37 



mineral. Even line, loamy and clay soils when examined with a 

 pocket lens or a microscope will be found to be composed of tiny 

 fragments of mineral. It is evident that in some way mineral has 

 been powdered up to form the soil ; and since the minerals come 

 from rocks, it is the rocks that have been ground up. The pow- 

 dered rock will make just such a substance as soil may be proved 

 by pounding a pebble to bits, or by collecting some of the rock dust 

 that is made when a hole is drilled in a rock. Much the same 



15. — A glacial soil, containing numerous transported pebbles and boulders, rest- 

 ing on the bed rock. 



substance is ground from a grindstone when a knife is sharpened on 

 it, making the water muddy like that in a mud hole. 



It will be an interesting experiment to reduce a pebble to powder 

 and plant seeds in it to see if they will grow as well as in soil ; l)ut 

 in preparing it try to avoid using a sandstone pebble, because sandy 

 soils are never very fertile. 



Not only is soil made up of bits of powdered rock, but it every- 

 where rests upon rock (Fig. 15). Some consider soil to be only the 

 surface layers in which plants grow ; but really this is. in most 



429 



