STRUCTURE AND COMPOSITION OF A BASALT FROM BONDI. 227 



On Plate 10, fig. 2, a large olivine is seen penetrated in this 

 way. The larger olivines show a disposition to develop in lath- 

 shaped forms rather than in forms familiar in basalts. Some of 

 the crystals will occasionally measure one-twelfth of an inch. 



The augite in the slides is abundant and characteristic. Idio- 

 morphic crystals can be seen, measuring from the one-twenty-fifth 

 to the one-twelfth of an inch. They show a warm brown border 

 which fades towards the centre of the crystals. A faint pleo- 

 chroism is noticeable, but not in every augite on a slide. Plate io> 

 fig. 1 shows an augite in the centre of the field. 



Plagioclase is not so well defined and distinct as in ordinary 

 types of our tertiary basalts, neither is it so abundant. Perfect 

 lath-shaped crystals are not common. There is no difficulty how- 

 ever in identifying them, as they are the first to catch the eye and 

 .give character to the rock in thin slices. 



The magnetite can be seen on every slide. It is a common 

 inclusion in the olivine and augite, and may be seen forming a 

 fringe or line around the faces of these minerals. A reddish 

 colouration is sometimes seen in the neighbourhood of the magne- 

 tite, evidently due to an incipient alteration of the magnetite 

 As a rule magnetite is seen in crystals, but also forms irregularly 

 shaped patches. It was one of the first minerals crystallized out 

 •of the magma. 



Besides occurring in porphyritic forms the minerals named ar e 

 also found as granules imbedded in the base. Microliths of 

 apatite can also be found in most of the slices as an inclusion in 

 other minerals. 



There is another mineral not determined in this basalt. It L 

 seen in minute six-sided forms measuring not more than the one 

 rseven hundredth to the one thousandth of an inch in diameter. 

 The mineral is clear and colourless. It may be a hexagonal or a 

 -cubical mineral. It is absolutely isotropic, showing no light 

 between crossed nicols. If a hexagonal mineral it should show 

 -an axial figure in convergent polarized light, but unfortunately 



