518 B. MALLET ON HIS THEORY OP VOLCANIC ENEEGY. 



equally diffused throughout the mass crushed, hut must be localized, 

 and that the circumstances of this localization must result in pro- 

 ducing a local temperature far greater than that due to crushing. 



Lastly he has shown that, after the highest temperatures have 

 been thus reached, a still further and great exaltation of temperature 

 must arise from detrusive friction and the movements of forcible 

 deformation of the already crushed and heated material. 



It appears to the writer, therefore, that he has sufficiently shown 

 that there is no physical difficulty in the conception involved in his 

 original memoir (Phil. Trans. 1873), but not there enlarged upon in 

 detail, that the temperatures consequent upon crushing the materials 

 of our earth's crust are sufficient locally to bring these into fusion. 



Discussion.' 



Prof. Duncan remarked that this reply to Mr. Fisher's paper, 

 which is not yet in print, was one which required careful thought 

 and consideration. He thought that Mr. Mallet had not considered 

 sufficiently the effects of tangential thrust. The curving of strata 

 takes place along great planes, producing main synclinal curves; but 

 there was another series of actions giving rise to thrusts over smaller 

 areas. In any case the action of the thrust would be slow, and 

 thus it can furnish no parallel to experiments by crushing rocks, in 

 which the effect was rapidly produced. He further pointed out that 

 volcanoes do not follow mountain-chains, and that the crust of the 

 earth has no doubt become more rigid than formerly, and that there- 

 fore tangential thrusts would now be less effective. Yolcanic cones 

 are found in older rocks than the Eocene. 



Prof. Ramsay said that he agreed with Prof. Duncan in general, 

 but thought that the meaning of quick and slow was difficult to de- 

 fine in connection with great thicknesses of strata. If the pressure 

 was sufficient to maintain motion, this might be quick in one sense 

 and slow in another. Alterations may be gradually going on over 

 great areas, by which means metamorphism may be evidenced in 

 different degrees in different parts. 



The Peesident remarked that, assuming gravity to be free to act, 

 the pressure at a depth of 20 miles would be about 120,000 lb. to 

 the square inch, and that but little was known as to the behaviour 

 of granite under such a pressure. 



