Metamorphism. 409 



there is change, and generally it is of a very marked kind, 

 between the original and present condition of rocks. This 

 change is metamorphism. 



Let us consider the principal varieties of rocks, one after 

 another, with reference to this point. Sand and pebbles, with 

 occasional thin beds of marl, are the rocks least altered, and 

 often these are hardly changed from their original state. They 

 are neither more compact nor of different texture. Sometimes, 

 however, they are converted into stone, the sand particles being 

 cemented by carbonate of lime, silica, or oxide of iron, all 

 clearly and unmistakably brought in and deposited by water. 

 Sometimes, parts are unchanged and other parts converted 

 into crystalline quartz rock, with no mark of heat. Some- 

 times, on the marly films are crystals of marl, occupying places 

 once occupied by crystals of salt. The marl has been carried 

 in by water to replace the salt crystals dissolved by water. 

 Scarcely any organic remains are preserved in such material 

 now, and there is not much indication of life in the older ex- 

 ample, but footprints of animals, worm casts, etc., prove that 

 life was present. Crevices in the sand rock are filled up with 

 crystalline quartz, certainly of water origin. 



Limestones are more varied and richer. Deposited as mud, 

 or coral, or shell sand, the particles are cemented by carbonate 

 of lime. Sometimes the whole mass is compact and crystalline, 

 sometimes parts are crystalline, sometimes the whole is semi- 

 crystalline. The shells are changed ; they preserve their form ; 

 they sometimes retain parts of their original structure, and 

 sometimes they are full of calc spar ; or the shells are gone, 

 and their place is occupied by casts, also of carbonate of lime, 

 or perhaps of iron oxide. The whole mass is uniformly tinted 

 with iron oxide, or the iron is collected into belts, and these 

 are often rich enough to be exceedingly valuable as iron ore. 

 The rich ores recently discovered and now chiefly used in the 

 north of England are of this kind. Any foreign substances 

 once mixed up with the limestone mud are now collected and 

 separated in veins or crevices. Large cavities and empty 

 spaces are common in the mass, and these are filled partly with 

 broken fragments of the same limestone, partly with valuable 

 ores of lead and zinc. The whole series of operations, thus 

 changing the limestone mud into important limestone rocks, 

 with mineral veins, is the work of currents of water. Water 

 has compacted the loose particles; water has opened large 

 cavities ; water has filled them with quartz and other crystals ; 

 and water has carried in the metals. A little heat would have 

 changed the whole mass, and caused fresh combinations. This 

 at least is the case as far as we can judge. It is the case 

 according to experiment, and no one, without conflicting 



