218 BROWN & HERON: GEOLOGY AND ORE DEPOSITS OF TAVOY 



deposit, crystals of about the size of a large coffee bean with rounded 

 edges can be picked out of the finer material. 



Further down still, finer, rather angular material is found, 

 while in the true alluvial sands and gravels the fine concentrated 

 or ■ takes on a rounded form. Here it is associated with magnetite, 

 ilmenite, topaz, garnet, zircon and sometimes with small amounts 

 of monazite and gold. 



Cassiterite crystallizes in the tetragonal system, it has an im- 

 perfect cleavage, a subconchoidal fracture and a grey streak. It has 

 a hardness of 6i and cannot be scratched with a knife. Its lustre 

 is adamantine to splendent. Its specific gravity is 7. 



It may be and is mistaken for many other minerals, and its den- 

 sity, hardness and the lustre of its crystal faces must be used to 

 distinguish it from them. When such characters are masked, 

 as they may be in the field, it is always safest to perform chemical 

 tests. Though the contrary has been asserted, it is often impos- 

 sible to distinguish small amounts of cassiterite in fine-grained 

 mixed wolfram concentrates and the only reliable means of identifica- 

 tion is a chemical test. 



VEIN MINERALS ASSOCIATED WITH WOLFRAM AND CASSITE- 

 RITE. 



Native Metals. 



The bright silver-white and very brittle native bismuth has been 

 found in situ at Kanbauk in one of the lower 

 workings, where beautiful specimens of wol- 

 fram in long thin crystals separated by narrow bands of the vein quartz 

 matrix occur. The edges of the crystals are lined with narrow 

 films of native bismuth, which also penetrates into cracks in the 

 wolfram and is scattered in the quartz between. The metal is 

 very characteristic with its well-marked cleavage, metallic lustre 

 and platy structure. Fresh surfaces tarnish rapidly in the atmos- 

 phere of Tavoy. Specimens of native bismuth, which appear to 

 come from a vein, have been obtained at the London and Burmese 

 Wolfram Co.'s Paungdaw mine, but the exact locality i; not known. 



Native bismuth also occurs in detrital deposits, in which irregu- 

 lar pellets re found surrounded by black decomposition crusts of 

 hydrous oxide, or. if the decomposition has proceeded further, by 

 a grey crust of other oxy-compounds in which a kernel of the native 



