1911 Encyclopædia Britannica/Slate

SLATE (properly ; in M. Eng. slat or sclat, from O. Fr. esclat, a small piece of wood used as a tile; esclater, to break into pieces, whence modern Fr. éclat, the root being seen also in Ger. schleissen, to split), in geology, a fissile, fine-grained argillaceous rock which cleaves or splits readily into thin slabs having great tensile strength and durability. Many other rocks are improperly called slate, if they are thin bedded and can be used for roofing and similar purposes. One of the best known of these is the Stonesfield slate, which is a Jurassic limestone occurring near Oxford and famous for its fossils. Slates properly so-called do not, except on rare occasions, split along the bedding, but along planes of cleavage, which intersect the bedding usually at high angles. The original material was a fine clay, sometimes with more or less of sand or ashy ingredients, occasionally with some lime; and the bedding may be indicated by alternating bands of different lithological character, crossing the cleavage faces of the slates, and often interrupting the cleavage, or rendering it imperfect. Cleavage is thus a superinduced structure, and its explanation is to be found in the rearrangement of the minerals, and the development of a certain degree of crystallization by pressure acting on the rock. Slates belong mostly to the older geological systems, being commonest in Pre-Cambrian, Cambrian and Silurian districts, though they may be found of Carboniferous or even of Tertiary age, where mountain-building processes have folded and compressed these more recent formations. The action of pressure is shown also by the fossils which sometimes occur in slates; they have been drawn out and distorted in such a way as to prove that the rock has undergone deformation and has behaved like a plastic mass. Evidence of the same kind is afforded by the shape of the knots and concretions sometimes present in the slate. If the bedding be traced, either in the slates or in the other rocks which accompany them, flexures will be frequently observed (the folding often being of an isoclinal type), while reversed faulting, or thrusting, is usually also conspicuous.

The origin of slaty cleavage is in some measure obscure. This structure is by no means confined to slates, though always best exemplified in them, owing probably to the finegrained, argillaceous materials of which they consist. Grits, igneous rocks, ashbeds and limestones may and often do show cleavage. Coarse rocks and rocks consisting of hard minerals are always imperfectly cleaved. The cleavage of slates must be distinguished from cleavage of minerals, the latter being due to different degrees of cohesion along definite crystallographic planes. The connexion of cleavage with pressure, however, is unmistakable. It is never exhibited except by rocks which have been sub jected to the tangential stresses set up in the earth's crust by folding. These stresses may operate in several ways. They will alter the shape of mineral particles by broadening them in a direction at right angles to the principal pressures, while they are thinned in the direction in which the pressure acted. Probably the size of the particle will be slightly reduced. This method of reasoning, however, does not carry us far, as the minerals of slates vary considerably in form. Pressure will also tend to produce an expansion of the rock mass in a direction (usually nearly vertical) at right angles to the compression, for such rocks as slates are distinctly plastic in great masses. This flowage will help to orientate the particles in the direction of movement, and, operating conjointly with the flattening above explained,will accentuate the liability to cleave in a definite set of planes. The recrystallization induced by pressure is probably of still greater importance. Slates consist largely of thin plates of mica arranged parallel to the cleavage faces. This mica has developed in the rock as it was folded and compressed. In the moist and plastic slate the mineral particles slowly enlarged by the addition of new crystalline molecules. Those faces which were perpendicular to the pressure would grow slowly, as the great pressure would promote solution, and inhibit deposition; the edges or sides, on the other hand, being less exposed to the pressure would receive fresh deposits. In this way thin laminae would form, lying at right angles to the direction of greatest stress. Micas and other platy minerals (such as chlorite), which naturally grow most rapidly on their edges, would show this tendency best, and such minerals usually form a large part of the best slates; but even quartz and felspar, which under ordinary conditions form more equidimensional crystals, would assume lenticular forms. In the necessary co-operation of these three causes, viz. flattening of particles by compression, orientation of particles by flow and formation of laminar crystals, the fundamental explanation of slaty cleavage is found. The planes of cleavage will be approximately perpendicular to the earth pressures which acted in the district; hence the strike of the cleavage (i.e. its trend when followed across the country) will be persistent over considerable areas.

Sketch (by Du Noyer) of a block of variegated slate from Devil's Glen, Co. Wicklow. The crumpled bands mark the bedding, and the fine perpendicular striae in front are the cleavage planes; the fine lines on the darkened side merely represent shadow, and must not be taken for planes of division in the rock. It will be observed that the cleavage planes do not pass through the white bands.

Where the rock masses are not homogeneous (e.g. slates alternating with gritty bands), the cleavage is most perfect in the finest grained rocks. In passing from a slate to a grit the direction of the cleavage changes so that it tends to be more nearly perpendicular to the bedding planes. A structure akin to cleavage, often exemplified by slates especially when they have been somewhat contorted or gnarled, is the Ausweichungsclivage of Albert Heim. It is produced by minute crumplings on the cleavage faces all arranged so that they lie along definite planes crossing the cleavage. These slight inflections of the cleavage may be sharp-sided, and may pass into small faults or steps along which dislocation has taken place. A secondary or false cleavage, less perfect than the true cleavage, may thus be produced (see, Pl. IV. fig. 7). The faces of slates have usually a slightly silky lustre due to the abundance of minute scales of mica all lying parallel and reflecting light simultaneously from their pearly basal planes. In microscopic section the best slates show much colourless mica in small, thin, irregular scales. Green chlorite is usually also abundant in flakes like those of the mica. The principal additional ingredient is quartz in minute lens-shaped grains. The size of the individual particles may be approximately one-five-hundredth of an inch. Minute rods or needles of rutile are also common in slates, and well-formed cubes of pyrites are often visible on the splitting faces. The brownish colour of some slates is due to limonite and haematite, but magnetite occurs in the darker coloured varieties. Other minerals which occur in the rocks of this group are calcite, garnet, biotite, chloritoid, epidote, tourmaline and graphite or dark carbonaceous materials.

By advancing crystallization and increased size of their components, slates pass gradually into phyllites, which consist also of quartz, muscovite and chlorite. In the neighbourhood of intrusive granites and similar plutonic igneous rocks, slates undergo "contact alteration," and great changes ensue in their appearance, structure and mineral composition. They lose their facile cleavage and become hard, dark-coloured, slightly lustrous rocks, which have a splintery character or break into small cuboidal fragments. These are known as "hornfelses" (q.v.). Farther away from the granite the slates are not so much altered, but generally show small rounded or ovoid spots, which may be darker or lighter in colour than the matrix. The spots contain variety of minerals, sometimes mainly white mica or chlorite. In these spotted slates andalusite, chiastolite, garnet and cordierite often occur; chiastolite is especially characteristic; cordierite occurs only where the alteration is intense. The chiastolite-slates show elongated, straight-sided crystals with black cores (see, Pl. IV. fig. 9), which, on transverse section, have the form of a cross constituting the two diagonals of the rhombic or squarish pattern of the mineral. These crystals may be half an inch to several inches in length; they are usually more or less completely weathered to white mica and kaolin. In other cases, especially near mineral veins, slates are filled with black needles of tourmaline or are bleached to pale grey and white colours, or are silicified and impregnated with mineral ores. Frequently in districts where slates are much crumpled they are traversed by numerous quartz veins, which have a thickness varying from several inches up to many feet, and may occasionally be auriferous. (J. S. F.)

Slates are widely used for roofing houses and buildings of every description, and for such purposes they are unequalled, the better sorts possessing all the qualities necessary for protection against wind, rain and storm. The finer varieties are made into writing slates, and in districts where cross cleavage exists slate pencils are made. Slabs are also manufactured, and, being readily cut, planed, dressed and enamelled, are used for chimney pieces, billiard tables, wall linings, cisterns, paving, tomb-stones, ridge rolls, electrical switch-boards and various other architectural and industrial purposes.