AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 129 
Iron-oxides and apat. j are abundant, and considerable areas of the slide are occupied 
by granular turbid semi-opaque analcite.”'* 
The susceptibility for the typical Whitwick rock is 391x10“^ as compared with 
the values for the marginal and central portions of the Owthorpe dolerite 434 and 
9 X 10“® respectb^ely. The diflPerences in the figures are evidently accounted for by 
difierences in the amounts and composition of the iron-ores in the three rocks. These 
differences are well brought out in the description of the Owthorpe dolerite by 
Dr. H. H. Thomas as follows :— 
“ In the hand specimen the typical rock (E. 6520) taken from well inside the 
mass, at a depth of 1966 feet from the surface is grey in colour and moderately 
coarse in grain ; crystals of augite and felspar are visible to the unaided eye. 
Towards its upper surface the mass becomes much darker, finer in grain, and more 
compact (E. 6518-19). A specimen taken from about nine feet below the upper limit 
at 1953 feet from the surface is almost black in colour and evidently rich in iron-ores. 
“Under the microscope the coarser rock (E. 6520) proves to be an ophitic olivine 
dolerite, composed of augite, olivine, plagioclase-felspars, and accessory iron-ores. 
“ The augite which is evidently a highly-titaniferous variety has a rich plum- 
colour, is pleochroic, and forms ophitic plates, enclosing and being penetrated by the 
felspar laths. 
“ Olivine replaced by calcite pseudomorphs occurs somewhat sparingly in granules, 
but a few idiomorphic crystals were noticed. 
“ The felspars apparently are all basic plagioclase approximating labradorite ; they 
are zoned, and occur chiefly as much twinned narrow laths. The iron-ores were 
originally almost all ilmenite, but are now converted into leucoxene pseudomorphs. 
“ There is very little base, and practically no apatite. A few cavities have been 
filled with radiating masses of calcite having a nucleus of silica. 
“ The finer-grained rock (E. 6518-19) presents properties which would without 
any other evidence lead us to regard it as occurring nearer to the margin of the mass. 
The minerals are the same as those in the coarse variety described above, but there 
are certain differences which have a bearing on the origin of the rock. It contains a 
large amount of glassy base rendered almost opaque by rods and skeleton-crystals of 
iron-ores. Olivine is more plentiful, and there is an increased prevalence of crystals 
with good outline. The augite occurs in smaller plates, and there is a marked 
tendency towards idiomorphism. 
“ Small chlorite-lined cavities contain colourless isotropic analcite, while a good 
deal of chlorite and serpentinous material has been localised in the decomposing 
felspar. 
“ From the above characters, especially the apparent segregation of the olivine and, 
iron-ores towards the margin of the mass, it would seem that the rock is intrusive in 
character. 
* “ Geology of the Leicestershire Coalfield,” ‘ Mem. Geol. Survey,’ 1907, p. 35. 
VOL. CCXIX.-A. 
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