34 



Marvels of the Universe 



can be at once seen by a comparison with that of 1820, when it was estimated that only 

 twenty milhons in all were raised. How long will our coal last ? This is a frequentlj-- 

 asked question. The Royal Commissioners reported in 1905 that at depths of less than four 

 thousand feet there were one hundred thousand millions of tons of coal available in the United 

 Kingdom, which means that at the present rate of output there wll be enough coal at home 

 to last for about four and a half centuries more. On the other hand, we must remember 

 that there is untold wealth of coal abroad, and it is not likely that the world's supply will ever be 

 exhausted. 



A coal-seam is really a most insignificant part of the coal-measures in which it is found ; for 

 instance, a South Wales coal-field gives us twelve thousand feet of \-ertical thickness, and of this 

 total, all the coal-seams put together would give us but one hundred and twenty feet of coal. Great 

 masses of shale and sandstone intervene between the seams, and the cost of coal-getting is brought 

 about to a great extent by the difficulties and dangers which they entail. A seam of coal less than 

 a foot in thickness is, as a rule, not worth the working, but many of the best seams are only a few 

 feet thick, and it will be seen how small a place they play in a series of measures such as we have 

 instanced. 



But why are the seams so thin ? The mystery of their origin is wrapped up in this question. 



Coal is the compressed product of many 

 years' growth of forests that were in existence 

 in ages that have long passed away. If the 

 whole of tfie growth of the thickest forest 

 which is now living were suddenly to be 

 overwhelmed and precipitated as coal, we 

 might get as a result a thin seam of coal, 

 but it would be unworkable, for it would be 

 but an inch or two in thickness at the most. 

 A single workable seam of coal represents to 

 us the steady annual sheddings of trees ex- 

 tending over a space of time only to be reckoned 

 by centuries, and these becoming buried deep 

 down in the crust of the earth, the great 

 pressure and heat brouglit to bear upon the 

 deposits have combined to bring about a sub- 

 stance which now takes the form of coal. 

 Dawson stated that he felt safe in asserting 

 that every foot of coal implied the growth 

 and fall of at least fifty generations of 

 sigillaria trees, and that this necessitated 

 many centuries of undisturbed forest con- 

 ditions. We are thus able to form a slight 

 estimate of the enormous mass of crushed 

 vegetation which has gone to form our familiar 

 coal. It is possible, too, to form an idea of 

 the amount of time necessary to enable those 

 changes to take place which have brought 

 about the finished product. Suppose we stir 

 up the black mud which is found at the 

 [r.jij. T.Ufni.ura. bottomof our ditches and stagnant pools and 



FOSSIL-TREE OF THE COAL PERIOD. , , ,, r i ., ,, Ti ^ • 



, , , canals, we shall find there all that i-s necessary 



round in position in the coal beds as It grew. -' 



