140 



G. D. OSBORNE. 



after some ferromagnesiao mineral. The structure of the 

 main mass of colourless mineral is typically subidiomorphic 

 granular under crossed nicols, the junctions between 

 adjacent grains being extremely difficult to discern in 

 ordinary polarised light. 



It is intended to investigate this mineral further, with 

 possibly the application of micro-chemical or staining tests. 

 At present the actual identity is left as a matter of doubt. 



To conclude this section the occurrence of xenocrysts of 

 quartz and olivine in the basalt should be noted. The quartz 

 is clearly foreign in nature. It is sometimes very shattered 

 and cracked, at other times more compact. Sometimes 

 it has the appearance of having been recrystallised. In- 

 variably surrounding the quartz xenocrysts there is a border 

 of tiny diopside crystals. 1 The olivine, which is often 

 decomposed, is of cognate nature, having been derived 

 either from the breaking down of peridotites or by original 

 fractional crystallisation from the ultrabasic magma. 



5. Relations between Host and Inclusions, — In the 

 Dundas papers by Prof. David, Watt, and Smeeth 2 and 

 Prof. Benson, 3 attention was drawn to the assimilation 

 effect by the basalt on the xenoliths and its bearing of their 

 rounded form. The former point was also mentioned by 

 Benson in his Gerringong notes. 4 



The Basin inclusions are often well rounded, an effect 

 due in part to magmatic corrosion. Some specimens of 

 basalt with inclusions simulate the appearance of a con- 

 glomerate. 



The mutual relations of the basalt and inclusions are 

 well shown in some of the slides, and descriptions follow. 



1 Cf. Lacroix, " Enclaves des Koches Volcaniques," p. 19, fig. 1. 



2 Prof. David, Watt, and Smeeth, Journ. Roy. Soc. N.S.W., 1893, p. 401. 



3 W. N. Benson, Journ. Roy. Soc, N.S.W., 1910, p. 542. 



4 P.L.S., N.S.W., 1914, p. 447. 



