I told you that the earth's crust had been worked over to a depth of
many miles since geological time first commenced. Subsequently, I have
referred to the growth of the continent in different geological periods.
All of our continents are being gradually worn down by the action of
rains, rills, rivulets, and rivers, and being deposited along the sea
margins, just as the Mississippi is gradually stretching out into the
Gulf, by the deposition of the muds of the delta. This encroachment on
the Gulf of Mexico may continue, yea, doubtless will, until that deep
body of water shall have been filled up by the remains of the continent,
borne down by the rivers; for the Mississippi alone carries annually 268
cubic miles of mud into the Gulf, according to Humphreys and Abbot. This
represents the valley of the Mississippi losing one foot off its whole
surface in 6,000 years. And were this to continue without any elevation
of the land, the continent would all be buried beneath the sea in a
period of about four and a half million years. But though this wasting
is going on, the continent will not disappear, for the relative
positions of the land and water are constantly changing; in some cases
the land is undergoing elevation, in others, subsidence. Prof. Hilgard
has succeeded in measuring known changes of level, in the lower
Mississippi Valley, and records the continent as having been at least
450 feet higher than at present (and if we take the coast survey
soundings, it seems as if we might substitute 3,000 feet as the
elevation), and subsequently at more than 450 feet lower, and then the
change back to the present elevation.

Let us now study the history of the great river in the last days of the
Cenozoic Time, and early days of the fifth and last great Geological
Time, in which we are now living--the Quaternary, or Age of Man--an
epoch which I have called _the "Great River Age_."