RocJ:!^ near HeaAJtcote. 339 



Traversing the gi'een " sehvynite " are veins of an almost 

 colourless niicaioeous mineral, described by Ulrich as " talcosite." 

 An analysis by Newbery gave the following results : — 



SiO, - - - - 49.01 



AlA- - - - 45.1 



Cr.,0, - - - - tr 



FeO - - - - tr 



MgO - - - - tr 



H.,0 - - - 4.98 



Total - 99.09 



It has been recognised for many years that the substance, 

 "selwynite," i.s not a true mineral species, but a mixture, and is, in 

 fact, a rock. Microscopic examination of a thin section of the sub- 

 stance (No. 566) (Plate XVI., Fig. 3) confirms this view, and shows 

 that at least four minerals are piesent. The groundniass forniing 

 the bulk of the rock is a mineral which is pro'bably chrome- 

 bearing, and imparts the green colour to the rock in the hand- 

 specinien. It occurs as a granular or scaly meshwork of micrti- 

 •scopio colourless crystals, polarizing in neutral tints as an 

 aggi'egate. Scattered through this background are a few opaque 

 grains, probably chromite, and mamy larger granular cry^stals of 

 a mineral showing rather high refraction, purplish-brown colour 

 and p(jlarization colours of the second order similar to augite. 

 The mineral is slightly pleochroic, and may be an altered. orthor- 

 hombic pjTOxene. The remaining mineral, the so-cailled " tal- 

 cosite," occurs in a vein through the rock, and consists of 

 radiating and parallel prismatic crystals of a colourless to 

 pale-gi-een micaceous mineral showing high polarization colours. 



Corundum, while not identified with certainty in section, 

 occurs with the selwynite in the outcrop by the Murray road, as 

 mentioned above. 



The seh\ynite out-crop, while near the junction of diabase 

 and Silurian on the surface, is certainly within the dial)ase 

 boundary, and there can be little doubt but that it represents 

 one type of alteration of a diabase rock formed from a magma 

 locally rich in chromium and aluminium oxides. Solutions 

 passing through this rock have leached from it most of the lime 

 xind magnesia, and have introduced water, leaving a rock con- 



