BY D. MAWSON. 607 



EXPLANATION OF PLATES. 



Plate, xlix. 



Fig. 1. — One of the smaller pegmatite veins of class (b), subclass 1, type a. 

 Magnified 2 diameters. The fine-grained texture of the Eegirine- 

 syenite traversed is noticeable by comparison. Though warped, 

 the sides are sensibly parallel. 



Fig. 2. — A vein, showing flow-structure and a well-marked border-zone, of 

 class (c), type 1. Eeduced to §-natural size. This variety has 

 characters relating it to both the pegmatites and the aplites. The 

 accompanying key-plate brings out more prominently its dominant 

 features. 



Plate 1. 



Fig. 1. — Larger-sized pegmatite vein of class (b), subclass 2, type a. 

 Magnified -| diameters. The abundance of dark-coloured amphi- 

 bole occupying the centre of the vein filling is well shown. 



Fig. 2. — A typical specimen of the segirine-syenite pegmatite. Natural size. 

 The dominant constituents, twinned idiomorphic sanidines and 

 the darker subordinate arfvedsonite are readily distinguishable. 

 It is proposed to adopt the locality name Bowralite to denote this 

 rock type. 



Plate li. 



Fig.l. — Uralitisation of asgirine to arfvedsonite- like amphibole. Magnified 

 13 diameters. The several light-coloured crystals are fegirine; the 

 amphibole, developing, is the large dark area. Note the clear gaps 

 in the amphibole; these are chiefly chalcedony. 



Fig. 2. — Idiomorphic arfvedsonite crystals subordinate to the large sanidine 

 individuals (parts of three of which show on the right), and 

 embedded in secondary silica, largely chalcedony. Before the 

 introduction of this silica, the cavity evidently had a drnsy lining 

 of arfvedsonite and sanidine. Magnified 14 diameters. 



Fig. 3. — A late stage in the decomposition of idiomorphic arfvedsonite. The 

 clear gaps are mainly carbonates. Magnified 12 diameters. 



Fig.4. — Illustrating the decomposition of amphibole. An earlier stage than 

 that represented in fig. 3. There are fewer clear spaces in the section 

 indicating carbonates and the like, and dark lines where partial 

 carbonation and oxidation are in progress are more abundant. 

 The general darker appearance of the marginal parts is original, 

 and is frequently observed in the primary mineral where meta- 

 chemisation is evidenced, as explained in the text. Magnified 13 

 diameters. 



