METAMORPHIC ROCKS. One of the three great divisions of the rocks (see RocK) char acterized generally V a foliated or schistose structure and including: (1) rocks which have been shown to be altered (metamorphosed) igne ous rocks (q.v.) : (2) rocks which have been shown to he altered sedimentary rocks and (3) rocks which, while resendding one or both of these types, do not allow of a definite determination of their origin. This implies that the product of metamorphisin acting upon a sedi mentary rock may lie indistinguishable from the result of the same agencies acting upon an igneous rock. There are, however, certain limits of composition fixed by the laws of consolidation of rock magmas that restrict somewhat the composition of metamorphic rocks which can have had an igneous origin, the processes of meta morphism having been shown in the great ma jority of instances not to have altered in an im portant way the ultimate composition of the rock as a whole. The metamorphic rocks are as a class those of which the processes involved in their alteration have been active within the crust of the earth, and are exclusive of those formed through the chemical action of the atmospheric agencies. the latter class being generally desig nated residual rocks (q.v.). See SMETAmoamftsNr.
AlErAmouenrc SEDIMENTARY ROCKS. The prin cipal metamorphic sedimentary rocks are definite ly related to the unaltered sedimentary rocks, principally, however, in respect to composition. Thus marble (q.v.) and crystalline hmestone (q.v.) are metamorphosed limestone. and dolo mitic marble is the product of metamorphism of dolomite. Quartzite (q.v.) and quartz schist re sult from the metamorphosis of arenaeeous rocks, and by the recrystallization of contained iron ore or by the impregnation by ferruginous ma terial they hecome jaspilite (q.v.) or hematite rock. The argillaccous rocks produced by the same processes are slate, phyllite, mica saki. hornblende or actinolite schist. garnetiferon3 schist. staurolitie schist. and the rarer types of hornfels of hornstones (q.v.), albite schist and ehiastolite schist.
Alr.T.vmortruto Icxmirs PocKs. The types in chided under this head may generally be reeng nized by the partial preservation of the peculiar textures of igneous rocks; such, for example. as
the granitic, porphyritic. vitreous or glassy', spherulitie. pearlitie, vie. (See lc.Nrot's RocKs.) These structures are, however. almost always oh seured by the presenee of parallel or approxi mately parallel sets of fissile planes which are eollectively referred to as the sehistosity of the rock. (See Al ET:014 oRIIISM.) There may be several sets of these fissile planes, but when two or more are present it is generally impossible to determine whether the rock had an igneous or sedimentary origin, and it would be relegated to the third class of metamorphic rocks. it has been found that igneous rocks which were origi nally glassy in texture arc liable to devitrifica lion, the product of which process is a rock of microcrystalline or crypto-erystalline texture. Metamorphic rocks of this derivation arc named in terms of the rocks from which they have been derived with the addition of the prefix ape; for example, aporhyolite from rhyolite If, however, the induced texture is that of schistositv or foliation alone, a structure which must be referred to the processes known as dy metamorphism, the original name of the igneous type is joined to the textural term gneiss; as, for example, granite-gneiss from metamor phism of granite (q.v.). Granitoirl gneiss is the equivalent of granite-gneiss. The third class of metamorphic igneous rocks owe their origin chiefly to the chemical alteration (recrystalliza tion) of igneous rock types with the abundant development of new minerals out of old ones. Sad! rocks are steatite or soapstone (q.v.), from the development of talc, and serpentine rock (q.v.), from the development of serpentine, in rocks of dominant magnesian composition.
Consult: Rosenbusch, Element(' der Gesteins lehre (Stuttgart, IS98) ; Zirkel, Lehrbiwk der trographie, vol. iii. (Leipzig, 1S94) : Diller, "The Educational Series of Rock Specimens Col lected and Distributed by the United States Geological Survey," in Bulletin 150, United Slates Geological Surrey (Washington, 1898).