546 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



on both sides of the river, but " even here, though the rapidity of 

 the current was very considerable, the thick sward of grass was 

 ' laid ' flat along the sides of the stream and the soil was not abraded 

 so much as to discolor the flood." In his later work,-" he offers an 

 explanation of the conditions. 



" The shallow valleys, along the sides of which the villages are clotted, 

 have, at certain seasons of the year, rivers flowing through them, which at 

 this time formed only a succession of pools, with boggy and sedgy plains 

 between. When the sun is vertical over any part of the tropics on his way 

 south, the first rains begin to fall and the effect of these, though copious, is 

 usually only to fill the bogs and pools. When on his way north he again 

 crosses the same spot, we have the great rains of the year, and the pools 

 and bogs, being already filled, overflow and produce the great floods whicii 

 mark the Zambesi, and probably in the same manner cause the inundations 

 of the Nile. The luxuriant vegetation, wdiich tlie partial desiccation of many 

 of these rivers annually allows to grow, protects these bottoms and banks 

 from abrasion, and hence the comparative clearness of the water in the 

 greater floods." 



Darwin and Airs. Agassiz tell a similar story respecting the 

 Parana and the Amazon ; Cameron and Stanley have shown the con- 

 ditions in the region of Lake Tanganika and the Sudd of the Nile 

 has been described by Baker, Willey and other travellers. Every- 

 where, the conditions are the same ; living vegetation and even 

 humus are practically proof against the action of floods. 



The Plant Materials Transported by Rivers. — While it is true 

 that a vegetable cover is an almost complete protection against ero- 

 sion and that neither rain nor floods have much ability to remove 

 rooted plants or to take off the superficial coat of decayed or decay- 

 ing plant-stufi^, still the fact remains that rivers do carry away great 

 quantities of plant materials in one form or another. The quantity 

 brought down by a single torrent may be insignificant ; even that 

 borne bv a river of considerable size may not impress an observer 

 as important ; but when one reaches the lower Mississippi, draining 

 an area of not less than 1,250,000 scpiare miles, fed by tributaries 

 from the Rockies at the west and the Appalachians at the east, 

 which flow for long distances in broad alluvial plains, he finds a 



-"■'Narrative of an Expedition to the Zambesi and its Tributaries," 

 New York, 1866, p. 554. 



144 



