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



but a foot or two above the high-water level of spring tides, and 

 when high tides coincide with heavy rain, the mud-laden flood- 

 waters are ponded up, overtop the banks and spread far and wide 

 over the surrounding country. This happens frequently during tho 

 monsoon, and the flats are in fact water-covered for five or six 

 months of the year. Deposition of the silt carried down from tho 

 hills is promoted by the mixing of salt water with the muddy 

 river water, and it is retained by the dense growth of dan.i (Nipa 

 fructans) and mangrove along the creeks and by the growing rice 

 on the flats. 



On the coast sand is almost tho only sediment. Granite is the 

 only rock which outcrops on the coast, and as it breaks up into its 

 component mineral fragments, there is rarely any gravel or material 

 intermediate in size between small boulders and sand. Tho latter 

 forms shelving crescentic beaches between promontories and con- 

 sists of shell-fragments, quartz and felspar. The finer sand, includ- 

 ing most of the mica and decomposed felspar of the granite, and 

 organic material, accumulates as a much flatter shelf which runs 

 out from the toe of the steep beach of coarse sand and is uncovered 

 rtt half tide. In the lagoons and mangrove swamps behind the 

 beaches are alternating deposits of fine mud (kaolinitic and organic 

 material) and coarse angular quartz-felspar sand, tho latter washed 

 directly from the granite. 



Physical Geography. 



The outline of the district and the main features of its topography 



are determined by the granito bosses, which 



Mountain masses. « ■ • . ^ ■ ^ t i 



all give rise to nigh ranges and rugged 



mountain masses. They are naturally classed in six groups (p. 184), 



each granite area being synonymous with an elevated tract. 



A.s to the exterior outline of the district, on the west the coast- 

 line is formed by the Coastal Range as far south as the mouth of 

 the Tavoy River ; southward from that point the Coastal Range 

 disappears beneath the sea for a space (to reappear as Tavoy and 

 King Islands) and the coast-line is shifted eastwards to the next 

 line of intrusions, a series of small bosses which flank the Mintha 

 mass. 



Eastwards, the political boundary and actual watershed between 

 British territory and Siam is the Frontier Range, which extends 

 at a height of from 2,000 to 4,000 feet from the Amherst District 



