338 Ernest W. Skeats : 



Fig. 2) it is seen that the original texture of the diabase is almost 

 completely lost owing to secondary silicification. In one place, 

 howe^'er, an original vesicular structure is still visible. The 

 secondary silica occurs both as allotriomorphic quartz crystals, 

 but maiinly in the form of radial chalcedonic aggregates. In 

 ordinary light dots of opaque iron oxide separate the radiating 

 fibres, and in polarised light the fibres give a black cross. 

 Scattered through the i-ock are long prisms Avith hexagonal cross 

 sections, which have precisely the habit of the corundum 

 previously described. The corundum has, however, been almost 

 entirely replaced by secondairy minerals, including opaque oxide 

 of iron, chalcedony, chlorite amd carbonates. This is only the 

 second record of corundum in Victoria, apart from its occurrence 

 in the deep leads and alluvial deposits. The first occurrence 

 was noted last year in association with the serpentine of the 

 Mt. Wellington district, in Gippsland. This occurrence has bee-n 

 described by Mr. Dunn^ amd by Mr. Thiele.2 The corundum 

 occurs as blocks on the surface of the serpentine, and may be 

 " in situ," but this cannot be proved. 



This occurrence of corundum at Heathcote appears to be the 

 fii'st in Victoria in which it has been traced to its parent rock. 

 Its crystallization evidently took place from a part of the diabaise 

 magma, not only locally rich in alumina, but also in chromium, 

 and it may represent a local segregation from the normal diabase. 



(c) The Selwynite and its Origin. — This substance, described 

 as a mineral, was first referred to in the Exhibition Essay for 

 1867. It was more fully dealt with by Ulrich.l Four quanti- 

 tative anal}''ses were made by Cosmo Newberj^ and one is here 



recorded. 



SiO, - - - - 47.15 

 AlA- - - - 33.23 



CrA- - - - ^-61 



MgO - - - - 4.56 



Na^O - - - - 



H,0 - - - - 6.23 



Total - 98.78 



1 ' Minin<r standard," Oct. 16th, 1907. 



2 Proc. Roy. Soc. Victoria (this volume). 



3 Contributions to the Mineralog:.v of Victoria, 1870, pp. 21-2;'). 



