618 



GREAT SERPEN-TINE-BELT OF NEW SOUTH WALES, V., 



Tertiary Basalt. 

 The basalt is extremely fresh. It consists of small laths of 

 labradorite, slightly zoned, granules of augite, large prismoid, 

 but not idiomorphic grains of olivine, and small irregular grains 

 and aggregates of ilmenite. There is a vesicle about 2 mm. in 

 in diameter, the structure of which is shown in Text-fig. 15. All 

 round the vesicle is a thick zone of minutely granular augite, 







A B 



Fig. 15. — A. Amj^gdule in Tertiary basalt( 1164). Upper and middle por- 

 tion opal, lower natrolite. The first and last of these contain prisma 

 of augite. B. Spherule of augite-prisms in the same rock as A. 



but within are comparatively large idiomorphic prisms of augite, 

 of the same character as that in the groundmass of the rock, but 

 lying more or less isolated in a matrix of opal and natrolite. 

 The prisms are particularly well formed in the opal. The de- 

 termination of this mineral is rendered certain by the very clear 

 evidence of Becke's line, which shows that the colourless isotropic 

 mineral has a distinctly lower refractive index than the natrolite. 

 It is quite clear, and crossed by irregular cracks, without any 

 sign, of cleavage, such as analcite might show. There can be 



