152 ON THE FORMATION OF COAL v 



have accumulated, subaerially, upon the surface 

 of a soil covered by a forest of cryptogamous 

 plants, probably tree-ferns. 



As regards this important point of the subaerial 

 region of coal, I am glad to find myself in entire 

 accordance with Principal Dawson, who bases his 

 conclusions upon other, but no less forcible, 

 considerations. In a passage, which is the con- 

 tinuation of that already cited, he writes : 



"(3) The microscopical structure and chemical composition 

 of the beds of cannel coal and earthy bitumen, and of the more 

 highly bituminous and carbonaceous shale, show them to have 

 been of the nature of the fine vegetable mud which accumulates 

 in the ponds and shallow lakes of modern swamps. When such 

 fine vegetable sediment is mixed, as is often the case, with clay, 

 it becomes similar to the bituminous limestone and calcareo- 

 bituminous shales of the coal-measures. (4) A few of the under- 

 clays, which support beds of coal, are of the^nature of the vege- 

 table mud above referred to ; but the greater part are argillo- 

 arenaceous in composition, with little vegetable matter, and 

 bleached by the drainage from them of water containing the 

 products of vegetable decay. They are, in short, loamy or clay 

 soils, and must have been sufficiently above water to admit of 

 drainage. The absence of sulphurets, and the occurrence of 

 carbonate of iron in connection with them, prove that, when 

 they existed as soils, rain-water, and not sea- water, percolated 

 them. (5) The coal and the fossil forests present many evi- 

 dences of subaerial conditions. Most of the erect and prostrate 

 trees had become hollow shells of bark before they were finally 

 embedded, and their wood had broken into cubical pieces of 

 mineral charcoal. Land- snails and galley- worms (Xylobius) 

 crept into them, and they became dens, or traps, for reptiles. 

 Large quantities of mineral charcoal occur on the surface of all 

 the large beds of coal. None of these appearances could have 

 been produced by subaqueous action. (6) Though the roots of 



