g6 Armitage, Country about West Essendon. 1^^%^^^^' 



sparingly present, and seem to have been the last original 

 mineral to crystallize out. These are seen to extinguish from the 

 centre outwards in such a way as to show a composition ranging 

 from oligoclase through albite apparent>y to orthoclase, the 

 edges consisting probably of orthoclase. There is also an in- 

 dication in parts of extremely minute cross-hatching or micro- 

 cline twinning. These broad crystals are most probably anortho- 

 clase. The other minerals present are pale violet-brown inter- 

 stitial grains of titaniferous augite and irregular coarse needles 

 and small patches of magnetite. Needle-shaped microlites of 

 felspar are also abundantly present. Apatite appears to be 

 quite absent. 

 No. 3. — Greywacke, from north cliff face, Horseshoe Bend. 



Macroscopically, this is an exceedingly indurated, firmly- 

 compacted rock of various light colours, as white, creamy- 

 coloured, and grey, in many cases banded with fine lines of a 

 darker colour. Its texture is exceedingly fine, no structure 

 being visible even with a lens. It rings under the hammer, 

 and breaks with a conchoidal fracture, giving sharp cutting 

 edges. For this reason it was, in the past, freely used by the 

 aborigines, who made flakes, chips, and skinning knives from it. 



Microscopically, it is seen to consist essentially of very minute 

 granules of quartz and a fine dust of decomposed turbid felspar. 

 Tiny flakes of brown mica and of white mica are present, and a 

 black dust, which may be ilmenite or magnetite, is disseminated 

 amongst the other granules. Numerous small particles of an 

 iron oxide are arranged so as to present a banded appearance. 

 Tiny crystals of zircon and tourmahne also occur. The whole 

 of the above are firmly cemented together by interstitial 

 chalcedonic and amorphous silica. 

 No. 4. — Quartzite, from spur south of the Sand Pits. 



Macroscopically, this is an extremely hard, ringing, ferriferous 

 rock, ranging in texture from very fine to conglomeratic, and 

 in colour from white, through greys and yellows, to dark red. 

 It breaks with a fracture varying from conchoidal to flat. 



Microscopically, thin slices show the rock to be mainly com- 

 posed of large rounded water-worn, and smaller angular grains 

 of quartz. Felspar grains are very sparsely present. In one 

 case a felspar grain is saturated with silica. Most of the quartz 

 grains are fragments of single quartz individuals, but in some 

 cases the grains consist of many differently orientated, inter- 

 locking areas, such as can be seen, for example, in a thin section 

 of a granodiorite. Several grains of the sand show a micro- 

 graphic intergrowth of quartz and felspar. In the quartz 

 fragments are inclusions of zircons, brilliantly polarizing and 

 showing high relief, numerous small crystals of apatite showing 

 low polarization colours, brown mica, small crystals of turbid 



