GEOLOGY. lg7 



It contains abundant quartz, both orthoclase and acid plagio- 

 elase, the former frequently as large phenocrysts, with sometimes 

 microcline and micro-pegmatite. The mica is usually biotite, but 

 towards the peripheries of the bosses, biotite becomes scarce or is 

 absent and muscovite occurs instead ; the proportion of quartz 

 also increases. Hornblende is rare and always in small quantity. 



Accessory minerals are remarkably scarce in the granite itself 

 apart from veins. Iron pyrites is locally common and is probably 

 an original accessory. Tn the numerous microscope sections examined 

 sphenc was seen only once, and zircon, enclosed in biotite, is as 

 infrequent, Apatite, ilmenite, etc., were not observed. The latter 

 however probably occurs, at least near the granite margin, as it 

 forms the chief constituent of heavy concentrates from stream 

 sands and granite soils near the contacts. In these concentrates, 

 magnetite, garnet, zircon and a greenish-white opaque monazite or 

 other thorium-bearing mineral are found in lesser amount and are 

 probably sparsely distributed in portions of the granite. Oassiterite, 

 wolfram and, rarely, topaz are found in stream gravels and surface 

 soil in or near mineralised areas, where they have originated mainly 

 from quartz veins and greisens ; they also may be present as 

 disseminated grains in true granite. 



Generally speaking the granite may be described as coarse- 

 grained and porphyritic towards the centre of 

 Texturo and struc- „„„!,•,• -T 



tmo. eacn intrusion, becoming finer and more 



uniform towards the edges. The transition is 

 very gradual and there is little in the nature of a chilled edge 

 at the actual contact, the rock there being much the same as it 

 is fifty or a hundred feet within the mass. Tn a few cases 

 pseudo-foliation has been developed near the contact, not a true 

 gneissic structure produced by pressure, but one more analogous 

 to fluxion-structure in a lava, and caused by slight motion in an 

 almost consolidated magma, the finer crystals being drawn out 

 in lines curving round the larger felspars. This is best seen 

 near Myekhanbaw, where the zone of pseudo-foliation is about 

 half a mile wide. Its real thickness measured in a direction normal 

 to the original contact surface of the granite is much less, for the 

 presence of a residual patch of the Merguis on an adjacent hill about 

 1,000 feet higher, shows that the granite had here a gently inclined 

 upp?>r surface and has not been deeply denuded. 



