If this theory were true, the river once embanked would

banked.

6

pushed forward in the course of 45,000 years to the end of the present delta. And the inevitable consequence of this hypothesis will then be that the slope of the plane must have been at successive periods, as represented.' 'The whole space embraced in the triangle H A B, became filled up by sedimentary accretions, and the river sloped off from H to the mouth of the Arkansas, with a descent represented by H B, more than twice as great as its present rate of descent.'

If alluvial plains were formed according to this diagram, solely down stream, or forward in the be always em- sea, from the sea shore backward on the land, the river once embanked would be always embanked, and there would be no difficulty about it. But nothing in nature ever happened like this diagram. The left end of the line of deposits never got up to H before the right end arrived

at D.

Ellet has only allowed for his house being on fire at one end, whereas it is on fire at both ends. He has only raised and lengthened the line of new deposits to the right, or forward into the sea, whereas nature raises it, and lengthens it to the left also, or backward up the valley. And the present line of deposits, H D, is not by nature stationary at H, as Ellet has made it for 45,000 years, but would be for ever rising throughout,

and lengthening at both ends, unless stopped for a time (in lengthening, not in rising) by the foot of a rapid to the left, above H, which it will progress up, while the water wears it down.

causes neces

Though artificial embankments will stop this But eternal rise of deposits altogether, and even expose them to denudation, the eternal causes of the

rise of deposits will be at work on the rise of the surface of the river, and of its bed, and consequently on the necessary rise of the artificial embankment for ever. And as long as man can play this game, the plain, instead of rising every year from overflow and deposit, will sink every year from denudation, while the surface of the river and its bed will rise every year.

The filling in of valleys is, as has been said, on the same principle as that which governs their scooping out. That is, it proceeds chiefly from the sea towards the hill. And the chief growth of alluvial plains in extent of area is backward or up stream on the land, not forward in the sea. That is, it is simply from the ponding back and overflow of water, which must happen where valleys which have been worn horizontal, or nearly horizontal, receive the flood-water from the inclined channels of unalluvial rivers. Commerce is nearly 300 feet above the level of the sea. The direct distance to the sea is about 500 miles. The slope of the

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sitate an

eternal raising of the em

bankments.

land is about eight inches in the mile. By the river, the distance is about 1200 miles, the slope of the banks is about three inches in the mile. Cut off the lower 250 miles of alluvial plain, or introduce a deep estuary that distance, there would be a fall of the river into the estuary of nearly 150 feet. This would soon cut a deep channel through the soft alluvial deposits, backward to Commerce, and the river could not overflow and deposit on its banks. Clap on the lower 250 miles of deposits again, and it will overflow and deposit on its banks. Why? because the lower 250 miles of deposits pond the river back on the upper 250 miles of deposits.

Artificial banks prevent this rising of the plain, that is, of the natural banks. While, by increasing the rise of the delta in the sea, they increase the ponding back, or rising of the surface of the river and of its bed. Art, therefore, is not only perpetually fighting against itself, but perpetually increasing its powers against itself; that is, in the case of the Mississippi, the deposit which nature would annually spread over the surface of 40,000 square miles to raise the land against the water, art annually sends to the end of the delta to increase the rising of the water against the land, while at the same time it exposes that land to denudation.

the longitu

lateral slopes

plains.

There are two slopes in perpetual formation in Principle of alluvial plains. The degree of the longitudinal dinal and slope (eight inches per mile) is the result of the of alluvial annual ponding back of flood-water. The degree of the lateral slope (say 40 inches per mile) results from the dying off of deposits from moving water. The longitudinal slope is so slight that to almost all practical intents it may be left out of calculation. But the effect of this slight difference between principle and practice, in such a basin as the Mississippi, may expose millions of men to the constant chance of death to themselves and destruction to their property from inundation, with the certainty that their actual estates are in process of becoming subterranean and themselves and their works fossil. For let the Po fill the Adriatic at a dead level, what signifies it to Ferrara? but give a slope of eight inches in the mile from Otranto towards the Alps, and Ferrara is subterranean.

When the Mississippi first began to cut its channel to the sea, which was when the land had just risen from the sea, there was no indenture in the sweep of the shore of the gulf, nor did that shore reach up to above the Ohio, as Ellet has supposed. On the contrary, the shores of the gulf (I do not allude to the delta) extended then much farther into the sea than they do now, and they have been worn back by the sea. But the

line even of the present sea shore would cross the delta about New Orleans. And the very least that can be abstracted from Ellet's ancient sea is the distance between New Orleans and Commerce.

This question is, as I have said, of importance, because, according to Ellet's theory and diagram, the level of the plain at the sea shore, and from thence upwards, is stationary, and it is only in the sea that the delta rises. And the rise of nearly 300 feet from New Orleans to Commerce is, according to Ellet, exceptional, as regards the plain above it, because it was raised in the sea. Whereas, in fact, it was raised on the land, by the same causes which are now raising it and all the alluvial parts above it. The only part of the delta formed in the sea is from New Orleans to the point D, in the diagram.

The young river made an indenture in the sweep of the shores; the mature river filled it in. The young river made an indenture as its estuary, or negative delta, as it is called, which must always precede a positive delta. For, as has been repeated too often, the direct operation of a river is only to cut down a deep channel for itself, till that cutting down is stopped by the sea. But since this estuary was cut, rain has formed the entire valley of the Mississippi, and the river has rolled the former contents of this valley into the