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. Of
course only an inch or a few inches will be taken from some regions where
the ground is harder or rockier, or where little rain falls, while many
feet will be washed away from other places. The waters of the Hoang-ho
come from about 700,000 square miles of country, from which one foot of
soil is washed away in 1464 years. The Ganges River, draining about
143,000 square miles, carries off a similar depth of eroded materials from
its basin in 823 years! Should we add to the above figures those that
specify the bulk of the chemical substances in solution carried by these
waters, the total would be even greater. We know that in the case of the
Thames River, calcareous substances to the amount of 10,000 tons a year
are carried past London, and all this mineral has been dissolved by
rain-water from the chalky cliffs and uplands of England, so that the land
has become less by this amount. Thus we learn that vast alterations are
being made in the structure of great continents by rain and rivers, as well
as by glaciers and other geological agencies. And at the same time that old
strata are undergoing destruction new ones are in process of construction
at other places, where animal remains can be embedded and preserved as
fossils. The forces at work seem weak, but they continue their operations
through ages that are beyond our comprehension and they accomplish results
of world-building magnitude.

Thus the whole process of geological construction is such that older
exposed strata continually undergo disintegration, but this involves t