Geology 



and Tourmaline (a complex boro-silicate of aluminium, 

 iron, and magnesium). 



Examining the surfaces of granite exposed in the 

 mountain-side, we came across numerous veins of quartz 

 containing masses of tin-stone. The quartz was arranged 

 in crystals, having their greatest length at right angles 

 to the sides of the vein, forming what is known as comby 

 structure, while the tin-stone occupied the centre of the 

 mass and varied in thickness from two to six inches. A 

 section across the vein and the surrounding granite was 

 as in Fig. 33. The granite on both sides of the vein 



FIG. 33. Section of Mineral Vein, a, Granite ; b, Greisen ; 

 r, Quartz ; d> Ore. 



had suffered considerable alteration, it was much harder 

 than the general mass and contained much secondary 

 quartz, Topaz (Al,F) 2 SiO 4 and Fluorspar (CaF 2 ). This 

 variety of altered granite is called greisen, and is con- 

 stantly associated with tin veins. As will be seen, it 

 throws much light on the mode of origin of the tin 

 deposits themselves. 



For some distance on each side of the zones of 

 greisen, the granite is very soft and highly decomposed, 

 the felspars having been converted into Kaolin probably 

 by the action of steam and carbonic acid. 



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