29 



process, are well seen, and recall the well-known appearances 

 in the troctolite of Volpersdorf . Where the olivine has been 

 completely serpentinised, the resultant substance is inert on 

 polarised light. 



3. Pyroxenite. 



Sp. Gr. 3-10. 



This rock is exclusively a pyroxene ; it contains no olivine, 

 and consequently conies within the group pyroxenite, as 

 established by Dana in L880. Pyroxenites are considered as 

 intimately related to the gabbro family ; and as pei'idotites 

 are ultra basic olivine representatives of gabbro, so we may 

 call these pure pyroxene rocks the olivine-free ultra basic 

 members of that family. They vary greatly, and the varieties 

 have been classified by G. H. Williams as websterite, bronzitite, 

 hypersthenite, diallagite. The Heazlewood rock appears to 

 occupy an intermediate position between the first two divisions. 

 Websterite typically consists of a rhombic pyroxene and augite 

 (or diallage) in equal quantities, but varies into bronzitite or 

 hypersthenite by losing its augite, or, on the other hand, the 

 rhombic pyroxene disappears and the rock becomes diallagite. 

 Our rock in thin section shows nearly exclusively large crystal 

 faces of enstatite, accompanied sometimes by a more ferriferous 

 variety (bronzite) with characteristic wavy cleavage lines and 

 bronzy lustre. There is, besides, a small quantity of laminated 

 augite. What is known as the mortar structure may be seen, 

 which is very common in this type of rock. The large crystals 

 are separated from each other by a channel filled with as 

 breccia of small crystal fragments of enstatite, a colourles 

 non-striated augite (diopside?), and sometimes with brightly 

 polarising talcose matter, which has infiltrated by cracks into 

 the substance of the large enstatites. The development of 

 this structure is due to rock crushing, and the appearance of 

 talc is a frequent feature in altered pyroxenites. The rock is 

 evidently a passage one between websterite and bronzite. 



MacroscopicaUy the rock is seen to be composed of closely 

 packed lamellar crystals of considerable size ; those of the 

 principal essential, enstatite, sometimes reaching several 

 inches in length and proportionate width. In colour the 

 mass of the rock does not vary to any material extent ; it is 

 in a general way of a greyish green to pale brown hue, with 

 occasional patches of a fairly bright green. The uniform 

 lustre is silky, vitreous, and slightly metalloid, but when 

 individual crystals are exposed parallel to the face of the 

 rhombic prism they are usually highly polished, and this 

 character lends itself to add to the peculiarly attractive ap- 

 pearance of the mass. The rock is very tough under the 

 hammer, with a hackly fracture when broken across the 

 embedded crystals. The interstices sometimes show bright 



