92 DR. A. H. COX: REPORT ON MAONETIC DISTURBANCES IN NORTHAMPTONSHIRE 
“ The observations show that with improved differential instruments, such as those 
referred to on pp. 82 and 83, it would be possible to map the distribution of even 
comparatively weak magnetic material.”—G.W.W. 
III. The Underground Structure. 
It will be shown in the second part of this section dealing with the Melton Mow¬ 
bray district, that the disturbances there do not arise from strata near the surface, 
but that they are deep-seated in origin and closely connected with faulting. It is 
necessary to consider, therefore, whether the same may not be true for the Irthling- 
borough district, where a fault is also present, and whether the agreement between 
the disturbances and the results to be inferred from a consideration of the surface 
geology may not be entirely fortuitous. 
On this point the magnetic observations themselves shed some light. Mr. Walker 
states:— 
“ The system of disturbances suggests as the most obvious distribution of disturbing 
magnetic material a flat sheet with a boundary as already indicated (p. 90), and 
situated at no great depth.”—G.W.W. 
Now within the first few hundred feet of strata we know of no rock other than the 
ironstones likely to cause such disturbances. This is apparent from the following 
considerations. The Northampton Ironstones and associated rocks belong to the 
Jurassic System. The divisions which immediately underlie the ironstone are the 
Upper, Middle, and Lower Lias. The Upper Lias, as already mentioned (p. 88), consists 
of stiff blue clays attaining, in the Irthlingborough district, a thickness of about 
180 feet. The susceptibility of these clays has been tested (p. 86) and is so small as 
to be negligible. Below the Upper Lias is the Middle Lias. This division contains an 
important ironstone bed in Leicestershire (see part B of this section, p. 97), but south¬ 
wards from Leicestershire the ironstone bed thins out, as shown by borings, and east¬ 
ward “beyond Wellingborough the rock bed, where it can be recognised at all, is 
merely a layer of pebbles from a few inches to a foot thick.”* Such a bed obviously 
could not cause the disturbances measured. Below this attenuated representative of 
the Middle Lias is the Lower Lias consisting entirely of blue clays with some limestone 
bands. The susceptibility of both clays and limestones is negligible. The thickness 
of this formation beneath Irthlingborough is probably at least 450, and perhaps as 
much as 600 feet, which would bring its lower limit down to between 400 and 
500 feet below O.D., that is to about 700 feet below the Northampton Ironstone. 
Throughout this thickness of 700 fe&t, it is practically certain that there is no roch 
that can give rise to distinct magnetic disturbances. 
As to what rocks come immediately beneath the Lower Lias clays our information is 
not so definite, for, unfortunately, there is no deep boring in the immediate vicinity of 
Bebby Thompson, Article on “Northamptonshire,” ‘ Geology in the Field,’ 1910, vol. IL, p. 460. 
