160 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



surrounded by a fringe of augite granules which have 

 resulted from corrosion by the magma. {See Plate XIII., 

 fig. 4.) It is noticeable that when an augite phenocryst 

 impinges on another augite crystal or on a plagioclase 

 phenocryst. the fringe does not exist, but occurs only along 

 the junction of the augite with the groundmass. The 

 fringe of granules is optically continuous with the crystal 

 so that it is a resorption border. 



The plagioclase phenocrysts are crystals of basic 

 andesine, AbgsAn^. for the most part, although some of 

 them show zoning with more acid shells ; these phenocrysts 

 are idiomorphic and have not been corroded. Magnetite 

 occurs as inclusions in the augite, and also through the 

 groundmass as very fine granules. The specific gravity 

 of this rock is 2-75. 



Normal Andesites. 

 These are all compact dark-coloured rocks and range 

 from porphyritic to non-porphyritic. They are perpatic, 

 and the phenocrysts are plagioclase, augite, and in some 

 cases hypersthene, the latter occurring sparingly and 

 having strongly marked resorption l)orders. 



The plagioclase phenocrysts range from medium ande- 

 .sine, AbyAug, to medium labradorite, Ab^Aug. and do not, 

 as a rule, show zoning. They show carlsbad, albite and 

 pericline twinning, and are usually sub-idiomorphic or 

 allotriomorphic ; in size they range from 2 mm. to 4 mm. 

 in length. 



In several flows, particularly the Tamborine, Obser- 

 vation Hill, One Tree Hill and Mount Meerschaum flows, 

 the plagioclase phenocrysts appear to be medium andesine, 

 Ab3An2, which is crowded with regularly arranged inclu- 

 sions. These inclusions are frequently studded all through 

 the crystal with the exception of a narrow band at the 

 edge, Avhich is quite clear. The inclusions in most cases 

 consist of an opaque material, but in others they are small 

 granules of augite. Both kinds of inclusion occur in the 

 same crystal however, and this peculiar character of the 

 plagioclase phenocrysts has also been noticed in some of 

 the andesitic basalts. A great deal of variation exists in 



