AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 119 
east-west direction. The change of direction is explained as due to a lagging of the 
southern ends of the folds as they approach the resistant barrier of Lower Palaeozoic 
and older rocks which existed to the south. 
Thus one may expect the southern end of the coalfield to have certain characteristics 
of its own as regards tectonic structure. Now it is highly significant that it is 
towards the southern end of the coalfield and where the bending of the strike of the 
beds into a south-east direction becomes most pronounced, and along the continuation 
of this south-east line and in the district east of it, that the east-west and south-east 
to north-west faults attain their greatest development, that the Owthorpe dolerites 
have been discovered, and that the magnetic disturbances at .Pempstone, &c., have 
been observed. The inference is that the dolerites came up as a direct accompaniment 
or result of the south-easterly twisting, with concomitant south-east fracturing, of 
the Coal Measure strata. Hence the restricted distribution of the dolerites is 
explained. And hence, also, the magnetic disturbances in their turn may be 
ultimately connected with the change in the strike, while the change of strike 
suggests an approach to the southern limit of the basin of the coalfield. 
If at this point of the concealed coalfield there are local occurrences of these 
highly susceptible intrusive rocks, an explanation could be given of an apparent 
anomaly in the course taken by one of the “ magnetic ridge-lines” of Rucker and 
Thorpe.* 
This line swings from a north—south to a north-west—-south-east direction along 
a course parallel, although not actually coincident, with the swing of the rocks. It 
runs first of all north—south approximately coincident with the Pennine Axis. But 
at a point north of Bakewell, Derbyshire, the line begins to swing olf south-east¬ 
wards, crossing the strike of the coalfield obliquely to a point east of Nottingham, 
when it resumes its original north—south direction, and passes through Owthorpe and 
between Melton Mowbray and Rempstone. The course of this “ ridge-line ” may be 
attributed to the interaction of two quite different sets of disturbing agencies. 
(i.) In the northern part of its course along the Pennine anticlinal axis, the 
disturbances are probably caused by more ancient rocks l^rought nearer to the surface 
by folding, so that the anticlinal region dominates magnetically the country on either 
side, where the older rocks are more deeply buried. The association of magnetic 
ridge-lines with anticlinal structures among the Palseozoic rocks was noted in several 
other districts by Rucker and Thorpe.! 
(ii.) The swing of the ridge-line away from the Pennine anticlinal axis, and its 
oblique traverse across the Coal Measure strata is apparently due to the incoming of 
the intrusive dolerites into the Coal Measures in the area between Southwell, 
Kelham, Owthorpe and Rempstone, that is, the area in which the south-eastern 
strike and the east-south-east faulting set in. The magnetic effects due to the high 
* ‘Phil. Trans.,’ Ser. A, vol. 188 (1896), Plate 13. 
t Op. cit., p. 656. 
