From these principles it follows, first, that a much smaller power is required to move a block of stone lying in the bed of a river, than if it were lying on the surface of the land; and secondly, that a very slight increase in the velocity of a current effects a very great increase in its moving power. We need not wonder, then, when we hear of the enormous masses of rocks and trees and mason-work which are carried away even by small rivers in times of flood.[23] Here are a few examples. In August, 1829, a fragment of sandstone, fourteen feet long, three feet wide, and one foot thick, was carried by the river Nairn, in Scotland, a distance of two hundred yards. On the same occasion the river Dee swept away a bridge of five arches, built of solid granite, which had stood uninjured for twenty years; the whole mass of masonry sunk into the bed of the stream and was seen no more. And the river Don, as we are assured on the authority of Mr. Farquharson, forced a mass of stones four or five hundred tons in weight up a steep inclined plane, leaving them in a great rectangular heap on the summit. A small rivulet called the College, in Northumberland, when swollen by a flood in August, 1827, “tore away from the abutment of a mill-dam a large block of greenstone-porphyry weighing nearly two tons, and transported it to the distance of a quarter of a mile.”[24] But it is needless to multiply examples of phenomena which are occurring every day around us, and of which many among our readers have probably been eye-witnesses. The transporting power of rivers must not always be estimated by the bulk and velocity of the current; for it is often greatly increased by some accidental obstruction, which for a time blocks up the channel through which the river flows. An instructive illustration is afforded by the river Dranse, which flows through the valley of Bagnes, in Switzerland, and empties itself into the Rhone above the lake of Geneva. In the year 1818 the avalanches which fell down from the mountain side formed a barrier across the valley, and thus effectually blocked up the course of the stream. The upper part of the valley was, in consequence, soon converted into a lake which gradually increased in size as the season advanced. When summer came, and the melting of the snows began, the ice barrier suddenly gave way with a tremendous crash, and the lake was emptied in half an hour. The mass of water, thus in a moment disengaged, burst with destructive violence over the lower valley, sweeping away rocks, forests, houses, bridges, and cultivated lands. Thousands of trees were torn up by the roots, fragments of granite as large as houses were rolled along, and the whole flood presented the appearance of a moving mass of ruins. [Illustration] [Illustration] _CHAPTER III._ THEORY OF DENUDATION--FURTHER ILLUSTRATIONS.