128 



THE MINERALS OF TASMANIA. 



The writer has already expressed the opinion that pro- 

 bably but one shower of these objects occurred (Pro. Roy. 

 Soc. Tas., 1905) in pasi Pliocene times, which impinged 

 upon the earth in a generally north-eastern track crudely 

 extending from this island to western Victoria, and thence 

 to West i^ustralia, thus coinciding with their line of pro- 

 fusion as now found. 



Dr. Siiess, in his treatise, proposes to call these natural 

 glasses tektites, and to divide the group into three sections, 

 viz. — (1) Moldavites, for those so long known as occurring 

 in Bohemia. These are often fairly large, very irregular 

 in shape, and all deeply indented and furrowed. They 

 are of a clear transparent green, and have been used as 

 gems after passing through the lapidary's hands. (2) 

 Billitonites, for those from the island of Billiton, and also 

 occurring in South-East Borneo and Java. These glasses 

 are of medium size, oval form to almost round, with 

 strongly but irregularly and deeply grooved surfaces. They 

 are transparent, of a greenish-brown colour, in splinters, 

 and nearly colourless, in extremely thin flakes. They 

 would seem to vary in colour to a light-green. (3) Austral- 

 ites^-or, as termed by R. H. Walcott, F.G.S., obsidian- 

 ites (" The Occurrence of S'o-called Obsidian Bombs in 

 Australia," Pro. Roy. Soc. of Victoria, 1898). 



Finally it may be said to be now almost generally 

 accepted that these obsidianites or australites, with their 

 congeners, are of cosmic origin, and that they must be 

 accepted as acid meteorites hurled from space over the 

 regions indicated ; those occurring throughout the Com- 

 monwealth having their parallels in the billitonites and 

 moldavites, as referred to so briefly. 



227. Oligoclase (Poly silicate of Aluminium, Sodii/zn, and 



Calcium ). 



This IS a triclinic felspar that is intermediate between 

 albite and anorthite. It often accompanies orthoclase in 

 many varities of granite and trachytes, and is an essential 

 constituent in porphyrites and andesites. It occurs in 

 well-formed crystals in the alkaline rocks at Oyster Cove. 

 The plagioclase felspars generally are characteristic of 

 rocks of intermediate and basic composition. 



228. Olivine. (See Chrysolite.) 



This is a common accessory constituent in most Tertiary 

 basalts, and is an essential in the ultra-basic rocks of the 

 Heazlewood district. (" On some Igneous Rocks from the 



