A part of the water which falls as rain, and a part of that which results from the melting of snow and ice, sinks into the soil and into the rock below, becoming _ground water_. It is this ground water which especially justifies the name _hydrosphere_, often applied to the waters of the earth, for it literally forms a spherical layer in the outer portion of the solid part of the earth. During the stay of the water beneath the surface it effects changes in the rocks through which it passes, dissolving mineral matter here and depositing it there, substituting one substance for another in this place, and effecting new chemical combinations in that. Slow as these processes are, they have worked wondrous changes in the course of the earth’s history. When the waters are gathered together in ponds, lakes, and oceans, they are still active, and the results of their activity are seen along the shores, where winds and waves produce their chiefest effects. Even the ocean currents, far from land, and the processes of the deep sea, are not without their effect on the course of geological history. The work of the _surface waters_, _ground_ (_underground_) _waters_, _standing waters_, and _ice_ will be considered in order. RAIN AND RIVER EROSION. Rain and river erosion began when the first rains fell on land surfaces. Neither the location nor the nature of the first land surface is known. There is little reason to believe that the ocean was ever universal, but there is reason to believe that most land areas have at some time or other been covered by the sea. The prevalent conception that land areas which were once submerged came into existence by being elevated above sea-level, should be supplemented by the alternative conception that submerged areas may have become land by the depression of the ocean basins, thus drawing off the water from the areas where it was shallow. Thus in Fig. 36 the sinking of the sea-bottom from _a_ to _b_ would lower the surface of the water from _cc′_, to _dd′_, and draw off the water from the surfaces _cd_ and _c′d′_. [Illustration: +Fig.+ 36.—Diagram to illustrate the origin of lands by the lowering of the sea-level due to depression of the sea bottom. If the bottom is depressed from _a_ to _b_ the surface will be drawn down from _cc′_ to _dd′_, and the surfaces _cd_ and _c′d′_ will become land.]