Although the swayings of the streams to and fro in their alluvial plains will give the reader some idea as to the struggle which the greater rivers have with the _debris_ which is committed to them, the full measure of the work and its consequences can only be appreciated by those who have studied the phenomena on the ground. A river such as the Mississippi is endlessly endeavouring to bear its burden to the sea. If its slope were a uniform inclined plane, the task might readily be accomplished; but in this, as in almost all other large water ways, the slope of the bed is ever diminishing with its onward course. The same water which in the mountain torrent of the Appalachians or Cordilleras rolled along stones several feet in diameter down slopes of a hundred feet or more to the mile can in the lower reaches of the stream move no pebbles which are more than one fourth of an inch in diameter over slopes which descend on the average about half a foot in a mile. Thus at every stage from the torrent to the sea the detritus has from time to time to rest within the alluvial banks, there awaiting the decay which slowly comes, and which may bring it to the state where it may be dissolved in the water, or divided into fragments so small that the stream may bear them on. A computation which the present writer has made shows that, on the average, it requires about forty thousand years for a particle of stone to make its way down the Mississippi to the sea after it has been detached from its original bed. Of course, some bits may make the journey straightforwardly; others may require a far greater time to accomplish the course which the water itself makes at most in a few weeks. This long delay in the journey of the detritus--a delay caused by its frequent rests in the alluvial plain--brings about important consequences which we will now consider.