118 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
displaced by later movements along the faults. Such displacement might either 
increase or decrease the magnitude of the magnetic disturbances according to the 
relations between the shape of the intrusive mass and the throw of the fault. 
To sum up, the evidence is all in favour of olivine-dolerites being the true 
disturbing agents, since such rocks are known to occur, to have a high susceptibility, 
and to be closely associated with faults. 
Faulting is, however, but one of many expressions of the yielding of rocks under 
the stresses to which they have been subjected, and we may well seek further light 
from the other tectonic features of the district with a view to accounting for the 
local character of the intrusions. 
5. The Steucture of the Concealed Coalfield. 
It will be noted that all the dolerites recorded from the Nottingham district have 
been discovered in deep borings into the concealed portion of the coalfield, whereas 
no single example is known to occur in the exposed coalfield, despite that in the 
nature of the case the exposed field is much better known than the concealed field. 
On this matter the structure of the coalfield has a decided bearing. The structure is 
explained by Dr. Walcott Gibson as due to the interaction of two movements of 
pre-Permian date. He writes : “ By far the most powerful thrust came from the 
east. It was accompanied, or ... . was preceded, by a secondary impulse from the 
south and south-east .... It is important to bear in mind that the impulse from 
the east was the most intense, and that to it is due the elongated north and south 
direction of the coalfield. At the southern end of the coalfield, east of Erewash 
Valley, it mastered the movement from the south, so that the east and west trend 
of the Coal Measures between Dale and Sandiacre is twisted round to the south-east, 
in which direction it is known that it extends past Buddington ....”* 
Structures ranging south-eastwards have been shown by Prof. FearnsidesI to 
play a prominent part further north in the coalfield between Mansfield and 
Sheffield. He has suggested that these structures were caused by the action of a 
pre-Permian thrust from the north-east which threw the Coal Measures into a series 
of folds arranged parallel with the north-west to south-east Charnian axis which 
lies to the south of the coalfield. He finds these south-east folds become more 
pronounced in the southern part of the coalfield, that is, in the part nearest to the 
main Charnian axis. As the south-east folds are followed towards the central 
trough-line of the coalfield, they curve round until they assume an approximately 
* Op. cif. 
t “ Some Effects of Earth-movement on the Coal Measures of the Sheffield District.—^Part II.,” ‘ Trans. 
Inst. Min. Eng.,’ vol. LI., part 3 (1916), p. 442 et seq. ■, see also Prof. Kendall in “Sub-report of the 
Concealed Portion of the Coalfield of Yorkshire, Derbyshire, and Nottinghamshire,” Appendix III. to the 
Geol. Committee’s Report in Part IX. of the ‘Report of the Royal Commission on Coal Supplies, 1905,’ 
pp. 18 seq. 
