40 SUPPLEMEXTAKY NOTES ON SOME 



Amongst the Rook specimens Bdsalts are well repre- 

 sented, and vary in texture from tine-grained, dense, 

 dark-coloured locks to scoriaceous, lighter-coloured 

 varieties. 



The first and smallest s])ecimen is that of a dense 

 black Basalt^ showing here and there a few black augites 

 and very small grains of olivine. Under the microscope 

 the augite (of which an excellent cross-section is present 

 in one slide) appears almost colourless. Prismatic and 

 a weaker pinacoidal cleavage are shown : prismatic 

 angle about 87°. The augites are quite free from cor- 

 rosion, and enclose a few magnetite grains. The olivine 

 ofrains show traces of crv^t;llline outline, and are altered 

 round the margins and along cleavage cracks into 

 ferruginous matter. Magnetite is present in large and 

 small grains, sometimes showing crystalline form. The 

 base consists chietlv of lath-shaped felspars, which show 

 what appear like Huxion phenomena round the porphy- 

 ritic constituents. The felspars,^which are of a basic 

 variety, are closely packed together, and in the 

 interstices come fine grains of magnetite and a little 

 glass. Fig. I. is a diagrammatic drawmg showing a 

 cross-section of an augite prism, and the base. 



The rock termed Hornblende Basalt is one possessing a 

 peculiar whitish-grey coat of weathering products, but, 

 on fracture, shows a very fine dense rock, with here and 

 there a few porphyritic crystals. Mr. Twelvetrees 

 suggests that, on account of these porphyritic horn- 

 blendes, the rock is an andesite. In the New South 

 Wales collection some doubtful andesites are mentioned. 

 An analysis of this specimen gives 45 per cent, of Si O2, 

 placing this rock amongst the Basalts." 



Under a high power the base of this rock is seen to 

 consist of long lath-shaped felspars, grains of magnetite, 

 and everywhere are scattered small needle-shaped 

 crystals, which do not extinguish straight, and probably 

 are felspathic microliths. These are set in a glass of a 

 light brown tint. Fig. IV. shows the arrangement of 

 felspars, microliths, and glass. 



In the scoriaceous Basaltic Ruck, augites and olivines 

 are clearly visible to the naked eye. The augites are 

 similar to those above mentioned, but the olivines are 



