For these reasons, then, we must conclude that the ravine or valley has been carved out by the stream; but perhaps the most convincing arguments are afforded by the furrows and miniature ravines so frequently met with on the sides of all mountains; and it is impossible to examine these without concluding that they have in every case been cut out of the solid rock by the little rapid torrents that run along them after heavy rain. If we are fortunate enough to see them on a thoroughly rainy day, we may derive much instruction from watching the little torrents at work as they run down the mountain-side, here and there dashing over the rocks in little cascades, and bringing down to the base of the hill much of the débris that forms higher up. In this way Nature gives us an "object lesson," and seems to say: "Watch me at work here, and learn from such little operations how I work on a larger scale, and carve out my ravines and big valleys. Only give me plenty of time, and I can accomplish much greater feats than this." The question of time is no longer disputed; and all geologists are willing to grant almost unlimited time, at least periods of time that seem to us unlimited. Most streams have been flowing for thousands of years; and when once we grant that, we find no difficulty in believing that all valleys are the work of rain and rivers. Surely no one would argue that the furrows on a mountain-side are all rents which have been widened by the action of water; for if they were rents, each must have been caused by some disturbance of the rocks composing the mountain, and we should of course be able to see the cracks for ourselves, and to find that the rocks had in some way been disturbed and rent open. Even the rain which falls on the road in a heavy shower teaches the same simple but important lesson, as it runs off into the gutters on each side; and we may often find the road furrowed by little miniature rivers, that carve out for themselves tiny valleys as they run off into the gutter, bringing with them much débris in the form of mud and sand. Sometimes a stream encounters in its course a layer of rock that is harder than the rock underlying it. In this case the softer rock is worn away faster, and the hard layer forms a kind of ridge at a higher level; the result is a waterfall. Waterfalls are frequently found in mountain streams. In this case, it is easy to trace the ridge of harder rock running unbroken across the path of the stream, showing clearly that it has not been rent in any way. First it showed merely as a kind of step, but gradually the force of the falling water told with greater effect on the softer rock below, wearing it away more rapidly than that above, and so the depth of the waterfall went on increasing year by year; and at the same time the hard layer was slowly worn away until the stream sawed its way through.