378 



HISTORICAL GEOLOGY 



that the coal seams, which average but a few feet in thickness, 

 are interbedded with thicker layers of sandstone, shale, and 

 other rocks. The details of this section hold good for a 

 single locality only. Elsewhere we may find fewer or more 

 coal seams, and the thicknesses of the individual beds vary 



from place to place. 

 But the general re- 

 lations are typical of 

 the whole region. 



Origin of coal. 

 There is ample proof 

 that coal is composed 

 of vegetable matter 

 much altered from 

 its original condition. 

 Stumps of trees are 

 sometimes found 

 standing in the coal 

 seams as they grew; 

 delicate leaves are 

 matted upon the 

 shales which accom- 

 pany the coal ; and it 

 is often possible to 

 identify, even with 

 the naked eye, the 

 cellular structure of 

 plant tissues in pieces 

 of the pure coal itself. 

 Vegetable substance is composed chiefly of carbon, hydro- 

 gen, and oxygen in very complex compounds. When wood 

 decays, chemical changes take place and new substances are 

 produced. If this decay goes on in the open air, the carbon, 

 hydrogen, and oxygen (most of this from the air) unite in such 

 a way as to form water and the gas carbon dioxide. As these 

 are volatile, the entire substance of the plant soon disappears. 



FIG. 399. Curves showing the changes which 

 take place in the alteration of wood through 

 coal to graphite. 



Why so little change in the ash? What 

 proportion of the changes may be passed 

 through while the marsh is still unburied ? 



