134 Reviews — Magnetic Distitrhances. 



examination, one lying between Melton Mowbray and Nottingham, 

 and the other in the ironstone district of Irthlingborough. 

 Additional magnetic observations were carried out by Mr. Walker, 

 and Dr. Cox was commissioned to make a geological and petrological 

 study of the two areas and to discuss the results, a duty which he 

 has accomplished with much skill and judgment. 



The problem is quite different from that presented by the much 

 greater magnetic disturbances in the Western Isles of Scotland, 

 which are manifestly due to permanent magnetization of rocks at 

 the surface. Here, on the other hand, we have to do with induced 

 magnetism, and Mr. Walker's observations indicate that the seat 

 of the disturbance in Northamptonshire is at a depth of not less 

 than 3,000 feet. The inference is that rocks of noteworthy magnetic 

 susceptibility are in place at some considerable depth beneath this 

 area, and that there is some irregularity in their distribution. 

 Discussion of underground geology necessarily involves an element 

 of the hypothetical ; but the author is able to muster a sufficient 

 number of data bearing on the question. He has made a list of 

 all the rocks which are likely to occur beneath the district, and 

 examined their petrographical characters, while determinations 

 of magnetic susceptibility have been furnished, by Professor Ernest 

 Wilson. Dr. Cox points out that this property in different rocks 

 is closely correlated with their content of magnetite. A granite 

 containing only a little magnetite has a much higher susceptibility 

 than a basic rock rich in iron in the form of silicates and in ilmenite. 



The author ascribes the disturbances to the presence of dolerites 

 as intrusions in the subjacent Coal-measures. Such dolerites are 

 well-known in other parts of the Midlands, and they possess a higher 

 magnetic susceptibility than any other rocks in that region. It may 

 be mentioned that the investigation was undertaken primarily 

 in the hope of detecting concealed iron-ores in this way, and the 

 possibility should certainly be kept in view in relation to like 

 magnetic disturbances elsewhere. Such cases as that of the Rosedale 

 ore in Cleveland show that an isolated body of bedded magnetite- 

 rock may occur in the midst of a formation \vhich elsewhere carries 

 only iron-oxides and carbonate. We have, however, no grounds 

 for conjecturing such an occurrence beneath Northamptonshire, 

 and intrusive dolerites seem to afford at least a good provisional 

 explanation of the facts. The inferred irregularity of their 

 distribution is attributed to faulting, and the author shows that the 

 observed magnetic disturbances in this area correspond closely 

 with faults which are mapped in the Mesozoic strata, and almost 

 certainly extend into the underlying Palaeozoic rocks. The 

 disturbances near Irthlingborough can be referred to the Jurassic 

 iron-ores of the district, which pass from ferrous carbonate m the 

 interior into hydrated ferric oxide at the weathered outcrop, with 

 a consequent diminution in magnetic susceptibility. 



A. H. 



