An Unrecognised Agent in the Deformation of Rocks. 389 



known, but at 1,000° C. or 1,832° F. Dr. Callendar- found that 

 rods of pure silica no longer returned to their original dimensions, 

 showing that a partial change of state, or an incipient viscosity, had 

 been produced at that temperature. Dolerite and quartz are probably 

 the two extremes in the nature of rocks common to the solid crust of 

 the earth, the varieties more basic than dolerite being of extreme 

 rarity. It is evident, therefore, that by heat alone all rocks would be 

 rendered plastic between 1,400° F. and 1,800° F., and this heat would 

 be obtained between 8 and 45 miles down from the earth's surface. 



The whole question of heat, in reference to the plasticity of rocks 

 is an immense subject by itself, and is one which I do not intend to 

 go into here. It has also been very ably treated by a number of 

 investigators such as Mellard Eead, ! Fisher,! van Hise,§ Barus,|| 

 and Doelter.^i I only in passing wish to point out that the internal 

 heat of the earth is a very real agent, which must be taken into 

 account in dealing with the deformation of rocks. We have seen 

 that the dead weight of the superincumbent material is of itself able 

 to crush rocks, but in estimating the depths at which this takes 

 place in Nature we find that the results obtained by crushing rocks 

 at the surface must be very much lessened owing to the incipient 

 plasticity induced by this internal heat. For the same reason I 

 must leave out all reference to the potential and actual heat produced 

 by the intense pressure, which led to Mallet's theory of the origin of 

 volcanic energy. ^'-^^ 



The next element to be taken into account in the deformation of 

 rocks is the action of water. Sir Humphrey Davy! f was the first 

 to propose the theory that water played a large part in the economy 

 of the earth's interior, and derived the internal heat from the 

 oxidation of metallic substances by means of water percolating down 

 to depths where these exist in a metallic state. The difficulty to be 

 overcome in this theory is to explain how the water gets down 

 to such great depths against pressure ; we see, for instance, that 

 if there is any chance of a crack going down through the earth's 

 crust, the intense pressure is sufficient to force up through it a mass 

 of molten rock with explosive violence. We have also seen that 



* See Shenstone, Nature, vol. Ixiv., 1901, p. 65. 

 t "Origin of Mountain Kanges," 1880. 

 + " Physics oi the Earth's Crust," 1889. 



§ " Metamorphism and Rock-Howage," Bull. Geol. Soc. of America, 1898. 

 il Bull. Geol. Survey, U.S.A., No. 103, 1893. 



^1 "Neues Jahrbuch," 1901, ii., pp. 141-157; Tschermak's " Mitth.," xx., 1901, 

 pp. 210-232, and pp. 307-330. 



** Phil. Trans., Roy. Soc, vol. clxiii., 1873, pp. 147-227. 

 ft See Humboldt's " Cosmos," Bohn's Edition, p. 234. 



