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



(4) Nomenclature.— The field occurrence and microscopic- 

 structure of this rock prove its position to be amongst the graphic 

 granophyres. 



Point Arkwright Rocks. 



Sp. No.i. ( Plate xii., figs.9-1 1). 



i. This specimen is of a bluish colour, and at first glance not 

 unlike diorite. Phenocrysts of black hornblende and yellowish- 

 green plagioclase are easily recognised imbedded in a fine-grained 

 base, which has a greasy lustre, and gives an oily appearance to 

 the rock. 



ii. Microscopic characters. — (1) Texture (a) crystallinity, 

 holocrystalline; (b) grain-size, variable, porphyrinic. The 

 phenocrysts reach a length of from several millime'ers to nearly 

 half an inch in the case of hoimblende. The base is ver}' fine- 

 grained, microcrystalline to cryptocrystalline. (c) Fabric, 

 porphyritic; cryptographic, micrographic and microspherulitic 

 structures are seen in the base. 



(2) Constituents (in order of decreasing abundance). — (a) 

 plagioclase, (b) hornblende, (c) orthoclase, (d) biotite, (e) quartz; 

 Minor, (f) magnetite, (g) sphene, (h) calcite, (i) apatite. 



(3) The plagioclase occurs in idiomorphic, often considerably 

 corroded phenocrysts, and also in the fine base. The phenocrysts 

 are composed essentially of labradorite of the composition Ab x An 2 

 though some consist of oligoclase, and most show beautiful zoning, 

 due to decrease in basicitj' from the centre outwards from 

 labradorite to oligoclase. Many show incipient decomposition, 

 and contain calcite in the centre where the decay commences. 

 Shadowy extinction from the above-mentioned variation in 

 basicity is almost universal. Twinning is very common, and 

 frequently Carlsbad, albite and pericline twinning occur together 

 in the same crystal. As inclusions in the phenocrysts we have 

 glassy material, hornblende and apatite, often zonally arranged. 

 In shape the plagioclase phenocrysts are tabular and rod-shaped. 

 The last kind have a six-sided, almost hexagonal cross-section. 



