304 



J. A. THOMSON. 



leucoxenic alteration on the borders of some grains. A 

 small amount of secondary magnetite is found in the olivine 

 of some of the specimens. 



Olivine occurs plentifully in the more basic rocks. Some- 

 times it is clear, with only a slight separation of iron ores 

 along the cracks, and is a variety with an axial angle 

 approximating 90° (No. 7), and occasionally shows a slight 

 schiller structure (No. 11). In No. 14 it is of a violet colour 

 and is always surrounded and apparently partially replaced 

 by iron ores. In the other rocks, when present, it shows 

 an incipient or complete alteration into a brown or green 

 biotite-like mineral of high birefringence that may be 

 referred to iddingsite, accompanied in No. 12 by a consider- 

 able amount of iron ores and talc. The olivine is sometimes 

 of early crystallisation (Pig. 1), but is sometimes ophitic to 



the plagioclase (Figure 6, 

 Plate XIV). It never 

 shows perfect crystal out- 

 lines, but occurs in more 

 or less embayed forms, 

 which are sometimes cre- 

 sentic when the mineral 

 is enclosed in a pyroxene. 

 There is in places a con- 

 siderable amount of hyper- 

 sthene interposed between 

 the olivine and felspars. 



The pyroxenes are very 

 interesting in their rela- 



Fig. 1. — Olivine dolerite with 

 hypersthene and enstatite-augite. 

 No. 13. Cavanagh Range. Crossed 

 nicols. Magnification 15 diams. 



tions. A rhombic pyroxene referable to hypersthene oti 

 account of its optically negative character but with some- 

 what variable intensity of pleochroism, is abundant in the 

 basic rocks. It is usually subordinate in amount to the 

 monoclinic pyroxenes, but in No. 7 it predominates in 



