68 ELEMENTS OF AGRICULTURE 



form soils, but such is undoubtedly the case. If a soil is 

 examined with a good magnifying glass it is found to be 

 made up of many small fragments of various shapes and 

 sizes. The fragments resemble those which are formed 

 when a coarse stone is pulverized, and this is just what 

 has happened. Coarser fragments of stone have been 

 broken up by various agencies, and the small particles 

 resulting have formed the soil. We may put it down 

 as an accepted fact that the mineral matter of all soils 

 has resulted from the breaking up of rocks and stones. 



68. Formation of Mineral Fragments in Soils. — Now, 

 how is it that these apparently indestructible rock 

 masses have been so broken up as to form the great soil 

 areas of the world? Soils are formed so slowly and 

 quietly that we are not aware that such a work is going 

 on about us, but; soils are forming to-day just as they 

 have been for thousands of years. The great soil-maker 

 of the world is water, and it has several different 

 methods of making soil. 



69. Mechanical Action of Water. — More soils are 

 formed by the mechanical action of water than by any 

 other agency, so we may best consider it first. 



The surfaces of all rocks, however smooth appa- 

 rently, are filled with minute cracks. The surfaces of 

 rocks long exposed to the weather are particularly full 

 of irregularities; an apparently smooth rock, if exam- 

 ined with a magnifying glass, will show many cracks and 

 joints. Into these cracks and joints water penetrates, 

 and if the temperature becomes low enough, the water 

 freezes, the cracks become enlarged, and often pieces of 

 rock break off. Water always expands when freezing, 



