MET'AMOR'PHISM. A term commonly used by geologists to indicate the profound changes which sonic rocks have undergone. The term is not applied to the changes induced by weathering and decomposition, but is appli cable only to the profound modifications that are usually accompanied by an increase in hard ness and degree of crystallization from the origi nal conditions. Metamorphism may lead also to the mineralogical reconstruction of rocks. In a way it may be contrasted with weathering, which tends to break down the surface strata and thus promotes their removal to the sea by rivers; metamorphism, on the other hand, con solidates the sediments when they are deeply buried. The phenomena of metamorphism are commonly met with in nature particularly in the regions underlain by ancient rocks, where the processes of upheaval and subsidence and vol yank forces have operated through long periods of time. The degree to which rocks have been changed varies from mere hardening in some eases to an extreme marked by complete :litera 1 tiara in structure and composition.
KiNns OF META7U ORM! I tir.‘1 . Strata in the vi cinity of igneous intrusions, as dykes and bosses , of granite, are more or less metamorphosed and show an increase in hardness and crystallization. In this kind of metamorphism, called contact metamorphism, the amount of change effected de pends upon the character of the invading rock and the rock that has been invaded, and also neon the proximity to the contact. Deep-seated igneous masses exert greater influence than sur face flows of lava, doubtless because of the greater beat and longer period of cooling: while those of acid composition in which there are large quantities of occluded gases and vapors are more effective than basic types. The character of the
strata invaded by the igneous rock largely de termines the extent to which new minerals are found; sandstone usually shows no change be yond a hardening or recrystallization of the quartz particles, but clay rocks, such as shales and slates, exhibit a complete rearrangement of the chemical constituents with the formation of new minerals.
A second form of metamorphism by which rocks have been influenced over wide areas de pends upon the energy developed by the great stresses and movements within the earth's crust this is called regional or dynamic metamorphism. The first stages of regional metamorphism are in dicated by hardening and the loss of volatile sub stances; but, as the strains of eompression and shearing increase, the rock assumes a schistose structure and its constituent minerals are more or less completely recrystallized. The change ef fected may amount to a thorough transformation, as from an unaltered sediment to a homogeneous crystalline mass resembling all igneous rock.
CAusEs of \lETA\IURpIt . Heat. pressure. and moisture are the most effective agencies in producing the changes known as metamorphism. The influence of heat is shown in igneous con tacts, but it is also an accessory in regional metamorphism, although pressure is here the dominant factor. Moisture which is present in all classes of rocks assists in decomposing min erals and in the formation of new compounds. The phenomena of metamorphism have been imi tated in an experimental way by subjecting speci mens of various rocks to the influence of heat and pressure. Consult Geikie, Test-Book of Geology (London, 1893). See GEOLOGY ; METAMORPHIC ROCKS.