Geological Stiuct.ure and Magnetic Disturbance. 427 



detailed magnetic survey of these areas, accompanied by a peno- 

 logical survey and an examination of the magnetic properties of 

 the rocks of the neighbourhood, should be made. He (the speaker) 

 had been approached with a view to the petrologieal work being 

 undertaken by the Geological Survey, and it had been arranged by 

 the Board of Education, with the consent of H.M. Treasury, that 

 a geologist should be temporarily appointed as a member of the 

 staff for the purposes of the investigation. Dr. Cox had received 

 the appointment, and the lecture which ■ he was about to deliver 

 would show that results of great significance had been obtained by 

 him. The new magnetic observations had been made by Mr. Walker, 

 and the examination of the specimens collected, in regard to their 

 magnetic susceptibility, had been conducted by Prof. Ernest Wilson. 



Dr. Cox then described the selected areas, which lay on Lias and 

 Keuper Marl between Melton Mowbray and Nottingham, and in 

 the neighbourhood of Irthlingborough, where the Northampton 

 Sands are being worked as iron-ores. The Middle Lias iron-ores, 

 consisting essentially of limonite, which crop out near Melton 

 Mowbray, have been proved incapable, by reason of their low 

 magnetic susceptibility, of causing disturbances of the magnitudes 

 observed, while the distribution of the disturbances showed no 

 correspondence with the outcrop of the iron-ores. Nor was any 

 other formation among the Secondary rocks found capable of 

 exerting any appreciable influence. It appeared, therefore, that 

 the origin of the magnetic disturbances must be deep-seated. 



Investigation showed that the disturbances were arranged along 

 the lines of a system of faults ranging in direction from north-west 

 to nearly west. The faults near Melton Mowbray have not been 

 proved in the Palaeozoic rocks, and, so far as their effects on the 

 Secondary rocks are concerned, they would appear to be only minor 

 dislocations. But farther north, near Nottingham, faults which 

 take a parallel course, and probably belong to the same system 

 of faulting as those near Melton Mowbray, are known from 

 evidence obtained in underground workings to have a much givater 

 throw in the Coal Measures than in the Permian and Triassic rocks 

 at the surface. It appears therefore that movement took place 

 along the same lines at more than one period, the earlier and more 

 powerful movement being of post- Carboniferous but pre- Permian 

 age, the later movement being post-Triassic. Accordingly, it is 

 probable that the small dislocations in the Mesozoic rocks indicate 

 the presence of important faults in the underlying Palaeozoic. 



The faults can only give rise to magnetic disturbances if they 

 are associated with rocks of high magnetic susceptibility. It is 

 known from deep borings that the concealed coalfield of Nottingham- 

 shire extends into Leicestershire, but how far is not known. Deep 

 borings have proved that intrusions of dolerite occur in the Coal 

 Measures at several localities in the south-eastern portion of the 

 concealed coalfield and always, so far as observed, in the immediate 

 vicinity of faults. It has been established that dolerites may exert 

 a considerable magnetic effect ; and the susceptibility of those that 

 occur in the Coal Measures is above the general average. Further, 

 no other rocks that are known to occur, or are likely to occur 

 under the area, have susceptibilities as high as the dolerites found 



