BY H. I. JENSEN. 897 



Note : this rock covers a great area towards the headwaters 

 of Oakey Creek, a couple of miles above its junction with BuUawa 

 Creek. It forms a large boss or Jaccolite which has been exposed 

 by erosion. Chemically it is closely related to the dark syenite 

 porphyries and labradorite porphy rites occurring as sills under 

 Mount Odin. 



N.32. Loc: one-half mile W.N.W. of Ningadhun Rock. 



Handspecimen a reddish, very vesicular rock, with large felspar 

 phenocrysts. It was sectioned to determine whether it is a 

 trachyte or andesite. 



Texture holocrystalline, with phenocrysts up to 05 mm. longv 

 and an extremely fine-grained microcrystalline base; fabric 

 pilotaxitic in the base. 



Constituents : andesine-labradorite felspar full of glass-inclu- 

 sions, forming phenocrysts, and lath-shaped and tabular microlites 

 of the same mineral; haematite replacing some hornblende; primary 

 magnetite in minute idiomorphic grains; kaolin; a honey-yellow 

 mineral, probably opal, and various other decomposition-products. 



This rock is undoubtedly an altered andesite closely allied to 

 the trachytes and phonolites which occur in the vicinity. Flows 

 of these diflferent differentiation-products occur frequently inter- 

 bedded, one eruption giving a phonolitic lava, the next perhaps 

 an andesite, the next a trachyte, and so on. 



A porphyritic vesicular trachy-andesite of grey colour caps 

 Ritter's Razorback between Bullawa Creek and Spring Creek. 

 This rock in handspecimen seems intermediate between the 

 andesite just described and the vesicular trachytes. 



A pilotaxitic porphyritic andesite (N.33, Pl.li., fig.4) occurs 

 around Deriah Mountain, which contains andesine-labradorite 

 felspar; corroded, colourless diopside and light brown titaniferous 

 augite; magnetite; ilmenite, and chlorite. This rock has also 

 been shown to be later than the trachytes. 



These andesitic rocks also form a link between the trachytes 

 and the final basalts of the alkaline series. The first generation 

 is felspar, usually a labradorite in highly corroded phenocrysts, 

 round which a zone of orthoclase has frequently developed. The 

 second generation of felspar consists of orthoclase and albite. 



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