40 THE PRINCIPLES OF AGRICULTURE. 



Is any of the first crust of the earth still in existence ? Name the 

 five natural agencies that have ground rocks into soil. 



What is the effect of the atmosphere? What work do streams and 

 rivers perform ? What has the ocean done ? Explain the effect 

 of ice upon rocks in winter. What is a glacier? Where are gla- 

 ciers to be found at the present time? How have glaciers assisted 

 in forming soil? What has been the effect of winds? 



Name the classes of substances of which soil is composed. Explain 

 the origin of humus, or vegetable mold. Give the history of the 

 formation of peat beds. What is the origin of limestone ? Name 

 the different varieties of soil. Give the peculiar properties of 

 each. 



