Page:The New International Encyclopædia 1st ed. v. 13.djvu/406

* METAL- WORKING MACHINEB.Y. 370 METAMORPHIC ROCKS. piece to be cut is made to revolve as well as the saw, but at a slower speed. By this means only a small surface of the bar to be cut is presented at a time to the circunifcience of the saw. The saw is about the same size as the cold saw de- scribed above, and is rotated at a velocity of about 25.000 feet per minule. The heat generated by the friction of this saw against the small sur- face of the bar exposed to its action at one time is so great that the particles of iron or steel in the bar are actually fused and the 'sawdust" wehls into a solji! mass as it falls. This disk will cut either east iron or wrought iron or steel. JllLLl.NG Machines. Milling machines are used for forming exact surfaces of various forms by means of revolving cutting tools or cutters. By means of dilTercnt shaped cutters an almost unlimited variety of surfaces can be cut by mill- ing machines. They are used extensively for sliaping small articles such as scwingiuachine and gim parts. Tlicy vary greatly in form, being vertical, horizontal, and universal, with one or several cutters. Fig. shows a form of ])Iain milling machine with ,a horizontal spindle, and Fig. 10 shows representative forms of cutlers and tools, of which there are many special shajjes. FlO. 10. MILI.INi; 1 MTTKRS ANIl TOOL9. Special Machines. The term special ma- chines includes all metal-working machines fur tapping nuts, threading bolls, and nutking jiins, screws, nails, rivets, pens, etc. The use of mclal- working machines has largely replaced hand labor in metal-working in all eounlries. Iiut it has become particularlv extended in the United States. METAMTERISM (from Gk. ^ni, nirla. after -^ /iz/ior, )/i<)(,s-. part). ,-V term equivalent to segmentation, as seen in the worms whose bod.v is divided along the prinniry or longitudinal axis into segments, homiilogiiu* with each other, which arc technically called 'somites' or 'metamercs.' Each metamere or segment contains a chamber or compartment of the body-cavity, and a section of the aliiucutaiy canal and other organs. The external appendages or the so-called 'segmental organs' are said to be segmcnlally or nu'tanieri- cally arranged. Metamerism is most obviously exhibited in worms and arthropods, as the lob- ster, myriapods, and insects. Vertebrate animals, also, in the vertebral column, and some of the other organs, show a tendency to a repetition of segments, i.e. to metamerism. METAM'NEH. town of Galabat (q.v.). METAMORPHIC ROCKS. One of the three great divisions of the rocks (see Rock) char- acterized generally by a foliated or schistose structure and including: (1) rocks which have been shown to be altered (metamorphosed) igne- ous rocks (q.v.); ("i) rocks which have been shown to be altered sedimentary rocks (q.v.) ; and (.3) rocks which, while resembling one or both of these types, do not allow of a definite (li'termination of their origin. This implies that the product pf metamorphism acting u])on a sedi- mentary rock may be 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 I'ock magmas that restrict somewhat the composition of metamorpbic rocks which can have bad 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 comjiosition of the rock as a whole. The metamorpbic 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 atmosiiheric agencies, the latter class being generally desig- nated residual rocks (q.v.). See ilETAiioni'ili.SM. Metamori'IIIC SEnniENTARY Koc'KS. The prin- cipal metamoriihic sedimentary rocks are delinitc- ly related to the unaltered sedimentary rocks, principally, however, in respect to composition. Thus marble (q.v.) and crystalline limestone (q.v.) are metamorphosed limestone, and dolo- mitic marble is the product of metamorphism of dolomite. Quartzito (q.v.) anil quartz schist re- sult from the nu-taniorphosis of arenaceous rocks, and by the rccrystallization of contained iron ore or by the impregnation by ferruginous ma- terial they become jas])ilite (q.v.) or hematite rock. The argillaceous rocks produced by the same processes are slate, phvUite, mica schist, hornblende or aetinolite schist, garnetifcrous schist, stanrolitie schist, and the rarer types of hornfcis or hornstones (q.v.), albite schist and chiastolitc schist. METAMORPHIC IGNEOUS RocKS. The types in- cluded under this head may generally be recog- nized by the partial preservation of the peculiar textures of igneous rocks; such, for example, as the granitic^ porphyritic, vitreous or glassy, spherulitic, pearlitic," etc. (See loNEor.s Rocks.) These structures are, however, almost always ob- scured by the presence of parallel or ai)proxi- matelv parallel sets of fissile planes which are collectively referred to as the schistosity of the rock. (Sec MhrrAMoiiiMiis>f.) There may be several sets of these l']s<iilc planes, but when two or more arc present it is generally impossible to di'teniiinc whether the rock had nn jgncoiis or sedimentary origin, and it would be relegated to