By the term metamorphism is meant the profound transformation of a rock from its original condition by means other than those of disintegration. The incipient changes of the latter class may very greatly modify a rock and its constituent minerals, but such changes are distinguished from metamorphism under the term alteration. Metamorphism usually implies an increase in hardness and in the degree of crystallization, and very frequently also the generation of an entirely new set of minerals, which take on a characteristic arrangement. The degree of metamorphism varies according to circumstances, and from the mere consolidation of loose sediments to the most radical reconstruction of the rock there is every possible transition. Fossils may be found in those metamorphic rocks of sedimentary origin which have not been completely changed. The more thorough the reconstruction of the rock, the more obscure do the fossils become, and in advanced stages nearly or quite all trace of them is obliterated.

It was long supposed that the metamorphic rocks were one and all transformed sediments, but later investigations have shown that many of them were originally igneous. Indeed, it is often quite impossible to decide whether a given metamorphic rock has been derived from a sedimentary or an igneous original. This is not surprising, for the ultimate chemical (not the mineralogical) composition of a basalt, a volcanic tuff, or a clay shale, may be the same, and the metamorphic processes may produce an identical rock from any one of these three as a starting-point. Much yet remains to be learned regarding the modes, causes, and results of metamorphism, and some of the most far-reaching problems of geology are bound up with these questions.

Metamorphism is of two quite distinct kinds: (1) contact or local, and (2) regional metamorphism.