208 REV. J. MILNE CURRAN. 



Slice 32. — On this will be found a section of augite nearly parallel 

 to the clinopinacoid ( oo P oo ). The extinctions from the vertical 

 axis is 36°, and the angle between oP and oo P oo is 74° 5'. Under 

 crossed nicols, the crystal is seen to be beautifully typical of the 

 "hour-glass structure," so rarely observed in Australian rocks. 

 It will also be noticed that the augite is partially penetrated by 

 a plagioclase. The granules in the base are not all augites. By 

 treating the slice with dilute hydrochloric acid, many of them are 

 removed; besides, in polarized light, the olivine granules are rather 

 more brilliant than those of augite. To observe properly these 

 augite granules, it is necessary to remove the secondary constituents 

 as well as the olivines and magnetite. This can be done very effec- 

 tively I find, by treating an uncovered slice with warm hydro- 

 chloric acid for about three hours. A few of these slices show 

 glassy matter, but not in any great quantity. The general struc- 

 ture of this rock is best seen under crossed nicols with a low 

 power. The slice is then seen with lath-shaped felspars streaming 

 round the porphyritic constituents. Both augite and olivine 

 polarize brilliantly, in fact they seem to stand out in relief in the 

 fine-grained ground-mass in which they are set ; and any of these 

 slices forms excellent examples of the micro-porphyritic structure 

 of basalts. The sharp contrast between this and other basalts is 

 well seen by an inspection of the figures on Plate xxi. 



Slice 34. — Basalt from Bald ELills, Perth, New South Wales. 

 In this slice, aggregations of augite are recognizable with the 

 unaided eye. Under the microscope, some of these are beauti- 

 fully zoned, and in one aggregation, a partial hour-glass structure 

 is clearly shown in polarized light. A cavity is seen in the slice 

 that has been filled with a zeolite radiating from three different 

 centres, and showing parts of three crosses under crossed nicols. 

 The magnetite grains often give the outlines of perfect cubes. A 

 few perfect forms included in the clear olivines measure the one- 

 six hundredth part of an inch in diameter, less perfect forms going 

 up to the one-hundredth of an inch. The serpentine into which 

 the olivines of this basalt alters is always of a greenish colour. 



