Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/436

Rh displaced, the direction of its lateral movement, and the width of the original river course between its steep clay banks, been a little different, a nearly staunch dam would have been at once formed, and the ponded back river would have begun to form a temporary lake. In fact, at hundreds of places, as well as at this one, along some 10 or 15 miles of the river thereabouts, the occurrence of just such a landslip, and of one not much bigger in volume, might have produced a temporary lake of many thousands of acres in water surface, but in almost every case, (probably not one case in a thousand to the contrary,) the lake could, by no possibility, have been more than temporary.

As soon as the accumulated waters reach the top of the lowest point of the dam, they begin to cut it through; their action upon loose and ill-compacted earth and stones is rapid, and increasingly so, as the waters enlarge their own channel of overflow, partial slippages from its sides soon occur, succeeded by partial debacles, and rapid sweeping down of more of the obstruction; and, at last, nought remains of it but a low, flat, curved-surfaced mud bank or bar, a few inches, perhaps only, above the level of the original stony river bed, and a few hundred feet lower down the stream than where the dam was originally launched into it. Even this mud bank is, in the end, bit by bit carried down to the sea.

The water, while ponded to its full depth, may have sapped the bases and flanks of the temporarily submerged clay banks, at both sides the river course, and partial slippages will then have taken place from them, to be in like manner gradually removed. And thus this secondary effect of earthquake, which looks to the eye so formidable, when

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