I9II-] STEVENSON— FORMATION OF COAL BEDS. 67 



Oscillations of level still continued as the north, but the land 

 constantly encroached on the shallowing sea, the mud encroaching 

 on the Carboniferous liinestone and the sandbanks following the 

 mud closely. ^Meanwhile " the terrestrial vegetation was spreading 

 from the old Lower Carboniferous land areas over the new Upper 

 Carboniferous marsh lands, from the mountains of Wales and 

 from the other Lower Carboniferous islands, now uplands. These 

 forests contributed in their decay, through many generations, the 

 accumulation which now, compacted by pressure and subjected to 

 earth heat, is familiar to us as a coal seam. Each coal seam repre- 

 sents a land surface, just as the sandbanks and mudbanks (sand- 

 stones and shales) above it point to submergence. The fact too 

 that the coal seams in a given section are parallel to each other or 

 nearly so, implies that the forests grew on horizontal tracts of land, 

 just as the associated sandbanks and mudbanks, with marine or 

 freshwater shells, prove that these horizontal tracts were near the 

 sea level or within reach of the waters of a mighty river. We may 

 learn also from the study of the isolated coal fields that this great 

 horizontal tract of forest clad alluvia occupied nearly the whole 

 area of the British isles in the Upper Carboniferous age, from the 

 Scotch Highlands southward, the dead flat being broken only by the 

 higher lands, the old islands of the Lower Carboniferous sea, which 

 I have already described. It was indeed the delta of a mighty river, 

 analogous in every particular to that of the Mississippi — a delta in 

 which from time to time the forest growths became depressed beneath 

 the water until the whole thickness (7,200 feet in Lancashire) was 

 accumulated of coal seams and associated sandstones and shales. 

 After each depression the forest spread again over the bare expanse 

 of sand and mud piled up in the depression." 



The great northern and western land, termed by Dawkins, 

 Archaia, whence came this mass of mineral deposits, occupied the 

 North Atlantic sea, stretching from the west coast of Ireland and 

 the Scottish Highlands to the American continent. To this great 

 land may be traced the pebbles and groups of pebbles found in the 

 Lancashire coal seams, mostly quartzites, which probably were 

 brought down in flood time in roots of trees from the shingle beach. 



67 



