139 



magma, and its occurrence here forms another link in the 

 chain of Pre-Cambrian intrusions. Macroscopically the rock 

 is chiefly composed of pale-pink felspar, with light-green 

 pyroxene, large and small masses of ilmenite, chiefly aggre- 

 gated into lenses. Brown sphene can be distinctly seen with 

 the aid of a handglass. Microscopically the felspar is ortho- 

 clase, in irregular, often interlocking, areas, sometimes per- 

 thitic, sometimes passing into microcline. Acid plagioclase is 

 also very abundant, and occurs either as large areas poikil- 

 itically enclosing potash felspar, or in small subidiomorphic 

 prisms. Diopside is present, almost colourless, though rather 

 ■clouded. On the periphery it passes into pale, weakly pleo- 

 chroic amphibole. Ilmenite is present in large amount in 

 irregular grains and aggregates. It was one of the first min- 

 erals to crystallize. Sphene is present in small amount, in 

 pale-brown, rounded grains. Quartz is a minor accessory. 

 Yellow-green epidote is scattered about. The texture does 

 not appear to be primary. Rather it suggests a recrystalliza- 

 tion, in which the ilmenite became aggregate in layers ; the 

 plagioclase, poikiloblastic ; and the potash felspar, diablastic. 



Name. — Diopside syenite. 



It is scarcely necessary to point out the close mineralogi- 

 cal affinity of this rock with those described above. 



APPENDIX. 



List of rocks occurring in South Australia which have 

 been petrologically described and which show points of simi- 

 larity to rocks of the Houghton magma : — 



Granites. — Palmer (ilmenite and sphene present) — C. J. Moul- 

 den, Trans. Roy. Soc, S.A., vol. xix., p. 76. 

 Kaiserstuhl, like that of Palmer, but crushed — C. J. 

 Moulden, loc. cit., p. 77. 



Many others contain magnetite, but it is not reported 

 whether or not this is titaniferous. 



Pegviatite. — Everai-d Ranges, with irregular secretions of 

 magnetite — H. Basedow, Trans. Roy. Soc, S.A., vol. 

 xxix., p. 77. 



This rock (if the magnetite is titaniferous) is very like 

 the ilmenite pegmatites of Yankalilla and the Grey Spur. 



Quartz Syenite. — Neale River, accompanied by magnetite — 

 J. J. East, Trans. Roy. Soc, S.A., vol. xii., p. 39. 



If the magnetite is titaniferous this may possibly belong 

 liere. 



Syenite porphyry. — ^Yarden, Gawler Ranges, augite pheno- 

 crysts with soda orthoclase in the groundmass — C. J. 

 Moulden, loc. cit., p. 75. 



Diorite — Hall River, 23'40 S., contains hornblende, plagio- 

 clase, epidote, augite, secondary quartz, apatite, and 

 sphene; the rock has become crushed, as indicated by a 

 tendency to become schistose — C J. Moulden, loc. cit., 

 p. 74. 



