Metamorphism

Metamorphism is a term employed in geology for the various changes in texture which certain rocks undergo subsequent to consolidation. These rocks, which are termed metamorphie, may be originally either aqueous or igneous. The alteration they have undergone may consist merely in a rearrangement of their particles, or may involve partial or complete crystallisation or even the introduction of new substances into the rock. The production of cleavage (q.v.), by which clay is converted into clay-slate; and of foliation, by which granite may be changed into gneiss (q.v.), or diorite into hornblende-schist, are examples of the first of these modes of change. Marmarosis, i.e. the conversion of chalk into marble (q.v.); and the formation of spotted slates by the development of crystals of andalusite, chiastolite, etc., are examples of the second mode; and dolomitisation, i.e. the conversion of ordinary limestone into magnesian limestone, exemplifies the third mode of change. Such metamorphism as the first or second modes may be the result of pressure and the heat that pressure generates. Shale (q.v.) seems to have originated in part from vertical pressure, and the direction of cleavage-planes points to cleavage as being generally the result of such tangential or horizontal thrust as would come in the general cooling of the earth's crust. Foliation and schistosity may be merely rearrangements of crystalline particles also due to such lateral pressure; but the development of scattered crystals in slate, and the formation of some marble from chalk, is often obviously the result, as at Rathlin Island, of the thrusting of molten igneous rock through rock not previously crystalline. Heat, however produced, seems sometimes to have reheated igneous rocks to such a point as to permit a rearrangement of crystals without actual fusion. The introduction of new substances into a rock cannot be explained by mere pressure or heat, but must be due to percolating water, mineral solutions or vapours, which sometimes accompany the thrusting in of molten igneous rock. Many metamorphic rocks, indeed, show signs of the combined action of all the various agents of metamorphism.