AAD A POPULAR EXPOSITION OF THE 
days’ freshet, about fifty years ago, greatly enlarged its channel, and 
added much in places, to the steepness of its banks. The amount of 
detrital matters borne down by some rivers to the sea, is, at first 
thought, almost incredible. This is well shown by the formation of 
deltas. The delta of the Mississippi, on this continent, for example, 
like all other deltas, is formed almost entirely out of the sandy and 
other matters brought down by the stream. On entering the sea, the 
velocity of the river is necessarily checked, and the sediments are thus 
thrown down. Much of the coarser matter is indeed deposited on 
the bed of the river itself, raising this, and compelling the formation 
of artificial banks, or levées, to prevent inundations. Finally, as a 
well-known illustration of the immense amount of sedimentary matters 
borne seawards by certain rivers, the case of the Ganges, as described 
so fully by Sir Charles Lyell, in his “ Principles of Geology,” may be 
here cited. That river, it has been demonstrated, by actual obserya- 
tion and experiment, conveys annually to the sea an amount of matter 
that would outweigh sixty solid pyramids of granite, each, like the 
largest of the Egyptian pyramids, covering eleven acres at its base, and 
standing 500 feet in height. A considerable quantity of sediment is 
also produced by the slow movements of glaciers in Alpine and other 
districts in which these remarkable ice-rivers prevail. The glacier of 
the Aar, which covers with its tributaries an area of only six or seven 
square miles, thus furnishes daily, according to some recent researches 
of M. Collomb, at least 100 cubic yards of sand. This is carried off 
by its terminal stream or torrent. 
Action of the Sea (and of large bodies of Water generally.)—Vast 
in amount as are the sediments collected by rivers, they are far sur- 
passed by the accumulation of detrital matters obtamed by the waves 
and breakers of the sea. All who have resided for any length of 
time on an exposed and rocky coast, must be well aware of the de- 
structive action of the waves. The cliffs subjected to this action, 
gradually become undermined and hollowed out; and thus large 
masses of rock are brought down by their own weight. These, 
sooner or later, are broken up, and spread in the form of sediment 
along the shore, or over the sea-bottom. On some coasts, the amount 
of land destroyed in this manner almost exceeds belief.* On some 
* It would obviously be out of place inan Essay like the’present, to enlarge on this point- 
The reader unfamiliar with geological details of this character, should consult, more es- 
pecially, Lyell’s Principles of Geology, and also the Cours Hlémentatre of the late 
Alcide d’Orbigny. 
