Page:The National geographic magazine, volume 1.djvu/45

 been in operation throughout geologic time, though they perhaps represent a diminishing series; they have added materially to the superficial crust of the earth; and it is fair to suppose that they have modified the geoid not only by additions to the surface but by corresponding displacements in their vicinity. The category may be tentatively (but rather improperly) called vulcanism, and the antagonistic classes of movements constituting it are extravasation and its antithesis. The vibratory movements of seismism probably result from both deformation and vulcanism under certain conditions.

The second subordinate category of processes is closely linked with all of the others. It comprises the various chemic and chemico-mechanical alterations in constitution and structure of the materials of the earth's crust. The processes have affected the rocks ever since the solidification of the planet, though probably in a progressively diminishing degree; and they have materially (but indirectly rather than directly) modified the internal constitution and external configuration of the earth. The processes may be collectively called alteration; and the antagonistic classes into which the category is divisible are lithifaction and decomposition in their various phases, or rock-formation and rock-destruction.

The third subordinate category of processes, viz: glaciation, is related to the second principal category; but since (1) it is probable if not actually demonstrable that under certain circumstances glacial grinding tends to accentuate preëxisting irregu- larities of surface, and since (2) it is well known that glacial deposition sometimes gives great irregularity of surface, it is evident that glaciation is not a simple process of gradation, but must be clearly distinguished therefrom. A considerable portion of the earth's surface has been modified by glaciation during later geologic times. The general process comprises glacial construction and glacial destruction.

There is a fourth subordinate category of processes, which is also allied to gradation, viz: wind-action, which may be made to include the action of waves and wind-born currents; but since the winds scoop out basins and heap up dunes, while the waves excavate submerged purgatories and build bars, it is evident that this category, too, must be set apart. The processes are only locally important as modifiers of the land surface of the globe. They comprise constructive action and destructive action.