156 VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



(v.) Dacites, 



Andesites are fairly common through the area, but in 

 only one or two localities do dacites occur. These localities 

 are near Bankfoot House in the Glass House Mountains 

 district, and at Mount Alford. 



The Glass House ^Mountains daeite which Jensen has 

 fully described and analysed is certainly younger than the 

 trachytic rocks with which it is associated, for it contains 

 frequent inclusions of trachytic material which bears a close 

 resemblance to the adjacent trachytic rocks. It is a variable 

 rock, and in some microscopic sections appears to be a normal 

 andesite while in the others it has abundant corroded 

 quartz crystals. Chemically the rock is very similar to the 

 daeite of Devonian age from the ]\Iacedon area in Vic- 

 toria and more particularly the rock from the Willimigon- 

 gong Creek near " Cheniston," " Upper Macedon."*" The 

 Macedon daeite is rather low in alkalies, particularly soda, 

 but otherwise the resemblance is very close. Microscopi- 

 cally, however, there is a great difference, as not only is 

 there a big difference in texture, but also in mineralogical 

 composition. The Glass House Mountains rock covers an 

 area of a few square miles. Init it is apparently a thin flow, 



^Microscopically it is seen to be holocrystalline. It is 

 perpatic having occasional rounded phenocrysts of plagio- 

 clase set in a groundmass with an average grain-size of 

 •15 mm. The groundmass is pilotaxitic and ophitic struc- 

 ture is common. The felspar present frequently shows 

 zoning, and also simple and lamellar twinning; it ranges 

 from labradorite to andesine, and occasional lath-shaped 

 crystals of orthoclase are present. A feature of the plagio- 

 clase phenocrysts is the abundance of bright-green glass 

 inclusions. Violet-coloured augite is frequent through the 

 groundmass, and in some sections brown hornblende is seen, 

 but augite is more abundant. ^Magnetite is abundant, and 

 greenish chloritic alteration products are also plentiful. 

 Quartz, when present, occurs as corroded phenocrysts. The 

 Mount Alford material is somewhat similar to the Glass 

 House ^Mountains material, but while the latter follows 

 the trachytes the former precedes the rhyolite, for the 

 rhyolite plugs at Glennie's Pulpit &c., on Mount Alford, 

 intrude right through the andesites and dacites. 



*»Geol. Surv., Vic, Bull. 24, pp. 17, 18. 



