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



and appearance. They are hard, white or greyish, rather fine- 

 grained rocks when fresh, characterized by an abundance of quartz 

 and light coloured mica. The dark coloured micas are not so 

 abundant in their peripheral portions and iron ores of any kind are 

 rare. We believe that all the isolated expanses of granite are 

 probably connected underground, and that they were formed by 

 the same processes in the same period in time. Tin 4 question arises 

 why large areas of the granite show no signs of mineralization, and 

 that in spite of careful search, no mineral-bearing veins have been 

 found in them. The geological map of Tavoy shows that all the 

 producing mines of the district are situated on or near a granite 

 contact ; that the larger ones are located in positions where the 

 granite bands are narrowed ; that Hermyingyi, the biggest mine 

 of all. is on a comparatively small granite exposure of its own. It 

 may be that the absence of wolfram and eassiterite from certain 

 areas is due to the absence of the elements tungsten and tin in those 

 portions of the original magma which gave rise to these particular 

 outcrops of the granite. The matter is a highly speculative one, 

 but it seems to us more reasonable to suppose that the original 

 magma which gave rise to a range of mountains, stretching at least 

 from the Southern Shan States to the islands of Banca and Billiton, 

 varied somewhat in composition from place to place, than that it 

 was of a perfectly uniform composition throughout. At the same 

 time the size of a granite outcrop reveals in many cases the relative 

 amount of denudation which the original intrusion has undergone. 

 The intrusions narrow towards their summits and broaden out 

 downwards. The wide expanses of granite have suffered severely 

 from denudation and they contain no minerals of economic value 

 because the veins, if they ever existed, have been entirely removed. 

 The depths to which the veins of any particular mine in granite 

 will extend downwards then depends partly on the amount of 

 erosion to which it has been subjected. The prospector is advised 

 to concentrate his attention on the acidic portions of the granite ; 

 by this we mean the fine-grained white varieties with much pale 

 mica and free quartz, rather than the coarser kinds which contain 

 a good deal of dark biotite, like the material from the southern 

 portion of the Coastal Range ; to pick out the narrower exposures 

 and in particular to examine most carefully the contacts where 

 isolated patches of sedimentary rocks still remain to prove that 

 denudation has just reached the granite itself. 



