19".] STEVENSON— FORMATION OF COAL BEDS. 547 



mass which seems to be ahiiost inconceivably great. On some 

 streams he finds or learns of huge log barricades, apparently afford- 

 ing ample confirmation of Ochsenius's barricade theory ; along others 

 the river bed is set with " snags," impeding navigation ; while along 

 the main stream the casual observer is apt to regard the floating 

 trees and other debris as an almost continuous mass. 



It is equally certain that a vast amount of finel}- divided vege- 

 table matter, derived from chafing of logs and trunks during their 

 voyage as well as from partial decay of the floating plants, is carried 

 by all rivers. It is true that studies of the Mississippi, in flood and 

 in ordinary stages, have shown that the quantity in the silts is utterly 

 insignificant when compared with the inorganic materials, but it 

 suffices, during decay, to give off a notable discharge of gas in the 

 outer area of the delta. The suggestion has been made that vege- 

 table matter, minutely divided, may explain the fertility of the Nile 

 deposits. According to Reclus, cited by Marsh,-" it has been com- 

 puted that the Durance river, fed by torrents of great erosive power, 

 carries down annually enough solid matter to cover 272,000 acres 

 with a deposit two fifths of an inch thick, containing more available 

 nitrogen than 110,000 tons of guano and more carbon than could be 

 assimilated by 121,000 acres of woodland in one year. The black 

 waters of the Scottish lakes, of several rivers in Florida, of great 

 rivers like the Congo in Africa, the Negro and others in South 

 America prove that an enormous amount of vegetable material is 

 leached from peaty deposits. 



When one considers the mass of transported timber, the- content 

 of organic matter removed by solution, and reads the more or less 

 crude estimates of organic stuff's in the detritus carried by rivers, 

 the mind is staggered and he is almost ready to concede that in this 

 transportation there is the process fully competent to bring about the 

 accumulation of coal beds. It is important, then, to ascertain, if 

 possible, what becomes of this material. 



The trees and shrubs carried by the rivers were not uprooted by 

 the torrents ; they come not from abrupt slopes but from lowlands 

 where meandering streams undermine their banks and the plants 



■' G. P. Marsh, "The Earth as Modified by Human Action," p. 245. 



145 



