200 BROWN & HERON: GEOLOGY "AND ORE DEPOSITS OF TAVOY. 



It is found that all the main valleys are occupied by rocks of 



_ . the Mergui Series, often in quite narrow strips 



Drainage system. .., , . , . , x . . , , r 



With high granite land on either side, and 



that the numerous granite bosses are seldom cut across by important 



streams. This preference for the sediment aries is so marked that 



the usual explanation seems inadequate, i.e., that the granite is 



more resistant to denudation than the Merguis. The possibility 



is suggested that the main drainage lines are still along tectonic 



hollows which originally lay between bulges in the upper strata 



caused by the intrusion of masses of granite, or, if not caused by its 



intrusion, were areas of low pressure, upraised during the folding 



movements, into which the granite found its way. 



The drainage systems are three : — the coast, the Tavoy River 

 valley and the Great Tenasserim valley. 



The steep seaward slopes of the Coastal Range are drained by 

 numerous small streams of trifling length and discharge, only at 

 the Zadi chaung and the Heinze Basin, where the range is breached, 

 does the drainage of a considerable area find its way to the sea. 



The Heinze Basin is a narrow-mouthed bay, in shape .resembling 

 the Bay of San Francisco. Its greatest length from north to south 

 is 18 miles and its greatest breadth about 6. Much of it is mangrove 

 swamp, with ramifying tidal channels of unexpected depth. 

 In the centre is a low island of Mergui rocks. The greatest depth 

 of the Basin is 13 fathoms, just inside the entrance, and channels 

 of 5 fathoms extend for some miles parallel to the longer axis of the 

 bay ; a depth of 5 fathoms is not met with in the adjacent sea for 

 6 miles outside the entrance, and the Basin is thus an area consider- 

 ably below the level of the adjacent sea-bottom. To the north and 

 south-east it is prolonged by valleys carrying fair-sized streams 

 and numerous shorter ones come in from all sides. The disturbance 

 of the elu vials in the adjoining mine of Kanbauk and the great 

 depth to which they go below the present sea-level (pp. 101-2), 

 are facts confirming the hypothesis that this neighbourhood has been 

 comparatively recently depressed. 



The Tavoy River rises in the extreme north, where the Coastal 

 and the Frontier Ranges join, and flowing slightly east of south 

 for some 90 miles, drains about half the district. On its western 

 bank it receives no streams of any importance from the Coastal 

 Range ; all its main tributaries come from the east, from the Frontier 

 Range and the western slopes of the high lands occupied by the 



