896 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



Constituents : phenociysts of orthoclase sanidine showing fine 

 Carlsbad and Baveno twinning, sanidine microlites, iron ores and 

 glass. 



Name : Hemivitreous Porphyritic Trachyte. 



Note : this rock is briefly described because it is important a& 

 one of the factors by means of which the relative ages of the 

 calcic dolerite and the alkaline intrusives were determined. 



Another dyke rock of the alkaline series cutting the doleritic 

 mass is numbered N.48. It is wholly aphanitic, but micro- 

 scopically porphyritic, having microlites of orthoclase sanidine^ 

 anorthoclase and oligoclase, and minute needles of greenish 

 diopside in a noncrystalline base. Limonite occurs as a decom- 

 position-product. This rock is a keratophyre. 



N.15. Loc: headwaters of Oakey Creek, branch of Bullawa 

 Creek. (Plate 1., fig.6; Plate li., fig.6). 



Handspecimen an even-grained rock of a light grey colour^ 

 being made up of a whitish felspar and a greenish hornblende. 



Texture holocrystalline, even-grained, with allotriomorphic 

 granular fabric. 



Constituents : the most abundant minerals by far are ortho- 

 clase and moiree microcliiie, both considerably kaolinised. 

 Plagioclase with sericitic decomposition-products is fairly 

 plentiful. Next in order of abundance is a greenish very pleo- 

 chroic hornblende which is undergoing decomposition to magnetite 

 and chlorite. Greenish segirine-augite and colourless diopside 

 are also present, as are also a few flakes of biotite with chloritic 

 decomposition-products. A little quartz occurs interstitially, 

 Albite is included in, and intergrown with, the orthoclase. The 

 chief accessories are stout prisms of bluish apatite terminated 

 with pyramids, titaniferous magnetite and ilmenite and fluorspar. 

 Chlorite, serpentine, leucoxene and kaolin are the chief decom- 

 position-products. 



This rock, from the composition, is seen to have affinities with 

 quartz syenite, quartz diorite, and the more alkaline augite 

 syenites. 



Name: Akerite. Magmatic name, Akerose (see Tables i. & ii.) 



