ANORTriOCLASE BASALT FROM MAPTjETON, QUEENSLAND. 1 fiS 



showing lamellar twinning, is seen, but anorthoclase much 

 predominates. 



The cleavage surfaces of the latter show an interference 

 in the reflection, owing to the abundant inclusions of 

 augite and magnetite, which are seen to better effect under 

 the microscope. 



The specific gravity is 2-725 and is rather lower than 

 usual in the basalts of this area. 



MICROSCOPIC CHARACTERS. 



Section No. 266.^ 



The rock is holocr^'stalline, witli phenocrysts of 

 anorthoclase up to 5 mm. in length. 



The ground-mass is composed of plagioclase laths up 

 to 0-35 mm. long, augite granules averaging 0-1 mm. in. 

 diameter, and abundant small octahedral crj^stals of 

 magnetite about -03 mm. in diameter. 



The fabric is porphyritic and perpatic' The minerals 

 present are anorthoclase as phenocrysts. plagioclase (acid 

 andesine to medium andesine), augite, olivine, magnetite, 

 and a greenish alteration product. 



The arwrthoclase phenocrysts, like many of the 

 phenocrysts of plagioclase in the basalts and andesites of 

 Upper Kainozoic age in Southern Queensland, have been 

 much affected by the ground-mass, as the corners are 

 rounded and a definite reaction rim has developed. 



The abundance of augite and magnetite inclusions in 

 these anorthoclase phenocrysts is a marked feature. In 

 all eases the reaction rim is free from inclusions. {See 

 Fig. 1, Plate IV.) 



A definite net-like arrangement of considerable 

 regularity characterises the in-:'lusions, and many instances 

 suggesting micrographic iutergrowth of the augite 

 ''inclusions" and the phenocryst ma}^ be seen. 



Simple twinning is common in the crystals, and in one 

 or two cases very fine mieroeline twinning occurs. 



The plagioclase crystals in the ground-mass furnish 

 lath-shaped and rectangular sections. 



- The number refers to the slide in the University of Queensland 

 collection. 



'J. P. Tdflings, "Igneous Rocks," Vol. 1., 1909, p. 199. 



