=By the growth of gullies.=—Had the slopes of the hypothetical island not been absolutely uniform the processes of erosion would have been different. Let the departure from uniformity be supposed to consist of a single slight meridional depression near the base of the island (Fig. 43). As the rain falls it will no longer run off equally in all directions. A greater volume will flow through the depression than over other parts of the surface having the same altitude, and the greater volume of water along this line will give greater velocity, greater velocity will occasion greater erosion, and greater erosion will deepen the depression. The immediate result is a _gully_ or _wash_ (Fig. 44). So soon as the gully is started it tends still further to concentrate drainage in itself, and is thereby enlarged. The water which enters it from the sides widens it; that which enters at its head lengthens it by causing its upper end to recede; and all which flows through it, so long as its bottom is above base-level, deepens it. The enlarged gully will gather more water to itself, and, as before, increased volume means increased velocity, and increased velocity increased erosion. As the gully grows, therefore, its increased size becomes the occasion of still further enlargement. [Illustration: +Fig 43.+—Diagram showing a slight meridional depression in the surface of an otherwise even-sloped island.] Continued growth transforms the gully into a ravine, though between a gully and a ravine there is no distinct line of demarkation. But growth does not stop with ravine-hood. Water from every shower gathers in the ravine, and, flowing through it, increases its length, width, and depth, until it reaches such proportions that the term _ravine_ is laid aside, as childhood names are, and the depression becomes a valley. [Illustration: +Fig.+ 44.—Diagram illustrating the development of a gully, starting from the condition shown in Fig. 43.]