The rain falling on a land area disappears in various ways; part of it evaporates, part of it sinks, and part of it runs off over the surface. If the island be composed of fine and unconsolidated materials, such as clay, the water which runs off over the surface will carry sediment down to the sea. If the island be composed of solid rock instead, exposure to the air will cause it to decay, and the products of decay, such as sand and mud, will suffer a like fate. For the sake of a clear understanding of the processes involved, two cases may be postulated; one in which the waters of the sea remove the sediment washed down from the hypothetical island as fast as it reaches the shore, and one in which they allow it to accumulate without let or hindrance. In both cases the wear of the waves will be neglected. 1. In the first case the water flowing off over the surface (the _run-off_) will descend equally in all directions. It will constitute a continuous sheet of surface-water, and both its volume and its velocity will be the same at all points equally distant from the summit. Erosion accomplished by sheets of running water, as distinct from streams, is _sheet_ (or _sheet-flood_) _erosion_.[26] Since the material of the surface is homogeneous, the wear effected by the water will be equal at all points where its velocity and volume are equal. For obvious reasons the depth of the run-off will increase from summit to base. The gradient (slope) also increases in the same direction, and the increase of volume and of gradient conspire to augment the velocity of the water, and therefore of the wear effected by it. If the thin sheet of water starting from the top of the island with relatively low velocity be able to wash off even a little fine material from the surface, the thicker sheet farther down the slope, moving with greater velocity, will be able to carry away more of the same sort of material, and the increase will be progressive from summit to base. It follows, therefore, that the surface will be worn equally at points equally distant from the summit, but unequally at points unequally distant from it. The first shower which falls on the island may be conceived to wash off from its surface a very thin sheet of material, but a sheet which increases in thickness from top to bottom. The run-off will not be stopped immediately on reaching the sea, but will displace the sea-water to some slight depth, and wear the surface some trivial distance below the normal level of the sea. The result of successive showers working in the same way through a long period of time will be to diminish the area of the island and to steepen its slopes. The results of a considerable period of erosion under these conditions are shown diagrammatically in Fig. 37, which illustrates both the diminution in area which the island has suffered, and the increase in the angle of its slopes. Immediately about it, at the stage represented by _aa_, Fig.