42 FARM DEVELOPMENT 



roots and of air on the soil are all of great interest and 

 lead in a most interesting way to the study of plants; 

 likewise of animals and men which depend upon the food 

 the plants obtain from the soil and air. 



Source of materials moved by glaciers. The source of 

 material moved by the glacier is difficult to determine. 

 Doubtless in the beginning of things, when the earth's 

 crust first became cool, it was much like the lava which 

 now flows from the craters of volcanoes. This glassy, 

 hard, rocky substance was not made up like marble or 

 limestone or slate, but elements of all kinds of rocks were 

 melted together in one mass. This rocky crust was 

 broken up by the water, air, sun and winds, literally 

 rotted, and thus made into soil; and later, as the earth 

 became cooler, ice became one of the greatest factors 

 in soil making, as seen in the immense grinding done 

 by the glaciers we have bjeen considering. During the 

 earlier geologic periods, doubtless even the interior heat 

 helped to break up the material of the earth's crust. 

 Water, running into fissures of the earth and coming in 

 contact with the heated inner part, formed such volumes 

 of steam as to cause great explosions, throwing about 

 and breaking up great masses of materials. We have, in 

 more recent times, illustrations of volcanic action; one, 

 for instance, when Vesuvius belched forth enough ashes 

 to completely bury the city of Pompeii, and, again, more 

 recently, on the island of Martinique. 



Some of the materials of the great area of glacial drift 

 have doubtless been transported many hundreds of miles. 

 Many of the bowlders and other rocks found in Minne- 

 sota can be traced to beds of similar materials far to the 

 northward in Canada. One of the proofs of the glacial 

 action is the fact that the glacier flowed southward and 

 carried down from the north many large fragments of 

 the rocks over which it passed, as well as much of tHe 

 finely pulverized materials. Usually these coarser ma- 



