Igneous rocks such as granites, syenites or gabbros, by process of metamorphism, may be altered into gneissic rocks. The chief meta morphic factor in the changing of igneous rocks to gneisses is, however, intense pressure, as a result of which even these most resistent of all rock-masses are made to flow like some viscous substance. This flowing, which is accompanied by more or less shearing, results, in producing schistosity or foliation, which is accompanied by a distinct banding closely simulating original bedding, so that the rocks lose their original massive form and come to resemble altered sed iments. A granite in which this secondary banding or foliation had been produced would be called a granite-gneiss; a gabbro similarly altered would be termed a etc, In general we can say that a gneiss is any halo crystalline, foliated, metamorphic rock, of either sedimentary or igneous origin, which has the mineralogical composition of some one of the plutonic igneous rocks. It is frequently im possible to determine whether certain rocks are true sediments or altered igneous rocks.
Sat/sta.—The term schist is applied to any metamorphic rock which, on account of the parallel arrangement of its crystal particles, tends to split more or less perfectly in parallel planes (schistosity or foliation). Clay upon metamorphosis yields mica. If little iron be present it furnishes muscovite (or sericite, its hydrated form), but if much iron be present it produces biotite. From this it follows that clay-beds on metamorphosis furnish mica of some sort or other, and as the rocks are always distinctly foliated, owing to the invariable par allel orientation of the mica-scales, they are called muscovite (or sericite) or biotite schists or, in more general terms, mica schist. Some
quartz and feldspar may be present, but if subordinate they are stilt reckoned among the schists. Rocks of this character mark the tran sition of the schists into the gneisses. Mica schists range in granularity from coarse- to fine-crystalline, the fine-crystalline varieties be ing called slates.
Quartz-schists are essentially quartz-rocks which contained originally some disseminated clay that altered to mica, as a result of the presence of which the rock has a schistosity. Many basic igneous rocks of the diabase order on metamorphosis change to hornblende-rocks. Where they are foliated they are called horn blende-schists. If more or less massive they are termed amphibolites. The diabases are very apt, however to alter to chlorite sc,hists, especially if the original augite was of the alummous variety, and they may alter to biotite schists.
Other Metamorphic Varieties.— Pure quartz-sand metamorphoses to a holocrystalline rock called quartzite. Limestone rnetainorroses to a more or less coarsely crystalline ro con sisting of grains of calcite. If fine, even-gran ular, it is known as marble. If coarse-granular, it is called crystalline limestone or simply lime stone. But the term marble is used to embrace all crystalline calcite rocks that are susceptible of a polish. Serpentine results from the meta morphosis of basic igneous rocks consisting largely of the non-aluminous varieties of horn blende or pyroxene and olivine. The titer ate especially susceptible to this change. Anthracite coal results from the metamorphosis of bituminous coal, which in turn is derived from peat or lignite. In certain extreme cases of metamorphism anthracite becomes altered to a graphitic material.