DESCRIPTION OF THE CHIEF TYPES. 167 



The augite is almost colourless and is far less abundant than 

 the brown hornblende, which is somewhat similar in its optical 

 characters to the hornblende occurring so commonly in the associated 

 norites, Crystals of the latter mineral give a maximum extinction 

 angle on (010) of 16 and are strongly pleochroic — 



a = pale yellow. 



fc = deep brown. 



t = deep greenish brown. 



b = £>a. 

 They are often schillerized by black needles and plates, the 

 needles being generally arranged parallel to a direction of extinction 

 and therefore inclined to the cleavage cracks. 



Olivine appears to be irregular in its distribution in these 

 rocks. In some slides (No. 1438) it appears in considerable abund- 

 ance as small, irregular crystals, which are cracked in characteristic 

 fashion, with formation of a yellowish-green serpentine. The high 

 index of refraction, strong double refraction, absence of colour and 

 the characteristic fracture and secondary alteration leave no doubt 

 as to the identity of this mineral, although its appearance as an 

 accessory mineral in pyroxenite is so exceptional. In a rock, 

 however, in which the large predominance of hypersthene shows 

 that the percentage of alumina is low, whilst the protoxides of iron 

 and magnesia are in large quantity, the appearance of olivine in the 

 least siliceous types would naturally be less surprising than in those 

 in which the predominating pyroxene is of the aluminous, mono- 

 clinic varieties. The present instance, therefore, forms an interest- 

 ing example of a link between the pyroxenites and the peridotites. 



Green spinel similar to pleonaste and hercynite in appearance 

 is very commonly found in these rocks, and appears to be a common 

 constituent or associate of such pyroxenic rocks all the world over. 

 As a constituent it appears in the pyroxene rocks and sometimes 

 in the basic norites. As an associate it occurs amongst the products 

 (apparently) of contact metamorphism ; under the latter circum- 

 stances it is a very constant associate of corundum. The late G. H. 

 E ( 49 ) 



