128 



earlier-formed constituents are imbedded. Subordinate to 

 the orthoclase in amount, plagioclase is present, an acid 

 oligoclase, generally in grains of a roughly prismatic out- 

 line, but often in large poikilitic patches like the orthoclase. 

 Microcline is fairly abundant, occurring both interstitially 

 and in the orthoclase. In the plagioclase some also occurs, 

 but the twinning-plane of the microcline is not necessarily 

 parallel with that of its plagioclase host, as was the case in 

 the Houghton rocks. A few grains of microperthite are pre- 

 sent. Diopside was originally present, but has now become 

 a pale uralite, whose fibres extend far beyond the limits of 

 the original crystal. The alteration was accompanied by the 

 formation of very fine epidote, which considerably clouds the 

 mineral. Idiomorphic crystals of magnetite are present in 

 large amount, and appear quite fresh, though haematite occurs 

 as a cementing material in parts of the slide, giving rise to 

 the brownish streaking of the rock. Quartz occurs in small 

 amount. Ilmenite is indicated by numerous oval patches of 

 titanomorphite. Name. — Diopside quartz syenite. 



A very interesting type of syenite was found included 

 in a pegmatite vein near the track leading down to Mr. 

 Smith's homestead (Section 1133, Onkaparinga) . It is grey 

 in hand specimen and obviously sericitic. Microscofically 

 (slide 555) it appears to be related to the rock that occurred 

 near Mr. Scrymonger's by the Torrens(see page 119). It is 

 much clouded ■ with sericite. The predominant mineral is 

 microperthite, occasionally microcline-microperthite, the 

 microcline lamellae, however, being very small. Biotite occurs 

 in a small amount, forming rather large flakes and inter- 

 grown with muscovite. Included in the biotite are grains of 

 ilmenite considerably leucoxenized. In one. instance sphene 

 was present in a biotite grain, probably secondary. A few 

 grains of quartz occur interstitially. Name. — Mica syenite. 



Opposite Mr. Melrose's house on (or near) Section 1133, 

 Hundred of Onkaparinga, there is another occurrence of 

 j^lutonic rock intruding the Pre-Cambrian schists, (20) but 

 absent from the overlying ilmenite grits. This is composed 

 predominantly of oligoclase, occurring in granular anhedrons 

 rather kaolinized, and with the formation of a great deal 

 of secondary mica, probably paragonite, in between the 

 grains. Orthoclase is present also, but is quite subordinate. 

 The femic silicate is a biotite almost completely changed to 

 chlorite, with the separation of secondary magnetite. Titan- 

 iferous magnetite is present in irregular grains fairly fresh,, 

 while rounded, clouded grains of titanomorphic occur. A 



(20) W. Howchin, op. cit., p. 252. 



