BY H. I. JENSEN. 899 



Similar rocks were met with over considerable areas at the 

 head of Bobbiwaa Creek, Thus N.55 occurring near the base of 

 Dripping Rock is a highly vesicular, holocrystalline; porphyritic 

 red trachyte with trachytic fabric in the base. The constituents 

 are felspar, probably anorthoclase, forming phenocrysts and 

 microlites; katophorite C?), a brown highly pleochroic hornblende 

 showing brown, red and deep blue or purple tints; segirine; 

 primar}^ and secondary haematite, quartz; magnetite and opal. 



Another specimen from this locality contains, in addition, a 

 little tridymite, fluorite, and nepheline. After gelatinising with 

 acid, the interstitial matter stains strongly. A little quartz is 

 also apparently present and also cancrinite. These colourless 

 minerals commonly contain as inclusions sagenitic rutile needles,^ 

 zircon segirite and tabular microlites of a brownish mineral. 



The brown amphibole of this rock when fresh exhibits very 

 strong pleochroism in very thin slices, but where the sections 

 are thick the pleochroism tints are masked by a deep colour. The 

 double refraction is likewise masked, and the mineral shows no posi- 

 tion of extinction. In outline the mineral has the shape character- 

 istic of arfvedsonite. It is therefore probably a katophorite. 

 A little brownish wohlerite (?) occurs sparingly in the'rock. 



All the rocks which contain the brown pleochroic mineral 

 contain also distinct haematite having the same crystalline form 

 and apparently pseudomorphic after it, not by decomposition 

 but by alteration by pneumatolysis in the period of consolidation 

 of the rock. Brownish minerals which are neither the one nor 

 the other of these but conform to the same habit (either pris- 

 matic or in dendritic aggregates) also occur. These may, and 

 probably do, belong to the allied mineral species of (1) mosandrite, 

 rinkite and johnstrupite, and(2) wohlerite,which are all zirconium- 

 bearing minerals, and therefore show bluish pleochroism tints in 

 some positions in thin sections. Those crystals which show both 

 green and blue as well as red-brown are probably true katophorite. 



N.59. Loc: Deriah Mountain. (Plate li., fig.2). 

 Handspecimen indistinguishable from the trachyte of Mount 

 Ngun-Ngun in the Glass House Mountains. It is a medium- 



