_Corrasion_ is accomplished chiefly by the mechanical wear of streams, aided by the hard fragments such as sand, gravel and bowlders, which they carry. The solution effected by the waters of a stream may also be regarded as a part of corrasion. Under ordinary circumstances solution by streams is relatively unimportant, but where the rock is relatively soluble, and where conditions are not favorable for abrasion, solution may be more important than mechanical wear. So soon as sea bottom is raised to the estate of land, it is attacked by the several processes of degradation. The processes of weathering at once begin to loosen the material of the surface if it be solid; winds shift the finer particles about, and with the first shower transportation by running water begins. Weathering prepares the material for transportation and transportation leads to corrasion. Since the goal of all material transported by running water is the sea, subaerial erosion means degradation of the surface. _Erosion without valleys._--In the work of degradation the valley becomes the site of greatest activity, and in the following pages especial attention is given to the development of valleys and to the phases of topography to which their development leads. If a new land surface were to come into existence, composed of materials which were perfectly homogeneous, with slopes of absolute uniformity in all directions, and if the rain, the winds and all other surface agencies acted uniformly over the entire area, valleys would not be developed. That portion of the rainfall which was not evaporated and did not sink beneath the surface, would flow off the land in a sheet. The wear which it would effect would be equal in all directions from the center. If the angle of the slope were constant from center to shore, or if it increased shoreward, the wear effected by this sheet of water would be greatest at the shore, because here the sheet of flowing water would be deepest and swiftest, and therefore most effective in corrasion. _The beginning of a valley._--But land masses as we know them do not have equal and uniform slopes to the sea in all directions, nor is the material over any considerable area perfectly homogeneous. Departure from these conditions, even in the smallest degree, would lead to very different results.