192 BROWN & HERON: GEOLOGY AND ORE DEPOSITS OF TAVO?. 



No wolfram, cassiterite nor sulphide minerals have been detected 

 in these veins and, as far as Tavoy district is concerned, the tourma- 

 line pegmatites have no connection with the metalliferous veins. 

 Tho latter have in Tavoy never been found to contain tourmaline 

 and all known occurrences of tourmaline pegmatite are far from 

 wolfram or tin bearing areas, with a single excoption— at tho remote 

 Zinba mine. 



In the Thaton district, however, tourmaline and wolfram occur 

 together in quartz veins and on Belugvun Island, Aloulmein, tourma- 

 line and cassiterite. In Mergui district these tourmaline pegmatites 

 are the principal source of the tin ore. 



Generally the granite is very sparingly jointed, with the result 



that in exposed situations from which 



wo J thorin fc ° S *** decomposed material is rapidly ' removed, as 



on the sea-coast and on steep slopes, it forms 

 great curving surfaces of bare rock with only minor inequalities, 

 and none of the fantastic sculpturing which characterises granite 

 weathering in tropic lands of low rainfall and considerable tempera- 

 ture variations. 



Occasional departures from the general paucity of joints have 

 been noted, as for example the small intrusion on the shore west 

 of Bok, where a coarse-grained and strongly biotitic granite is 

 traversed by close, irregular joints and numerous slippage planes 

 faulting the veins ; Shittaunggyi, the next boss to the south, is 

 an exceptionally pale-coloured quartzose and homogeneous rock, 

 with a system of distinct, persistent, parallel and straight vertical 

 joints, a foot or so apart, trending north-east — south-west and a 

 much less definite set at right angles, also vertical. 



Where weathered material is not readily removed, the granite 

 is thoroughly rotten to a depth of in places as much as a hundred 

 feet, becoming soft and reddish-yellow from the decay of the felspars 

 and biotite, with the liberation of iron oxides and hydrous aluminous 

 silicates. The surface soil is much lighter in colour and more sandy 

 than that formed by the Merguis, so that it is not difficult to tell 

 what is the underlying rock. 



Between the softened outer layer and the solid rock there is a 

 zone where decomposition is proceeding unevenly in ramifying 

 channels, leaving less altered cores of all shapes and sizes, which 

 ultimately succumb to decomposition. Hill torrents cut rapidly 

 through this down to hard rock and choke their beds with great 



