GEOLOGY. 201 



Central intrusion. It is tidal to 60 miles from its mouth and its 

 lower valley comprises most of the inhabited and cultivated land 

 of the district. 



To the east of the Central intrusion is the valley of the Great 

 Tenasserim, which is formed by the confluence of the Ban and the 

 Kamaungthwe. The Ban rises in the mountain mass of Myinmo- 

 letkat (6,800 feet) and flows northward between the Amya intrusion 

 and the southern portion of the Central intrusion. 



The Kamaungthwe drains the high country between the Sinbo- 

 Sinma mass and the Frontier Range, flowing south to meet the 

 Ban at Myitta. The united stream finds its way eastwards round 

 the north end of the Amya intrusion and then south-east and south 

 between it and the Frontier Range. The Ban-Kamaungthwe and the 

 Tenasserim valleys date from Tertiary times, as the fluviatile deposits 

 occurring in them show. 



The coast is fringed entirely by granite and is a series of head- 

 lands alternating with bays, the latter now 

 Coa3t ' largely filled up with accumulations of sand 



and mud. Cli fYs are absent. On headlands dense forest comes 

 to 20 or 30 feet above high-water level, then there is a bare slope 

 of curving surfaces of granite to about half-tide level and below 

 that a pile of great boulders (p. 193). 



Every bay has a crescentic sandspit with a lagoon behind it ; 

 the entrance to the lagoon is almost always at the south end of 

 the spit. One description serves for all. The sandspit is composed 

 of coarse sand, partly wave-borne and partly wind-blown, and it 

 extends with rather a steep slope from half-tide level to the highest 

 point to which waves can reach. At the toe of this slope is a flat 

 of mud or very fine sand with an almost imperceptible fall out to 

 sea (p. 108). The sandspit becomes bound by wiry grass and a 

 purple creeping convolvulus and supports groves of casuarina trees 

 and salt-loving shrubs. 



The lagoon behind may be open water, mangrove swamp, or 

 a salt marsh flooded only at spring tides, according as to how far 

 silting up has proceeded. Twice a day the tide carries in and out 

 its burden of sand and mud, leaving a certain amount entangled 

 amongst the mangroves and other vegetation, and every rainstorm 

 brings down rock and vegetable debris from the hills behind. My- 

 riads of molusca, hermit-crabs and other burrowing Crustacea 

 live in the lagoons, bringing up mud from below and adding their 



