CHAPTER XV.

ELLET ON THE MISSISSIPPI.

ELLET FORMS THE ALLUVIAL PLAIN SOLELY FORWARD AND SOLELY IN THE SEA. THE BEDS OF RIVERS AND VALLEYS ARE FORMED CHIEFLY BACKWARD, OR UP THE VALLEY. RIVERS MAY HAVE PATCHES OF ALLUVIAL PLAIN ANYWHERE. AND THESE PATCHES PROGRESS BACKWARD, OR UP THE VALLEY. THE PRACTICAL MAN'S MONSTROUS THEORY OF THE FORMATION OF THE DELTA. IF THIS THEORY WERE TRUE, THE RIVER ONCE EMBANKED WOULD BE ALWAYS EMBANKED. BUT ETERNAL CAUSES NECESSITATE AN ETERNAL RAISING OF THE EMBANKMENTS. PRINCIPLE OF THE LONGITUDINAL AND LATERAL SLOPE OF ALLUVIAL PLAINS. THE BED OF THE RIVER RISES WITH THE ALLUVIAL PLAIN. WHY ALLUVIAL RIVERS RECEIVE TRIBUTARIES WITHOUT INCREASE OF WIDTH. CAUSE OF THE SHAPE OF DELTAS.

was

THE second edition of the Tree-lifter ' scarcely printed before Ellet's work on the embankment of the alluvial part of the Mississippi came into my hands. His views of the mode of formation of alluvial plains differ widely from those which I have stated in the Tree-lifter,' 1853.

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This practical man abjures and bars 'geological speculations,' and at the same moment that he abjures these, he brings forth geological specu

lations of a character more monstrous than any which have been perpetrated in modern times. But, doubtless, Ellet's opinions are practice, and his neighbour's opinions speculations.' This is a very usual and very useful definition of the difference between theory and practice.

Ellet calls the whole of the alluvial plain the delta. I should restrict the delta to that small

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part of an alluvial plain from the sea to where the river first breaks into branches to the sea, the two outside of which, with the sea shore, often form a A. I do not, however, fight about a name, nor should I if it was the hardest and longest that Humboldt ever coined. But Ellet makes the head of the delta' and the line of the ancient sea shore the same, and he places these as far north or up the river as Commerce, above the Ohio. I should place the line of the ancient shore at least as far south as the present shore east and west of New Orleans. Ellet, indeed, deprecates the mooting the question, and designates it as an irrelevant geological speculation.' But, as will be seen by the diagram, page 223, Ellet forms his delta solely forward and solely in the sea. And according to him and his diagram, it is only the sea-formed delta which rises. From the sea shore upward or backward, the level of the landformed alluvial plain has been stationary for

Ellet forms

the alluvial

plain solely forward, in the sea.

The beds of rivers and valleys are formed chiefly backward, or up the valley.

45,000 years! It is therefore of vital importance even to Ellet's extraordinary theory, to know which part of the plain is sea formed, and which is land-formed, that is, where the ancient sea shore ran. I shall refer to this question hereafter,

and shall here endeavour to show that all alluvial plains, including their deltas, increase and rise every year everywhere, backward up the valleys, as well as forward in the sea, at the ends of the deltas.

Rain, or a river, works at the whole length of the valley or river course at the same time. But the chief formation is backward, that is from the sea to the hill. For, as the beds of rivers, if not horizontal, slope down to the sea, it is evident that the cutting down of the parts farthest from the sea must depend on the cutting down of the parts nearest the sea; that is, the parts farthest from the sea cannot be cut lower than the parts next the sea. So far the cutting of the parts farthest from the sea is dependent on the cutting of the parts next the sea. So that the chief longitudinal cutting of a valley or river-bed may be said to proceed from below upward as regards the valley, or backward as regards the stream of the river; that is, it proceeds from the sea to the hill, not from the hill to the sea.

No one will doubt that the Niagara gorge is

cut backward; but take the case of a barrier without a lake behind it. Suppose a barrier of rock to run across any valley or river-bed; when the bed of the valley or river on the upper side of the barrier has been worn down to a horizontal level with this barrier, it can not go lower. The barrier is in this particular case what the sea is in all cases, a negative key to the level of the valley. Thus deep shalt thou go and no deeper. A river-bed cannot run down before it comes to the barrier and then up again over the barrier. But as the barrier is cut through, the bed of the valley or river will be deepened backward, or from below upward, or towards the hills. This principle is true of the filling in of valleys as well as of the scooping them out. For when the scooping out or deepening is stopped at the lower end by the sea or a barrier of rock, denudation of the inclined parts above still goes on, and makes that lower part horizontal. And the passage of the detritus and soil from the inclined upper parts of valleys is checked in the horizontal lower parts of valleys, and soil accumulates there. This is the origin of alluvial plains; and a river of any size, or any rapidity, may, at any distance from the sea, have patches of alluvial plain, where no lakes have ever been; that is, above every rapid

or accidental barrier of hard ground. For as the

Rivers may have patches of alluvial plain any

where,

and these patches pro

gress backward, or up the valley.

barrier ponds the flood-water back on the horizontal valley above it, deposit will take place, that deposit will increase the ponding back up the valley, and, as long as overflow takes place, these patches will rise and progress backward up the valley. The only difference in the laws for the growth and gradient of these patches from those which regulate the growth and gradient of the plain at the level of the sea, is that they have no increasing cause for rising equivalent to the forward lengthening of the delta of the lower alluvial plain.

These flat alluvial patches may be seen even in torrents, sometimes reaching from one cascade to the other; and though each cascade digs a hole deep in proportion to the height of the cascade, as the cascade recedes, that part of the hole or rather groove which is farthest from the cascade is filled with débris. It is easy to perceive that these patches must be liable to constant change. They must be perpetually shortened by the recession of the lower barrier, and lengthened by the recession of the upper one: also that the wearing down of a barrier or shallow from below will expose the alluvial patch above it to denudation while the rise of an alluvial patch from below, over a rapid above it, may join two alluvial patches in one. These principles are eternally