36 



is noncrystalline, the rest of the field appearing to consist of 

 smaller felspars and augite, and feathery chlorite after augite. 

 This rock in its primitive state was an ophitic dolerite, but 

 it has undergone a thorough chloritic alteration. The term 

 diabase in Tasmania, as elsewhere, should be reserved for such 

 occurrences, and not applied to comparatively fresh rocks 

 like the mesozoic dolerite of Launceston. 



12. Websterite. 

 Sp. Gtr. 2-94. 



This is a second type of websterite, a species of the 

 plutonic group of distinctly pyroxene rocks, of which several 

 . peculiar varieties occur in the Heazlewood district. They 

 form, in conjunction with other members of this group, one 

 of the most characteristic features of the locality. They 

 are almost invariably intimately connected with tlfe serpen- 

 tine area, and consequently soon attract attention. That at 

 present under consideration has a strongly noncrystalline 

 granular structure throughout. The enstatite is readily 

 discernible scattered throughout the substance of the rock in 

 the form of translucent lamina3 of a yellowish-brown colour, 

 which have a pseudo-metallic lustre, in this respect closely 

 resembling its ally bronzite. The more plentiful monoclinic 

 diallage is apparently of a darker tint, with little or no pro- 

 nounced lustre. 



The two forms of pyroxene are closely packed together, so 

 that they give the fractures a distinctly roughened surface 

 by the projecting angles of the numerous crystals. The em- 

 bedded individual crystals of both pyroxenes are fairly 

 uniform in size, rarely exceeding 5 millimetres in length, and 

 are entirely without the scattered large flakes of diallage 

 which form such a pronounced feature in the variety of 

 websterite already described. This rock is sometimes found 

 to enclose elongated, crudely lenticular blocks of perfectly 

 serpentinised material of an intensely black colour ; these 

 often reach a considerable size, wdiich then tend to give the 

 mass a brecciated appearance. It is often met with in a par- 

 tially decomposed condition, in which case the components 

 are more readily reduced to powder, and the general appear- 

 ance of the rock is considerably altered. In this condition it 

 becomes of a dull greenish-brown colour, with here and there 

 thin patches of ferrous oxide, and has occasionally an indis- 

 tinct fissile structure. On weathering, the exposed surface of 

 the rock generally becomes of a dark rusty brown colour. 



Microscopical Structure. That of a pure pyroxenite, being 

 solely pyroxene, mostly monoclinic in elongated forms accom- 

 panied by enstatite in sufficient quantity to rank it as the 



