37, there is a narrow marginal platform, or submerged terrace, in place of the land area which has been worn away at or just below the level of the sea. Long successions of rains working in the same way will give the island steeper slopes, a smaller area, and a wider marginal terrace. Successive stages are shown by the lines _bb_ and _cc_, Fig. 37. [Illustration: +Fig.+ 37.—Diagram to illustrate the effect of rain erosion on an island where there is no deposition or wave erosion about its borders. The uppermost curve represents the original surface, while _aa_, _bb_, and _cc_ represent successive surfaces developed by sheet erosion, on the supposition that no material is deposited along the shores.] If rain falls on such an island until it completes the work which it is possible for running water to do, the island will be reduced essentially to the level of the sea, and in its place there will be a plain, the area of which will be equal to that of the original island. Its central point will be at the level of the sea, and its borders a trifling distance below it (Fig. 38). The island is gone, and in its place there is a plain as low as running water can wear it. Other agencies might come in to defeat the result just outlined, but if the island did not rise or sink after its formation, rain falling upon it would, _under the conditions specified_, finally bring about the result which has been sketched. The plain (Fig. 38) which succeeds the island is a _base-level of erosion_, though this term is also used in other ways. Under these conditions the slope of the land would remain convex at all stages, but the convex erosion profile of the land would meet a nearly straight line just below sea-level. The relative lengths of these two elements of the profile, the curve above and the straight line below, vary as erosion progresses, the convex portion becoming shorter and the other longer, The two parts of the profile taken together are concave upward at the lower end all the time, and for a greater distance from its lower end in all the advanced stages of erosion (Fig. 37). [Illustration: +Fig.+ 38.—Diagram to illustrate the final effect of rain erosion under the conditions specified in the text. The diagram expresses the final result of the processes suggested by Fig. 37.] In the destruction of the land under these conditions _neither valleys nor hills would be developed_, nor would the topography of the land be fashioned to correspond with the surfaces with which we are familiar. It is to be distinctly borne in mind that the foregoing is a hypothetical case; it is not probable that such an island ever existed, or ever will; but that does not diminish the value of the illustration, since the principles involved are operating on every land mass, though in less simple relations.