INEQUALITIES OF SEDIMENTATION 351 



If with the Httoral zone of western AustraHa such an area as 

 the Bay of Bengal is contrasted, the enormous difference in the 

 volume of clastic sediments which comes to rest on different parts 

 of the sea bottom is clearly seen. The upper part of the Bay of 

 Bengal receives the drainage from the southern slopes of the Hima- 

 laya Mountains where in places the rainfall amounts to 472 inches 

 or nearly 40 feet per annum." 



Role of vegetation in estuaries. — Closely related to the relative 

 aridity of the sea coast in different parts of the world is the variable 

 rate at which sedimentation on different coasts transforms shallow 

 estuaries into marshes and eventually into dry land. When 

 sedimentation has developed to the stage of shifting sand banks 

 in an estuary, vegetation becomes the chief factor in fixing these 

 and in extending land conditions. In an arid climate the virtual 

 absence of this factor may cause the last stages of filling to proceed 

 with extreme slowness while in a moist chmate they will develop 

 with relative rapidity. The role of vegetation as a factor in late 

 stages of sedimentation is well illustrated in Professor Oliver's 

 study of the plant life of a salt marsh in Brittany called the Bouche 

 d'Erquy. 



Sand litters the beach, and in part is blown by the winds to form the 

 barrier of sand dunes. Another part is brought into the estuary and is driven 

 hither and thither by every tide. So restless are the shifting sands in the lower 

 part of the estuary that plants cannot gain a footing, though seed be scattered 

 far and wide. Higher up as we saw stability has been reached, and the ground 

 is held by a turf of halophytes, and half way up the estuary where the bare and 

 covered parts adjoin, the restless sands are tamed and bound. The transition 

 from the fixed marsh to the mobile sand is a gradual one. The continuous 

 carpet of the former gives place to scattered hummocks or tussocks of Obione 

 portulacoides and the perennial Salicornia radicans. Beyond this the Obione 

 dies out and the Salicornia radicans carries on its colonizing work alone. This 

 it does by throwing out each year its seeds which germinate on the treacherous 



sand In some places the pioneers are advancing at a rate of 10 feet 



a year, elsewhere less rapidly; whilst in spots where the sand is very mobile 

 they seem perfectly stationary but the local conditions are always changing 

 and the most unpromising surroundings may give place at any time to such 

 as favour a rapid advance. A rough estimate based on the rate of progress 

 noted during the last three years and on the assumption that this rate will 



^ G. T. Bonney, The Structure of the Earth, p. 40. 



