184 



THE WORLD'S LUMBER ROOM. 



The mineral composition of the mass would, in fact, be 

 identical with that of coal ; and much coal has evidently 

 been formed in the manner just sketched, for we can see 

 the soil in which the ancient trees and plants grew, as well 

 as the soil heaped on their heads ; and the trees themselves 



Fig. 36. DIAGRAM OF COAL-SEAM AS SEEN IN THE FACE OF THE 

 WORKING OF A COAL-MINE. 



(a) Under clay with roots passing through it ; (b) bed of coal ; (c) "roof" of the 

 coal, composed of sand and shales, with upright trunks of trees passing through it. 



are often found standing upright and still rooted in the spot 

 where once they nourished (Fig. 36). 



Coal is found in the Manchester coal-field at intervals 

 through a thickness of 6,800 feet, 60 feet of which are 

 workable coal ; but the seams are separated by so many 

 other deposits that the swamp, if swamp it was, must have 

 undergone many such subsidences as that just described. 



Coal-seams, however, appear more often to be the 

 remains of dense jungles growing along the coast, such, 



