DIHC'USSIOX OF PLANT INDUSTRIES 435 



may (/I'usli the stones, or streams fif water may wear them into 

 smaller pieces. In ancient times great glaciers crushed and wore 

 the stones, reducing enormous masses to smaller ones, gravel, 

 and finely pulverized material. All these agencies and others 

 have reduced the rocks so that in soils we are sometimes unable 

 to find sand particles except by means of the microscope. 



Fig. 342. Production of humus in tlie soil 



A partially reclaimed swamp in which dead plant material several inches deep is 

 decaying. In the foreground is a cluster of young skunk-cabbage leaves, and just 

 back of these and in front of the tree is a cluster of unfolding leaves of Clayton's fern 



In a yravelly soil there are present small pebbles which 

 usually show by their form and sometimes by their markings 

 the kind of treatment they have undergone. In mncly soil the 

 reduction of the rock is more uniform and has gone further. 

 In clayey soil the particles are so small and fit together so 

 compactly that the rock origin is not very evident. Peaty soil 

 contains comparatively little rock material but much more of 

 the results of partial decay of plant and animal bodies. There 

 are all possible gradations between these different kinds of soils. 



