Many authorities have demonstrated that the hardness of ice varies inversely as the temperature, which is, of course, to be expected by analogy with metals. Recently Dr. E. Blackwelder1 caused some experiments to he made at a temperature of −78.5° C., the temperature of solid carbon dioxide. At this temperature the hardness of ice was found to be approximately 6, or that of orthoclase felspar. Hitherto it had been assumed that aerial corrosion of rocks close to glaciers and snow-fields was in the main due to rock dust. It now seems likely that when air temperatures fall very low, as for instance during blizzards, ice fragments or snow could abrade certain limestones and shales and even some igneous rocks. Ice and snow may therefore play a more important part in aerial erosion than had been suspected.