132 



•cally. The latter often encrusts large crystals 10 to 15 mm. 

 across of magnetite, which forms in rough, almost cubical 

 masses. It has a good cleavage, and contains a little titan- 

 ium, as shown by colouration of the sulphuric acid solution 

 with hydrogen peroxide. Its specific gravity is 4'93. Micro- 

 scopically it is composed chiefly of acid plagioclase, the gi'ains 

 often containing grains of quartz. Orthoclase is less abund- 

 ant, and a great deal of magnetite is scattered about in grains 

 and idiomorphic sections. A little muscovite occurs, and 

 some intergranular limonite. 



B. PETKOLOGY OF THE SEDIMENTAKY ROCKS. 



Only a few of the sedimentary rocks have been sectioned 

 irom the districts described. These were collected almost at 

 random, and are very different macroscopically. Microscopi- 

 cally, however, most of them show the closest relationship, 

 and can hardly be other than portions of a single Pre-Cam- 

 brian sedimentary series. The exceptions are of interest as 

 showing other formations than the one mostly represented 

 occurring in the Pre-Cambrian rocks. 



In the Houghton district in a small quarry on Sections 

 560 and 3420, Yatala, there occurs a green fine-grain seri- 

 citic schist, which microscopically is composed of a fine grano- 

 blastic gToundmass of quartz, in which are imbedded small 

 irregular porphyroblasts of quartz and microcline. These 

 porphyroblasts are considerably strained, and appear to have 

 suffered to some extent a marginal granulation, indicative of 

 a return to mass-mechanical conditions after the mass-static 

 ■conditions under which the predominant structure was pro- 

 duced. The quartz has suffered most from the straining. The 

 microcline, being more elastic, has to a large extent escaped 

 granulation. Occupying the greater part of the slide is a 

 mass of sericite in very small flakes, among which are larger 

 partially chloritized biotites, very irregular in outline. These 

 micas give to the rock its dominant structure (the lepido- 

 blastic structure described by Grubenmann), the texture 

 being the lamellae texture as defined by the same author. 

 Magnetite partially altered to haematite and a few rutile 

 needles are also present. 



Somewhat similar to this is the schist occurring in the 

 creek opposite the Pound near Aldgate Station. It is com- 

 posed of angular grains of quartz about A mm. in diameter 

 set in a groundmass of colourless mica and larger flakes of 

 Motite. It is impossible in the slide to get a flake of the mica 

 large enough to determine by its axial angle whether the 

 mica be miiscovite or sericite. The latter appears to be the 

 more probable. Scattered all through the micaceous ground- 



