227 



another. Wherever the refractive index is measurable it is 

 less than that of Canada balsam, so that the felspar has become 

 acid, perhaps as acid as albit-e. The plagioclase of the second 

 generation occurs in the usual lath-like form. Except in a 

 few cases, twinning is not recognizable. The refractive index 

 is less than that of Canada balsam. Chlorite is the predom- 

 inant coloixred silicate, having completely replaced the ori- 

 ginal augite, though occasionally a few fibres of actinolite 

 are recognizable. A little biotite is present among the 

 chlorite, almost certainly of secondary origin. The chlorite 

 occurs between the felspar laths in cloudy areas of exceed- 

 ingly low birefringence. It is associated with much magne- 

 tite, and possibly haematite, forming a black border on nearly 

 every felspar lath, and particularly segregated about the bor- 

 ders of vesicles. A number of rounded red rixtile grains are 

 present also. Quartz has been intruded into cracks in the 

 rock in some amount, also fairly clear felspar (anortho- 

 clase?)(3^j scraps of chlorite, and a few strips of colourless 

 mica. The vesicles are lined with matted-green chlorite, asso- 

 ciated with quartz. Very often the centre of the vesicle is 

 occupied by siderite or ankerite, which is markedly pleochroic. 

 This carbonate is also present in cracks in the rock. As a 

 rule it is accompanied by some limonite. yamr. — Amygda- 

 loidal melaphyre. 



Allikd Rocks. — Apparently belonging to the same 

 magma and eruption period as the Blinman rock is one 

 occurring in a dyke at the Victory Mine near Leigh Creek, 

 a hundred miles north of Blinman. For a specimen of this 

 I am indebted to Mr. M. W. Judell, B.Sc. It is of a light-grey 

 colour, fine-grained, and containing a few vesicles filled 

 chiefly with chlorite. Microscopically the fabric is slightly 

 porphyritic, by reason of the larger size of some of the fel- 

 spars, though this disparity is hardly sufficient to prove two 

 generations of plagioclases. The texture is almost intersertal. 

 Plagioclase is the joredominant mineral, slightly dusty by de- 

 composition. The smaller laths are rather ragged in outline. 

 Twinning is well developed, usually on the albite law, but 

 occasionally the Baveno law appears to be present. The 

 species is albite, shown by the high symmetrical extinction 

 angles (IG"^) and low refractive index, being in all cases 

 lower than Canada balsam. The pyroxene is partly urali- 

 tized and partly converted to chlorite. The uralite is actin- 

 olite, often optically continuous across a break, such as a 

 felspar lath. It is pseudomorphotis after augite that was 



(3) Compare E. B. Baily and G. W. Grabham. "Albitization 

 of Basic Plagiocla.se Felspars," Geol. Mag., June, 1909, p. 254. 



