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



rock, and more sparingly as tabular phenocrysts. Carlsbad and 

 Baveno twinning only have been noticed. Most of the felspars 

 show shadowy extinction, probably due to strain in consolidation. 

 The felspar forms at least 85 % of the bulk of the rock. The 

 hornblende occurs in corroded phenocrysts. Frequently the blue 

 variety envelops grains of brownish barkevicite or of clear 

 reddish-brown katophorite or cossyrite. From pleochroism, 

 absorption, and extinction angle (see p. 170), this mineral was 

 identified as arfvedsonite. Smaller grains occur abur. dan tly, aid 

 these display closer affinities with riebeckite. Hornblende forms 

 an important constituent (between 10% and 14%) .iEgirine 

 occurs very sparingly in small acicular crystals. Quartz foims 

 an infilling of some of the interstices between crystals. The 

 other minerals are present only in minutest amount. Magnetite 

 occasionally s-erves as a nucleus for hornblende. Zircon occurs 

 in minute geniculate needles. 



(4) Magnetite, zircon, and apatite were the first minerals to 

 consolidate, and occur as inclusions. .ZEgirine crystallised out 

 next. The amphiboles commenced next, but continued to 

 separate out after the felspar had commenced. Felspar, quartz, 

 and a few minute patches of isotropic mineral (garnet or sodalite ?) 

 were the last to finish crystallising. 



(5) Nomenclature. — This rock is a " soda-trachyte " (" Arfved- 

 souite- riebeckite trachyte"). The magmatic name resulting 

 from the calculating of the norm is phlegrose, clos-e to nord- 

 markose. For analysis see Anal, v., and Norm v. (pp. 168, 170). 



(ii.) Ccnowrin Comendite. (Plate xiv. fig.20). 



i. In handspecimen this is a fine-grained grey rock, with 

 minute black specks. Some specimens show flow-structure. 



ii. Microscopic examination. — (1) Texture: (a) holocrystalline; 

 (b) fine-grained, microcrystalline, having an aphanitic base, with 

 small plianeric phenocrysts less than 1 mm. in diameter; (c) 

 fabric orthophyric. 



(2) Constituents. — Essentially felspar and riebeckite in a 

 micro- to cryptocrystalline, felsitic base which contains quartz. 

 The felspar occurs in porphyritic crystals; it consists of sanidine 



