DECAY AND REPKODUCTTCXN". 41 



just as rich in organic matter now as at the beginning, and 

 much richer on the surface, where a thick stratum of black 

 mould forms from the repeated fall and decay of leaves and 

 wood ; all the substance which the forest shall have drawn 

 from the earth (with the exception of certain salts and 

 earthy matters) must therefore have been derived from the 

 air, which contains every ingredient necessary for its 

 formation, while in the earth itself no kind of organic 

 matter is ever found. 



There was formerly a time when the carbonic acid of the 

 air was in much greater abundance, and favoured the growth 

 of those plants which thrive where there is plenty of water, 

 as in swamps and marshes. They grew and decayed for a 

 vast period of time, till a thick stratum of carbonaceous 

 matter was deposited, which, after being buried (by some 

 convulsion of nature) at a great depth, and pressed by the 

 enormous weight of the superincumbent earth into a hard 

 solid substance, is now being dug up by man, and forms that 

 most valuable of all products of the mine, coal (fig. 10). 



PIG. 10. 



That coal is derived from decayed and altered vegetable 

 matters, is pretty well proved ; for many pieces of coal, if 

 ground thin and subjected to the microscope, present a 

 texture exactly such as can be seen in wood of the present 

 date, and not only the ordinary structure, but in very many 

 cases certain " dotted fibres," indicating that the wood 

 belonged to the order of cone-bearing trees (Conifer ce), and 

 there is but little doubt that the constant deposit of such 

 wood and its slow and gradual decay was the real source of 



