SCHISTS, in petrology, metamorphic rocks which have a fis sile character (Gr. to split) ; in all there is at least one mineral which crystallizes in platy forms (e.g. mica, talc, chlorite, haematite), or in long blades or fibres (anthophyllite, tremolite, actinolite, tourmaline), and, when these have a well marked par allel arrangement in definite bands or folia, the rock will break far more easily along the bands than across them. The platy minerals have a perfect cleavage parallel to their flat surfaces, while the fibrous often have two or more cleavages following their long axes ; hence a schistose rock may split not only by sepa ration of the mineral plates from one another but also by cleav age of the parallel minerals through their substance.
Schists in the common acceptance of the term are completely recrystallized rocks ; fissile slates, shales or sandstones, in which the sedimentary structures are little modified by recrystallization, are not included in this group by English petrologists, though the French schistes and the German Schiefer are used to designate also rocks of these types. The differences between schists and gneisses are that the latter have less highly developed foliation, are, as a rule, more coarse grained, and contain far more quartz and felspar—two minerals which rarely assume platy or acicular forms, and hence do not lead to fissility in the rocks in which they are important constituents. Schists, as a rule, are found in regions composed mainly of metamorphic rocks, e.g., the Central Alps, Himalayas and other mountain ranges, Saxony, Scandinavia, the Highlands of Scotland and N.-W. Ireland. They are typical products of "regional" metamorphism, and are in most cases older than the fossiliferous sedimentary rocks. Transitions be tween schists and normal igneous or sedimentary rocks are often found. The Silurian mica-schists of Bergen are f ossiliferous; in the Alps it is believed that even Mesozoic rocks pass laterally into mica-schists and calc-schists. These changes have probably been produced by the operation of heat, pressure and folding. Igneous rocks also may be converted readily into schists (e.g., serpentine into talc-schist, dolerite into hornblende-schist) by the same agencies.
There are two great groups, viz., schists derived from (a) sedi mentary and (b) igneous rocks, or, as they have been called, the "paraschists" and the "orthoschists." The first is the more impor tant and includes some of the commonest metamorphic rocks. In the paraschists, though fossils are exceedingly rare, sedimentary structures such as bedding and the alternation of laminae of fine and coarse deposit may frequently be preserved ; the foliation is often parallel to the bedding, but may cross it obliquely or at right angles, or the bedding may be folded and contorted while the foliation maintains a nearly uniform orientation. When the
foliation is undulose or sinuous the rocks are said to be crumpled, and have wavy splitting surfaces instead of nearly plane ones. The development of foliation in shaly rocks is undoubtedly closely akin to the production of cleavage in slates.
The sedimentary schists or paraschists have three great sub divisions, the mica-schists (q.v.) and chlorite-schists (which corre spond in a general way to shales or clay rocks) the calc-schists (impure limestones) and the quartz-schists (metamorphosed sand stones). The chlorite-schists are often of igneous derivation, such as ash-beds or fine lavas which have been metamorphosed. Many of them contain large octahedra of magnetite. Others are probably sedimentary rocks, especially those which contain much mus covite. Calc-schists are usually argillaceous limestones in which a large development of biotite or phlogopite has occasioned folia tion. Often they contain quartz and felspar, sometimes pyroxene, amphibole, garnet or epidote. Pure limestones do not frequently take on schistose facies. The quartz-schists consist of quartz and white mica, and are intimately related to quartzites. Many of them have been originally micaceous or felspathic sandstones. We may mention also graphitic-schists containing dark scaly graphite (often altered forms of carbonaceous shales), and haematite schists which may represent beds of ironstone.
The orthoschists are white mica-schists produced by the shear ing of acid rocks, such as felsite and porphyry. Some of the "porphyroids" which have grains of quartz and felspar in a finely schistose micaceous matrix are intermediate between porphyries and mica-schists of this group. Still more numerous are ortho schists of hornblendic character (hornblende-schists) consisting of green hornblende with often felspar, quartz and sphene (also rutile, garnet, epidote or zoisite, biotite and iron oxides). These are modified forms of basic rocks such as basalt and dolerite. Every transition can be found between perfectly normal ophitic dolerites and typical hornblende-schists, and occasionally the same dike or sill will provide specimens of all the connecting stages. A few hornblende-schists are metamorphosed gabbros ; others have developed from dikes or sills of lamprophyre. Under extreme crushing these basic rocks may be converted into dark biotite schists, or greenish chlorite-schists. Tremolite-schist and antho phyllite-schist are in nearly all cases the representatives of the ultra-basic igneous rocks such as peridotite in regions of high metamorphism. Talc-schists are of the same category. They are soft and lustrous, with a peculiarly smooth feel. They are also known as the products of metamorphism of siliceous dolomitic limestones.