METAMORPHISM. The phenomena of mountain building and igneous activity are favorable to that alteration of rocks which is included under the term metamorphism. Heat, heated water, and great pressure are effective in changing the char acter of rocks. This alteration may be local, through contact with intruded masses of igneous rock, when it is called contact metamorphism; or it may be widespread, through intense and ex tensive mountain-building, when it is known as regional metamorphism. In each case the result ing changes are similar, though the alteration is usually carried to a far greater degree in regional than in contact metamorphism. Metamorphism has also been subdivided, according to the agency which has predominated, into hydrometamor phism, thermometamorphism, and dynamometa morph ism.
All rocks in a region of metamorphism are in volved, and the resulting changes are independent of the origin of rock, being determined by the nature of the metamorphism and the composition of the rock subjected to the change. Sometimes the alteration is so complete that no trace is left to tell the original character of the rock, not even the general class to which it belonged; and there are some geologists who believe that in some cases metamorphism has been carried to the extreme of actual melting, or, at least, to the reduction of the rock to a plastic condition. On
the other extreme, some rocks are so slightly altered that their original condition is easily recognized; for example, pebbles of conglom erate, elongated and stretched out of shape, are sometimes found; bedding planes in some of the slate are still observable crossing the planes of cleavage; distorted fossils may he present; and beds of marble may be traced to their origin from limestone strata, or quartzite to a previous con dition of sandstone. The genesis of even the highly metamorphosed schists and gneisses may at times be traced by following along the beds to some less intensely metamorphosed section con taining fossils, or other indications of their origin. Thus it is known that some of the highly altered beds of metamorphic rocks in the Alps were deposited in the Tertiary sea and meta morphosed during the building of the Alps in late Tertiary time. See METAMORPHISM.