518 



WITHIN THE EARTH'S CRUST 



fuel of that country. That such bogs are filled lakes or 

 swamps and that it has taken thousands of years for the 

 peat to accumulate, is shown by the fact that hollowed 



Courtesy of Taylor Coal Co. 

 COAL MINING IN SOUTHERN ILLINOIS 



Using a pneumatic drill, preparatory to blasting. Notice the horizontal 

 layers in which the coal lies. 



logs used as canoes by prehistoric men are sometimes found 

 buried in the peat at a depth of thirty feet or more. 



If these peat accumulations should at some time be grad- 

 ually submerged and covered with sand and silt, the ever- 

 increasing pressure of the water and of the layers of sediment 

 would gradually compress the spongy mass of vegetation 



