BY D. MAWSON. 



591 



the cai'bonates from the unaltered mineral; under high power 

 this belt was found to consist chiefly of very fine granular deep 

 blue amphibole. 



Direct carbonation, or what appeared to be such, was noted 

 only in one or two rare instances. 



In zoned crystals, a strongly coloured portion with extinction 

 angle about 12° and intermediate in composition withstands 

 decomposition most effectively, often remaining as the sole 

 remnant of the former composite individual. 



The segirine-augite kernels, when decomposed, are more usually 

 found altered chiefly to a yellowish-brown chlorite-like substance 

 with a small admixture of carbonates. The text figure (fig.2) 



JCompound chlorite-like^ (Carbonate decomposition 

 {decomposition products, (products. 



Arfvedsonite . 



Aegi'rtne . 

 Ulntermediate bright green zone, 

 Aegirine-Augi te (seldom attacked by decomposition. 



Fig.2. — Illustrating a common form of decomposition in the case of zoned 



pyroxenes. 



represents an actual case noted in one of the smaller veins, and 

 is very instructive as showing all the various peculiarities above 

 described. 



Arfvedsonite. — The arfvedsonite is found in association with 

 well developed fegirine crystals and plentiful glassy sanidines; 

 zirconium and titanium minerals are present as accessories; 

 chalcedony and quartz, the products of a second cycle of deposi- 

 tion, are usually abundant (Pl.li., fig.2). In some of the veins 

 the chalcedony is brownish in colour, due to the presence of 

 ferric iron, and is then indicative of certain changes having taken 

 place in the accompanying minerals. In such locations much of 



