THE STORY OF COAL 519 



into compact layers. In the course of ages these layers 

 would harden and change into seams of bituminous (soft) 

 coal. The overlying layers of sand and mud would change 

 into layers of sedimentary rock. 



This process has been repeated many times in the history 

 of the earth. In fact there are some sections where it has 

 happened more than once over the same area, and has re- 

 sulted in the formation of several seams of coal, one above 

 the other, with layers of sedimentary rock between. If in 

 after ages such seams of coal were heated by the folding of 

 the earth's crust, or by some other means, the bituminous 

 coal was changed into anthracite (hard) coal. 



Sometimes miners find the roots of ancient trees, now 

 changed into coal, projecting from the bottom of a seam of 

 coal into the underlying rock layers that formed the soil 

 in which this ancient vegetation grew. Sometimes the 

 impressions of leaves and plant stems are found in the 

 underlying or the overlying rock layers and even in the coal 

 itself. 



How dependent the greater part of the civilized world 

 is upon nature's supply of coal, for comfort and for com- 

 merce, was shown during the coal famine of the winter of 

 1917-1918 in our Eastern and Middle states. Coal is a 

 plentiful commodity in normal times, and in many sections 

 is very cheap ; but considering that nature has required 

 ages to form and preserve it, and that what now seems an 

 unlimited supply must some day be exhausted, the prodigal 

 waste of coal of which recent generations have been guilty 

 is a serious matter. 



Petroleum is probably the result of the decomposition of 

 animal and plant remains which have been subjected for 

 ages to heat and enormous pressure. By distilling petroleum, 



