THE GROWTH OF COAL 245 



earthy bitumen, as well as for the occurrence of scales of fish 

 and other aquatic animals in such beds. Lyell's interesting 

 observation of the submerged areas at New Madrid, keeping 

 free of Mississippi mud, because fringed with a filter of cane- 

 brake, shows that the areas of coal accumulation might often 

 be inundated without earthy deposit, if, as seems probable, 

 they were fringed with dense brakes of calamites, sheltering 

 them from the influx of muddy water. It seems also certain 

 that the water of the coal areas would be brown and laden 

 with imperfect vegetable acids, like that of modern bogs, and 

 such water has usually little tendency to deposit any mineral 

 matter, even in the pores of vegetable fragments. The only 

 exception to this is one which also occurs in modern swamps, 

 namely, the tendency to deposit iron, either as carbonate (Clay 

 Ironstone), or sulphide (Iron Pyrite), both of which are 

 products of modern bogs, and equally characteristic of the coal 

 swamps. 



Where great accumulations of sediment are going on, as at 

 the mouths of modern rivers, there is a tendency to subsidence 

 of the area of the deposit, owing to its weight. This applies, 

 perhaps, to a greater extent to coal areas. Thus the area of a 

 coal swamp would ultimately sink so low as to be overflowed, 

 and a roof shale would be deposited to bury up the bed of 

 coal, and transmit it to future ages, chemically, and mechanically 

 changed by pressure and by that slow decomposition which 

 gradually converts vegetable matter into carbon and hydrocar- 

 bons. The long continuance and great extent of these alterna- 

 tions of growth and subsidence is perhaps the most extraordinary 

 fact of all. At the South Joggins, if we include the surfaces 

 having erect trees with those having beds of coal, the process 

 of growth of a forest or bog, and its burial by subsidence and 

 deposition must have been repeated about a hundred times 

 before the final burial of the whole under the thick sandstones 

 of the Upper Carboniferous and Permian. 



