AND LETCESTEESHIEE AND THEIE EELATIONS TO GEOLOGICAL STEUCTUEE. 113 
Thus, in every case at which a marked magnetic disturbance was recorded, the 
station stands on or near a line of fault or its probable prolongation. Moreover, it is 
suggestive that of these stations the one recording the smallest disturbance, that is. 
Pen Hill, should stand on the fault of smallest throw. 
3. The Probable Effect of Dolerite Intrusions. 
Faults could only cause magnetic disturbances if (l) they dislocate rocks of high 
susceptibility, or (2) if rocks of high susceptibility have invaded the fault-planes. 
It has been suggested by Naumann* that the connexion sometimes observed 
between lines of faulting and of magnetic disturbance depends not on rock-magnetism 
but on the elfects produced on earth-currents by dislocations of the strata. This 
view was, however, opposed by Rucker and ThorbeI on the grounds that the 
intensities and directions of flow of earth-currents as measured between various 
localities, including Melton Mowbray and Asfordby, were not such as could cause 
the observed magnetic disturbances. On the other hand, these authors showed that 
at other localities the susceptibility of certain igneous rocks was such that, assuming 
the rocks to be magnetised by the earth’s induction, the magnetic disturbances would 
be explained. 
The susceptibility determinations made by Prof. Wilson (p. 84) indicate that 
such olivine-dolerites as are known to occur in the Coal Measures of the concealed 
coalfield of Nottinghamshire have higher magnetic susceptibilities than any other 
rocks known in the district. It is, therefore, necessary to survey the evidence bearing 
upon the possible extension of such olivine-dolerites under the Melton Mowbray area, 
and their probable relations, if present, to the faulting and other tectonic features of 
the area. 
(i.) A dolerite, intrusive into the Coal Measures, has been proved by boring to 
occur at —1750 feet O.D. at Oivthorpe, three miles north-west of Pen Hill, which is 
a centre of minor disturbance. It is true that the thickness of dolerite passed 
through in the Owthorpe boring was only 40 feet, but the mass may well thicken 
laterally and so be the originator of the Pen Hill disturbance. It is noticeable that 
at Pen Hill the ^'relative” horizontal disturbing forces are directed straight at 
Oivthorpe with its dolerite intrusion. 
Similar dolerites have been proved again by deep bores (p. 98) to occur in the 
concealed Coal Measures at several localities, Southwell, Kelham, and Whitwick, all 
within 20 miles of Melton Mowbray. Also similar rocks are common as intrusions in 
the Coal Measures of other districts such as South Staffordshire, Shropshire, North¬ 
umberland, and Scotland, passing in places below the Coal Measures into the 
Millstone Grit, Carboniferous Limestone, or even with the Old Red Sandstone. 
* ‘ Die Ersclieinungen des Erdmagnetismus,’ Stuttgart, 1887. 
t ‘Phil. Trans.,’ Ser. A, vol. 181 (1890), p. 315. 
VOL. CCXIX.-A. 
Q 
