Though varying very much according to circumstances, the underground supply is perhaps very much greater than is generally supposed. In permeable soil, especially if the watershed be fairly level and the surface protected by bush, there is a constant percolation into the ground, and, except in the case of a very heavy rainfall, by far the greater portion of the water goes through the earth before it finds its way to the river-bed. Even in hard rocks, underground streams, starting originally in some fault or fissure, wear for themselves well-defined channels, when, after running sometimes quite considerable distances, they emerge in the form of springs about the head-waters and sides of the creeks. It is by this underground supply that the average volume of a river is maintained. But if the bush has been removed, and nothing but a hard, bare surface remains on the watershed, then the rain, as before mentioned, runs off at once; and, unless the ground be of a very porous nature, there is no water left to feed the underground supply, and the river is starved. Unfortunately, in many extensive forest-areas the land is of clay or “papa rock,” both of which are almost impervious to water. Little or no percolation can take place, and practically the whole of the rain runs off as soon as it falls on the ground. The consequence is that in wet weather we have a succession of floods, and in dry weather a dwindling streamlet, or even an empty watercourse. On a small scale this sequence of cause and effect may be seen in the dry creeks that bring such trouble to the grazier and the dairyfarmer; while on a larger scale it may be witnessed in some of the small river-ports, where for weeks—or it may be for months—there is not sufficient water to clear the channel on the bar.