SOIL 269 



G. Glass scratches limestone, and is scratched by quartz; which is 

 the hardest of these three? How does the finger nail compare with 

 these substances in hardness? 



7. Suppose that the rock directly under the Niagara River (Fig. 

 225) were suddenly to become a soft shale; what would be the effect 

 upon Niagara Falls? Do you think the Falls have always been where 

 they are now? Why? 



8. Compare the composition of granite and of porcelain. Which 

 rock may be called the cementing rock of granite? 



9. In what forms besides clay tablets did ancient peoples leave their 

 records? 



294. Soil. Soils have as their foundation weathered 

 rock. Usually this is a mixture of sand and clay (cf. 

 285), and is called loam. Besides loam, a fertile soil 

 requires humus, or vegetable mould. This is the material 

 formed by the growth and decay of plants and trees; we 

 see it in the black top-soil of a forest. Growing plants 

 need humus as a food; but it is more than this, for it is 

 the means by which the minerals of the loam are prepared 

 for the use of the plant. The subsoil lies below the culti- 

 vated soil. On its qualities the qualities of the upper soil 

 depend. 



Soil has not only the chemical materials of loam, humus, 

 air, and water, but it contains multitudes of the lower 

 organisms, such as bacteria (cf. 56 and 324). One who 

 studies the soil carefully soon realizes that it is not a dead, 

 inorganic mixture, but a world of life and activity. He 

 also learns that a fertile soil required centuries for its 

 development, and that if it is once destroyed, it cannot be 

 restored in a year or two. Every agriculturalist needs to 

 know what his soil contains, what it lacks, for what crop 

 it is suited, and how it ought to be handled, so that it will 



