BY H. I. JENSEN. 101 



east of the Range, and on top of it. The basalts are typical, 

 normal olivine basalts. In addition, great areas of country 

 between the Fassifern district and the Birnani Range, near Beau- 

 desert, are covered with coarse-grained dolerites which closely 

 resemble the gabbros of the D'Aguilar Range, north of Ipswich. 

 These dolerites represent sills intruded into tuffy sandstones of 

 Trias- Jura age. One of them (F1.21) is selected for description. 



F1.21. Dark, coarse-grained gabbroic rock occurring as 

 xenoliths in a decomposed dolerite-laccolite near Milora State 

 School. The xenoliths are of the same magma as the main rock. 

 These rocks are probably older than the trachyte-series. 



Texture : holocrystalline, coarse and uneven-grained, magno- 

 phyric. Hypidiomorphic granular fabric. The rook looks fresh 

 in hand specimen, and is very hard to break; but the abundance 

 of zeolites in the section sliows that it is somewhat decomposed. 



Composition — Felspar : some of the more decomposed crystals 

 have the composition of anorthite (extinction angle 45°), but 

 other fresher crystals (one of which showed symmetrical Carlsbad 

 and albite twinning) have an extinction angle of about 24° in 

 symmetrical sections, indicating labradorite. The decomposing 

 anorthite crystals contain strands and irregular masses of analcite; 

 they are probably only altered labradorite. Analcite is abundant, 

 amounting to probabl}^ 10% of the rock in area. Some of it 

 appears primary, just as in the Prospect dolerite, and shows 

 crystalline outlines. Anomalous double refraction sometimes 

 occurs in it. The felspars possess Carlsbad, albite, and sometimes 

 pericline twinning. The zeolites, thomsonite, mesolite, and 

 natrolite, all occur in fibrous radial aggregates associated with 

 analcite. Sericite also occurs. These are all decomposition- 

 products of the felspar. 



Augite occurs in hypidiomorphic crystals of a light brownish 

 colour inclining to salmon-pink. The extinction angle is about 

 45°, but in some crystals undergoing uralitic decomposition and 

 developing hornblendic cleavage it is only about 20°. The augite 

 is studded with idiomorphic magnetite inclusions. It appears 

 somewhat titaniferous. 



