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(2) Atmospheric agency acts on rocks chiefly in four 

 ways. First, the carbon dioxide gas renders the calcium 

 carbonate soluble. Limestones, chalk and kankar thus get 

 dissolved and available as plant food, and the rain 

 water from calcareous rocks charged with calcium carbonate, 

 flows into the sea where shell-fish and corals and fora- 

 minifera utilise the lime in building up their own bodies, 

 which in time settle in the form of dead shells and form new 

 rocks. Secondly, the dew and water vapour of the atmosphere 

 getting into the interstices of rocks in cold regions become 

 congealed and the expansion resulting from this has the 

 effect of disintegrating particles of rocks. Thirdly, the 

 Oxygen of the Atmosphere is a very potent agent for oxidiz- 

 ing and disintegrating surfaces of hard rocks. Fourthly, 

 strong currents of wind carry sand and finer particles of 

 matter from sea-shore and dry beds of rivers into the in- 

 terior, 



(3) Physical agency operates in disintegrating soils 

 chiefly in the form of heat. Earthquakes, hot-springs and 

 volcanoes have the tendency to alter even the superficial 

 layers of the earth's surface more than we think they do ; but 

 these agencies were more potent in past ages than now. 

 Electric agency is also at work. The water-vapour and the 

 free nitrogen of the atmosphere combine in the presence of 

 lightning and thunder in the form of nitric acid which being 

 brought down by rain acts on the rocks and helps to dissolve 

 their particles more quickly. 



(4) Organic agencies are at work in various forms. 

 Minute bacteria are continually at work in soils and on the 

 surface of rocks. Higher forms of vegetation, lichens, 

 mosses, grasses, shrubs, creepers and trees are also most 

 potent in disintegrating rocks. Animal life also is at work 

 chiefly in the sea in the formation of soils. Coralreefs, 

 Chalk cliffs, Nummulitic and other limestones and marls, 

 consist of dead shells chiefly of marine animals, large and 



