BY 11. I. JENSEN. 895 



Name : this rock is apparently a Cossyrite-Nepheline Trachy- 

 Andesite, the high TiO^ percentage shown in the analysis being 

 confirmatory of the presence of cossyrite and wohlerite. Mag- 

 matic name, Monzonose {c}). Tables i. and ii.). 



Note : it is significant, in connection with the occurrence of 

 quartz in the calculated norm, that the nepheline occurs as 

 corroded phenocrysts, the base being andesitic. Further, the 

 occurrence of opal shows that secondary silicification has taken 

 place. 



Both N.31 and N.49 are closely related chemically and miner- 

 alogically to the remarkable corundum basalt found at Billy 

 King's Creek, south of Coonabarabran, in the Warrumbungles. 

 They are all the basic differentiation-product of a magma exceed- 

 ingly rich in AUO.,, Na^O, and TiOo, and very poor in MgO 

 and FeO, just as the arfvedsonite trachytes (as N.25 and N.30) 

 form the more acid differentiation-product. 



The richness of the magma in TiOo is especially striking for 

 the Nandewar Mountains. In many of the rocks where condi- 

 tions have not been favourable for the formation of titaniferous 

 amphiboles and pyroxenes, the titanic acid has crystallised out 

 in the form of an abundance of sagenitic rutile needles. 



N.20. Loc: a hill west of Dingo Creek. 



Handspecimeu a dark greenish-brown rock, which occurs as a 

 dyke in a hill near the doleritic laccolite at Dingo Creek. It 

 contains numerous vesicles infilled with calcite. Under the 

 microscope the main constituent is seen to be oligoclase-andesine 

 showing Carlsbad and Albite twinning (R.I. greater than Canada 

 balsam, extinction angle 0°-10°). Haematite is fairly abundant 

 through the decomposition of the original femic minerals. Opal 

 and magnetite occur, and also some brownish, highly pleochroic 

 aggregates and grains referable to katophorite and brownish 

 needles referable to wohlerite. 



Name : Decomposed Phonolitic Andesite. 



N.21. Loc: dyke cutting dolerite laccolite, Dingo Creek. 

 Handspecimen a dark porphyritic rock witli an aphanitic base. 

 Texture hemihyaline and porphyritic. 



