M r . E. W. Hilgard, Professor of Geology in the University of Michigan, in a paper “On some points in Mallet's Theory of Vulcanicity” (Amer. Journ. Sci. vol. vii. June, 1874), has remarked that that author's experiments (“On the Nature and Origin of Volcanic Heat and Energy,” Phil. Trans. 1873) “fail to carry conviction as to the efficacy of this particular modus operandi in reducing large masses of solid rock to fusion—unless essentially supplemented by friction , and the heat produced within more or less comminuted, detrital, or igneo-plastic masses by violent pressure and deformation.” The author's crushing-experiments had not for their object to prove how high a temperature could be by this process and its consequences attained in nature; and he has in his paper above referred to entered into no details on this point of the subject; to have done so would have too largely extended an already long paper. Moreover he considered that every physicist interested in the subject would follow out for himself the conditions and consequences of the work of crushing in our globe's crust, and discern that there was no physical impossibility in a temperature of rock-fusion resulting therefrom. The object of the author's experiments was to fix what was the annual minimum amount of heat available for vulcanicity in our entire globe upon the mechanism which he has assigned. For this purpose the crushing of cubes of rock in air was alone available: his experiments prove what total minimum amount of heat must be so