426 



Prof. J. Prestwich. 



[June 18, 



the force required to crush portions of various rocks of given size, 

 and to determine the quantity of heat evolved by the process. For 

 this purpose, as I have before mentioned,* the work done was measured 

 by the proportion of water at 32° F. that could be converted into steam 

 of one atmosphere (or at 212° F.) by the estimated heat evolved by 

 the crushing of 1 cubic foot of each class of rock. 



The crushing weight in the case of the specimens of the Sedimentary 

 Strata was found to vary in round numbers from about 2 to 18 tons 

 per square inch, while for the Crystalline and Igneous rocks it reached 

 to over 30 tons (each class of rock showing considerable variations) ; 

 and the temperature resulting from crushing 1 cubic foot of rock 

 varied from 8° F. in Caen Oolite to 217° F. in Guernsey Granite. 

 In conclusion Mallet estimated that the heat of liquefaction of 

 1 cubic mile of ice at 32° melted is equivalent to the crushing work 

 of 1*277 cubic mile of mean rock when transformed into heat, or that 

 1 cubic mile of crushed mean rock would fuse 0*108 cubic mile of 

 volcanic material. 



With all the harder rocks the heat produced in the metal surround- 

 ings by the complete crushing was easily perceptible by the hand, and 

 was so great with some of the granites and porphyries as to necessi- 

 tate a delay for the apparatus to cool. Both Mr. Mallet and Professor 

 Rankine were of opinion that in the crushing of a rigid material such 

 as rock, almost the entire mechanical work (with the exception of a 

 small residue of external work) reappears as heat. It was further 

 shown that, even in the most rigid bodies, crushing begins by com- 

 pression and yielding, and that at this stage heat begins to be evolved. 



Mr. Mallet, applying these results to the deformation caused in 

 the earth's crust by the contraction of the cooling nucleus, observes 

 that the compression will be greatest along lines of fault and moun- 

 tain-upheavals, so that the chief amount of the work of compression 

 will be transferred to those lines of fracture or weakness ; and the in- 

 crease of temperature produced by the greater part of the internal 

 work will cause the parts of the crust about those lines to become 

 much hotter than the intervening parts, where the crust is undis- 

 turbed. 



Consequently the work thus developed being transformed into heat, 

 that heat will be greatest along those lines or planes at places where 

 the movement and pressure, together constituting the work, is 

 greatest ; whence Mallet concluded that along or about such axial 

 lines of concentrated compressive and crushing work, the temperature 

 may locally rise to a red heat, or even to that of fusing the rocky 

 materials crushed and the pressing-together- w T alls themselves adja- 

 cent to them. This was, in his opinion, the real nature and origin of 

 the volcanic heat as now produced on the globe. 



* " Proc. Roy. Soc." for May, 1885. 



