deltas--Conglomerates--Constant interchange of land and sea.

In the last chapter several examples were given of the deltas of inland
seas, where the influence of the tides is almost imperceptible. We may
next consider those marine or oceanic deltas, where the tides play an
important part in the dispersion of fluviatile sediment, as in the Gulf
of Mexico, where they exert a moderate degree of force, and then in the
Bay of Bengal, where they are extremely powerful. In regard to
estuaries, which Rennel termed "negative deltas," they will be treated
of more properly when our attention is specially turned to the
operations of tides and currents (chapters 20, 21, and 22). In this
case, instead of the land gaining on the sea at the river's mouth, the
tides penetrate far inland beyond the general coast-line.

BASIN AND DELTA OF THE MISSISSIPPI.

_Alluvial plain._--The hydrographical basin of the Mississippi displays,
on the grandest scale, the action of running water on the surface of a
vast continent. This magnificent river rises nearly in the forty-ninth
parallel of north latitude, and flows to the Gulf of Mexico in the
twenty-ninth--a course, including its meanders, of more than three
thousand miles. It passes from a cold climate, where the hunter obtains
his furs and peltries, traverses the temperate latitudes, and discharges
its waters into the sea in the region of rice, the cotton plant, and the
sugar-cane. From near its mouth at the Balize a steamboat may ascend for
2000 miles with scarcely any perceptible difference in the width of the
river. Several of its tributaries, the Red River, the Arkansas, the
Missouri, the Ohio, and others, would be regarded elsewhere as of the
first importance, and, taken together, are navigable for a distance many
times exceeding that of the main stream. No river affords a more
striking illustration of the law before mentioned, that an augmentation
of volume does not occasion a proportional increase of surface, nay, is
even sometimes attended with a narrowing of the channel. The Mississippi
is half a mile wide at its junction with the Missouri, the latter being
also of equal width; yet the united waters have only, from their
confluence to the mouth of the Ohio, a medial width of about half a
mile. The junction of the Ohio seems also to produce no increase, but
rather a decrease, of surface.[356] The St. Francis, White, Arkansas,
and Red rivers are also absorbed by the main stream wi