408 Metamorphism. 



work that goes on very slowly and imperceptibly at first, is 

 more rapid and energetic in proportion as the depth is greater, 

 and the most impracticable and refractory rocks are those 

 which are more thoroughly influenced by the action of water in 

 the interior of the earth. 



That water is capable of reaching the separate minerals of 

 crystalline rocks, acting specially on some one of them, removing 

 one member of a group, and replacing minerals by others, with- 

 out in the smallest degree altering or affecting the external 

 form, is a fact of which there is absolute proof. That water 

 alone is capable of replacing the limestone of a shell by galena 

 and other sulphides of metals, and even by native metals, such 

 as silver or copper, is equally certain. That water alone has 

 dissolved and removed, in one case, silica ; in another, car- 

 bonate of lime ; in another, felspar, and probably almost every 

 familiar mineral, placing in the stead of these other minerals, 

 which it can also again remove, is one of those curious and 

 almost inexplicable facts that nothing but absolute and direct 

 proof could make credible ; but facts are stubborn things, and 

 must be accepted when they present themselves. 



When, then, in crevices of rocks, whether wide or narrow, 

 and whether filled completely or partially, we find groups of 

 strange minerals and metals, quartz, and other crystals, and 

 even granite and porphyry (mixed masses of crystals), we need 

 not at once assume that these have been once in a molten state ; 

 we should rather inquire how far they may have been separated 

 out from the mass, or introduced by currents of heated water 

 acting during long periods of time under certain chemical con- 

 ditions. Some of the appearances it is now certain may have 

 been produced by water ; most of them cannot be proved to 

 be brought about by fusion under any known contingencies. 



But if this is the case with veins, much more is it the case 

 with the masses of rocks loaded with organic remains still 

 retaining their intimate structure. In them there is often a 

 wide departure from the normal condition of soft mud, sand, 

 pebbles, and animal or vegetable remains ; and yet, though so 

 great a change has supervened, there has certainly been no 

 exposure to heat, for none of the results of heat are in any 

 way traceable. Where, as in the case of leaves and wood, heat 

 would convert ihe whole mass into an ash, the result is the 

 production of a combination of carbon, hydrogen, oxygen, and 

 other gases, capable of serving still as an admirable fuel. It 

 is not necessary to point out what the result would be if heat 

 had been applied to such a combination ; but where the case is 

 less manifest, it is often not less certain, and thus abundant 

 proof exists of the possibility of enormous and complete change 

 Avithout any exposure to burning heat. In all cases, however, 



