191 1.] STEVENSON— FORMATION OF COAL BEDS. 561 



either the rock over which they flow or the vegetation growing on 

 the floor or on the walls. If they carry sand, clay or even fine 

 gravel, vegetable growth on islands, formed by aggradation, resists 

 the flow and causes increased local deposit without material injury 

 to itself. Even fierce mountain torrents sweeping their load of very 

 coarse materials over a dejection cone do not clear the surface from 

 trees. 



2. Water of rainfall has practically no ability to remove a cover 

 of living vegetation, even on steep declivities, except indirectly — as 

 by finding access to unconsolidated material below, which may be 

 rendered semi-fluid, so as to move down as a landslide. The heaviest 

 rainfall barely disturbs the cover of litter in a forest; that material 

 breaks the force of the falling drops, it absorbs much of the water, 

 it obstructs the formation of rivulets and protects itself as well as 

 the underlying soil from erosion. Forests are practically uninjured 

 by the heaviest rainfall ; even tiny plants, growing in clefts of rocks 

 are equally undisturbed. Rain does not remove the petty deposit 

 of soil on a projecting point of rock, if a tuft of grass has thrust 

 roots into it. 



3. River floods in great lowland areas rise slowly, as is shown 

 by the floods of [Mississippi, Amazon, Orinoco, Nile and the rivers 

 of central Africa. In passing through forests, those floods lose 

 speed and become merely a quiet overflow* with sluggish movement, 

 so that they disturb neither the living growth nor the decomposed 

 litter on the surface. They move the loose dried twigs and leaves, 

 but even those in great part, are transported only a short distance, 

 unless swept into the stream from the bank. The main current 

 itself cannot uproot trees; it cannot even tear loose a floating tree 

 which has lodged against a sandbar directly in the line of strongest 

 flow ; but, unless the sandbar be washed away and the tree be set 

 free, that will remain as a " snag," to become more firmly fixed 

 during each succeeding flood. In most cases, the areas subject to 

 these vast floods are prepared beforehand by heavy rains, whereby 

 the humus cover is soaked and, so to say, cemented to resist the 

 moving water. A dense growth of vegetation forms in the channels 

 of tropical rivers and ofi'ers such resistance that even the mightiest 



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