28 



equidimensional grains to which the irregular grains of micro- 

 cline (averaging about '8 mm.) act as a kind of mosaic back- 

 ground. The rock is speckled with little flakes of biotite and 

 granules of magnetite, and patches of secondary muscovite are 

 developing from the felspar. The rock may be termed a 

 granophyric potash aplite. 



Specimen No. 3 is from a 'pipe" in the porphyritic 

 granite at Port Elliot, on the Victor Harbour side; with this 

 pipe there is a quartz-tourmaline "nest." 



Under the microscope the rock is seen to have a curiously 

 uneven grain; the greater part has an average grainsize of 

 about '2 mm., but there are irregularly distributed patches, 

 composed mostly of a few quartz grains surrounded by felspar, 

 and in these patches the grainsize may reach 1*5 mm. The 

 dominant felspar is potassic, but lacks the characteristic 

 twinning of microcline; a good deal of albite is also present. 

 Very subordinate biotite and muscovite, and a little magnetite 

 and apatite form the remainder of the rock. Felspar is alter- 

 ing into muscovite in a manner which suggests some other 

 process than mere surface weathering. 



A pipe of biotite-bearing microcline aplite cuts through 

 the coarser granite at Rosetta Head on its western side. It is 

 comparable in grainsize with the coarser of the Port Elliot 

 aplites, and is very poor in albite. 



(7) SODA APLITE. 



Contrasting with the strongly potassic aplites just 

 described there are to be found at Port Elliot another series 

 of very acid minor intrusions in which the predominant alkali 

 is, or was originally, soda, this being expressed mineralogically 

 by the almost complete absence of potash felspar. 



The microscopic description of the soda aplite is as fol- 

 lows : — 



The grainsize is about 1 mm. and fairly even, although 

 some of the felspar attains a length of 2'5 mm. A Rosiwal 

 measurem.ent showed 32*5 per cent, by weight of quartz and 

 67*5 per cent, of albite, neglecting very small amounts of apatite 

 and iron ore, mostly altered to haematite. Both of the main con- 

 stituents are thoroughly allotriomorphic as a rule, showing the 

 usual aplitic type of simultaneous crystallization, but in one 

 part of the slide examined there is a tendency to columnar 

 habit in the plagioclase, to which the quartz plays in part an 

 interstitial role. No traces of granophyric intergrowth are 

 visible. Quartz is fairly free from inclusions but is markedly 

 strained. Albite is twinned according to the ordinary albite 

 and occasionally to the carlsbad and pericline laws, but occurs 



