134 



No. 592, which also occurs at the hill above the gorge, 

 is micro ^rojncolh/ a very handsome rock (plate iv., fig. 9). 

 It contains the same minerals as 593, but the micas 

 are present in much smaller amount. The structure is thor- 

 oughly schistose. The predominant mineral is quartz, occur- 

 ring chiefly in long-drawn-out patches, with a most irregular 

 outline, while between the streaks of crystal the small amount 

 of groundmass is almost granulitic. The qiiartz is highly 

 strained, so that two jjortions of one and the same grain may 

 have their vibration directions as much as 60° apart. Small 

 crystals of microcline, somewhat less strained, also occur in 

 the groundmass. Increase of size of the irregularly-bordered 

 quartzes would give rise to an augen structure with regard 

 to quartz, but in the present rock its blastoporphyritic growth 

 has not proceeded as far as this. This augen structure is in- 

 duced by the presence of large crystals of microcline forming 

 eyes slightly lenticular, the diameter perpendicular to the 

 schistosity, being much smaller than that in the par- 

 allel position. The ratio is about 3 to 5. (That of the quartz 

 patches is nearer 1 to 6.) The microcline is plainly blasto- 

 porphyritic. It is not marginally granulated to any extent, 

 and includes quartz grains and portions of sericite still 

 arranged in the position of schistosity, which is due to a 

 growth of pseudo-porphyritic grains (blastoporphyritic of 

 Grubenmann) affe/- the formation of a schistose structure in 

 the rock by metamorphosis under mass-mechanical conditions. 

 That mechanical conditions have been present after the for- 

 mation of the blastophenocrysts is shown by the cracking of 

 these and the formation in them of granulitic quartz veins. 

 Microcline in considerable amount also occurs in smaller 

 grains in the groundmass of the rock. The characteristic 

 twinning is often much obscured (moire) ; it is moulded on 

 quartz, sometimes taking on almost a blastopoikilitic struc- 

 ture. Sericite and biotite form strings passing in a roughly 

 parallel direction through the rock, bending to one side or 

 the other around the phenocryst, though sometimes passing 

 through it. Rutile grains also occur. Slightly oxidized 

 titaniferous magnetite is also present. 



There are also in other Pre-Carabrian areas schists 

 closely related to these. In the Humbug Scrub there is a 

 large area of augen gneiss. As one works up the creek run- 

 ning westward from the Princess Alice Mine, for instance, 

 the schists are noticed to get gradually more and more meta- 

 morphosed, and eyes of pink felspar appear till at the mine 

 the rock is a perfect augen gneiss, the eyes being more 

 than an inch in diameter. Facts so far learned are insuffi- 

 cient to decide how far these "eyes" are porphyroblasts deve- 



