46 



Prof. J. Prestwich. 



long periods of time, and it was only when the tension had reached a 

 certain point that fracture and disruption, accompanied by a more 

 rapid motion of the parts, took place. 



What the force of the pressure may have been in these cases is 

 shown by the compression of the strata in the Alps, by the extra- 

 ordinary folds and inversions of the rocks, and by the vertical 

 cleavage (a resultant of pressure) which the whole mass of rocks has 

 undergone. We may illustrate this point by the following generalised 

 section across the central axis of the Alps along the line of tunnel. 



FlG-. S. — Section across St. H-otTiarrl (rerhic^rl from the large section of "Dr. Sta-pff). 



gr. Granite, gn. Grneiss. m. Metamorphic schists. t. Temperature curves. 



But although the compression may have been excessive, and the 

 actual mechanical displacement great, the crushing was not so complete 

 nor so sudden as to produce the extreme effects indicated by the expe- 

 riments. Complete crushing is not, however, necessary for our object, 

 since the experiments show that on the first yielding of the rocks, 

 which takes place when the weight is rather more than one- third of 

 the crushing weight, a large portion of heat is given off. Consequently 

 not only would the heat be developed very gradually (and much of it 

 might be dissipated daring the long periods that the disturbances 

 lasted), but also the major effects obtained artificially would never be 

 realised in nature. Nevertheless, although fusion may not have taken 

 place, there were molecular and chemical changes produced in the 

 rocks which indicate the existence of very considerable heat ; and there 

 is reason to believe that this heat was often due to mechanical causes, 

 and not to the protrusion of the molten granitic centres. In fact 

 M. A. Favre and other Swiss geologists now consider the granite in 

 those ranges to have been in existence in its present relative place 

 when the elevation and crushing occurred. 



Mallet further showed that the quantity of heat developed varied 

 greatly in different rocks, and that, although compressed by the same 

 force before their elastic limits were passed, yet, when released, it 

 would render a quartz rock nearly three times as hot as a slate rock. 



