132 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
of the other minerals, but rather irregularly distributed. It constitutes a little under 
1 per cent, of the rock. . 
The pink variety of the granite shows no essential difference as far as the micro¬ 
scopical characters are concerned, but its susceptibility appears to be lower than that 
of the grey granite. 
The granite differs from the dolerites described above and from the granophyres 
described below (p. 134) in the way in which its TiOa is combined. In the dolerites 
and in the granophyres a large part of the TiOg has combined with FeO to form 
ilmenite, while in the dolerites a part of the TiOg may also be present in the augite 
molecule. But in the granites part of the TiOg entered the biotite, from which, as 
the result of secondary changes, it has since separated as rutile. The remainder 
of the TiOg entered into combination with CaO to form sphene, although the CaO 
percentage in the granite is less than that in the dolerites. The result is that 
ilmenite is absent from the granite. 
The basic granite of Brazil Wood is a marginal facies of the main mass of the 
Mount Sorrel intrusion. In the hand specimen it is finer grained and much darker 
than the normal granite, resembling rather a dolerite at first sight. Occasional pink 
felspars approach in size the felspars of the normal pink granite, giving a pseudo- 
porphyritic character to the rock. Under the microscope the constituents are seen 
to be the same as in the normal granite, but plagioclase and the dark minerals, 
especially hornblende, are present in greatly increased amount and are more idio- 
morphic than in the parent rock, while orthoclase and quartz are in decreased 
amount, being reduced to interstitial constituents, the quartz sometimes acting 
poecilitically as host to all the other minerals. The plagioclase is zonal and is 
slightly more basic than that in the normal granite, having the composition of an 
andesine-labradorite, approximately AbiAnj. TiOg is again combined with CaO to 
form sphene. 
Original iron-ores occur (l) as distinct granules of magnetite as in the normal 
granite, and (2) as a fine dust irregularly distributed in the other minerals, and 
abundant in certain areas, but absent from some of the larger felspar crystals. 
This dust may consist in large part of sulphides, not of magnetite. Pyrites also 
occur aggregated in patches as in the normal granite and is probably formed by 
secondary changes. On account of the fine state of division of much of the iron- 
ore it is not possible to get a satisfactory estimation of the amount of magnetite 
present. The susceptibility is considerably lower than that of the typical grey or 
pink granite. 
Basic patches or “ heathen ” are abundant in both types of the normal granite. 
They are of all shapes and sizes, and in the hand specimen the junctions with the 
normal rock appear to be quite sharp. They are darker in colour and much finer 
grained than the surrounding matrix of normal granite. 
Under the microscope the texture is seen to be quite different from that of the 
