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Page:Popular Science Monthly Volume 34.djvu/528

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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 immediate 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 spots on the crust of the globe. Instead of preserving the horizontal position which they assumed when they were deposited, these beds have often been thrown up, folded, and contorted in various ways; and the resultant dislocations are observable 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 considerably 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 resistant. We have then reason to believe that in nature, when rocks more coherent and less plastic than ordinary clay have been submitted 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, contrary 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 minerals produced, of the hydrated silicates, like chlorite, for example, no less than the uniformity of their disposition in vast masses,