86 



There is one feature wliich is only occasionally exhibited, 

 and that is a glomero-porphyritic *''' aggregate of angite and 

 olivine. 



Idiomorphic grains of magnetite are present in notable 

 quantities. There are other similar undecomposed grains and 

 crystals of a dark, opaque mineral, which is probably ilmen- 

 ite. It has a dark, semi-glistening surface when seen by 

 reflected light, but not so intense as the magnetite. It is 

 difficult to ascertain which is magnetite and which is ilmen- 

 ite ; only a few grains exhibit the white decomposition pro- 

 duct (leucoxene), which is characteristic of ilmenite or titan- 

 iferous magnetite. 



Order of Consolidation. 



a. Magnetite and ilmenite ... 



h . Olivine 



c. Augite 



d. Labradorite 

 €. Augite 

 /. Magnetite ... 

 (/. Glass 



III. — Chemical Composition. 



A chemical analysis of the rock was made by the author 

 m the laboratory of the University of Adelaide, by the kind 

 permission of Professor Rennie. 



The rock selected was a typical representative of the 

 compact variety of basalt occurring in the Blue Lake. 



The methods of analysis were those commonly employed 

 by Washington and Hillebi-and. 



Evaporations were made in porcelain dishes, where 

 platinum dishes were unavailable. 



The Laurence Smith method for the alkalies and the 

 colourometric method for titanium were employed, and 

 the usual corrections made in each case. 



Sulphur was not determined, for there was very little 

 evidence of the occurrence of pyrites in the rock. 



Barium and strontium were not separated from the lime. 



The mean of three very concordant analyses has been 

 taken, and is represented by the column marked A. The 

 analyses marked B, C, D, E have been inserted for com- 

 parison. 



(7)Judd, Q.J.G.S., vol. xlii., pt. i. 



