AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 1 I 5 
Olivine-dolerite, Rowley Regis, South Stafford- ^ 
shire. 454x 10“’'' I 
Olivine-dolerite, Glee Hill, Salop 
445x 10“® Determination by 
,, „ (with nepheline), intrusive in ' 
Trias, Butterton, North Staffordshire . . . 236x 10“’® 
Average of 45 dolerites and basalts from various 
parts of the British Isles. 255x10“’® 
Rucker 
White.* 
and 
Rocks of such high susceptibilities are certain to cause magnetic disturbances. 
Further, such rocks need not occur in exceptionally large quantity in order to cause 
disturbances such as those at Rempstone and Melton, measured by I70y. 
A difficulty arises from the fact that the central coarser portion of the Owthorpe 
dolerite shows the extremely low susceptibility 9x 10“’®. This value is exceptionally 
low for a doleritic rock, but it may be purely local and due to the particular form 
taken by the iron-ores present in the rock (see p. 105). The figure will also be 
brought belovf the normal because of the extent to which the rock is impregnated 
with calcite. The figure should be compared with the higher values 391, 279, and 
210x10“® for the Whitwick, Kelham, and Southwell dolerites, and with 255 x 10“’® 
the average value determined for basaltic rocks by Rucker and White. 
(vi.) Intrusions being of quite local character, and often irregular in shape, are 
more likely to cause sharp changes in the character of the magnetic disturbances 
than sedimentary rocks occurring as horizontal or gently dipping sheets. 
(vii.) Further, it is a well-known fact that in many areas the distribution of 
intrusions is intimately related to the faulting, and there is evidence that similar 
relationships hold in Nottinghamshire and Leicestershire, so that it is quite possible 
that the intrusions really represent fault-intrusions. In such case the sharp nature 
of the changes in the character of the disturbances between Rempstone and Lough¬ 
borough, and again between Melton and Asfordby, might be readily explained. 
The point will be further elaborated below. 
4. The Relationships between the Faults, the Intrusions, and the 
Magnetic Disturbances. 
If it be granted that there is a strong probability that the origin of the magnetic 
disturbances is to be looked for in dolerites associated with the buried Coal Measures, 
it becomes necessary to enquire why they should only cause marked magnetic 
disturbances when they occur in the neighbourhood of faults. Intrusions may be 
related to faults in one of three ways ;— 
(а) The Intrusions may be earlier than, and if so, may be displaced by, the faults. 
(б) The intrusions may be fault-intrusions, that is, they may have arisen along the 
fault-planes during or after the progress of the earth-movements. From the fault- 
‘ Roy. Soc. Proc.,’ vol. Ixiii. (1898), p. 466. 
Q 2 
