AND LEIOESTERSHIEE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 77 
the magnetic method of surveying iron-ore bodies is well known. The procedure, 
therefore, was not so simple as that adopted for the Swedish cases, where magnetite 
occurs in great masses at or near the surface, resulting in disturbances which 
approach in order of magnitude the normal effect of the earth’s magnetic field. 
The general positions of the magnetic observation-stations were chosen (l) round 
Irthlingborough, with the help of Mr. Thos. Falcon, with due regard to the outcrop 
and distribution of those rock-strata that, in view of their iron-content, might be 
expected to exert a disturbing influence ; (2) round Melton Mowbray, in order that 
by means of a system of triangulation it should be possible to gain some idea of the 
distribution and depth of origin of the local disturbing forces. The geographical 
positions and azimuths of the observation-points were accurately fixed by Mr. Gibson, 
Chief Observer of the Ordnance Survey, whose services were kindly lent for the 
occasion by Colonel Sir Charles Close, Director General of the Ordnance Survey. 
The field magnetic observations were made concurrently by Mr. Walker. After 
the control magnetic data from Greenwich had been obtained from the Astronomer 
Royal, and the other necessary data from the Ordnance Survey Office, the reduction 
of the observations was completed and the disturbing forces calculated. 
A geological examination of each district was then made by Dr. Cox. Specimens 
of the typical rocks, and of any other rocks which by reason of their exceptional 
characters appear likely to shed light on the problem, were sent to Prof. Ernest 
Wilson, who determined their magnetic susceptibilities. By these means it was 
possible to ascertain the rocks which were in any way capable of exciting the magnetic 
disturbances, and those which were certainly incapable of causing the disturbances. 
The results so obtained were compared with information derived from a chemical and 
petrological examination of the most susceptible rocks, and the geological structure 
was then considered in its bearing on the known facts. 
In the following pages, Mr. Walker and Prof. Wilson give separate accounts of 
their own parts in the investigation, and Dr. Cox has endeavoured, in Part III., with 
the help of contributions from Mr. Walker, to show how far the results may be 
explained by the known geological structure. Contributions from Mr. Walker or 
Prof. Wilson included in Part III. are indicated by quotation marks and initials. 
Dr. Cox wishes to acknowledge his great indebtedness to Dr. Strahan, Director 
of the Geological Survey, for much valuable advice as to the carrying out of the work, 
and for his help in the preparation of this report. He has also to thank other 
officers of the Geological Survey, more especially Dr. H. H. Thomas and Dr. Walcott 
Gibson, for their assistance in obtaining specimens and for help on various points. 
He is also indebted to Prof. Watts, Secretary of the Committee, for reading the 
manuscript and for many helpful suggestions as well as for the loan of certain 
specimens of Charnwood Forest rocks. Finally, he has to ofier his thanks to 
Dr. Ronald Burrows, Principal of King’s College,- London, to Prof. Sir Herbert 
Jackson, and to his colleagues in the Chemical Department of King’s College, for 
