Slates are exceedingly fine-grained rocks which have a remarkable cleavage which permits them to be split into thin and broad sheets. Their color is commonly gray to black, but may be green, yellow, brown, red, etc. They have been formed commonly by the metamorphism of shales. Their characteristic slaty cleavage may or may not be parallel to the bedding planes of the original shales – the cleavage is formed perpendicular to the direction of the metamorphic pressure. They are quite common in occurrence all around the globe. Slates are dense and homogeneous rocks, of such fine texture that the individual mineral particles composing them cannot be distinguished by the eye or lens. The most valuable property is their ready cleavage, by means of which they split readily into broad, thin sheets, which, as is well known, may be used for a variety of purposes. The slates represent the metamorphic form the finest material of the land waste formed by erosion, which, the equivalent among the unmetamorphosed stratified rocks, would be clay shales of various kinds, etc., as previously described. With such material more or less volcanic dust and debris, or tuffs, may be mingled. The cause of the slaty cleavage is discussed further in the following paragraphs. The difference between slate and shale has been discussed in the description of the latter rock.
Mineral Composition and Other Properties. The mineral particles are so fine in slate that the composition from the megascopic standpoint is not a matter of importance. It may be mentioned, however, that since the clays, silts, etc., from which they are formed come from a great variety of sources, so the microscope detects in them many and varied minerals, the chief of which are quartz, mica, chlorite, carbonaceous substance, etc. The kaolin and feldspar particles, which one might naturally expect, are rare and appear to have been converted into other minerals. They not infrequently contain crystals of pyrite, readily seen with the eye or lens, which may attain large size, sometimes as distinct crystals, sometimes as concretions, or replacing fossils. Veins, lumps, and lenses of deposited quartz are also common in them, those of calcite being much rarer. The color is chiefly gray, to dark gray, to black, according to the amount of carbonaceous substance, but they are often green from chlorite, or red, purple, yellow, or brown, from the oxides of iron. The surface of the slaty cleavage is apt to have more or less of a silky luster, sometimes scarcely perceptible; the cross fracture has a dull surface.
While the rock is firm and never friable, it is also rather soft, so that it may be quite readily cut, a feature of great value for technical purposes. The specific gravity of an average slate is about 2.8. The predominance of magnesia over lime shows that the soluble lime silicates have been mostly dissolved out of the silt in the process of erosion and laying down of the sediments. The presence of carbon in the black varieties, and of ferric iron in the red, is to be noted.
Varieties. Roofing slates are compact, very fissile varieties which split with a smooth, even cleavage. All the different colors are used, but the most common is a dark gray. Some slates fade when taken from the quarry, on continued exposure, through incipient alteration and the possibility of this can only be determined by practical trial. The presence of pyrite in any notable quantity is very prejudical, as this substance on exposure quickly alters and gives rise to rusty stains. The slates used for blackboards and ciphering are the blackest and most compact kinds. Calcareous slates are those which contain a good deal of intermingled calcite, or chalky material, which may rise to 30 per cent of the whole; they represent slates which have been formed from original marls.
Cleavage of Slates and its Origin. The cause of slaty cleavage has occasioned much speculation and has been the subject of investigation, both experimental and mathematical, as well as geological, by a number of scientists. From this work it has become clear that it is the result of great pressure upon the material and that the planes of cleavage are at right angles to the direction of pressure. When the fine-grained sediments are subjected to intense pressure, unevenly shaped particles tend to rotate, so that their longer axes are perpendicular to the direction of pressure; they also tend to become flattened perpendicularly to it. This tends to give the rock a grain, an arrangement of particles, by which it tends to split more readily along such a direction than in any other. Moreover the rock minerals, which naturally tend to be flattened or elongate in the shape of their particles, such as the micas, kaolin, hornblende, chlorite, etc., possess an excellent cleavage parallel to the elongate or flattened directions, and this is a great help in promoting the capacity of the rock cleavage. Slaty cleavage is thus partly molecular, or mineral cleavage, where it passes through a single mineral particle, and partly mechanical where it passes between arranged, unlike mineral particles. Not necessarily all of the minerals whose cleavage and arrangement induce the slaty cleavage are original; some of them, micas for example, may have been formed by the metamorphism accompanying the pressure. The planes of cleavage do not necessarily bear any definite relation to those of original bedding. The beds were laid down horizon tally and the direction of pressure is also usually horizontal; the cleavage planes are at right angles to this, and may therefore cut the bedding at right, or highly inclined, angles. But, as the beds may be folded before the pressures become intense, the cleavage planes may pass through the bedding at various angles, although they themselves are strictly parallel. Slates, in addition to their cleavage, are intersected by cross joints which are frequently so numerous as to divide them into small blocks and prevent their technical use. They generally form systems intersecting at definite angles. In the older mountain ranges the slates are frequently crumpled by repeated movement and show this upon their cleavage surfaces.
Occurrence. Slates are common rocks in metamorphic regions and range geologically from the Algonkian up to recent periods. In eastern North America they are chiefly Paleozoic and have an extensive development in Maine, in Vermont, in Pennsylvania, and in Georgia. They are also extensively distributed in the Lake Superior region and in the older ranges of the Rocky Mountains Cordillera. They are found in southern England, in Wales and in many other parts of Europe.
Phyllite. Closely connected with slate by intermediate types are a group of rocks to which the name of phyllite has been given. The name means "leaf stone" and is used on account of the remarkable cleavage of the rocks, by means of which they split into exceedingly thin sheets, in typical examples. The surface is sometimes flat, sometimes curved, folded, or crumpled by crustal movements. It differs from ordinary slate in containing a larger amount of mica, or at all events the mica is in larger flakes, and is more evident, giving the surface of cleavage a shimmering or micaceous appearance, and thus furnishing a transition form between slate and mica schist.
The mica is a fine, scaly, silky variety of muscovite to which the name of sericite has been given. Quartz is the other chief mineral and may sometimes be seen on the cross fracture. Rocks, which in this country have been called "hydromica-schists," are in large part such phyllites. Their color is sometimes pure white, more often tinged with reddish, yellowish, or greenish tones, and sometimes dark colored, or black, from pigments, like those of slate. They are apt to have a soft talcy or greasy feel, and to be more brittle than slate, and to lack its toughness and firmness. Sometimes they contain visible crystals of pyrite, garnet, and other minerals. The origin of phyllites, as shown by the researches which have been made upon them, is a varied one; in some cases they represent sedimentary material which has been metamorphosed, like the slates, but has attained a more complete degree of recrystallization than they have. On the other hand a considerable part of the phyllites represent original felsites igneous rocks which have been subjected to the energetic operation of metamorphism through dynamic forces, to pressure and great shearing, aided probably by liquids and heat. Their feldspars have been largely, if not entirely, converted into mica, and a thin schistose or slaty cleavage has been imposed upon them. In some extreme cases the rock appears as if wholly composed of this silky mica. The chemical analyses of these rocks show them to have compositions similar to that of many felsites or felsite tuffs.
Porphyroid Sheared Felsites. In many places where phyllites occur, they may be traced into types which are firmer, with less pronounced but yet distinct cleavage, and which contain visible phenocrysts of quartz and feldspar, similar to those in felsite-porphyries (embedded in the phyllitic ground mass). Such rocks have been termed porphyroid. These again may be further traced into undoubted felsites which still retain the phenocrysts, flow structures, spherulites, etc., characteristic of lavas, or the broken, angular, fragmental features of tuffs and breccias, in spite of the slaty cleavage, which to a greater or lesser degree, has been imposed upon them by the dynamic movements and shearing to which they have been subjected. These again may be followed into undoubted, unsheared felsites. Rocks with these characters, in their varied types as described above, occur in various places among the older metamorphosed Paleozoic areas of eastern North America, in Maine, at South Mountain, Pa., in Virginia and North Carolina, in Wisconsin, the Lake Superior region, etc. They have been found of various ages in Great Britain, Germany, the Alps and other places in Europe. It is only in comparatively recent years that such altered igneous rocks, with more or less schistose appearance and cleavage, have been recognized and their significance appreciated. The older geologists, confused by their cleavage, regarded and mapped them as slates and considered them as of sedimentary origin. They are of interest, because, as stated in the introduction to metamorphic rocks, these latter comprise material both of igneous and sedimentary origin.
Of the feldspathic, igneous rocks, the coarser-grained ones, like granite, as we have seen, on being subject ot metamorphic forces, yield gneiss; the compact felsites and their tuffs under the metamorphic agencies of pressure, shearing, etc., are turned into phyllites, porphyroids, and compact slaty rocks, according to the degree to which these agencies have acted. The igneous ferromagnesian rocks we shall see later among the amphibolites and other schists.
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