most abundant ; but pyrite, chalcopyrite, and more rarely 

 leucopyrite, occur in patches of variable size throughout the 

 rock. These are decomposed on the surf ace to iron-oxide, 

 thin films of native copper, and small quantities of the car- 

 bonates of the same metal. In thin sections, when examined 

 under the microscope, the bulk mass of the rock is seen to 

 consist of axinite, augite, and calcite, with some hornblende. 

 The other minerals appearing in lesser quantity are quartz, 

 chlorite, actinolite, tourmaline, ami granular sphene. 



Details of Micro Examination : Axinite. — This is in large, 

 irregular, and also sharply defined crystals, in section of a 

 pale lavender colour to a deeper shade of the same tint. 

 Interference colours, lavender, yellow, blue, sometimes inter- 

 penetrating twins. Cleavage lines irregular, Pleochroism 

 scarcely perceptible. The axinite has enclosures of quartz 

 and fibrous augite, and has been replaced occasionally by 

 clear quartz and vermicular chlorite, the latter light green in 

 colour, pleochroic, showing fixed dark cross, and polarising 

 steel grey. The quartz is very clear, and contains small 

 prisms and needles of strongly absorptive tourmaline. The 

 tourmaline-bearing quartz is probably original, and the tour- 

 maline may be looked upon as resulting from the same boracic 

 acid emanations which were involved in the crystallisation of 

 the axinite. Where the quartz is secondary, replacing axinite 

 and augite, it contains long needles of actinolite. 



Augite. 



This mineral is in large plates and crystals, sometimes 

 twinned. The sections out of the zone of the vertical axis 

 give an extinction angle as high as 40deg. The augite is 

 changing to hornblende ; sometimes it is quite uralitic, i e., 

 the crystal form is still that of augite, though its substance 

 has been converted into fibrous hornblende. The substance 

 has often been replaced by calcite and quartz. Iu the 

 granite contact zone of Cornwall the augite of the foliated 

 diabase is often uralitic, becoming hornblende at the margins, 

 whereas in the Colebrook rock it has suffered uralitisation all 

 through in patches. The uralitic fibres, greenish in colour, 

 are parallel and pleochroic, and between them are parts of 

 the non-pleochroic augite which have remained unaltered. 



Hornblende. 



There are a few lozenge-shaped sections, colourless without 

 the cleavage lines or pleochroism of hornblende, but which 

 appear to be that mineral. They resemble many of the 

 sections occurring in amphibolites. 



