74 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
Part III. (continued)— 
(B) Leicestershire (continued)— Page 
HI. Correlation of Magnetic Results with the Composition and Tectonics of the Rocks 109 
1. Effect of the (Jurassic) Iron-ores.110 
2. Coincidences between the Fault Lines and the Magnetic Disturbances. . . Ill 
3. Probable Effect of Dolerite Intrusions.113 
4. Relationships between the Faults and the Intrusions.115 
5. Structure of the Concealed Coalfield.118 
6. The Presence of an Anticlinal Structure.. 120 
7. Evidence for the Presence of an East-West Anticlinal Axis in the Concealed 
Palaeozoic Rocks. 122 
8. Other Possible Causes of the Disturbances.124 
Part IV.^—Summary and Conclusions ..126 
Addendum.—Petrology of the Igneous Rocks, by A. H. Cox.. 128 
Introduction. 
History of the Besearch. 
During the course of the magnetic re-survey of the British Isles, recently carried 
out by Mr. G. W. Walker under the auspices of the Royal Society, it was 
found that Melton Mowbray, Leicestershire, was a centre of pronounced magnetic 
disturbance, both vertical and horizontal. This result was in conformity with that 
obtained by Rucker and Thorpe"^ in their surveys carried out 29 and 24 years 
earlier. 
So far as is known the only element that gives rise to magnetic phenomena on a 
large scale is iron.f It appeared probable, therefore, that the local disturbances 
were to be connected with the distribution of iron in the rocks, and accordingly the 
origins of such disturbances were likely to be ititimately related to the geological 
structure of the district. 
The subject was brought to the notice of the Council of the Royal Society by 
Mr. Walker, and v^as referred by the Council to the Conjoint Board of Scientific 
Societies, who appointed a Committee to consider the bearing of magnetic disturbances 
on the possible occurrence of iron-ores. 
* ‘Phil. Trans.,’ Ser. A, vol. 181 (1890), p. 288j and vol. 188 (1896), pp. 602 and 626. 
t The fact that there exist substances, as, for example, cobalt, nickel, and the Heusler alloys^—CuAlMn 
—that contain no iron and yet are, or can be rendered, strongly magnetic, has not been overlooked. It 
has also been claimed that magnetism may be induced in certain minerals, such as kyanite AloSiO^, 
dolomite CaMg(C 03 ) 2 , and leucite KAlSiOs, but it has not been satisfactorily proved that in these latter 
cases no trace of iron is present. In any case the susceptibilities of such mineral substances are small as 
compared with those of materials containing an appreciable amount of iron, and are not sufficient to 
excite the disturbances measured by Mr. Walker. The whole results of this work go to show that the 
disturbing element must be iron in some form or other. 
