BY D. MAWSON. 



599 



blue with only moderate absorption-variation. Modifications 

 may even appear in the uralite of a single individual. 



An interesting case was noted in a section prepared from one 

 of the smaller pegmatite veins where, in a primary amphibole, a 

 deep blue boi'der zone appeared sharply 

 differentiated from a lighter-coloured central 

 area (see fig. 4). The exact orientation of 

 the section was not ascertainable, though 

 evidently in the prism zone near the b face. 

 The border zone, with pleochroism bluer in 

 the b direction, corresponds more nearly to 

 riebeckite, and the lighter-coloured central 

 area to arf vedsonite. The deeper blue colour 

 is very likely significant of relatively in- 

 creased ferric iron molecules. 



Felspar— The felspars are large well- 



Fig. 4. — Arf vedsonite 

 with a border-zone of 



formed idiomorphic orthoclase crystals which, riebeckite. 



from their clear and glassy characters, are 



true sanidines. They are beautifully tabular on the b face, show 



a strongly marked cleavage parallel to the a face, and almost 



universally exhibit carlsbad twinning. 



In the hand-specimen, they are seen to be an early product of 

 crystallisation, other minerals appearing, for the most part, 

 pilotaxitically arranged in the interspaces between the sanidine 

 tablets. A microscopic examination shows occasional enclosures 

 of both arfvedsonite and segirine fragments always, however, 

 very small. At other times, there is evidence that the latest 

 stages of felspar-growth have been concurrent with the earlier 

 development of the larger arfvedsonite individuals. There is a 

 tendency for decomposition to take place along the cleavage 

 planes, and when sufficiently far advanced, by the deposition of 

 opaque dust-like matter often of a reddish hue due to iron oxide 

 stain; twinned individuals present a herring-bone appearance. 

 Concurrently with the deposition of decomposition products 

 along the cleavage traces, corrosion of the border has been noted. 



