I9II.] STEVENSON— FORMATION OF COAL BEDS. 23 



or from transported vegetable material. The answer to the first 

 question is certain — a great period of time was required for forma- 

 tion of the coal beds and their associated strata; but the second 

 question is more complex and he is inclined to believe that both 

 methods are possible, though there may be difficulty ip determining 

 which prevailed at a given locality. Vertical stems are not decisive, 

 for they are found at times in rocks formed by transport, while 

 prostrate stems occur in deposits clearly made in situ. 



He believed that there were no continental areas during Carbonif- 

 erous times, that the dry land consisted only of islands. For this 

 reason, it is impossible to accept the hypothesis that coal was formed 

 in great lakes or at the mouths of rivers. The only method of 

 formation by transport would be the driving of great masses of 

 vegetable matter against an island, which would collect in the quiet 

 eddy on the opposite side, where, becoming waterlogged, they would 

 sink and be covered with mud. He clearly prefers the doctrine of 

 origin in situ. 



An island, heavily forested for an indefinitely long period, be- 

 comes covered by a mass of bark, wood, etc., and similar remains 

 of small plants. If the island be flooded by the outburst of granite 

 and consequent elevation of the sea-level, the vegetation will be pro- 

 strated. By frequent outbursts the sea-level will be raised perma- 

 nently and the island remains submerged. Deposits of sand and mud 

 bring the island again to the surface of the water : a new forest rises 

 on the grave of the old one. He thinks the alternation of strata and 

 the formation of coal in situ can be explained very simply in this 

 way. 



IMurchison,-"^ after his study of the Donetz field in Russia, was 

 convinced that the doctrine of transport alone could explain the 

 conditions. The sections in southern Russia show "that the hypo- 

 thesis of the formation of coal beds by masses of vegetation which 

 there grew having subsided //; situ (the truth of the application of 

 which to some basins we do not deny ) cannot be applied to the cases 

 in question any more than to the pure marine coal beds of the north- 

 ern districts, Northumberland and the northwestern parts of York- 



^' R. L Alurchison, " The Geology of Russia in Europe and the Ural 

 Mountains," London, 1845, Vol. L, pp. 112-114. 



23 



