110 DR. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
Other disturbing rocks might possibly lie concealed under newer Formations, though 
their presence within the district has not hitherto been proved. We must, therefore, 
consider also the probable magnetic eifects of:— 
(VIII.) Ferruginous dolomites in the Carboniferous Limestone. 
(IX.) Haematite deposits ,, ,, 
(X.) Basalt lavas ,, ,, ,, 
(XI.) Basic intrusions (Camptonites) in Cambrian Bocks. 
It will only be necessary to consider (I.), (II.) and (III.) in any detail ; (I.) and 
(II.) because the Jurassic iron-ores have a wide outcrop near the areas where the 
magnetic disturbances were observed, (III.) because of the high magnetic suscepti¬ 
bility of the dolerites. The others will be dealt with briefly in a later section. 
1. The Effect of the {Jurassic) Iron-ores. 
The limonite iron-ores of the Middle Lias Marlstofie may at once be ruled out as 
incapable of originating the main disturbances. 
In the first place their magnetic susceptibilities, which range from 10 to 
20 X 10~®, are not high enough, according to the susceptibility rule (p. 82), to cause 
disturbances measuring up to 170. 
Secondly, the iron-ores occur as a nearly flat sheet capping a ridge of high ground 
that runs about four miles north of Melton Mowbray. The geological structure, 
as regards the Mesozoic (Triassic and Liassic) Formations, is almost symmetrical 
on the two sides of the ridge (see section, fig. 8, p. 121). But although the magnetic 
observation stations were more or less symmetrically disposed on either side of the 
ridge, the distribution of the disturbing forces on the two sides was found by 
Mr. Walker to be markedly unsymmetrical; the disturbances north of the ridge 
were comparatively small, whereas those south of it are very large. 
Further, the sharpness of the changes as between the stations Melton Mowbray— 
Asfordby and Ptempstone—Seagrave respectively, seem to bear no relation to the 
outcrop of the iron-ores, while the disturbances themselves seem to originate at levels 
much below the surface (see p. 82). 
What has been said with regard to the Marlstone iron-ores applies with even 
greater force to the small development of iron-ore (Northampton Iron-ore) in the 
Inferior Oolite at Waltham, north-east of Melton Mowbray. The small patch there 
seen obviously cannot be the cause of the large disturbances measured at Bempstone, 
seven miles away to the west. 
We must therefore look for some other agent if the cause of the asymmetry of the 
disturbances, and also their intensity, are to be explained. It will be seen in what 
follows that a study of the faulting of the rocks brings out a remarkable and 
suggestive cbrrespondence between the tectonic and magnetic features of the region. 
