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



Conclusions. — The later work of the Geological Survey in Tavoy 

 has not led to any important modifications of the views already 

 published by one of us. While we do not deny the important part 

 played by water in the vein formation, we prefer to regard it as of 

 magmatic origin rather than to assume its meteoric derivation, as 

 Dr. Morrow Campbell has done. We conclude : — 



(1) That the columbite-tantalite group of minerals is absent. 



(2) That tourmaline does not occur in the ore mineral associa- 



tion in Tavoy. 



(3) That fluorite is a widely distributed accessory mineral in 



the pegmatites and veins, though it is not found in large 

 quantities. It does not appear to exhibit any preference 

 for veins in granite over those in the sedimentary rocks. 

 Since the earlier report was written fluorite has been 

 found at numerous new localities, and for the supporters 

 of the pneumatolytic theory, as it is generally understood 

 there may be sufficient fluorite present to account for 

 the formation of both cassiterite and wolfram in those 

 veins in which it occurs, provided that the flourine 

 element acted after the manner of a catalytio agent. 



(4) That sulphides are common accessory vein minerals, their 



relative order of importance being sulphides of iron, 

 copper, molybdenum, bismuth, lead and zinc. We are 

 also of the opinion that sulphur played some part in 

 the pneumatolytic reactions that took place. 



(5) That wolfram, cassiterite and scheelite occur in pegmatites. 



(6) That some of the veins are of hydrothermal origin. 



(7) That many of the mineralised veins are of pegmatitic origin. 



(8) That in the formation of these pegmatites and veins, water 



of magmatic origin played a leading part. 



Hydrothermal veins a phase of pegmatitic development. — Recent 

 geological literature adds support to this contention. According to 

 Thomas and MacAlister, " a hydrothermal phase of some pegmatites 

 is now recognised, and is generally represented by siliceous deposits 

 (often containing rare minerals), formed in fissures as a continua- 

 tion or prolongation of the pegmatite. This ultimate phase is 

 explained by the fact that the pegmatites with which it is found con- 

 tained a considerable amount of water, which was given off as solu- 



