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mass are numerous short colourless prisms, with occasional 

 cross fracture, high refractive index and birefringence. The 

 extinction angles are in general very small, but sometimes 

 nearly 30°. The prisms are often terminated by prism 

 faces. These are most probably cyanite. There are also 

 areas of brownish material, which on close examination appear 

 to consist of highly-refractive short crystals set in a brownish- 

 cloudy material. These are rutiles, after ilmenite. Rutile 

 occurs separately in single crystals, brown in colour and 

 strongly pleochroic. 



Similar to the last rock, but more closely allied to the 

 Houghton rock described previously, is the schist intruded 

 by rocks of the Pre-Cambrian titaniferous series at Yanka- 

 lilla. It consists of angular grains of quartz, with a sub- 

 ordinate amount of microcline, whose general outline indi- 

 cates a certain schistosity set in a groundmass quite subor- 

 dinate to the larger grains of mica sericite and biotite, with 

 fine granulitic quartz. Both this slide and the last appear 

 to present an approach to the blastopsammitic structure of 

 Grubenmann. 



From the hill above the gorge on the road from Norman- 

 ville to Second Valley (Section 1103, Hundred of Yanka- 

 lilla) there occurs a number of beautiful gneisses, very 

 diflFerent in macroscopic appearance from those described 

 above, but microscopically plainly related to them. The 

 Yankalilla rock was very siliceous in nature — almost a quartz- 

 ite — and showing only to a small extent its schistose charac- 

 ter. The rock from the gorge is a dark-green, and showing 

 a strongly schistose "augen" structure, the "eyes" of pink 

 felspar or quartz being several millimetres in diameter. Its 

 base consists of quartz and mica in alternate narrow, layers. 

 Microscofically it is completely schistose, the quartz occur- 

 ring in long lenses of granulitic structure between long bands 

 of fine sericite and biotite. The quartzes are somewhat 

 strained, and the mica lamellte bent. The eyes are porphyro- 

 blasts of quartz and microcline, the latter predominating. 

 Both are considerably granulated at the edges. The micro- 

 cline contains perthitic inclusions of albite, the host being 

 oriented apparently without reference to the schistosity plane 

 of the rock. The quartz would appear to have recrystallized 

 before the microcline, or to have exerted stronger power of 

 crystallization, as wherever the two minerals are in contact 

 as porphyroblasts the quartz intrudes the felspar. A few 

 crystals of magnetite are present, arranged in parallel bands. 

 Some oval grains of rutile occur also. The structure of the 

 rock as a whole may be defined as porphyroblastic, with a 

 iepidoblastic groundmass. 



