As a rule, however, rain acts through rivers. The power of rivers to denude and transport is exemplified daily. Even a comparatively small stream when swollen by rain may move rocks tons in weight, and may transport thousands of tons of gravel. The greatest damage is done when rivers are dammed by landslips or by ice. In 1818 the River Dranse was blocked by ice, and its upper part became a lake. In the hot season the barrier of ice gave way, and the torrent swept before it rocks, forests, houses, bridges, and cultivated land. For the greater part of its course the flood resembled a moving mass of rock and mud rather than of water. Some fragments of granite rock of enormous size, which might be compared to houses, were torn out and borne down for a quarter of a mile. The rivers of unmelted ice called the glaciers act more slowly, but they also have the power of transporting gravel, sand, and boulders to great distances, and of polishing and scoring their rocky channels. Icebergs, too, are potent geological agents. Many of them are loaded with 50,000 to 100,000 tons of rock and earth, which they may carry great distances. Also in their course they must break, and polish, and scratch the peaks and points of submarine mountains. Coast ice, likewise, may transport rocks and earth. Springs also must be considered as geological agents affecting the face of the globe. But running water not only denudes it, but also creates land, for lakes, seas, rivers are seen to form deltas. That Egypt was the gift of the Nile was the opinion of the Egyptian priests, and there can be no doubt that the fertility of the alluvial plain above Cairo, and the very existence of the delta below that city, are due to the action of that great river, and to its power of transporting mud from the interior of Africa and depositing it on its inundated plains as well as on that space which has been reclaimed from the Mediterranean and converted into land. The delta of the Ganges and Brahmapootra is more than double that of the Nile. Even larger is the delta of the Mississippi, which has been calculated to be 12,300 square miles in area. TIDES AND CURRENTS. The transporting and destroying and constructive power of tides and currents is, in many respects, analogous to that of rivers, but extends to wider areas, and is, therefore, of more geological importance. The chief influence of the ocean is exerted at moderate depths below the surface on all areas which are slowly rising, or attempting, as it were, to rise above the sea; but its influence is also seen round the coast of every continent and island. * * * * *