BY H. C. RICHARDS. ] G7 



in these latter flows plagioclase phenocrysts up to half an 

 inch in length may occur. Olivine also occurs as j^henocrysts 

 in some of the lavas, and in particular at Toowoomba and 

 Spring Bluff. 



In crystallinity the rocks are either holoerystalline or 

 hypocrystalline. As far as the grain-size is concerned, 

 in the groundmass it varies from extremely fine to medium, 

 although a fine groundmass averaging -1 mm. in diameter is 

 the most common, and in the phenocrysts there are plagio- 

 clase crystals ranging up to 14 or 15 mm. long, although the 

 average length of plagioclase phenocrysts is 2 mm. The 

 groundmass is either pilotaxitic or hyalopilitic ; fiuxion 

 structure may be well developed, but ophitic structure is 

 very common and indicates that in most cases the lavas 

 had come to rest before the augite crystallised out. The 

 minerals present as phenocrysts are plagioclase and olivine ; 

 the former shows simple and broad lamellar twinning as a 

 rule and ranges from medium andesine, AbgAuo, to acid 

 labradorite, Ab^^Anjj, although basic andesine is the most 

 conuuon. Zoning is seen rarely but it has been noticed 

 in some of the large phenocrysts in an olivine-basalt from 

 Lamington Plateau. Olivine occurs frequently as pheno- 

 crysts and it may occur up to 2 mm. in length. It is 

 usually sub-idiomorphic to rounded and alters in several 

 different ways; the alteration to the red lamellar mineral 

 iddingsite is very well seen in some of the Lamington 

 basalts. The minerals of the groundmass are plagioclase, 

 augit(^ olivine and iron ores. The plagioclase crystals 

 usually show albite twinning, though only simple twinning 

 is seen in very fine lath-shaped crystals. 



Usually the groundmass felspar is more acid than the 

 phenocrysts, and ranges from acid andesine, Ab^Aug, which 

 is fairly common, to medium labradorite, AbaAug ; the most 

 abundant plagioclase is andesine. 



The augite appears either colourless in thin sections 

 or as the violet-tinted titaniferous variety. It occurs in 

 granules, sub-ophitic patches, or as definite ophitic patches. 

 It is frequently a matter of great difficulty to distinguish 

 between the colourless augite and olivine in the granules. 

 Hypersthene has not been seen in any of the basalts. 



