6 GENERAL CONSIDERATIONS 



and extend their roots is called the soil. It consists 

 of more or less minute rock particles mixed with de- 

 composed vegetable matter. The rocks that formed 

 on the cooling of the earth's crust in the early geologi- 

 cal ages soon began to be disintegrated by the action 

 of rain and wind. As the cooling process continued 

 the action of frost contributed largely to the same 

 end. The loosened particles, swept along by the 

 torrential rainfalls of this stormy epoch, were carried 

 to the sea, where they were deposited to form the 

 thick beds of stratified rocks which by subsequent 

 elevation and erosion have served to furnish the 

 material for most of our agricultural soils of to-day. 

 During the long ages of the glacial period, when so 

 much of the present temperate zone was covered with 

 an immense sheet of ice and snow, the slow, irresistible, 

 glacier-like southward movement of the mass also 

 served as a poAverf ul means for crushing and pulveriz- 

 ing the buried rocks below. From the earliest times 

 running streams have been efficient agents not only for 

 eroding rocks, but for transporting the disintegrated 

 particles from one place and depositing them in an- 

 other. As vegetation of various kinds began to ap- 

 pear on the sandbars and other exposed deposits of 

 these crude rock particles, the remains of the dying 

 plants became mingled with the sand and silt and by 

 its modifying influence served little by little to form 

 a true soil. Agricultural soils may, therefore, be 

 classified according to their origin as alluvial de- 

 posits along streams; lacustrine deposits in the bottom 

 of ponds and lakes, or the bottom of the sea, that 

 have subsequently been elevated or drained by the 



