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joint action of heat and pressure. Granites, though consisting 

 of felspar, mica and quartz, vary very much in composition 

 according to the proportion in which these minerals occur. 

 The larger the proportion of felspar in granite the richer it 

 is for the purpose of formation of soils. Trap-rocks and 

 volcanic tuffs form the richest soils, and a study of the 

 minerals which compose these is of great importance. 



39. The agencies operating in the disintegration of soils 

 are : (i) Aqueous, (2) Atmospheric, (3) Physical and (4) Organic. 

 (i) Aqueous agency in the disintegration of rocks and 

 soils is the most potent of all. What enormous quantities of 

 solid matter in large and small sizes are dislocated by rain and 

 brought down by streams and cataracts and rivers, may be 

 judged from Everest's calculation of silt carried down by 

 the Ganges alone. Everest calculated that 355,361,464 tons 

 of solid matter are carried down annually to the sea by the 

 Ganges. If 1000 ships laden with about 1000 tons of mud 

 daily were employed in emptying their contents into the sea, 

 they would perform the same work which is done by the 

 Ganges. The Brahmaputra carries to the sea a still larger 

 quantity of silt. The hardest and heaviest rocks become 

 converted into rounded boulders and pebbles by the action 

 of the moving water containing sand in motion. Water acts 

 not only mechanically in denuding rocks but it is also a sol- 

 vent. The potash, soda, silica, and lime get dissolved in water 

 and rocks may be denuded simply by the solvent action of 

 water. The solvent action of water or minerals is increased by 

 its containing salts in solution and gases in suspension. Be- 

 sides disintegration due to rainfall and the denudation due to 

 rivers and waterfalls, we have a third form of aqueous agency 

 in operation : sea waves beating against cliffs help in the for- 

 mation of soils. The action of glaciers in tearing down rocks 

 and in the formation of moraines and erratic boulders may be 

 also included under this head. The hydration of rocks in the 

 presence of water should be also mentioned here. 



