Page:The New International Encyclopædia 1st ed. v. 13.djvu/407

* METAMORPHIC ROCKS. 371 METAMORPHOSIS. tlic lliird class of iiietamorphic rocks. It has been found that igneous rocks which were origi- nally glassy in te.xture are liable to devitrifica- tion, the ])roduet of which ])roccss is a rock of niicrocrystulline or crypto-ervstalline texture. Jletaniorphic rocks of this derivation are named in terms of the rocks from which they have been derived with the addition of the prefix apu; as, for example, aporhyolite from rhyolite (q.v.). If, however, the induced texture is that of scliistosity or foliation alone, a structure which nmst be referred to the processes known as dy- namic metamorphisra, the original name of the igneous type is joined to the textural term gneiss; as, for exam])le, fininite-r/nciss from metamor- pbism of granite (q.v.). G-ranitoid gneiss is the equivalent of granite-gneiss. The third class of inet.Tmorphic igneous rocks owe their origin ehielly to the chemical alteration (recrystalliza- tion ) of igneous rock types with the abundant development of new minerals out of old ones. Such rocks are steatite or soapstone (q.v.), from the development of talc, and serpentine rock (q.v. I, from the development of serpentine, in rocks of dominant magnesian composition. Consult: Rosenbuscli, Elemeiitc dcr dcstcln.'!- lehre (Stuttgart. 1898) ; Zirkel. Lehibmh der I'etrofirdiiliic, vol. iii. (Leipzig. 1894): Diller, "The Educational Scries of Rock Specimens Col- lected and Distributed by the United States Geological Survey," in Bulletin 150, United States Geologieal Survey (Washington, 1898). MET'AMOR'PHISM. A term commonly used liy gcolugists to indicate the profound changes which some rocks have undergone. The term is not applied to the changes induced by weathering and decomposition, but is appli- cable onjy to the profound modifications that are usually accompanied by an increase in hard- ness anil degree of crystallization from the origi- nal conditions. Metamorphism may lead also to the mineriilogical 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 wMth in nature particularly in the regions underlain by ancient rocks, where the processes of upheaval and subsidence and vol- canic forces have operated through long periods of time. The degree to which rocks have been changed varies from mere hardening in some cases to an extreme marked by complete altera- tion in structure and composition. KixD.s OF Metamorphism. 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 cryst:ilIization. In this kind of metamorphism, called eoiilaet meinmnrphism, the amount of change effected de- pends upon the character of the invading rock and the rock that has been invaded, and also upon the proximity to the contact. Deep-seated igneous masses exert gi-eater influence than s;ir- face flows of lava, doubtless because of the greater heat and longer period of cooling: while those of aciil 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 cla.y 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 develojjed 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 compression 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 tliorough transformation, as from an unaltered sediment to a homogeneous crystalline mass resembling an igneous rock. Causes of Metamorphism. Heat, pressure, and moisture are the most eft'ective agencies in producing the changes known as metamorphism. The influence of heat is shown in igneous con- tacts, but it is also an acces.sory 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 G!eikie, Text-Book of Oeology (London, 1893). See Geology; Metamorphic Rocks. MET'AMOR'PHOSIS (Lat., from Gk. ^ra- ^pcpuiais, from ^€Taixop(pou(r6at^ nietamorphousthai. to be transformed, from /jfTi, meta, over + uop0i), morphe, form) . In the mythology of the ancients, those transformations of human lieings into bea.sts, stones, trees, and even into fire, water, etc., in fables of which that mythology abounded. See Ovid; Folklore; Werwolf. METAMORPHOSIS (in animals). A change of form in the ])ost-embryonic life of an indi- vidual animal. The term is also applied to the change in form of homologous parts in difl'crent species. The young of many animals pass through a series of changes of form, in each of which the animal is adapted to changes in its surround- ings, involving alterations in its mode of life. METAMORPB08I8 OF OIL-BEETLE. n. First larva : b, second larva ; c, third larva ; d, pupa; e, mature beetle. sliglit if the change of body-form is slight, thor- oughgoing and radical if its body becomes pro- foundly modified. As examples of a complete metamorphosis may be cited the life-histories of the jellyfish (q.v.), the starfish, midlusks, crus- taceans, insects, and also the salamanders, toads, and frogs. Most shrimps and crabs undergo a complicated metamorphosis, for in the different