28 SOILS AND MANUEES 



the sea or lake into which it flows. The channel becomes 

 filled up with sediment, and the stream spreads itself out 

 in numerous smaller rivulets. These, in turn, become 

 blocked by the silt and the process is repeated indefinitely. 

 The delta of the Ganges has an area of some 40,000 square 

 miles, and at Calcutta the deposit is at least 500 feet thick. 

 Great deltas have been formed at the mouths of the Nile, 

 Ehine and other rivers. The Holderness in Yorkshire 

 has been formed in this way, and the Wash is being gradu- 

 ally filled up. 



A deposit of river silt is called alluvium, whether 

 formed at the mouth of a river or along its course. 

 Alluvial deposits give rise directly to important classes 

 of soils which, owing to their mixed character, are usually 

 very fertile. In past ages it has frequently happened, 

 as a result of various geological changes, that alluvial 

 deposits have become cemented together, hardened by pres- 

 sure and so converted into what are now called sedimentary 

 rocks. Owing to the more or less complete separation of 

 the larger from the smaller particles, which takes place 

 in the course of their formation, the sedimentary rocks 

 can be conveniently classified according to their texture. 

 Those which consist of the finest material (clay) are 

 described as argillaceous, and those of coarser (sandy) 

 material as arenaceous rocks. 



The soluble matter taken up by the water consists of 

 salts, such as gypsum, common salt, etc. These crystal- 

 lise out when the water evaporates and are deposited in 

 layers according to solubility. Large deposits of this kind, 

 formed by the drying up of lakes, are called solutionary 

 rocks, but, as soils are not derived from them, they are 

 of no importance in this connection. Both solutionary and 

 sedimentary rocks are, it will be seen, of aqueous origin. 



Organic Rocks. Organic rocks are so called because 

 they do not consist of minerals at all but of organic 



