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



The constituents are felspar (orthoclase) and quartz, with some 

 kaolin from the decomposition of felspar, and iron ore from the 

 decay of a little original biotite. Silica percentage 75-20. 



Sp. No.104 is a hypocrystalline rock, consisting of a crypto- 

 crystalline to microcrystalline base, occasionally microspherulitic, 

 containing a few phenocrysts, and sometimes little areas of 

 glass. Axiolitic and microspherulitic structures are beautifully 

 represented. 



The constituents are orthoclase, quartz and decomposition 

 products (chiefly kaolin). The phenocrysts are very broken and 

 corroded; some are of orthoclase, some of quartz. The base 

 composed chiefly of orthoclase has become nearly entirely crypto- 

 crystalline by devitrification. A few grains of quartz, like those 

 of Sp. No. 75, microcrystalline granular, are present. 



Both the rocks may be termed Spherulitic Rhyolite. 



Sp. No. 75B, from Mt. Archer, is a reddish rock which from 

 handspecimens one would judge to be practically quartz-free. 

 This rock is found interbedded with rhyolites on Mt. Archer. 

 Microscopic examination proves it to consist essentially of ortho- 

 clase felspar and magnetite. The orthoclase occurs partly in the 

 form of highly corroded phenocrysts, partly as minute laths con- 

 tributing to form a trachytic base. The magnetite is highly 

 allotriomorphic. Much of it is dusty. It is probably for the 

 most part secondary after hornblende. This rock is practically 

 a decomposed porphyritic trachyte. This demonstrates that at 

 Mount Archer we have true rhyolites and true trachytes (ortho- 

 clase porphyries) erupted together, and it appears probable from 

 microscopic resemblances that these rocks were contemporaneous 

 with the Glass House Mountain and Maroochy district trachytes 

 and rhyolites, but have undergone greater subsequent alteration. 



Maroochy Rhyolites. 

 Sp. No.6. Loc: Nindherry Mtn. near Yandina. 



A light-coloured rock varying from white to yellowish-grey, 

 and indistinguishable from a trachyte in handspecimen, but much 

 more aphanitic and harder than the trachytes usually are. So 



