114 



In general the investigation of the felspar in this group 

 of rocks is I'endered somewhat difficult, especially when the 

 crystals are untwinned, by the presence between the diflferent 

 grains of their films of chlorite or other secondary material 

 which obscures the bright line test for relative refractive 

 indices, while in most cases every felspar present has a refrac- 

 tive index below that of Canada balsam. 



Following the felspars the most important mineral is a 

 ■pyroxene. It occurs in oval, oblong, or less regular grains, 

 up to 5 mm. in length, sometimes also idiomorphi- 

 cally, the elongation of the grains being usually roughly 

 parallel to the general direction of banding of the rock. In 

 colour the mineral is a pale-green ; by transmitted light, how- 

 ever, it is almost colourless and non-pleochroic. It is in only 

 a few specimens tliat the pyroxene remains in its 

 primary state. It is then a diopside with an ex- 

 tinction angle c' to t of 40°. Chemically it must be 

 chiefly magnesian, iron being present in but small amount. 

 Generally, however, the diopside has been changed to uralite. 

 This change takes place along the cleavage of the diopside 

 grains, the amphibole fibres being parallel to the vertical 

 axis of the pyroxene crystals. The colour changes to a light 

 or rarely a strong green with a faint or marked pleochroism, 

 namely- — 



a light-yellowish green 



ir green 



t green 



absorption li < b = C 



while the extinction angle falls to 22°. This indicates that 

 actinolite is the amphibole developed. Basal sections of the 

 uralite are generally rather confused, but appear to show a 

 pyroxene cleavage net still remaining. 



That the change to amphibole is not a true example of 

 paramorphism is indicated by the presence in the altered 

 rocks of much secondary epidote and some highly refracting 

 colourless grains, probably calcite. 



In the jDrocess of change there is an increase in volume, 

 and outgrowths frequently occur of actinolite fibres parallel 

 to those in the place of the original crystals, but extending 

 beyond their boundaries. 



The diopside and uralite often occur in association with 

 the titaniferous magnetite or ilmenite ; sometimes there is a 

 dusting of secondary ilmenite included in them. In general, 

 however, the primary iron ore appears to have crystallized 

 after the pyroxene. 



