THE HARDENING OF ROCKS 



of the same texture and material. Now this change or 

 metamorphism does not appear to be the same as that 

 produced by the intrusion of hot eruptive rocks. Let 

 us take a simple instance. We have seen that limestone 

 is changed by heat into marble. Sometimes its fossils are 

 preserved ; sometimes they completely disappear. Some- 

 times it is threaded by veins of harder and more crystal- 

 line rocks. But in the case of the white marble of 

 Carrara, which was once a bed of coral, the change seems 

 to have taken place less violently, less suddenly, more 

 gradually. The change was due, therefore, not to violent 

 heat suddenly applied, but to the penetrating action of 

 water, probably aided by sustained heat, and certainly 

 aided by pressure. When a rock is subjected to suffi- 

 cient pressure its very structure will alter ; its original 

 constituents may be torn out of it, pressed out of it, 

 filtered out of it, and afterwards rearranged. 



Once more let us call attention to the astounding 

 effects which great pressures can have. If pressure 

 enough be applied iron can be made to flow like 

 treacle ; and the pressure of two or three miles of strata 

 is enough to crumple or shear or tear any rock however 

 hard. Now we have shown that in the earth's long 

 history some regions are always being denuded of mate- 

 rials in order that these materials may be laid down as 

 sediments elsewhere. These movements may be com- 

 pared to those of a pair of scales, in which we are 

 continually taking weight from the scale pan that is 

 weighted in order to put it into the scale that is empty. 

 The scale that is weighted is the land from which mate- 



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