A History of Agriculture in India v1.c8.s1.5

v1.c8.s1.5

The atmospheric agency acts on rocks as follows: The carbon dioxide gas in the atmosphere renders the calcium carbonate soluble. Limestones, chalk and kankar thus get dissolved and become available as plant food, and the rain-water from calcareous rocks charged with calcium carbonate flows into the sea, where shell-fish, corals and foraminifera utilize the lime in building up their own bodies, which in time settle in the form of dead shells and form new rocks. The dew and water-vapour of the atmosphere which get into the interstices of rocks in cold regions are frozen and the resulting expansion disintegrates the particles of rocks. When water freezes, it expands, and the volume of ice formed is increased by nine per cent. This increase in volume exerts a force of 1,640 tonnes per square metre. The oxygen of the atmosphere is a very potent agent in oxidizing and disintegrating the surfaces of hard rocks. Strong currents of wind carry sand and finer particles of matter (such as common salt) from the sea-shore and the dry beds of rivers into the interior.