GEOLOGY, 191 



vite, in a granophyric ground- mass of quartz and felspar ; black 

 iron-ores are present. In hand specimens the green biotite strongly 

 resembles hornblende, but the latter mineral was not found in any 

 of the sections examined. 



A connecting link between the Bok porphyry and the usual 

 granite is seen at the right-angle bend of the Pe chaung, close 

 to the margin of the Mintha intrusion but within it, surrounded 

 by, and passing gradually into, normal granite. It is a coarse 

 porphyritic granite with phenocrysts of orthoclase (of large size 

 and including plates of green biotite), quartz showing corroded 

 crystal outlines, acid plagioclase and large plates of green biotite, 

 in a granitoid ground-mass of quartz and felspar without granophvre. 

 In the ground-mass are small isolated hornblende crystals and 

 dusters which appear to be largely hornblende, but containing 

 also green biotite, epidote, apatite and iron-ores. In hand specimens 

 this rock is very like the Bok porphyry, even to the large pink 

 orthoclase and green hornblende-like biotite phenocrysts, and differs 

 from it only in that hornblende and plagioclase are present in 

 addition, and that the ground-mass is granitoid instead of grano- 

 phyric. 



Tourmaline pegmatite occurs in isolated veins on the coast near 



the entrance to the Heinze Basin, at various 

 ^Tourmaline pegma- ^^ near ^ summit of tho Frontier Range 



and on the Tavoy river below Yapu, but it 

 is only on the coastal plain in the south, between the Mintha intru- 

 sion and the sea, that it is common or conspicuous. Here it out- 

 crops as parallel veins of all sizes, usually in groups, running in the 

 direction of the strike, and invading both Merguis and granite. 

 The veins are often clustered closely together, as for instance at 

 Kamyaing, where they are so crowded as to give the appearance 

 of a single mass. The minerals composing it are orthoclase, quartz 

 and tourmaline, the last often in large crystals or in groups of 

 smaller ones. Muscovite is a subordinate constituent ; sometimes 

 both muscovite and tourmaline are absent, and the rock then 

 becomes a quartz-orthoclase aggregate, with graphic intergrowth 

 of the two minerals. This type is, in part at least, subsequent to 

 the tourmaline bearing variety, as it is seen to cut the latter, e.g., 

 on the shore, 1 mile north-west of the mouth of the Kyan chaung. 

 Garnet is an infrequent accessory. 







