HINTS ON SOILS AND FERTILIZERS 7 
on these rocks, they are slowly, gradually, but 
nevertheless surely, broken up into fine parti¬ 
cles which go to make up the soil. Sudden 
changes of temperature, such as heat or cold, 
cause rocks to split and crack. Frost has a 
similar effect, when water freezes in small 
rock crevices, the expansion of water into ice 
causing the rocks to break into smaller pieces. 
Water also aids in soil formation, as it not 
only dissolves soil particles that are small 
enough, but it also has a wearing effect on 
rocks, as it constantly flows past a rock par¬ 
ticle. Wind is another agency that does its 
part in crumbling the giant rock. Sharp par¬ 
ticles of soil, such as sand, for instance, by 
being blown against rocks, tend to wear off 
gradually the surface of the rocks thus ex¬ 
posed. Certain gases, such as oxygen and car¬ 
bon dioxide, also have their effect upon the 
rocks, largely through chemical changes. Thus 
it can be seen that soil formation is not a 
quick process, but that it has taken millions 
of years to bring this disintegration about, 
and that soil is constantly being formed, and 
will continue to be formed for a long time yet. 
Not all soils found at a certain place were 
necessarily made from the rock that lies un¬ 
derneath it. Water, wind and ice all contribute 
to the transporting of soil particles, after they 
are small enough to be carried by these agen¬ 
cies, and deposit them on other rocks. We 
are all familiar with the running streams of 
water that carry small particles of soil in 
solution. These soil particles are not all car¬ 
ried a very great distance, unless extremely 
