108 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
magnetite present is not a measure of the basicity of the rock. For example, one of 
the most basic rocks—the hornblende-diorite—contains the smallest amount of 
magnetite and accordingly gives the lowest susceptibility. Again, it is noticeable 
that magnetite is not necessarily present in increased amount in the basic patches, 
or “ heathen,” as compared with the normal granite, hut tests on other specimens are 
needed before we are justified in analysing the results in detail. 
(f) Dolerites Intrusive into Coal Measures. 
The susceptibilities of the dolerites mentioned above (p. 101) are as follows :— 
Ophitic analcite dolerite, Kelham. 
,, ,, ,, Whitwick. 
Basaltic dolerite, lower portion, Southwell .... 
Basalt, upper portion, Southwell. 
Ophitic dolerite, centre portion, Owthorpe .... 
Basaltic dolerite, marginal portion, Owthorpe . 
Basaltic dolerite, Bowley Begis. 
„ Glee Hills. 
of 45 dolerites and basalts from various 
parts of the British Isles. 
Thus the olivine dolerites which occur in the Coal Measures have in general a 
high susceptibility, averaging about 400, but in some cases they exceed that figure. 
These high values are evidently due to the presence of magnetite in considerable 
amounts in the rocks. The lower figures sometimes given by apparently similar 
rocks are explained by the fact that in such rocks the iron-ore is in the form of 
ilmenite instead of magnetite. 
In the dolerites and similar basic rocks the two minerals, magnetite and ilmenite, 
are frequently intergrown, t but the amount of each present in the intergrowth varies 
considerably. It is this fact that causes the wide difierences observed in dolerites 
from difierent localities, and even in different specimens from the same mass, as in the 
Owthorpe example. BtioKER and White* determined the susceptibilities of four 
specimens from different parts of the Whin Sill (quartz-dolerite) as :— 
263, 280, 345, 541, 
whereas the petrologically similar rock at St. Davids had susceptibility 0. 
* ‘ Roy. Soc. Proc.,’ vol. LXIII. (1898), p. 466. 
t J. J. H. Teall, “ On Chemical and Microscopical Characters of the Whin Sill,” ‘ Q. J. G. S.,’ vol. XL. 
(1884), ji. 651, -ivith references. E. B. Bailey, “The Geology of the Glasgow District,” ‘Mem. GeoL 
Survey,’ 1911, p. 128. 
279 
391 
210 
7 
9 
434 
454 1 
443 ! 
Figures by 
Prof. Wilson 
(p. 84). 
255 
Figures by 
V BticKER and 
White.* 
