19"] STEVENSON— FORMATION OF COAL BEDS. 533 



are on a larger scale. The almost vertical walls of railroad cuts 

 through hard rock are adorned by small plants growing in clefts or 

 even by trees in similar position. These have grown in spite of 

 rains, which threatened to wash away the little soil on which they 

 depend. But the rains are as powerless against plants in railroad 

 cuts as they are against plants growing in like conditions on the walls 

 of Alpine gorges or of canyons in the Sierra and the Rockies. 



River Floods. — The floods of rivers have much in common with 

 those of torrents, for most rivers are more or less torrential in their 

 upper reaches ; but there are noteworthy differences, aside from those 

 due to volume. The topographical conditions required for torrents 

 are wholly unlike those amid which great rivers exist. Torrents 

 flow, for the most part, in narrow valleys with here and there some 

 wider portions in which are insignificant floodplains ; but rivers 

 usually flow in broader valleys, have less rapid descent and are bor- 

 dered frequently by extensive floodplains. Rivers entering the At- 

 lantic along the eastern coast of the United States empty in most 

 cases into estuaries, which occupy the drowned lower portion of the 

 valley and conceal the floodplain ; but the condition is different in the 

 vast interior basin where many great rivers find discharge through 

 the Mississippi channel. Each important tributary of that stream 

 flows for long distances through broad lowlands, which fuse with 

 those of the Mississippi, extending from above Cairo to the Gulf of 

 Mexico and constantly increasing in width toward the south. The 

 coast and the interior types must be considered separately. Illustra- 

 tions of river floods will be selected mostly from those of the United 

 States, partly because the conditions seem to be unfamiliar to many, 

 and partly because the topographical relations of the central Missis- 

 sippi region are much like those supposed by some to have existed 

 during the coal-forming periods. 



Rivers of the Atlantic Coast. — Shaler,^^ in describing several 

 northward flowing streams of eastern Massachusetts, says that the 

 floodplain is in direct communication with the present margin of the 

 river, so that a very slight rise sends water over the whole of it. 



^^ N. S. Shaler, " Fluviatile Swamps of New England," Amer. Journ. Sci., 

 3d Ser., Vol. XXXIII., 1887, p. 203. 



131 



