SCIENCE OF COMMON THINGS. 225 



How charcoal is prepared. "What is mineral coal. 



resin, etc., when, during the combustion of these sub- 

 stances, there is an insufficient supply of air. 



1499 How is charcoal prepared f 



By charring wood in mounds or pits, covered with 

 turf or soil in such a way as to exclude in a great degree 

 the admission of air, and thus prevent complete com- 

 bustion. 



1500 What is mineral or hard coal? 



Coal is the product of a vast accumulation of vege- 

 table matter, deposited during a remote geological 

 period in beds or layers in the earth, and which, by the 

 action of pressure, heat, and other causes, has become 

 consolidated and hardened into its present form. 



15 Ol How does the coal occur in the earth ? 



In strata or layers, varying from a few inches to 

 several feet in thickness, inclosed between other strata 

 of limestone, clay, or iron ore. 



15OS In what manner is it supposed that this great accumulation of 

 vegetable, material took place 1 



The vegetable matter of which coal is composed is 

 supposed, by some, to have grown in immense swamps 

 or marshes. By others, the vegetable matter is sup- 

 posed to have been swept down ~by rivers, and deposited 

 at their mouths or estuaries in immense beds. 



1503 Are such accumulations of vegetable matter, through the agency 

 of rivers, going on at the present day ? 



At the present time the Red River of Texas is abso- 

 lutely choked up with a raft composed of trunks of 

 trees and other vegetable matter, many miles in extent, 

 and of unknown thickness. Other rivers of the South- 

 western United States bring down vegetable materials 

 annually, sufficient for the production of vast beds of 

 coal. 



1504 How do we know that coal is all vegetable matter ? 



Because in every coal mine we find the leaves, 

 trunks, and fruits of trees in immense numbers, many 

 of them in a most perfect state of preservation ; so 

 much eo, that the botany of the coal period can be 



