237 



to be composed of a felspathic paste, sericite kaolin, etc.,. 

 scraps of biotite, and dusty haematite. Set in this 

 paste are elongated calcite crystals, angular quartz-grains, 

 flakes of graphite (?), grains of magnetite, and (rarely) 

 flakes of muscovite. The parallel disposition of these 

 shows that the rock has undergone some schisting 

 pi'ocess. The limonitic segregations are merely areas 

 of the rock, which are particularly rich in iron oxide. The 

 angular quartzes continue right through the coloured patches, 

 their long axes remaining in the direction of schistosity. It 

 would appear that these limonitic patches were formed by 

 the partial or complete replacement in these areas of the 

 felspathic paste by iron-bearing carbonates, siderite, or ank- 

 erite, such a replacement or segregation being roughly spheri- 

 cal or ellipsoidal. In oxidation this passes to limonite, with 

 the result that the segregation becomes merely loose quartz- 

 grains in an uncemented paste of limonite, which washes out 

 very rapidly on exposed surfaces. The alternation of densely 

 with slightly limonitic concentric areas indicates perhaps an 

 original alternation in concentric layers of the iron content 

 of the carbonate, or progress weathering inwards, with slight 

 segregation of the limonite into layers. An explanation 

 similar to this has been advanced by Mr. R. S. Bonney for 

 the far larger clay ironstone "nodules" of the Sydney Wian- 

 amatta shales. 



GNEISSES. 



Two specimens of gneiss are among Mr. Howchin's col- 

 lection, labelled "1 mile west of the Blinman Mine." One 

 is a pink felspathic rock with bands of biotite between layers 

 of pink felspar and quartz occasionally slightly lenticular. 

 Microscopically (see fig. 4, pi. xv.) these bands are seen to 

 be chiefly quartz with a subordinate amount of moire potash 

 felspar. The biotite is slightly chloritized and clouded with 

 limonite, while some chlorite is present in the leucocratic 

 areas. Thin bands of muscovite are present among the 

 quartzes. The amount and disposition of the quartzes sug- 

 gest that the rock is not of igneous origin, and I would ten- 

 tatively place it in Rosenbusch's subfamily of paragneisses, 

 classing it as a conglomerate gneiss. 



The other specimen of gneiss is not so suggestive of a 

 sedimentary origin. It is more felspathic in appearance, bio- 

 tite is quite subordinate, and magnetite is very common in 

 crystals 2 mm. in diameter. Microscopically the gneissic 

 structure is not a very well-marked feature. The predomi- 

 nant minerals are orthoclase and plagioclase, the latter sub- 

 ordinate, and both are dusted with limonite, and slightly 



