222 A. Tylor on Changes of the Sea-Level effected by 
Hutton, in 1795, has remarked, that there is evidence of denu- 
dation in every country where at any time of the year the streams 
carry off any particles of the superficial soil.* The Mississippi 
must derive its vast supplies of mud from thonsands of such tribu+ 
taries; for it could obtain them from no other source, unless we 
suppose it abstracts them from its own plains. Certainly in many 
places soil is being'removed from one part or other of its plains; 
but an equal quantity must be added to some other part, for the 
river could not make a permanent inroad into its plains without 
enlarging its channel. ‘This it does not do, or it would be able 
to carry off the winter-freshets without overflowing, and the pres- 
ent artificial bank would be unnecessary. : 
e thus briefly referred to observations made by British 
engineers which may throw some light on the causes of periodical 
floods, and changes of channel in rivers, and also upon the for 
mation of alluvial plains along their course. These questions 
need not further be entered into, because the limited growth of 
alluvial plains and deltas may be best illustrated by tracing the 
alteration in the mean level of a large part of North America that 
would be consequent upon a denudation sufficiently extensive t0 
furnish the alluvium said to exist in the valley of the Mississ!ppl- 
On the borders of the Gulf of Mexico at the present time marine 
strata are forming within a short distance of the finviatile, and 
frequently alternate with them, because spaces of the sea-shore 
are enclosed by banks of river-mud and converted into lakes of 
dinarily communicating with the river, but sometimes with the 
sea after high tides. 
he 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 00 be 
bottom of lakes supplied by the stream before it enters the Gul 
of Mexico. An idea of the amount of denudation that has taken 
place in the interior of North America might be either obtaines 
from the extent of the marine deposits formed of mud that he 
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. al 
But it is also necessary to estimate what proportion of the tol#” 
quantity of mud brought down by the river is carried completely 
t to sea, compared to what is left either upon the marine | 
fluviatile portion of the delta. pee: 
Sir Charles Lyell has remarked, that the alluvium now pre 
ing in the valley of the Mississippi can only represent a fragme” 
* Our clearest streams run muddy in a flood. The great causes, therefore, 5 as 
degradation of mountains never stop as long as there is water to run ; alt be more 
the heights of mountains diminish, the progress of their diminution may = 
and more retarded. Op. eit. vol. ii, p. 205. 
