117 



district. In some instances, particularly in the case of those 

 rocks which form the eastern vein on the Gumeracha Road, 

 the texture is coarse and granitic, the grain being between 3 

 and 5 mm. in diameter and the various constituents evenly 

 distributed. Other rocks are much finer in grain, the size 

 being only 1 or 2 mm., or even less. The majority of 

 the rocks, however, has a marked gneissic banding, the col- 

 oured constituents being concentrated into parallel bands, 

 which may be from several centimetres to only a millimetre 

 in width. Examined microscopically the white bands (fel- 

 spar) show little or no parallelism of grain disposition, either 

 crystalloblastically or in optical orientation. The coloured 

 minerals, however, show a strong parallelism, the vertical 

 axes of the pyroxenes and uralites, the longer diameter of 

 the sphene grains, and the elongation of the irregular ilmen- 

 ite masses, as well as the plates of crystallized ilmenite, being" 

 in general parallel to the general direction of banding. This 

 banding direction varies a great deal on the field, but roughly 

 is parallel to the schistosity of the Pre-Cambrian rocks, dip- 

 ping easterly and striking generally slightly west of north. 



The effects of pressure are to be seen in the shadowy ex- 

 tinctions of the felspars and quartzes, the bending and occa- 

 sional faulting of the plagioclase lamellae, and, chiefly, in 

 the presence of a finely granular mixture of felspar and some- 

 times quartz that surrounds the grains of those minerals. 

 This marginal crushing is present in most, though not in all, 

 of the rocks examined. The finely granular aggregate has 

 taken on a structure resemblinsf the granoblastic structure of 

 Grubenmann, and the rockmass, being of a slightly altered 

 granite texture, may be said to possess the blasto-granitic 

 structure defined by that author. (i^) In some cases, however, 

 the relict texture is so slightly developed as to render the 

 simple term granitic texture more applicable than blasto- 

 granitic. 



In the gneissic aplite which occurs in the quarries just 

 behind the Houghton school-house (Yatala, Section 5519), the 

 texture shows most clearly the effect of recrystallization under 

 pressure. Here the falspar and quartz, much strained, are 

 disposed in long parallel lines clearly visible macroscopically. 

 These bands are not due to original graphic intergrowth 

 merely, as the lines extend beyond the boundaries of indi- 

 vidual felspar crystals. Rather, it must be taken as an effect 

 of rock flowage, the "schistose by crystallization" (Krystalli- 

 zationschieferung structure of Grubenmann) . (is) 



(14) Die Krystallinen Schiefer. Bd. i., p. 83. 

 '15) Op. cit. sup., p. 88. 



