neighbourhood of Melton Mowbray, have suffered a local twist 

 due to the development of an east-and-west anticlinal structure. 



In view of the evidence that later movements have, in this district, 

 followed the lines of earlier and more powerful movements, it 

 appears possible and even probable that this post- Jurassic (probably 

 post- Cretaceous) anticline is situated along the line of a more pro- 

 nounced post- Carboniferous but pre-Permian anticline. In this 

 connexion the isolated position of Charnwood Forest has a consider- 

 able significance. The Forest is situated on the prolongation of the 

 east-and-west line of uplift, and just at the point where this uplift 

 crosses the line of the more powerful north-westerly and south- 

 easterly (Charnian) uplift. Where the two lines of uplift cross 

 the elevation attains its maximum, and the oldest rocks appear. 



The main line of faulting and of magnetic disturbance is parallel 

 with and on the northern side of the east-and-west anticline, and 

 the faulting is of such a nature that it serves to relieve the folding 

 while accentuating the anticlinal structure. It is possible that 

 this belt of magnetic and geological disturbance marks the southern 

 limit of the concealed coalfield. The results obtained by joint 

 magnetic and geological work have thus served to emphasize the 

 real importance of a structure which, when judged merely from its 

 effects on the surface-rocks, appears to be of only minor importance. 



A further series of observations was carried out on the Jurassic 

 iron-ores of the Irthlingborough district of Northamptonshire. 

 The ores occur in the form of a nearly horizontal sheet of weakly 

 susceptible ferrous carbonate partly oxidized to hydrated oxides. 

 They give rise to small magnetic disturbances which are quite 

 capable of detection, and these may be of use in determining the 

 boundaries of the sheets in areas not affected by larger disturbances 

 of deep-seated origin. 



The results obtained by the joint magnetic and geological work 

 in the two areas show that this method of investigation may be 

 used to extend our knowledge of the underground structure. It 

 appears also that an extension of the method to other parts of 

 the country would yield information of considerable scientific and 

 economic importance. 



