BY W. H. TWELVETREES AND W. F. PETTERD. 9 



idiomorphic columnar felspar here and there. It is essen- 

 tially a potash-felspar rock, but there is very little proper 

 orthoclase in it, for the orthoclase is intergrown perthiti- 

 cally or rather micro-perthitically with striped felspar, 

 often showing very fine striae indeed, giving the extinction 

 angles of oligoclase or oligoclase-allilte. These twinning- 

 lines are generally short, not continuous, and are sometimes 

 interrupted by similar sets at right-angles. Here and 

 there in the slide is water-clear albite, pellucid as qur.rtz ; 

 but apart from this, and an exceptionally clear crystal or 

 two of orthoclase, tire felspars are turbid. 



Some analcime in the felspars points to the former 

 l^resence of elaeolite, and its existence may be suspected, 

 though we cannot oj)tically demonstrate it. 



An important feature is the garnet, which is very plenti- 

 ful, in brown irregular grains and ill-formed crystals, 

 having a corroded appearance. These are characteristically 

 intergrown or associated with augite, biotite, and apatite, 

 and in their neighbourhood is often seen a yellowish 

 transparent haky or zeolitic-looking mineral, with low 

 refractive index, and giving in polarised light the appear- 

 ance of a soda decomposition product. The abundance of 

 garnet warns us that the rock is allied to the elaeolite- 

 . syenites. 



There is some grass-green augite (malacolite) in granular 

 irregular forms. It has slight pleochroism, and where 

 vertical sections could be found they gave extinction 

 angles of 33° and 34°. The mineral is often bleached in 

 the interior. Besides being specially intergrown with 

 garnet, it is associated occasionally with dark green horn- 

 blende. This hornblende is intensely absorptive. Its 

 pleochroism is a = yellowish-green, c = very dark green. 

 The 6 direction could not be ascertained. A little light 

 Iwown idiomorphic mica gave a = light greenish-yellow, 

 i) =■" dark dirty green. Zircon, present in all sj'enites, 

 occurs in small quantity, likewise a little sphene in 

 elongated wedges. Very little quartz can be definitely 

 recognised. 



This syenite cannot be considered quite identical with 

 any of those syenites, rich in alkali, which Rosenbusch has 

 erected into types under the names (after J. F. Williams) 

 Pulaskite, Albany, and (after Brogger) Laurvikite types : 

 but it is evidently related to these and to their allies, the 

 •elaeolite-syenites. The syenites which are known in other 

 parts of Tasmania have quite other relationships, being 

 more closely connected with the granite family. 



