226 



THE Basic Rocks of Blinman, South Australia, with 



NOTES ON associated OR ALLIED ROCKS. 



By W. N. Benson, B.Sc. 



[Read October 5, 1909.] 



Plate XV. 



In his valuable paper entitled "A General Description 

 of the Cambrian Series of South Australia," Mr. Howchin 

 refers to the occurrence of a large series of basic rocks in 

 volcanic dykes and necks near Blinman, in the Flinders 

 Ranges, some 260 miles north of x\delaide.(i) Through his 

 courtesy I have been enabled to examine a number of these 

 rocks, and here present the results of my studies, together 

 with some notes on allied or adjacent rocks. 



The basic rocks may broadly be divided into two groups 

 — the melaphyres and the diabases. The melaphyres are in 

 general fine-grained, often amygdaloidal. The diabases are 

 coarse-grained, with recognizable white or faintly-coloured 

 crystals of felspar in a dark-green base, sometimes determin- 

 able as pyroxene. Occasionally the rock is slightly porphy- 

 ritic. In both groups, as will appear in the following notes, 

 considerable alteration has taken place both of the felspars 

 and of the ferromagnesian minerals. 



I. — The Melaphyres. 



An example of this type of rock occurs in a volcanic 

 neck at Blinman South. It is a vesicular, light-grey rock, 

 the cavities being filled with siderite, or ankerite, limonite, and 

 chlorite. It is rendered porphyritic by idiomorphic felspar 

 phenocrysts. Microscopically (see pi. xv., fig. 1) it appears 

 probable that this rock on solidification was hypocrystalline 

 and had a structure rather resembling the microgranulitic 

 structure described by Judd '2)^ but differing by the presence 

 of a little glass. The felspar of the phenocrysts is very much 

 altered by the formation of white mica, so that determina- 

 tive extinction angles cannot be obtained. Sometimes, also, 

 a plagioclase crystal is broken up into small areas, the ex- 

 tinction directions of which are slightly inclined to one 



W Proceedings Aust. Assocn. for the Advancement of Science 

 xi., Adelaide, 1907, p. 418. 



(2) Quart-. Journ. Geol. Soc, 1885, p. 68. 



