156 Conservation Reader 



which promotes the growth of plants. Thus the plants 

 in the warm, moist parts of the earth grew more densely 

 and luxviriantly than they usually do today. 



In the decay of this vegetation deposits similar to the 

 peat marshes were formed, but they differed in being much 

 thicker and more extensive. If the story of these ancient 

 peat marshes had stopped here, we should never have had 

 any coal. Fortunately it did not, for some of the swamps 

 sank beneath the water of a lake or ocean and thick beds 

 of gravel, sand, or clay were deposited over them. While 

 buried deep in the earth, the decaying vegetation was heated 

 and pressed together by the great weight of the earth above, 

 and was finally changed to shining, black coal. 



After the coal was made, but before men came to the 

 earth, parts of the sea bottom with its buried treasures 

 were raised to form hUls and mountains. Then the rain- 

 water began its work upon the slopes, and after a time 

 washed away so much of the overlying material that the 

 coal was exposed at the surface. At last through some 

 accident, such as lightning perhaps, men learned that this 

 black substance would burn. Coal was httle used, however, 

 as long as there was an abundance of wood and the needs 

 of people were few. 



As manufacturing and the use of the steam engine in- 

 creased, coal grew in value. The business of mining coal 

 finally became one of the great industries. The mining 

 operations were carried on as carelessly as though the supply 

 in the interior of the earth were inexhaustible. In the 

 underground working it is customary to leave about one 

 quarter of the coal in the form of pillars for the purpose of 

 supporting the roof. At a little more expense other mate- 



