538 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



structure very perfectly in places, and is variable in 

 quantity, ranging from 3 to 8 per cent, in different parts of 

 the normal pegmatite, whilst basic patches occur here and 

 there much richer in ilmenite. Apatite is abundant and 

 its prisms frequently surpass 3 mm. in length. 



Aplites. 

 Megascopic Description, — Colour : White, grey or pale 

 green. Grainsize: (a) the fine-grained type 0*1 to 0*15 mmJ 

 (o) the medium-grained type 0*25 to 0*75 mm.; (c) the 

 coarse-grained type 1 to 2 mm. Minerals visible: felspar, 

 sometimes pyroxene, rarely biotite. Miarolitic structure 

 is not uncommon, particularly in the coarser types. 



Microscopic Description, — Texture : Holocrystalline, 

 almost panidiomorphic-granular. Grainsize, see above. 



Minerals present: — 

 Original — Albite 



Orthoclase (? anorthoclase) 

 Pyroxene (diopside,augite, and segyrine-augite) 

 Biotite (rarely) 

 Ilmenite 

 Apatite 

 Secondary — Analcite 



Ohloritoid and chlorite 

 Calcite 



The Felspars, 

 These, which consist of albite and orthoclase (? anortho- 

 clase), make up from 70% to 85% of the rock. They are 

 always idiomorphic and thick-tabular in habit. Under the 

 microscope they are seen to be much kaolinised, while 

 some crystals show alteration into analcite. The chloritoid 

 decomposition products are frequently present, sometimes 

 marginally arranged. The albite crystals have a R.I. 

 distinctly lower than that of the Canada balsam, and in 

 sections symmetrical to the albite lamellae, give extinction 



