108 STEVENSON— FORMATION OF COAL BEDS. [April 2.. 



sank immediately. But all alike were deposited at last on the soaked 

 talus of the delta. The lake basin, in which the deposition was 

 made, is conceived to have been quite deep, for Lemiere's diagram 

 shows curves to a depth of 350 meters and the last is still at consid- 

 erable distance from the bottom ; it is supposed also to be large in 

 comparison with the breadth of the tributary streams. The impor- 

 tant source of plant material is the space along the streams between 

 the average low water line and that reached by high floods ; but the 

 still higher portions of the drainage area, being exposed to rain and 

 wind, would contribute. 



During a long period of low water, little aside from inorganic 

 matter would be carried to the basin ; but when that was followed 

 by a period of heavy rains, the forested area was invaded, the vege- 

 table contributions were increased, while inrjrganic contributions 

 were decreased. The forest soil was covered with humus, which 

 had been accumulating without cessation. The soil, thus covered, 

 became increasingly unfavorable to vegetation, whose roots as 

 Grand' Eury says, hate to penetrate it deeply. Lemiere thinks this 

 a "peremptory argument against formation siir place of coal beds 

 formed by aerial plants very different from those which have formed 

 peat bogs. That the forest might continue and might renew itself 

 after destruction, it was necessary that the soil be cleared away at 

 intervals by winds, rains and especially by floods." 



The humus, already macerated and denser than living plants, 

 was swept off first ; afterwards, the living plants would be uprooted 

 and broken. The macerated humus, being denser, was deposited on 

 the convex surfaces of the delta, while the living plants had to 

 become watersoaked before sinking, so that they were superimposed 

 upon the other plant material. They would come to rest more 

 abundantly in the bays between deltas, so that one should find more 

 of volatile matters in coal laid down within the bays than in that 

 deposited on the delta slopes, along the axes of the currents. The 

 volatile should increase as one departs from those axes but it should 

 decrease with the depth at which the vegetable matter was deposited. 



Floating islands are possible, since a flood might tear oft' bodily 

 part of a forest, which, carried down, might float for a while and 



108 



