30 



Assuming the potasli to be contained in the albite this 

 gives the approximate mode : — 



Quartz 287 



Albite (Abgg) 68-31 -i.i 



Kaolin 2-8/^^^ 



Ilmenite ... ... ... ... '2 



neglecting the small amounts of magnesia and hydrated iron 

 oxides. The non-determination of PoO^ introduces no appre- 

 ciable error. 



The rock is then a true soda aplite. Its principal 

 variation from the majority of similar rocks lies in its very 

 small percentage of the iron oxides, and perhaps of titania 

 also. 



(8) GREISENISED SODA GRANOPHYRE. 



As mentioned above, this rock outcrops at the north-east 

 end of Port Elliot at the back of the red aplite, and only 

 touches the coast at Middleton Beach. 



The relative proportions of the constituents are found to 

 vary from place to place in the mass, and two specimens have 

 been selected and sectioned which illustrate this variation. 



Specimen A, from the more southerly part of the outcrop, 

 is of a pink colour, very fine grained, and shows well the spor- 

 adic distribution of muscovite, which occurs as little clumps or 

 ''books" irregularly scattered, and often associated with 

 quartz which is somewhat coarser than usual. 



Microscopically the minerals visible are quartz and albite 

 in roughly equal proportions, with muscovite in very subordin- 

 ate amount. The original fine granophyric fabric is still 

 strikingly characteristic of the rock ; generally the quartz, in 

 irregular radiating vermicular stringers ramifies through the 

 felspar, but occasionally the converse holds. The felspar, a 

 pure albite, is very heavily kaolinized as a rule, and stained 

 a brownish colour. 



The occurrence of the mica is interesting; for the most 

 part it is in irregular grains of medium size closely associated 

 with medium-sized quartz grains. This coarser mica forms 

 irregular patches and veins in the finer granophyric portions 

 of the rock, and is occasionally in radiate or plumose aggre- 

 gates. But the mineral is also seen to be replacing felspar : in 

 some places the mica-replaced parts of a felspar grain appear 

 as though in graphic intergrowth with the rest of it, and 

 again where the felspar of a graphic quartz-felspar inter- 

 growth has been replaced there is produced the effect of a 

 quartz-muscovite intergrowth. But for the undoubted evi- 

 dence of replacement by muscovite one might be tempted to 



