prevented him from perceiving it[196]. [Footnote 196: The instance mentioned in the Geological Essays, from the travels of the Abbé Fortis, concerning urns thrown into the Adriatic, upwards of 1400 years ago, and not yet covered with mud, must be explained from peculiar circumstances, or local causes, with which we are unacquainted, as it makes against the deposition of earth near the shore, and in narrow seas; a general fact which, I think, every body admits.] 379. A remark which Major Rennell has made concerning the mouths of rivers, in his Geography of Herodotus, deserves Mr Kirwan's attention, though perhaps he may not be able to put on it an interpretation quite so favourable to his system. The remark is, that the mouths of great rivers are often formed on principles quite opposite to one another, so that some of them have a real delta or triangle of flat land at their mouths, while others have an estuary, or what may not improperly be called a _negative_ delta. Of the latter kind are some of the greatest rivers in the world, the Plata, the Oroonoko and the Maranon, and by far the greatest number of our European rivers. Nobody can doubt, that the three rivers just named carry with them as much earth as the Nile, or the Euphrates, or any other river in the world. All this they have deposited in the sea, and committed to the currents, which sweep along the shore of the American continent, and by these they have been spread out over the unlimited tracts of the ocean. Indeed, nothing can be more just than Dr Hutton's observation, that where low land is formed at the mouths of rivers, there the rivers bring down more than the sea is able to carry away; but that where such land is not formed, it is because the sea is able to carry off immediately all the deposit which it receives. 380. Mr Kirwan has denied on another principle the power of the sea to carry to a distance the materials delivered into it: "Notwithstanding," says he, "many particles of earth are by rivers conduced to the sea, yet _none are conveyed in any distance_, but are either deposited at their mouths, or rejected by currents or by tides; and the reason is, because the tide of flood is always more impetuous and forcible than the tide of ebb, the advancing waves being pressed forward by the countless number behind them, whereas the retreating are pressed backward by a far smaller number, as must be evident to an attentive spectator; and hence it is that all floating things cast into the sea, are at last thrown on shore, and not conveyed into the mid regions of the sea, as they should be if the reciprocal undulations of the tides were equally powerful "[197] [Footnote 197: Kirwan's Geol. Essays, p. 439.]