Shale is the name given to rocks consisting of compacted muds and clays which possess a more or less thinly laminated, or fissile structure. Their parting is parallel to the bedding, and is the result of natural stratification. When such rocks have been subjected to folding and pressure, they assume a slaty cleavage which has nothing to do with stratification; they are then slates or phyllites and are described among the metamorphic rocks. This distinction, that rocks showing slaty cleavage are not shales, should be clearly noted, as the two are often confused. When both rocks are weathered, they have a very similar appearance. Shales are, in general, too fine grained for the component particles to be determined with the eye, or even with the lens. By microscopic and chemical analysis they are known to be formed mostly of kaolin and related substances, with which may be associated much white muscovite mica, but these are often accompanied by tiny fragments of quartz and other minerals. As the amount of quartz present increases, and also the size of grain, the shales pass over into sandstones, and such intermediate rocks represent deposited silts. There are also all forms of transitions between clays and shales, depending on the relative firmness and fissility of the mass.
Clay when dry is a fine, earthy, lusterless mass, giving a characteristic odor when breathed upon. It clings to the tongue, and when strongly rubbed to a powder between the fingers, it finally produces a soft, greasy, lubricated feeling, usually thus differing from loess, adobe, and similar appearing deposits. It absorbs water eagerly and becomes plastic. When pure it is white, but it is generally colored red or yellow by iron oxides, forming the red and yellow ochers, or gray, blue or black by organic substance. The colors are sometimes evenly distributed, and sometimes irregularly blotched, through the mass. Shales are apt to be soft, cut more or less readily with the knife, and are brittle and crumbly, so that taken in connection with the fissility, it is often difficult to prepare hand specimens of them. Like clays they exhibit a great variety of colors, white to buff or yellow, red to brown, purple, greenish and gray to black, and from the same causes.
Different shades of gray are perhaps the most common. They often contain various accessory mineral substances, such as carbonates, gypsum, halite (rock-salt), pyrite, etc. Some of these are frequently seen in the form of concretions, which may attain large size, up to several feet in diameter.
The chemical composition is somewhat variable, depending on the relative proportions of clay and other minerals. There are many varieties of shale which are normally distinguished by the presence of accessory minerals. Thus there may be a large amount of organic matter, mostly carbon, present, and such are called carbonaceous shales. They are black in color, and by increase of carbon, grade into coaly shales, shaly coals and so on into coal. They are a very common type, and are found associated with coal and also independently of it, sometimes covering wide areas and of great thickness. From the nature of the organic matter they are sometimes called bituminous shales. It is probable that the total amount of carbon in the shales far exceeds that existing in coal beds.
In other varieties of shales large amounts of carbonates, especially calcite, are present, and these are known as calcareous shales. By increase of this substance they pass into shaly limestones. They are apt to be associated with limestones and these calcareous varieties are detected by their ready effervescence with acids. Or the carbonate present may be chiefly carbonate of iron and thus produce transition forms between shales and clay ironstone previously described. The connection between clays, shales and marls has been mentioned on a previous page. Alum shale is a variety full of pyrite, or of sulphates resulting from its alteration; it has been used for the manufacture of alum.
The use of clay in the making of bricks, tiles, pottery, etc., is too well known to need further comment. Shale has no value for structural purposes, but in recent years, along with clay, it has become of value and is used in many places as a material for the manufacture of Portland cement, when mixed with the proper proportion of limestone and burned. Clays and shales are such common rocks in all parts of the world, where the unmetamorphosed stratified formations are found, that their occurrence needs no special description.
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