Under great pressure, many substances which otherwise appear to be solid, exhibit the characteristics of plastic bodies. Among the substances exhibiting this property, ice is perhaps best known. Brittle and resistant as it seems, it may yet be molded into almost any desirable form if subjected to sufficient pressure, steadily applied through long intervals of time. The changes of form thus produced in ice are brought about without visible fracture. Concerning the exact nature of the movement, physicists are not agreed; but the result appears to be essentially such as would be brought about if the ice were capable of flowing, with extreme slowness, under great pressure continuously applied. In the assumed ice-field, there are the conditions for great pressure and for its continuous application. If the ice be capable of moving as a plastic body, the weight of the ice would induce gradual movement outward from the center of the field, so that the area surrounding the region where the snow accumulated would gradually be encroached upon by the spreading of the ice. Observation shows that this is what takes place in every snow-field of sufficient depth. Motion thus brought about is glacier motion, and ice thus moving is glacier ice. Once in motion, two factors would determine the limit to which the ice would extend itself: (1) the rate at which it advances; and (2) the rate at which the advancing edge is wasted. The rate of advance would depend upon several conditions, one of which, in all cases, would be the pressure of the ice which started and which perpetuates the motion. If the pressure be increased the ice will advance more rapidly, and if it advance more rapidly, it will advance farther before it is melted. Other things remaining constant, therefore, increase of pressure will cause the ice-sheet to extend itself farther from the center of motion. Increase of snowfall will increase the pressure of the snow and ice field by increasing its mass. If, therefore, the precipitation over a given snow-field be increased for a period of years, the ice-sheet's marginal motion will be accelerated, and its area enlarged. A decrease of precipitation, taken in connection with unchanged wastage would decrease the pressure of the ice and retard its movement. If, while the rate of advance diminished, the rate of wastage remained constant, the edge of the ice would recede, and the snow and ice field be contracted.