32 The Great World's Farm 



preparation thousands and millions of acres have been 

 provided which would yield abundant harvests with less 

 than half the labor. 



QUESTIONS FOR REVIEW 



1. Describe the earliest forms of vegetation which are found 

 upon rocks. 



2. How do these lichens receive nourishment? 



3. Show the steps by which soil accumulates upon the rock. 



4. By what different means do roots break up the rocks? 



5. Why is the soil of a granite district apt to be poor? 



6. Explain the difference between the granite soil of the 

 Scilly Islands and of Dartmoor. 



7. What is the character of the soil of the chalk downs, 

 and why? 



8. What process is necessary before a soil can be very 

 fertile? 



