GEOLOGY. « X97 



are believed to come from these, and belong to species now living. 



If really from these beds, their age would be established as sub-recent. 



Laterite, which has been referred to as a decomposition product 



of the Mergui rocks (p. It) is probably of 



both recent and sub-recent age. It is 



doubtless in progress of formation at the present day, while the 



layers of it already in existence and cut through by the streams are 



of a slightly earlier date. 



" Eluvial " deposits on the hillsides, derived from the under- 

 lying rocks by weathering in situ or trans- 

 ported a little distance downhill by surface 

 creep, have been described (pp. 11, 26). Broadly speaking they consist 

 of a dark red clayey soil on the Mergui Series of rocks and a light 

 red or yellow soil, more friable and much intermixed with angular 

 quartz fragments, on the granite. Near the surface they are full 

 of vegetable matter and pass downwards into variegated litho- 

 marges and partially decomposed rock. 



In the Upper, torrential portions of stream courses, erosion 

 predominates and deposition is merely local and temporary, and 

 the stream passes through a jumble of angular blocks of the prevailing 

 rock, with pebble-banks in pools and eddies. In a few exceptional 

 places, where a temporary diminution of gr idient occurs, a bench 

 of coarse detritus may form in the steep portion of a valley, as for 

 example at Hermyingyi, where the monitor workings are in such a 

 bench, accumulated in a cup-shaped depression, and cut through 

 by subsequent deepening of the channel of the present stream. 

 These possibly owe their preservation to a landslide from the hill 

 above. 



Lower down the valleys, where deposition is more in balance 

 with erosion, beds of gravel and sand tend to round off the inequali- 

 ties of the valley floor. Through these the stream meanders, cutting 

 them away in some places and simultaneously depositing at others. 

 Flood-waters overtop the banks and spread fine sand and mud 

 on any flat land on either side. 



Transport of gravel ceases shortly below the highest point to 

 which neap tides reach, but the scour of the ebb carries sand much 

 further down the river. 



The lowest sections of the valleys, where they join the estuarine 

 flats of the Tavoy River, are level rice-plains traversed by ramifying 

 tidal creeks. Here a fine silt is the only deposit. The J and is 



