276 Mr. A. Tylor on Changes of t/ie Sea-Level effected by 



verted into lakes ordinarily communicating with the river, but 

 sometimes with the sea after high tides. 



The present marine or fluvio-marine deposits must be composed 

 of mud that has passed the mouth of the river, or washed up by the 

 sea, while the freshwater strata must be entirely formed from sand 

 and mud carried over the river banks, or deposited on the bot- 

 tom of lakes supplied by the stream before it enters the Gulf of 

 Mexico. An idea of the amount of denudation that has taken 

 place in the interior of N. America might be either obtained from 

 the extent of the marine deposits formed of mud that had passed 

 the mouth of the river, or from that of the purely fluviatile and 

 contemporaneous deposits formed from mud which had never 

 entered the Gulf of Mexico. 



But it is also necessary to estimate what proportion of the 

 total quantity of mud brought down by the river is carried com- 

 pletely out to sea, compared to what is left either upon the 

 marine or fluviatile portion of the delta. 



Sir Charles Lyell has remarked, that the alluvium now 

 remaining in the valley of the Mississippi can only represent a 

 fragment of what has passed into the Gulf of Mexico ; and this 

 can readily be believed when we reflect upon the depth and 

 breadth of the channel, and upon the short period of the year 

 that the stream would throw any large quantity of mud into the 

 plains even if there were no artificial banks. We must also bear 

 in mind that only the coarse mud could settle near the shore, 

 for the finer particles could not deposit except in very deep water. 

 For these reasons, even if the mud carried beyond the mouth of 

 the river is only ten times the quantity left behind on the fluvia- 

 tile portion of the delta and plains of the Mississippi, this amount 

 of detritus could not be obtained without the mean level of one- 

 fifth part of North America being reduced 100 feet by denuda- 

 tion affected by the action of rain, the atmosphere, and running 

 water*. But Hutton (vol. ii. p. 401) remarks, in 1795, that 

 wherever any stream carried off particles of soil in its waters at 

 any period of the year, it might be said that denudation was 

 taking place in that country ; yet he particularly obseiTcd that 

 the waste of land was very unequal, being much more rapid in 

 the elevated than in the more level parts of any district. It is 

 therefore possible that, during the reduction of the mean surface- 

 level of the land drained by the Mississippi to the amount of 100 

 feet, some portions of the area might be lowered many times that 

 amount, while other portions might suffer little, or be positively 

 raised by the superposition of alluvial deposit. We are, however, 



* The data for calculating the annual quantity of detritus carried over 

 the river's banks, in relation with that carried down to the sea, are very 

 imperfect. Further information on this subject is much needed. 



