886 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



The mineral was determined to be biaxial and optically negative 

 with a dispersion less for red than for blue (p<lv). The brown 

 spinel which occurs in crystals and fragments of all sizes in both 

 the inclusions and the base appears to be picotite. The brown mica 

 which occasionally forms huge phenocrysts occurs also in minute 

 grains in the base. The fine-grained base consists of allotrio- 

 morphic olivine and augite grains, picotite grains, a little biotite, 

 all evidently xenogenic, and of autogenic minerals including laths 

 and needles of brownish titaniferous augite, needles of colourless 

 diopside, rutile in needles, zircon in grains, and felspar. Both 

 albite and orthoclase occur, and in addition there is nepheline 

 and analcite. These last four minerals mentioned were the last 

 product of consolidation. Yet they have a strong tendency to 

 idiomorphism, except the analcite which is mainly secondary. 



To sum up: the specimen N. 18 is a lamprophyric rock; it 

 contains xenoliths of enstatite-peridotite, and xenocrysts of a 

 species of rhombic pyroxene, of biotite (lepidomelane 1), of picotite, 

 and of olivine and augite. The base has an almost panidiomorphic 

 structure, the so-called camptonitic fabric, and contains, in 

 addition to the xenocrysts, titaniferous augite, diopside, rutile, 

 zircon, albite, orthoclase, nepheline and analcite, and possibly a 

 little glass. The decomposition-products are margarite, bastite 

 (schiller spar), serpentine and haematite. Calcite is particularly 

 abundant, especially in the form of amygdules. 



Name : Monchiquitic Lamprophyre. Magmatic name, Ross- 

 weinose (see Tables i. and ii.). 



N.54. Loc: hill on the Spring Creek Road, 2 miles N.W. of 

 Narrabri. 



Handspecimen has the appearance of typical porphyritic basalt. 

 The age of the mass, which is associated with basic tuffs and 

 breccias, is probably later than the alkaline series. This rock 

 probably belongs to the late Tertiary — Pliocene — series of basaltic 

 eruptions. 



Texture almost holocrystalline, very uneven-grained, with 

 hyalopilitic fabric. 



