18 Ail Account of the Soil [pt. i 



hour the clay is seen to yield a markedly coloured 

 solution while the sand only gives a slightly yellow 

 solution: filter these and add ammonia to each until 

 the liquid turns red litmus blue : the solution from the 

 sand gives only a shght precipitate while that from the 

 clay gives a much denser one. Thus we conclude that 

 sand is much more resistant to the attack of acids than 

 clay. The same result is obtained when the sand and 

 the clay are exposed to the weathering agencies: the 

 sand resists more than the clay and therefore is less 

 completely broken down. Silt comes in between sand 

 and clay in point of resistance. 



We must now proceed a stage further and try to dis- 

 cover how the particles got there and what their history 

 has been. 



The origin of the soil particles 



The soil particles have originally been derived from 

 the rocks, and their present state is the outcome partly 

 of the nature of the rock from which the}^ arose and 

 partly of the circumstances through which they have 

 passed. The original rock gradually crumbled by alter- 

 nate warming and cooling and by the action of water 

 or ice; the particles formed were carried by wind, by 

 streams, rivers or glaciers for a greater or less distance 

 and ultimately found their way to the sea and there 

 they were deposited. In course of time the pressure of 

 the great accumulation of material caused some of it to 

 be converted again into rock and, when the sea-floor 

 was uplifted to form dry land, this new rock thus ex- 

 posed went through the same processes of disintegration, 

 and again the particles were exposed to air, to water 

 and to ice. Sometimes they remained where they were, 



