142 G. D. OSBOKNE. 



which was partly assimilated in such a way as to produce 

 a flask-shaped area, the neck of the "flask" being the 

 entrance through which the basalt made its way aud the 

 body of the flask representing partially dissolved anorthite. 

 Crystallisation of the host prevented the complete solution 

 of the felspar, and the molten material in the "flask" 

 solidified as felspathic basalt. The forces of crystallisation 

 probably de-oriented the undissolved felspar a little, wilh 

 the result that a broad pericline twin striation which was 

 originally continuous across the "flask" area now shows a 

 marked deviation. 



6. Comparison with the Dundas Rocks, — In the preced- 

 ing pages the general geological and penological features 

 have been described in some detail. It is proposed here, 

 briefly, to refer to the Dundas rocks by way of comparison. 

 Through the kindness of Prof. Benson the author has been 

 able to examine personally, his slides of the Dundas and 

 Gerringong inclusions. As one might expect the Basin 

 rocks are essentially similar to those at Dundas. Among 

 the points of distinction between the two sets, we must 

 note the absence of troctolites at Dundas. The felspars 

 of the gabbros at this locality show effects of alteration 

 akin to schillerisation, in Judd's sense, and secondary twin 

 lamellation, which are not met with at the Basin. 



The crystallisation of the spinel picotite in the ultra- 

 basic rocks has been different in some of the rocks considered 

 in this paper. 



The rock analyses show that in the dunites the ratio of 

 the bases to eacli other and to silica, and the content of 

 the alkalies, lime and the accessory metallic oxides, are 

 fairly constant at both localities. Nickel oxide is perhaps 

 an exception being '41% and *30% respectively in two of the 

 basin rocks, such values being much higher than those in 

 the Dundas rocks. At both localities it would appear that 



y 



