154 VOLCANIC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



ii. Microscopically it is a holocrystalline, micro- to crypto- 

 crystalline, even-grained rock, with a micropegmatitic fabric. 

 Felspar forms the main bulk of it. In addition there is a 

 brownish highly pleochroic mineral, probably a soda-amphibole, 

 sparingly represented in minute grains. The felspar is apparently 

 orthoclase, but may be partly anorthoclase and albite. The grain- 

 size is so extremely fine that the presence or absence of quartz 

 cannot be established with certainty. Hence 3 term it pro- 

 visionally Felsitic Trachy-rhyolite. 



It intrudes Trias-Jura rock, and underlies basalt. 



Orthophyric Comendite. Loc: Mt. Cooran (Plate xv. fig.25). 



This rock forms the pinnacle known as Mt. Cooran at Cooran, 

 about 16-m. from Gympie and 46-in. north of the Glass House 

 Mountains. 



i. Handspecimen exactly like Mt. Conowrin rock. 



ii. Microscopically it is a holocrystalline, tine-grained, uneven,, 

 orthophyric rock. It consists essentially of isometric felspars, 

 and a felsitic base containing minute felspar laths. A deep blue 

 soda-amphibole, and a very dark brown, highly pleochroic horn- 

 blende are present in subordinate amount. The isometric felspars 

 belong to the variety sanidine or soda-sanidine (anorthoclase). 

 There are also lath-shaped phenocrysts of this mineral up to 1 mm . 

 in length. They exhibit the characteristic cross-cracks, and 

 twinning on the Carlsbad and Baveno laws. The deep blue 

 amphibole is highly pleochroic, changing from deep indigo-blue 

 to greenish-blue, and in some grains to greenish-yellow. It is 

 probably more closely allied to riebeckite than to arfvedsonite. 

 The dark brown soda-amphibole is allied to cossyrite, and occurs 

 in dendritic aggregates of minute crystals, forming a kind of 

 graphic intergrowth with felspar in the base. Quartz also occurs. 



This rock is so like the Conowrin and Coolum comendites that 

 one feels safe in calling it Orthophyric Riebeckite (Cossyrite *?) 

 Comendite. 



Note. — In many of the orthophyric rocks like the above, the 

 brown amphibole was the last of the ferro-magnesian minerals to 

 consolidate, forming dendritic intergrowths with the felspar of 



