130 DR. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
“ Compared with the rock met with in the shaft of the Whitwick Colliery, the 
Owthorpe dolerite presents a general similarity ; there are, however, several features 
in which it differs. It has a smaller proportion of olivine, and this mineral at 
Whitwick is altered to serpentine instead of being replaced by calcite. The Owthorpe 
rock has a deeper coloured augite, less apatite, more iron-ores and more ilmenite than 
magnetite”^ 
The rock from the Southwell boring (p. 101) also shows a considerable variation in 
different parts of the mass. A specimen from the lower portion (L. 113) is a granular 
non-porphyritic olivine dolerite or basalt. Olivine is abundant in large sharply 
idiomorphic crystals now mainly replaced by serpentine; it includes scattered 
granular and idiomorphic iron-ores. Augite is likewise abundant in small crystals, 
some idiomorphic, others with only the prism faces developed, and others quite 
irregular in shape. It is pale green in colour and there is no marked dispersion of 
the opti axes. Plagioclase gives the usual lath-shaped sections with no sign of flow- 
structure. It is strongly zonal, the average composition being that of an acid 
labradorite between AbiAn^ and AbgAiig. It is remarkably fresh and shows in this 
specimen no sign of analcitisation. There is also a green pleochroic chloritic material 
with fairly high birefringence occurring in radiating fibrous masses of later formation 
than the felspars. Where only present in small patches, and also along the borders 
of the larger areas, the substance is rendered almost opaque by iron-ore microlites, 
but the centres of larger areas are remarkably free from iron-ores. The substance is 
probably analogous to delessite and represents an altered glass. Calcite is also of 
local occurrence as an interstitial material, or as replacements of original minerals. 
The iron-ores occur ’mainly as long rods with ragged edges cutting the other 
minerals. Occasionally the rods swell out into larger masses which have variable 
relations to the other minerals, sometimes being posterior to felspar and to augite. 
There is evidently a larger relative amount of the ilmenite molecule present in the 
Southwell rock as compared with the Whitwick rock (p. 128). The iron-ores comprise 
about 6 per cent, of the rock, the susceptibility being 210 x 10“^ 
Other slides (E. 11,442-11,444) taken from nearer the top of the mass show rocks of 
different aspect. The rock is now considerably altered, hand specimens having a light 
green colour instead of the usual dark basaltic appearance. The rock is a vesicular 
fine-grained olivine-basalt, consisting of decomposed basic plagioclase felspars, often 
with skeletal terminations, set in a once glassy base which is full of skeletal growths 
of felspar, and rendered almost opaque by finely divided iron-ore. The olivine has 
completely altered to serpentine, and any augite that may be present has also been 
completely altered. The percentage of iron-ore cannot be estimated under the 
microscope on account of the fine-grained character of the rock. The susceptibility is 
7 X 10“^, the low value probably being due to the altered condition of the rock. 
* “ The Geology of Newark and Nottingham,” ‘ Mem. Geol. Survey,’ 1908, p. 19. 
