98 ME. C. E. TILLET ON THE GEANITE-GNEISSES [vol. Ixxvii, 



cannot be regarded as antecedent to the crushing ; but it is certainly 

 produced in the period immediately following mechanical movement, 

 this movement affording channels for the circulation of solutions, 

 possibly at a locally slightly-elevated temperature developed fric- 

 tionally. This myrinekite is thus of purely metamorphic origin. 



YI. The Petro&eaphy or the Amphibolites aivd Allied 

 Types in the Flindees Seeies. 



(a) The Amphibolites and Hornblende-Schists. 



The dark basic bands of the Flinders gneisses can be separated 

 into two divisions, these divisions depending on the nature of the 

 dominant ferromagnesian mineral. Those bands which are defined 

 as amphibolites contain hornblende as the predominant ferro- 

 magnesian mineral, although it may be accompanied by pja-oxene, 

 of diopsidic constitution, and occasionally by garnet. In the 

 second division, the dark hue of the rock is usually less pronounced, 

 and pyroxenes form the dominant magnesian minerals. Usually 

 both rhombic and monociinic pyroxenes are present, and hornblende 

 may appear. 



If we turn now to the rocks of the first class, the amphibolites 

 may best be described as dark basic masses bearing the stamp of 

 mietamorphosed rocks, the essential constituents of which are 

 plagioclase and hornblende. The nature of the amphibolites can 

 be sufficiently indicated by a description of one type, and a detailed 

 account will therefore be given of an amphibolite occurring on the 

 shore at Kirton Point, Port Lincoln. 



This amphibolite is intercalated in the gneisses as a long band, 

 the orientation of which is dependent on the ])rimar3^ foliation of 

 the orthogneisses — the longer axis of the inclusion being parallel 

 to this foliation. It is a dark rock with glistening crystals of 

 hornblende, and has itself a distinct foliated structure. Under the 

 microscope the constituents seen to be present are hornblende, 

 plagioclase, with accessory quartz, magnetite, apatite, and a few 

 isolated grains of pyroxene. The foliated structure of the rock is 

 apparent in thin section by the parallel orientation of the horn- 

 blende. 



The hornblende is subidioblastic, with well-developed amphibolic 

 cleavages, Zac=:20°. The pleochroism is 



X= yellowish-green, Y= brownish-green, Z=green, and the intensity Z >^Y>X. 



In the plagioclase, twinning after the albite, Carlsbad, and 

 pericline laws can be observed, but the two last-mentioned laws are 

 not characteristic for these grains. Some of the plagioclase is 

 wholly untwinned. Sections perpendicular to (001, 010) give an 

 extinction of 13°, thus corresponding to an andesine of composition 

 Ab^An^. Most sections examined give positive birefringence, but 

 a few indicate a negative character. The refractive index has 

 nearly the same range as quartz. Grains showing negative optical 

 character indicate a transition to oligoclase-andesine. 



