Such areas are of paramount importance in metamorphic stud ies, for the gradational changes in structure, texture and mineral composition which their rocks spatially display are the resultant of the imposition of continuously varying temperatures and pressures. Two such model metamorphic regions are the High lands of Scotland and the Caledonian mountain chain of southern Norway.
The detailed examination of such regions reveals a progression of metamorphism in which—selecting one particular sedimentary type—a series of zones can be mapped out according to the entry of new-formed metamorphic minerals. Successive zones are thus characterized by special index minerals. Study along these lines was first carried out by Barrow (1893) in the south-east highlands of Scotland. Argillaceous rocks, owing to their ubiquity, are the foremost types in which such zones can be recognized. In them, successive zones of increasing metamorphism are charac terized by the index minerals, chlorite, biotite, almandine, stauro lite, kyanite and sillimanite. The boundary surfaces of these zones are at once isothermals and isodynamics, and as referred to the grade of metamorphism imposed, their intersections with the earth's surface have been termed isograds. Other sediments show corresponding zones, limestones for example have as index, min erals, zoisite, tremolite, diopside and wollastonite.
A complete study of such a metamorphic region would include the laying down upon the map of all the zones distinguishable among the varied sediments, and it is clear that from studies of this kind the data for the development of an ideal classification should be realized. In place of the three depth zones of Gruben mann's classification, we should recognize many zones. In this sense the facies classification suggested by Eskola is a refine ment of the existing classificatory scheme. A facies is defined to designate a group of rocks characterized by a definite set of minerals which, under the conditions obtaining during their formation, were in perfect equilibrium with each other. The quantitative and qualitative mineral composition in the rocks of a given facies varies gradually in correspondence with variation in the chemical composition of the rocks. The number of facies
expresses the variable physical environment under which rocks have been formed and any given facies may include assemblages (isofaciel) of widely different bulk composition. A distinctive facies is the hornfels facies characteristic of the inner zones of thermal aureoles. Other facies are recognized by the name of some important constituent assemblage, as green schist, amphi bolite, eclogite or sanidinite facies. The number of facies is not limited by a priori considerations, and may be increased as ad vancement of knowledge may require. The elucidation of the different zones in a region of progressive metamorphism is clearly a first step towards the determination of the facies of metamorphic rocks, and thus a guide—apart altogether from experimental study —to the goal of deeper understanding of metamorphic processes.
subject of metamorphism is treated in most elementary text books of petrology. (See PETROLOGY: Bibliography.) The principal detailed treatises are C. R. Van Hise, A Treatise on Metamorphism, Monograph U.S. Geol. Survey, No. 47 (19o4) ; U. Grubenmann, Die Kristallinen Schiefer (2nd ed., 1910) ; C. K. Leith and W. J. Mead, Metamorphic Geology (1915). The more recent advances are incorporated in U. Grubenmann and P. Niggli, Die Gesteinsmetamorphose, vol. i. (1924). Reference may also be made to R. A. Daly, "Metamorphism and its Phases," Bull. Geol. Soc., America, vol. xxviii. (1917) ; A. Harker, Presid. Address Geol. Soc., London, vol. lxxiv. (1918). On the classification of metamorphic rocks see U. Grubenmann, Fortschritte der Mineralogie . . ., vol. iii. (1913) ; P. Eskola, "The Mineral Facies of Rocks," Norsk. Geol. Tidskr. vol. vi. (1920). On the structure and texture of metamorphic rocks see especially F. Becke, "Mineralbestand and Struktur der Kristallinen Schiefer," Denkschrift d. Wiener Akad. d. Wiss. (1903) ; and for a summary of the more recent advances, F. Becke, Struktur and KlUftung, Fortschritte der Mineralogie . . ., vol. ix. (1924).
(C. E. T.)