28 



theoretically non-pleochroic, bronzite slightly so in the slices 

 usually prepared for the microscope. The colour, pleochroism 

 and enclosures, point to the mineral being the ferriferous 

 enstatite called bronzite. The boundary between enstatite 

 and bronzite is vague : there has been great divergence 

 of opinion as to wha/fc constitutes enstatite and what 

 bronzite. Professor Eosenbusch f unites them, while pre- 

 serving their separate names. Generally enstatite is applied 

 to the non - ferriferous and bronzite to a ferriferous 

 variety. In the rock from the Heazlewood the colour 

 and pleochroism which are sometimes relied upon as 

 distinguishing characters J vary even in the same plate. 

 It will suffice, however, if we assign to this mineral the 

 name bronzite. Everywhere it encloses rounded serpentinised 

 grains of olivine, and has familiar interpositions, consisting of 

 lamellar intergrowths of nionoclinic pyroxene. In some places 

 it is in course of change into its serpentinous modification, 

 bastite, in which there is some general approach to parallelism 

 of the fibres, quite distinct from the open meshed arrange- 

 ment of the serpentine in the olivine crystals. 



Felspar. — This is abundant in the broad forms usual in 

 gabbros. The albite plan of twinning prevails and the extinc- 

 tion angles exceed 40°, pointing to anorthite. The pericline 

 type of lamellations, traversing the albite twinning lines, is 

 also observable. The felspars often not only penetrate the 

 pyroxene, but are to be seen occasionally wholly included in 

 its substance. They frequently enclose olivine grains, and 

 consequently the order of consolidation of the three constitu- 

 ents of this rock is very plainly revealed. They are often 

 traversed by cracks filled with serpentine proceeding from 

 included and adjacent olivine grains, but whether the fels- 

 pathic substance itself has been attacked and serpentinised is 

 doubtful. On the theoretical possibility of this see Teall, 

 British Petrography, p. 107. 



Olivine. — This appears to have been the first mineral to 

 separate, and it occurs chiefly in the form of rounded grains, 

 sometimes in nests, but also scattered abundantly through 

 the rock. This mineral is always meshed with serpentine, and 

 is enclosed in the pyroxene and felspar, the latter also being 

 often embraced by the bronzite. It illustrates the lustre- 

 mottling structure in a very striking way. The olivine is 

 colourless, the serpentine reticulations pale green and greenish 

 yellow, and their course is often indicated by magnetite 

 grains. The fibrous serpentine forming the meshes polarises 

 faintly and extinguishes at 90°. The cracks in adjoining 

 crystals, due to expansion, caused by the serpentinisation 



t Mikroskopische Physiogr : der Petrogr. wichtigen Mineralien, 1892, p. 449. 

 t Hatch. Intr. Study of Petrology, 1891, p. 43. 



