104 DR. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
by the dolomites from the Carboniferous Limestone and the Permian, with suscepti¬ 
bilities 0’38, 0‘48, and 0‘18* respectively. 
As soon as iron in any form appears in a rock the susceptibility rises, as instanced 
by the ferruginous sandy limestone from the Marlstone with a susceptibility 4'3, the 
iron being present as limonite. 
Similarly, in those clays or slates, the blue colour of which testifies to the presence 
of a certain amount of iron as pyrites and chlorite respectively, the susceptibility 
is slightly increased as seen from the figures for the Upper Lias Clay and for 
Charnwood Slates, 2'0 and 3‘9 respectively. A comparison of the latter with the 
contact-altered slates (3’2 to 6‘l) near the Mount Sorrel granite shows that there has 
been no noteworthy increase in the susceptibility of the altered rocks, and presumably, 
therefore, no appreciable development of magnetite as the result of the contact- 
alteration, 
(b) The Mesozoic Iron-Ores. 
(i.) The Marlstone Iron-ores occupy the actual surface or occur under at most 
a thin cover of Boulder Clay or Upper Lias Clay, the workings being abandoned 
wherever the thickness of cover exceeds 16 feet. As a consequence of the absence 
of cover the ores are almost completely oxidised, and are in the form of the hydrated 
ferric oxide limonite, while ferrous iron is wanting. Accordingly the susceptibility 
remains relatively low (10 to 23), even though the iron-content (reckoned as metallic 
iron) may attain to over 40 per cent. 
(ii.) The iron-ores in the Northam'pton Sands round Waltham also occur for the 
most part under a comparatively thin cover, and again their oxidation is almost 
complete. Their susceptibility may be expected to compare closely with that of the 
exactly similar rocks which form the outcropping edge of the Northamptonshire 
Iron-ore in the Irthlingborough district (p. 89). The oxidised rocks of the outcrop 
consist essentially of limonite, and their susceptibility (6'0 to 8’3) is of the same order 
of magnitude as that of the Marlstone iron-ores mentioned above. 
(c) The Clay-Ironstones of the Coal Measures. 
These rocks consist of argillaceous material impregnated with ferrous carbonate. 
They occur as definite bands and as concretionary nodules interbedded with fire¬ 
clays, &c., at numerous horizons in the Coal Measures, and they are sufficiently rich 
in iron to have been worked near Dale in former years. It is well known that the 
presence of carbonaceous material exerts a retarding influence on the oxidation of 
ferous carbonate, and the iron in these clay-ironstones has remained in the ferrous 
condition in accordance with the general rule. The susceptibility is therefore 
considerably higher tharT that of the various limonite iron-ores of the Jurassic, and is 
* Throughout this section the susceptibilities are given in units 10“^ of C.G.S. units. 
