224 BROWN & HERON: GEOLOGY AND ORE DEPOSITS OF TAVOY. 



rare mineral in Tavoy, as far as our knowledge at the time of writing 

 goes. 



Tn the southern portion of the Hermyingyi lease and close to 

 its western boundary with the Hermyingale 

 Topaz-fluorito rock at y(i j n ^.j^ a un i ( j lie mineral association 



Hcrmyineyi. _ __ , , n 



is w(M-ked. The Kanbanonaung flows across 

 its strike. As the locality is very thickly wooded and raining 

 operations have been insignificant, a oomplete account cannot be 

 given, but the outcrop of the vein can be traced from the bottom of 

 the valley for some distance up its steep southern slope. The 

 country rock is decomposed argillite of the usual type. The vein 

 itself is 10 oi L2 inches in thickness, of dense, opaque, white quartz 

 carrying altered mica,, cubes of pyrite, wolfram, often in bladed 

 crystals, and brown cassiterite. On the hanging wall side there 

 is a band of topaz rock containing fluorite, about 12 inches in 

 thickness, in contact with the vein but separated from it by a definite 

 plane along which movement has taken place, as sliokensiding is in 

 evidence. The vein itself has also Buffered various movements, 

 as is proved by the character of the quartz, the contortion of the 

 mica and the fracture and recementing seen in some of the large 

 pyrite cubes. The country rock is a silicified argillite throughout 

 which very numerous small crystals of pyrite are scattered. The 

 topaz-fluorite rock is a loosely compacted, fine-grained aggregate 

 of the two minerals. The topaz is usually white and either in 

 granules or more or less imperfect crystals, in which the perfect basal 

 cleavage and the vertical striatums of the faces in the prism zone are 

 well developed. The fluorite is colourless and semi-translucent as a 

 rule, though pale brown and faint green tints are also seen in some 

 hand specimens. The rock often shows a tendency towards band- 

 ing in which one mineral is developed at the expense of the other. 

 Certain bands, upwards of i inch thick, are practically pure granular 

 topaz. Where there has been more room for development larger 

 crystals are the rule, the topaz prisms then being up to £ inch across 

 and the colourless fluorite pieces larger. It would appear as if the 

 topaz was the first mineral to crystallize and that the colourless 

 fluorite filled in the interstices afterwards. Between the topaz- 

 fluorite rock and the vein wall there is sometimes an edging of a grey- 

 ish-green, fine-grained mica-rock, which contains small scattered 

 crystals of brown cassiterite and bands of coarsely crystalline mica 

 of muck the same colour. Cassiterite and wolfram both occur 



