306 J. A. THOMSON. 



only by Osann 1 in a Tasmanian dolerite. They are fairly 

 common, according to my observations, in the Mesozoic 

 dolerite sills of Tasmania, specimens of which have been 

 kindly given me for this purpose by Mr. Twelvetrees, 

 Government Geologist, and slides prepared by Mr. ^ 

 Priestly. They are also not uncommon among the later 

 dykes of dolerite and quartz-dolerite traversing the gold- 

 fields of Western Australia 



Wahl 2 has also pointed out the presence of intergrowths 

 of various pyroxenes with one another, and after the analogy 

 of the felspar group has given them the name of pyroxene-, 

 perthites. As it has not been possible to obtain a copy of 

 this paper in Sydney, the writer cannot make further com- 

 parisons between those observed in these rocks and those 

 described by Wahl. 



As the size of the optic axial angle can be observed only 

 on suitably oriented sections, it is impossible by its obser- 

 vation alone to ascertain the relative amounts of common 

 augite and eustatite augite. Moreover it is not uncommon 

 to find that there are great variations of axial angle in the 

 same crystal. Enstatite augite has often a characteristic 

 basal striation and a peculiar alteration along the basal 

 plane, which is sufficient to distinguish it from augite in 

 the absence of such a striation in the latter. In the rocks 

 under consideration, hypersthene curiously enough never 

 shows schiller structures. The monoclinic pyroxenes are 

 also in the finer grained rocks quite free of any such struc- 

 tures, while in the coarser rocks, in which rod-like inclus- 

 ions both parallel to the A-pinacoid (diallagic schillerisation) 

 and parallel to the basal plane are abundant, no distinction 

 can be made out between the enstatite-augite and common 

 augite in this respect. 



1 Osann, A., Ueber cinem Enstatitaugit Piihrenden Diabas von Tas- 

 manien. Centralbl. F. Min. etc., No. 23, pp. 705-11, 1907, 



2 Wahl, W., Analogien Zwischen die Pyroxen und Felspathgruppe. 

 Quoted from memory. ? Ref. a Finnish Journal. 





