752 



'JlIK TIN-DEPOSITS OF NEW ENGLAND, N.S.W , I., 



shows that they pass over almost imperceptibly from vein-stuff 

 into the soft decomposed granite. A type-section through one of 

 these veins is given in text-fig.9. The centre of the vein is com- 

 pased of translucent quartz, as opposed to milk-white, opaque 

 quartz on the one hand, and clear transparent quartz on the 

 other hand. This central quartz passes rather abruptly, yet 

 without discontinuity in composition, into a quartzose vein-stuff. 

 This latter is highly siliceous, and is composed mainly of granular 

 quartz, the grains being from 1-2 mm. in diameter. Intimately 

 distributed through this matrix of quartz, and abundantly 

 present, is a variety of mica. This is light to dull green in 

 colour on a fresh fracture, but weathers reddish-brown on 

 exposure. These mica-flakes are quite small, the largest attaining 

 a diameter of about 2 mm. Inter-crystal cavities are quite 

 abundant, and these are frequently filled with limonite (see 

 Plate Ixi., tig. 2). The central quartz usually constitutes of 

 about one-fifth, but occasionally occupies as much as one-half of 

 the entire width of the vein. Vughs are common in both the 

 central quartz md quartzose, and in some of the larger veins are 

 as much as a foot in diameter. They are typically lined with 

 crystallised quartz, the long axes of the crystals being normal to 

 the walls of the cavities. Mica and hydrous iron-oxides are also 

 common fillings of these vughs. 



The central quartz baud of the veins usually exhibits well 

 developed comb-structure, and ca.ssiterite may be found crys- 

 tallised between the quartz-crystals. The cassiterite was 

 invariably found near the centre of the veins, either irregularly 

 distributed through the central quartz, or, as is more frequently 

 the case, forming a casing on eitiier side of it. Cassiterite also 

 occurs in vughs. It is usually crystallised, the average length of 

 the crystals being about one-quarter of an inch. In many' 

 instances crystallised quartz may be seen abutting on to crys- 

 tdlised cassiterite, the latter preserving its normal form, and 

 thus indicating its prior crystallisation. The cassiterite is dark 

 brown to black in colour, and assays from 73 to 76 per cent, 

 metallic tin, after being cleaned by the " Willoughby." Fractured 



