54 STEVENSON— FORMATION OF COAL BEDS. [April 21. 



would certainly carry also mineral matter. The coal and its ash 

 may, both of them, be of vegetable origin. Logan's discovery of 

 the underclay or Seatstone under nearly every coal bed was the first 

 great step in the right direction toward solving the problem. Bin- 

 ney's study of an erect stump discovered by Hawkshaw near Man- 

 chester was the next, for there a Sigillaria with Stigmaria roots was 

 rooted in a seat clay, while the stem was surrounded by rock. Many 

 similar cases were discovered. The underclay was the old soil sup- 

 porting plants which produced a layer of nearly pure vegetable 

 matter. When the surface was lowered beneath the water, sand 

 and clay were laid on top and the band of dead plants was converted 

 by pressure and chemical change into a seam of coal. 



When sinking ceased, the shallow water was filled up and a 

 swampy plain was made. \'egetation spread out from the land and 

 a second coal bed began to accumulate. This process repeated many 

 times over gave a succession of sandstone and shale with coal beds 

 at intervals. The great swampy expanses in the delta of the Ganges 

 and Brahmapootra must bear close resemblance to the marshy flats 

 in which the coal was formed. The nearest approach, however, is 

 in the accumulations on the coast of Patagonia, described by Lady 

 Brassey in " A Voyage in the Sunbeam " ; " To penetrate far inland 

 was not easy owing to the denseness of the vegetation. Large trees 

 had fallen and, rotting where they lay, had become the birthplace of 

 thousands of other trees, shrubs, plants, mosses and lichens. In 

 fact in some places, we might almost be said to be walking on tops 

 of the trees, and first one and then another of the party found his 

 feet slipping through into unknown depths." 



There are, however, deposits of subacjueous coal, derived from 

 driftwood carried cjown and buried amid mechanical deposits, but 

 they are irregular and are apt to be impure. It is probable that the 

 patches of canncl coal mark sites of pools or lakes in which vege- 

 table matter lay until it was maceratefl into a \n\\\). This passes 

 gradually by increase of earthy admixture into well-stratified carbo- 

 naceous shale. 



Green had already presented the same suggestions, though briefly, 

 in his work on the Yorkshire coal-field published in 1878. 



54 



