11 H DR. A. H. COX ; REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
planes the intrusions are likely to spread out into the surrounding strata wherever 
there was a plane of weakness to give passage to the magma. 
(c) There may be a combination of both the previous cases, due to earth-movements 
having been renewed at a later period along previously established lines of weakness. 
This last case represents the probable conditions in the Leicestershire and Notting¬ 
hamshire examples (see below, p. 117). 
In any event it can be shown that the faults would increase the power of the 
intrusions to cause magnetic disturbances. 
(«) Rucker and Thorpe* have shown by experiments that a normal sill or dyke 
may cause but little magnetic disturbance at a distance, even if composed of a 
susceptible rock. This is because the two sides of the intrusion, when parallel and 
not too far apart, neutralise each other’s effect at a comparatively short distance from 
the intrusive mass. For example, a dyke with parallel walls would give rise to 
a purely local vertical disturbance, but not to any appreciable horizontal disturbance. 
Thus intrusions might be present in non-faulted parts of the district without 
betraying their presence by any considerable magnetic disturbance. When, however, 
the intrusive mass is displaced by a faidt, new faces are produced and a magnetic 
disturbance is liable to be set up. The amount of disturbance will depend not only 
on the susceptibility of the rock, but also on the hade of the fault and upon its throw. 
(h) The second case, that in which the intrusions have arisen along the faidt- 
planes and have spread thence into the surrounding strata, may next be considered. 
Intrusions of this type are generally found to be less regular in shape than normal 
sills and laccolites. The more irregular the mass the greater is the probability, 
ceterihus parihus, of its exciting magnetic disturbances. Now the olivine-dolerites 
in the English Coal Measures, although frequently sill-like in habit, are so commonly 
found in the immediate proximity of faults that there would appear to be more than 
a chance relationship between the two. For example, the proximity of the South 
Staffordshire dolerites to the faults in that coalfield has already been mentioned. 
The Whitwick dolerite (p. lOl) apparently comes up along a line of north-west to 
south-east faulting which forms the eastern boundary of the Leicestershire coalfield.! 
Again, the occurrence of a north-west fault at Owthorpe, where a dolerite was proved 
by boring, has been described above (p. 112), while at Kelham a deep boring, almost 
certainly situated on an important line of faulting proved the presence of a dolerite 
which was pierced for 87 feet.J 
The close association of dolerites with faults is then so frequent that apparently in 
some cases the dolerites may act as fault-intrusions. As already stated, such 
intrusions are liable to be irregular, and hence to be potent causes of magnetic 
* ‘ Phil. Trans.,’ Ser. A, vol. 188 (1896), p. 6.39. 
t “ The Geology of the Leicestershire Coalfield,” ‘ Mem. Geol. Survey,’ 1907, p. 33. 
I A. Strahan : Lecture to Royal Institution, “ The Search for New Coalfields in England,” March 17, 
1916, p. 6. 
