existing Physical Causes during stated Periods of Time. 273 



Let us now turn to fig. 9, which exhibits Sir Charles LyelFs 

 transverse section of the channel and plains of the Mississippi, 

 and at all points throughout a course of several hundred miles. 

 The dotted lines are introduced to show the variation of the 

 water-level in the wet and dry seasons, b, h represent the arti- 

 ficial Levee ; d, d the banks and plains ; tw, m the swamps of the 

 Mississippi. " The banks* are higher than the bottom of the 

 swamps, because, when the river overflows, the coarser part of the 

 sediment is deposited on the banks, where the speed of the current 

 is first checked ^^ (Lyell) . The channel, however, is so wide and 

 deep, that even if there were no artificial banks to prevent floods, 

 the river would carry into the Gulf of Mexico the principal mass 

 of the mud it had received with the water of its tributaries ; for 

 it is only for a short time in the year that the level of water in 

 the river is above that of the adjoining plains. The swamps and 

 the numerous lakes formed by deserted river-bends communicate 

 at all times of the year with the main stream. In these places 

 mud could be constantly deposited mingled with the remains of 

 the vegetation which grows luxuriantly in the swamps. The only 

 supply of inorganic matter for raising the level of the vast plains 

 through which the river winds for hundreds of miles, must be 

 the mud deposited upon them during the periodical floods. These 

 are very much prevented by the artificial levee ; but when they 

 do occur, their force is augmented by the water being artificially 

 dammed up. 



"I have seen, says an eye-witness, when the banks of the 

 Mississippi burst, the water rush through at the rate of ten miles 

 an hour, sucking in flat boats and carrying them over a watery 

 waste into a dense swamp forest^' (Lyell). It would appear 

 that the Mississippi diff*ers in size and proportion more than in 

 other respects from our rivers. For instance, when floods occur 

 upon our own alluvial plains, they are most conspicuous at a 

 distance from the stream which caused them, indicating that the 

 parts of the plains nearest the banks are higher than those at a 

 distance from it, and therefore that fig. 9 would also represent 

 the transverse section of slow rivers generally. The similarity of 

 the physical features presented by the lower parts of all rivers 

 was particularly remarked by Huttonf. 



It has been observed by engineers J, that in all rivers in this 



* There is a similar section of the Nile and its banks published in the 

 fourth volume of the Quarterly Journal of the Geological Society, p. 344, 

 but communicated by Lieut. Newbold in 1842. 



t Theory of the Earth, vol. ii. p. 205-211. 



X On this and the following points see First Report of the Tidal Har- 

 bours Commission, above referred to, which contains the opinions of our 

 most celebrated engineers on the phaenomena presented by tidal and other 

 rivers. 



