156 ON THE FORMATION OF COAL v 



probably fell in successive generations from natural decay ; and 

 making every allowance for other materials, we may safely assert 

 that every foot of thickness of pure bituminous coal implies the 

 qniet growth and fall of at least fifty generations of Sigillarice, 

 and therefore an undisturbed condition of forest growth enduring 

 through many centuries. Further, there is evidence that an 

 immense amount of loose parenchymatous tissue, and even of 

 wood, perished by decay, and we do not know to what extent 

 even the most durable tissues may have disappeared in this way ; 

 so that, in many coal-seams, we may have only a very small 

 part of the vegetable matter produced. " 



Undoubtedly the force of these reflections is not 

 diminished when the bituminous coal, as in Britain, 

 consists of accumulated spores and spore-cases, 

 rather than of stems. But, suppose we adopt 

 Principal Dawson's assumption, that one foot of 

 coal represents fifty generations of coal plants ; 

 and, further, make the moderate supposition that 

 each generation of coal plants took ten years to 

 come to maturity — then, each foot-thickness of 

 coal represents five hundred years. The super- 

 imposed beds of coal in one coal-field may amount 

 to a thickness of fifty or sixty feet, and therefore 

 the coal alone, in that field, represents 500 X 50 

 = 25,000 years. But the actual coal is but an 

 insignificant portion of the total deposit, which, as 

 has been seen, may amount to between two and 

 three miles of vertical thickness. Suppose it be 

 12,000 feet — which is 240 times the thickness of 

 the actual coal — is there any reason why we should 

 believe it may not have taken 240 times as long to 

 form? I know of none. But, in this case, the 



