512 THE POPULAR SCIENCE MONTHLY. 



the increase of heat downward may have been at a more rapid 

 rate than now. Thus the regular propagation of the heat of the 

 globe has been competent to act upon entire formations. 



There is, however, another source of heat, at once more imme- 

 diate and more energetic, for the transformations with which we 

 are occupied, although it has been long misunderstood. Heat is 

 engendered by the mechanical actions that have left their marks 

 at numerous sjDots on the crust of the globe. Instead of preserv- 

 ing the horizontal position which they assumed when they were 

 deposited, these beds have often been thrown up, folded, and con- 

 torted in various ways ; and the resultant dislocations are observ- 

 able through several thousand metres of thickness. At every 

 step, in the Alps, for example, in the face of escarpments where 

 the rock shows itself to the quick, the least observing eye is 

 attracted by the boldness of the inflections, and the mind pauses 

 stupefied before the grandeur of the. forces that have produced 

 such effects. Not all the labor put in play in these colossal 

 upthrows has been employed in actions purely mechanical. A 

 part of it has been transformed into heat, and it is the effects of 

 this heat that we have been studying. 



Experience has come to confirm the last induction also. Clay 

 has been forced to flow either between cylinders like those of 

 iron-rolling mills, or under trituration in malaxating tubs, such 

 as are used in some brick-yards. In either case the rock is con- 

 siderably heated up after a very short time, without subjecting 

 it to any material pressure. In these operations the heating is 

 greater in proportion as the clayey part is harder and more resist- 

 ant. We have then reason to believe that in nature, when rocks 

 more coherent and less plastic than ordinary clay have been sub- 

 mitted to mechanical actions powerful enough to determine an 

 interior movement, even if it be of little amplitude, they will be 

 found in conditions still more favorable to their being heated. It 

 has, therefore, been enough for argillaceous masses to undergo a 

 lamination under the effect of dislocations in the crust of the 

 earth for their temperature to be notably raised. 



But heat alone, however intense it may be, can not explain the 

 most characteristic effects of metamorphism, nor the uniformity 

 with which they have been produced over considerable spaces ; 

 for the conductivity of the rocks is extremely weak. Then, con- 

 trary to what would be the case were the action simply a calorific 

 one, the effects have not always been most energetic in the parts 

 in contact with the eruptive rocks. The water included in all the 

 rocks, whether in their pores or in combination, has of necessity 

 intervened as an auxiliary to the heat. The nature of the min- 

 erals produced, of the hydrated silicates, like chlorite, for exam- 

 ple, no less than the uniformity of their disposition in vast masses. 



