BY H. I. JENSEN. 863 



characteristics of this rock are : the typical trachyte lustre, 

 trachytic texture, predominance of plagioclase felspar (probably 

 andesine, with which it agrees best in optical properties), and 

 the tendency of the rock as a whole to split in slabs. 



A few deep red to brown, strongly pleochroic grains, answering 

 to the description of cossyrite, w^ere observed as a nucleus to a 

 crystal aggregate of the green hornblende (arfvedsonite). 



Conoivrin Trachyte. — This is a white or greyish-white rock in 

 which a pocket lens reveals scattered black specks. A few felspar 

 phenocrysts are usually present. Some specimens show flow 

 arrangement to the naked eye. This trachyte forms the fine 

 rectangular columns of which Mt. Conowrin is entirely made up. 



Microscopically examined the Mt. Conowrin rock is seen to 

 consist almost entirely of sanidine. In fact the other constituents 

 -do not form 3% of the bulk of the rock. The texture is holocrys- 

 talline and orthophyric. The ferromagnesian minerals present 

 include ajgerine in minute green pleochroic rods; scattered crystals 

 of a blue hornblende which in transverse section show stronsf 

 pleochroism from deep green to deep blue; in longitudinal section 

 some of the crystals appear perfectly opaque. This amphibole is 

 probably allied to arfvedsonite or riebeckite. It is identical with 

 the deep blue pleochroic hornblende in the trachj^tes of Mount 

 Jellore. near Mittagong, recently investigated by Messrs. Mawson 

 and Taylor. This hornblende was the first mineral to crystallise 

 from the magma, occurring often as inclusions in the centre of a 

 sanidine phenocryst. Its crystalline form is never preserved, 

 corrosion and resorption having taken place. A colourless non- 

 pleochroic pyroxene is present in a few very minute grains. 



A section made of a specimen of trachyte from the dyke at the 

 landslip on the W. side of Conowrin is somewhat difl^'erent in 

 mineral constitution. The amphibole with the strong absorption 

 in one direction is absent, and its place is taken by very numerous 

 minute, acicular crystals of an oli^'e-green colour. They are 

 pleochroic in brownish and green tints. A few good crystals of 

 a greenish-brown hornblende (allied to barkevicite) are present. 



