BY V7. N. BEXSOX;, W. S. DUX^ AXD W. R. BROWXE. 415 



exhibits occasional carbonate alteration. Ilmenite shows hexagonal and irregular 

 plates. Tiny apatite prisms are quite abundant either in the grouudmass, or as 

 inclusions in the phenocrysts, notably in the felspars. Little zircon prisms, too, 

 sometimes appear. There is a fair amount of chlorite in rosette-shaped aggi'e- 

 gates, often pseudomorphous after what appears to have been felspar, judging 

 by its outlines, and by the abundant enclosed apatite needles. The groundmass 

 is largely crj-ptocrystaUine, with irregularly sinuous aggregates of secondary 

 quartz and chlorite. Magnetite dust may be fairly abundant locally. An im- 

 portant feature of the groundmass is the presence of numerous irregular patches 

 of brown glass showing marked fluidal fabric, and with streaks of cryptocrystalline 

 material through it. These patches pass rather quickly into the ordinary gi'ound- 

 mass, and it is hard to say whether they represent remnants of a once completely 

 glassy base, or whether the present texture as a whole is due to heterogeneity in 

 the original magma. 



Specimen 1441, from the centre of Portion 117, evidently represents a slightly 

 different phase of the same type. There is a much gi-eater proportion of glass iu 

 the groundmass, tlie felspar phenocrysts are rather more numerous and more 

 altered, while hornblende is also more abundant. Further there is another ferro- 

 magnesian mineral present, which is now represented by clusters of magnetite 

 granules, with or without an indeterminable greyish or brownish substance. The 

 sections are idiomorphic and very elongated, almost rod-like, and the original 

 mineral appears to have been biotite. Apatite is less abundant than in 1491 and 

 zircon is infrequent. This rock corresponds rather more closely to the Martin's 

 Creek type than does 1491. 



A rock of somewhat similar habit to the hornblende andesite outcrops in 

 Portion 88, about half a mile east of Rocky Creek, but the place of hornblende is 

 taken by biotite, and a single phenocryst of corroded quartz was observed in the 

 slide examined. 



(b) Pyroxene Andesites. These present many variations in texture, as regards 

 both crystallinity and fabric, and in composition are evidently more basic than the 

 hornblende andesites . 



The rocks may be conveniently subdivided into tlie litlioidal. vitrophyric. and 

 pilotaxitic types, the first of these terms being applied to those rocks of which the 

 groundmass. as seen under the microscope, consists of a mosaic of spongy-looking 

 felspar grains. 



The first two types are closely associated in the field, combining to form the 

 great series of sills which have invaded the tuffaceous grits at the ba.se of the 

 Rocky Creek Series, whereas it is rather significant that all the pilotaxitic rocks 

 of which specimens are available have quite a distinctive appearance, and occur 

 as dj-kes connected with the Warragundi complex. It is quite possible that, al- 

 though all three types are linked together by mineralogical similarities, there are 

 chemical differences between them, but the data available do not warrant a positive 

 statement on this point. It is difficult to say whether the mosaic groundmass of 

 the lithoidal type is jirimary, or results from devitrification of an original pitch- 

 ^ stone. Anderson and Radley have put forward the suggestion for certain pitch- 

 stones of Mull, that the stony types have been derived from the glassy by de- 

 vitrification due to the escape of some of the chemically combined water. This 

 suggestion is based on field observations supplemented by determinations of the 

 water present in the rocks (24) . In the case of the Currabubula rocks special 



