All over the continents these things
are going on, and indefatigable forces are at work that slowly but surely
shear from the surface almost immeasurable quantities of earth and rock to
be transported far away. In some instances it is possible to find out just
how much effect is produced in a given period of time, especially in the
case of the great river systems. For example, the mass of the fine
particles of mud and silt carried in a given quantity of the water of the
Mississippi as it passes New Orleans can be accurately measured, and a
satisfactory determination can also be made of the total amount of water
carried by in a year. From these figures the amount of materials in
suspension discharged into the Gulf of Mexico becomes known. It is
sufficient to cover one square mile to the depth of 269 feet; in twenty
years it is one cubic mile, or five cubic miles in a century. Turning now
to the other aspect of this process, and the antecedent causes which
produce these effects, it appears that the area of the Mississippi River
basin is 1,147,000 square miles--about one third of the total area of the
United States. Knowing this, and the annual waste from its surface, it is
easy to demonstrate that it will take 6000 years to plane off an average
of one foot of soil and rock from the whole of this immense area.
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