150 TOURMALIKK-HKAKIXd ROCKS, MT. UKEMSKlRhL. 



which are of common occurrence in tin veins are present in 

 small quantities, viz., bismuthinite, molybdenite (rare), 

 arsenopyrite, chaicopyrite, galena, and zinc-blende (both of 

 these rare, and in very small quantities), fluorite (in small 

 quantities), and smoky quartz. 



The vein rock differs very greatly in the amount of tour- 

 maline present. Often it is composed almost wholly of 

 quartz, but at other times the only quartz present appears 

 to be that which formed the original quartz of the granite, 

 the whole of the felspar being replaced by tourmaline. In 

 other cases the felspar has been replaced by white mica, 

 with or without the addition of quartz and tourmaline. 

 The vein rock then forms a typical greissen. The tour- 

 maline is of two varieties, the black or iron tourmaline and 

 the green or alkali tourmaline. Although tin ore is asso- 

 ciated with both varieties, the green tourmaline appears to 

 be much the more favourable indication for tin. 



The veins vary in width from a few inches up to 20 or 30 

 feet ; in the latter case there are usually a number of 

 parallel fissures filled with tourmaline manning through the 

 vein rock, and the mineralisation has evidently spread 

 outwards from these fissures until the adjacent zones of 

 replacement met in the centre. Often bands or lenticular 

 bodies of unaltered granite exist within the reefs between 

 two such zones. The veins generally run in parallel groups, 

 but often there are several main directions of strike in the 

 same locality. When the veins cro:^s one another there is 

 usually no faulting, pointing to the fact that the fissures 

 were produced by contraction of the granite on cooling. 

 The proof of the metasomatic nature of the vein rock is 

 very conclusive, and may be deduced both from the field 

 evidence and from the examination of thin sections under 

 the microscope. When the veins occur in granite, the 

 vein rock is granular, and the size of the grains is the same 

 as that of the adjacent granite or aplite ; there is no parting 

 between the granite and the vein rock, the one seeming to 

 pass over into the other. When the veins traverse sedi-- 

 mentary rocks, the vein rock, or metamorphosed wall rock, 

 retains the original laminated structure. Original differ- 

 ences in porosity or composition have led to deposition of 

 quartz along certain layers, and tourmaline along others, 

 with the result that a black-and-white striped rock is pro- 

 duced, of very striking appearance. Perhaps the most 

 striking field evidence of replacement is to be seen when a 

 vein passes through a granite or aplite which is rich in 

 quartz tourmaline nodules. The nodules are seen in the 

 same abundance and of the same size in the vein rock as in 



