•9«'-J STEVEXSOX— FORMATIOX OF COAL BEDS. 558 



the Mississippi river. Humphreys and Abbot''- have described the 

 process of formation and destruction: 



" Driftwood is lodged upon a sandbar. Deposition of sediment follows. 

 A willow growth succeeds. In high water, more deposition is caused by the 

 resistance thus presented to the current. In low water, the sand blown by the 

 wind lodges among the bushes. An island thus rises gradually to the level 

 of highwater and sometimes even above it, sustaining a dense growth of 

 cottonwoods, willows, etc. By a similar process the island becomes con- 

 nected with the mainland, or, by a slight change of direction of the current, 

 the underlying sandbar is washed away, the new made land caves into the 

 river, and the island disappears." 



When in the temperates such an island, whicli in spite of current 

 and flood, had grown and had become coated with trees, disappears 

 through undermining, the vegetation floats away piecemeal ; but in 

 the tropics the whole mass is bound together firmly by climbing and 

 other plants, whose roots are interlaced and whose stems embrace 

 the trees ; so that, when the underlying loose material has been re- 

 moved, the entangled vegetable matter floats away to be broken up 

 gradually. Many travellers have referred to floating island of plants 

 and plant material. Humboldt saw them on the Orinoco ; Mrs. 

 Agassiz was astonished by their size on the Amazon, where some 

 were like " floating gardens, sometimes half an acre in extent." 

 Kuntze saw patches of moderate size floating down the Parana sys- 

 tem. Miss Kingsley"'-' relates that during high water, the Congo 

 and Ogowe tear away their banks in the region above brackish water, 

 where there is no network of mangrove to protect them. Along the 

 Ogowe, the banks are of "stout clay" and the blocks hold together, 

 so that they often go sailing out to sea and are seen far from land 

 with shrubs or even trees on them. Xot all reach the open water, 

 for many are stranded in the delta region, where they collect debris 

 from the flood water and become matted with floating grasses. 

 Eventually they all go to pieces. 



De la Beche^° cites Tuckey's Expedition to the Zaire (Congo) 



^ Humphreys and Abbot, op. cit., pp. 97, 98. 



^° M. W. Kingsley, "West African Studies," 2d Ed., London, 1901, p. 87. 

 *" H. T. de la Beche, "The Geological Observer," Philadelphia, 1851, 

 p. 370. 



151 



