

ORIGIN OF SOIL 



93 



of "rock weathering." Their disintegrating effects upon 



rocks are both chemical and physical. Roots may enter 



crevices of rock, and by the great pressure of their growth 



crack these rocks 



apart (see Fig. 41). 



Burrowing animals 



produce important 



effects ; their bur- 



rows -increase the 



aeration (exposure 



to air). Earth- 



worms are particu- 



larly important in 



increasing fertility 



of soil by stirring 



it up and improv- 



ing its aeration 



The roots of plants 



excrete substances which produce corrosive effects upon 



rock, especially limestone. 



V. Glaciers. The very important action of glaciers 

 upon rock and soil has already been described (see Chapter 

 VIII). 



- 4I< Showing how the roots of trees may split 



QUESTIONS 



1. What is soil made of? 



2. What is our greatest mineral resource? 



3. What is humus? 



4. Name and explain the different factors in rock weathering. 



5. What is "talus" and how is it formed? 



6. How does water act on limestone? 



