Geology 



before and after high tide, that most of the deposition of 

 mud took place, for about half-ebb the current became 

 so strong and the waters so disturbed that the channel 

 was scoured and kept open. By this time, however, 

 considerable areas of the banks were already dry, and so 

 the film of mud laid down upon them was not removed, 

 and thus film by film the flats were being gradually 

 raised. 



The deposition of the mud is largely influenced by 

 the mingling of the fresh and salt water in an estuary. 

 The mud-laden waters of the river, when they mix with 

 the salt tidal waters of the estuary, at once commence 

 to drop their burden of solid particles. For every 

 strength of current there is a certain size of particle 

 which is critical, and any grains which are larger than 

 this are left behind, while any that are smaller are held 

 in suspension and carried away. Two things may 

 influence the deposition. In the first place, if the current 

 is checked, as is that of the river when it enters the 

 wider channels of the estuary, and meets the incoming 

 tide, matter will be deposited. In the second place, any- 

 thing which tends to increase the size of the particles 

 will bring about a similar result. 



If we take two tall glass jars, such as are in use in 

 chemical laboratories for the collection of gases, and fill 

 them, say, two-thirds full of water with which some finely 

 powdered china clay has been mixed, and add to one of 

 them a small quantity of a solution of common salt 

 (sodium chloride), we shall find that the clay soon com- 

 mences to settle, and after an hour or two the water 

 becomes quite clear, while that in the jar into which no 

 salt was put remains milky, it may be, for several days. 



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