Here then may be found the whole history of the ravine. Originally the streamlet wound its devious course through a flat alluvial country with a channel sunk but a foot or two below the level of the plain. Such continued its character till it reached a low bluff, down which the water flowed more rapidly to gain a second level undulating region. The part of this bluff crossed by the stream was ere long bared of its covering of soil and clay, and the rock below came to be washed by a group of little cascades. Once exposed to the decomposing and disintegrating forces, the stone soon began to decay and the cascades ere long merged into one. By slow degrees the rock gave way and the waterfall retreated from the bluff. For perchance thousands of years the same process has been going on, now with greater, now with less rapidity, according to the nature of the rocks encountered and other modifying causes, until the fall has eaten its way back for well-nigh three miles and scooped out a wild rocky gorge some fifty or sixty feet deep. This is but a solitary and insignificant instance of what may be seen all over the world, for the process remains the same whether we stand beside a tiny rivulet in some lone Highland glen or listen to the roar of the falls of Niagara. There is but one other principal agency at work in the demolition of rock-masses, the waves and currents of the ocean. But we have already noted the effects thus produced, and need not now retrace our steps further than to recall the vast amount of devastation which can be shown to have been effected in our own country by marine causes, both in breaching the existing shores and in scooping out valleys and grinding down hills at former periods when the land was either rising above or sinking below the level of the sea. Having now satisfied ourselves that there goes on all over the world an incessant waste of the solid lands, that the disintegrated debris is washed down by rains and transported seawards by rivers, and that the waves are ever eating their way into the iron-bound coast-line as well as into the low alluvial shore, we naturally come to ask the result and end of all this decay. What becomes of that vast amount of mineral matter annually removed from the land? To be able to answer this question clearly and distinctly, let us look for a little at what takes place in lakes, at river-mouths, and in open sea.