188 BROWN & HERON: GEOLEGY AND ORE DEPOSITS OF TAVOY. 



On the western side of the Coastal Range the granite has been 

 deeply trenched by the sea and excellent sections of deep-sealed 

 portions of the mass are available. There a banded arrangement 

 of its constituents, similar to the one already described but on a 

 much larger scale, is sometimes seen. Zones of extremely coarse 

 material, poor in biotite, alternating with others of fine-grained, 

 highly biotitic stuff, run in a direction approximately parallel with 

 the general strike, but in detail undulating ; there are also irregular 

 veins of both types. This structure is not a gneissic banding due 

 to intense crushing of a solidified rock, but is a fluxional effect of 

 movement in a partly consolidated magma after segregation of 

 the biotite. In both modifications the minerals show no signs of 

 strain nor deformation, and are essentially the same, the only 

 difference being in the proportion of biotite. The basic veins and 

 bands are quite distinct from the greisen found on the granite 

 peripheries ; their mica is biotite and not a white mica and they 

 carry no minerals such as wolfram and cassiterite. 



The minerals of the granite generally are not dynamically 

 metamorphosed ; faults of small throw and slippage planes are 

 common, but their effects reach only a few inches away from them 

 and there is no general crushing nor re-arrangement of minerals. 



The veins characteristic of the margins of intrusions are described 

 in the section on ore deposits, and repetition 

 is unnecessary. Much less known regarding 

 those peculiar to the deeper seated granite, as this has not been 

 touched by mining operations and it is practically only on the 

 seaward face of the Coastal Range that good sections are available. 

 They comprise — 



(a) Acid pegmatite veins. These are both coarse and fine in 



texture ; in large veins (they are as much as 15 feet 

 wide) the pegmatite is usually coarse, in small veins 

 fine. They consist of quartz, white or grey felspar 

 (rarely pink) and a little biotite. The last may be 

 absent. 



(b) Basic pegmatite veins. The same minerals as the granite, 



but with biotite in excess. Basic segregation patches 

 of this composition occur. 



Both " a " and " b " are represented in the light and dark 

 banding described above, and, like it, are differentia- 



