276 



A Tholeiitic Basalt from Eastern Kangaroo Island. 



(Communicated by Professor Walter Howchin.) 



The evidence of igneous action of Tertiary date in the 

 Kangaroo Island area has, so far, been indicated only in the 

 neighbourhood of Kingscote, and to the west. 



The rock, which is shortly described in this note, repre- 

 sents a further member of the Tertiary suite — developed in 

 the vicinity of Cape Willoughby. Through the kindness of 

 Professor Howchin, the writer has been able to study this 

 rock, collected by him from a dyke cutting the older Palaeozoic 

 or Proterozoic schists near Cuttle Fish Bay, Hundred of 

 Dudley (Trans. Roy. Soc. S. Austr., vol. xxvii., 1903, pt. I., 

 p. 82). 



The hand specimen is a grey-black, fine-grained rock con- 

 taining phenocrysts of felspar, visible with a low-power lens. 

 There are also present a number of small spherical masses of 

 brown colour which repiesent infilled amygdales. These do 

 not exceed one-sixteenth of an inch in diameter. 



Viewed in thin sections, the rock has the mineralogical 

 composition and texture of a basalt. The phenocrysts con- 

 sist of plagioclase and augite set in a ground-mass of hypo- 

 crystalline nature. The constituents of the ground-mass are 

 augite, plagioclase, magnetite, brown glass, and the amygdale 

 minerals, opal, and an undetermined zeolite. 



The augite phenocrysts are usually subidiomorphic and 

 reach in size 1J mm. They are elongated parallel to the 

 vertical axis. In convergent light they are seen to be of two 

 types, a dominant one, biaxal, and in less amount an enstatite 

 augite of uniaxial character. They are both colourless to 

 greyish with well-developed prismatic cleavages, and com- 

 monly show twinning on 100. The plagioclase phenocrysts 

 reach a similar dimension but are usually smaller. They 

 show twinning after the Carlsbad and Albite laws, and less 

 commonly the pericline. The composition approximates that 

 of bytownite, with a refractive index of 1*57. A glomero- 

 porphyritic texture is often apparent. 



The ground-mass of augite, plagioclase, and glass has the 

 typical intercertal fabric, the brown glass occurring as angular 

 patches between the plagioclase laths. The felspar is a plagio- 

 clase of labradorite composition, with a lath-shaped habit, 

 and the augite granules still preserve a tendency towards 

 elongation parallel to the vertical axis. Grains of magnetite 

 are uniformly distributed through the base. The amygdales 

 of spherical shape are filled with a very low refracting sub- 

 stance, partly isotropic, and partly birefringent. 



The isotropic material usually borders the vesicle, and 

 the central part is filled with a birefringent mass of radiating 



