III. THE SOIL 39 



Soils resting on the rocks from which they have been formed are 

 known as sedentary or indigenous soils and depend, for their fertility, 

 upon the stores of plant food present in those rocks. 



Transported soils. Many soils are produced from the decay of 

 rocks other than those upon which they rest. The rich, alluvial soils 

 of wide valleys contain inorganic materials which have been trans- 

 ported from a distance by the river and deposited there. The materi- 

 als, in many cases, have been brought from various rock formations, 

 and the resulting soil consequently possesses a greater fertility than 

 would be shown by a soil formed exclusively of the debris of any one 

 kind of rock. 



Other means of effecting transportation are provided by glaciers. 

 Large areas of land have, in many places, been covered with a thick 

 deposit of debris brought from a distant source by moving ice. Such 

 deposits are known as glacial drift and often consist of a finely divided 

 clay matrix holding blocks of hard rock, which, in some cases, show 

 the marks of ice scratches and have been brought from an immense 

 distance. 



Wind is sometimes a means of transporting matter from a distance 

 and depositing it so as to form a soil. This occurs with sand near 

 the sea coast or on the shores of large lakes, and also, over greater 

 distances, with the ashes ejected from volcanoes. 



The formation of soils is mainly brought about by the following 

 agencies : 



1. Water. This acts in various ways: 



(i) Mechanically. The flow of water over a rock subjects it to 

 slight abrasion; this is greatly increased by the pebbles and stony 

 fragments, which are urged by the current over its surface. In this 

 way, rapid- streams and rivers carry down large quantities of materials 

 from high ground and deposit them in the low- lying plains, giving rise 

 to alluvial deposits. This action becomes most evident after heavy 

 rains, when the water becomes muddy and discoloured. 



(ii) By alternate frost and thaw. Ice, as is well known, occupies 

 more space than the water from which it is formed. .The increase in 

 volume amounts to about 10 per cent, and the force exerted by water 

 in freezing is almost irresistible. Indeed, freezing cannot take place 

 without this expansion, and if it be prevented, the water remains 

 liquid, though its temperature be reduced much below C. It is 

 found that if an additional atmosphere of pressure be exerted upon 

 water its freezing point is lowered by -0075 C. The bursting of water 

 pipes in winter is a familiar consequence of this expansion in 

 freezing. 



In nature, the disintegration of rock is greatly aided by this action 

 of water. During the warm part of a winter's day, the crannies and 

 crevices of a rock may become filled with water. As the temperature 

 falls, the water begins to freeze, at first on the outside, so that every 

 crevice becomes stopped with a plug of ice, the still fluid water behind the 

 plug continuing to lose heat and therefore tending to solidify. This it 

 can onlv do if it can increase its bulk by about 10 per cent. In order to 



