90 SCIENCE IN 8UORT CHAPTERS. 



ceeding on a small scale in the Aaehensee, and on a larger 

 in Norway, we have, I think, a natural history of the for- 

 mation, not only of coal seams, but also of the Coal Meas- 

 ures around and above them. 



The theory which attributed our coal seains to such vege- 

 table accumulations as the rafts of the Mississippi is now 

 generally abandoned. It fails to account for the state of 

 preservation and the position of many of the vegetable re- 

 mains associated with coal. 



There is another serious objection to this theory that I 

 have not seen expressed. It is this: rivers bringing down 

 to their mouths such vegetable deltas as are supposed, 

 would also bring considerable quantities of earthy matter 

 in suspension, and this would be deposited with the trees. 

 Instead of the 2 or 3 per cent of incombustible ash com- 

 monly found in coal, we should thus have a quantity more 

 nearly like that found in bituminous shales which may thus 

 be formed, viz., from 20 to 80 per cent. 



The alternative hypothesis now more commonly accepted 

 that the vegetation of our coal-fields actually grew where 

 we find it is also refuted by the composition of coal-ash. 

 If the coal consisted simply of the vegetable matter of buried 

 forests its composition should correspond to that of the 

 ashes of plants; and the refuse from our furnaces and fire- 

 places would be a most valuable manure. This we know 

 is not the case. Ordinary coal-ash, as Bischof has shown, 

 nearly corresponds to that of the rocks with which it . is 

 associated; and he says that "the conversion of vegetable 

 substances into coal has been effected by the agency of 

 water;" and also that coal has been formed, not from dwarf- 

 ish mosses, sedges, and other plants which now contribute 

 to the growth of our peat-bogs, but from the stems and 

 trunks of the forest trees of the Carboniferous Period, 

 such as Sigittarm, Lepdodendra, and Coniferm.* All we 

 know of these plants teaches us that they could not grow 

 in a merely vegetable soil containing but 2 or 3 per cent of 

 mineral matter. Such must have been their soil for hun- 

 dreds of generations in order to give a depth sufficient for 

 the formation of the South Staffordshire ten-yard seam. 



* Hull, " On the Coal-fields of Great Britain." 



