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



impossible to devote to them the time necessary for 

 working them out in detail. 

 (e) Tourmaline pegmatite. The tourmaline pegmatites of the 

 district arc later than the granite in time of intrusion 

 and appeal to be distinct from it so far as is known up 

 to the present. They penetrate the latter and by frac- 

 tional crystallisation of their minerals give rise to quartz- 

 felspar veins and pure quartz veins. These are indis- 

 tinguishable from the quartz-felspar pegmatites and 

 quartz veins belonging to the granite, except in so far 

 as they are of later age and cut across them. The 

 relative ages of the basic dykes (<1) and the tourmaline 

 pegmatites are unknown, but both are subsequent to 

 the granite. 



Besides veins of quartz, pegmatite and greisen, which are treated 

 of in connection with ore-deposits, the granite 

 sends off into the surrounding Mergui sedi- 

 mentaries a few veins of fine-grained granite and felspar porphyry. 

 They are seldom seen in the field, partly on account of their 

 scarcity and also because they are small and occur in situations 

 where they are liable to be covered by the extensive debris shed 

 from the granite intrusions, but their fragments are sometimesfound 

 in stream gravels. 



Only three exposures of porphyry were seen in situ, on the Chi da w- 

 law near the Great Tenasserim, near Natkyizin and on the shore 

 at Bok (Kadwe). They consist of phenocrysts of quartz, kaolinised 

 felspar and in some cases biotite. in a cryptoorystalline and grano- 

 phyric ground-mass. 



The Bok porphyry forms massive Un jointed dykes on the shore, 

 with smooth surfaces on which the largo zoned pink orthoclase 

 phenocrysts show with great distinctness. It bears fine-grained 

 dark basic patches, in which the phenocrysts are much less numerous. 

 These have sharp margins and are surrounded by rims of normal 

 rock, but devoid of phenocrysts. The dyke-margins are sharp and 

 without a fine-grained selvedge. The adjoining Bferguis are locally 

 indurated and turned pink at the junction and xenoliths of them 

 are included in the dyke. The orthoclase phenocrysts are very 

 large, zoned in various shades of pink, and include small plates 

 of biotite. The other porphyritic minerals arc corroded euhedral 

 crystals of quartz and ragged green biotite intergrown with musco- 



