90 DR. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
reduced to four feet. E is thus very near but not quite on the edge of the outcrop. 
The fourth station C is on the valley alluvium below the iron-ore outcrop, while the 
E.S.E. 
A 0 
0_1_2_3 Miles 
Fig. 2. 
N. 
S. 
Fig. 3. 
Figs. 2 and 3. Sections to illustrate geological structure of Irthlingborough district. 
fifth station D is on the opposite side of the valley and on the iron-ore bed, 
the depth of the bed at this point being unknown. 
II. Interpretation op the Reselts. 
The figures for the various observation-stations have been given in an earlier 
section (p. 79). Regarding their correlation, Mr. Walker writes as follows :— 
“The ironstone bed may, I understand, be regarded as a flat sheet of ferrous 
carbonate, considerably oxidised at the outcrop. The material is very slightly 
susceptible and may be expected to be magnetised by the earth’s induction so that 
the upper side has south magnetism.”—G.W.W. 
“ The adjoining diagram (fig. 4) illustrates the lines of force in a vertical section 
through such a flat plate. If we are above the line XY the horizontal force always 
tends to the centre of the plate. In magnitude it is small near the centre, rises to a 
maximum as we get just over the edge, and beyond the edge diminishes again to 
zero. The vertical disturbance is relatively downwards near the centre, diminishes 
to zero as we approach the edge and then changes sign to an upward disturbance, 
attains a maximum and then rapidly diminishes.”—G.W.W. 
“ If we compare this with the map, it will be seen that the relative disturbances 
at A, B, and E are exactly what we should expect from the flat sheet, and we may 
even argue that the edge runs a little to the north of E, a little to the north of B, 
and rather more to the north of A.”—G.W.W. 
