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.

As an alluvial plain is constructed, we generally find at the base
pebbly material which fell to the bottom in the current of the main
stream as the shores grew outward. Above this level we find the
deposits laid down by the flood waters containing no pebbles, and this
for the reason that those weightier bits remained in the stream bed
when the tide flowed over the plain. As the alluvial deposit is laid
down, a good deal of vegetable matter was built into it. Generally
this has decayed and disappeared. On the surface of the plain there
has always been growing abundant vegetation, the remains of which
decayed on the surface in the manner which we may observe at the
present day. This decomposing vegetable matter within and upon the
porous alluvial material produces large quantities of carbonic acid, a
gas which readily enters the rain water, and gives it a peculiar power
of breaking up rock matter. Acting on the _debris_, thi