1885.] 



Regional Metamorphism, 



427 



Although the hypothesis fails for various reasons* in its application 

 to vulcanicity, for, as a matter of observation, the great lines of dis- 

 turbances and compression of the Alps, Pyrenees, and other mountain- 

 chains are free from either active or extinct volcanoes; there is 

 nevertheless reason to believe that this source of heat may have been 

 adequate to produce great molecular changes in the rocks along the 

 lines of disturbance and upheaval. It is precisely along such lines 

 that not only are the older rocks metamorphosed, but rocks of creta- 

 ceous and tertiary age — which usually have not been affected by 

 normal metamorphism — coming, in these mountain-chains, under the 

 influence of the disturbing forces, have undergone a change analogous 

 to that produced by normal metamorphism. 



[Unfortunately no other experiments than those of Mr. Mallet, to 

 determine the heat developed by the compression of .rocks, have been 

 made ; and valuable as his experiments are, his full conclusions cannot 

 be accepted, because in nature the complete crushing upon which his 

 calculations are based does not obtain, nor can the heat be localised 

 in the way assumed. 



On the other hand, his experiments do not take into separate con- 

 sideration friction and deformation, the influence of which in raising 

 the temperature during earth-movements must be very considerable. 

 No special experiments have, in fact, been made on the work of these 

 forces on rocks, but it has been proved experimentally with metals. 

 Iron can be raised by those means to a low red heat, and it has been 

 estimated that with lead the rise of temperature under deformation is 

 equivalent to 700° F. 



Further, Mallet's estimates were based on an initial temperature of 

 32°, whereas the initial temperature in the underground rocks affected 

 by the earth-movements would necessarily be high. Consequently, 

 although we cannot accept the extreme estimates of Mallet — nor can 

 we hope to ascertain with the imperfect data at present in our posses- 

 sion the exact heat developed by rock movements — still it is certain 

 that his experiments, combined with what is known of the heating 

 effects of friction and deformation, indicate that a large amount of 

 heat must be developed in the underground rocks by these causes. — 

 July 6, 1885J 



If the disturbances had taken place at once, or suddenly, and the 

 rocks had been wholly crushed, the results calculated on by Mr. Mallet 

 were more likely to have been attained. But the movements were in 

 all probability of extreme slowness for very long periods — and this 

 might be an argument that they were so — and it was only when the 

 tension had reached a certain point that disruption took place, and 

 the movement of the parts became more rapid, pending the restora- 



* For some other of the physical objections, see the Rev. O. Fisher's " Physics of 

 the Earth's Crust," Chapter XVIII. 



