Shale is any argillaceous rock which splits into thin laminae in the plane of sedimentation—a kind of lamination not to be confounded with the fissile structure seen in clay-slate (see SLATE). Shale is composed mainly of alumina and silica; but some shales are rich in carbonate of lime (calcareous shale), others contain much ferric oxide (ferruginous shale). Those shales which contain much quartz pass into shaly sandstone. When carbonaceous matter is abundantly present shale often passes into gas-coal, or, it may be, bituminous shale. When it is remembered that shale is of sedimentary origin, and was washed down in the form of fine silt from the land to be accumulated in lakes and quiet areas of the sea-bottom, as in estuaries, protected bays, &c., its variable composition will be readily understood. Shales are frequently highly fossiliferous—the fossils, owing to the impermeable character of such argillaceous rocks, being generally well preserved.
The importance of certain decomposing shales, through which sulphuret of iron is disseminated, for the manufacture of alum has been long known, and the quantity raised for that purpose from the Carboniferous beds of Lancashire and Lanarkshire and the Lias beds of Yorkshire is very considerable, yielding about 16,000 tons of manufactured alum annually. Shales of a similar kind are worked in France, Germany, and North America.
Bituminous shales—i.e. shales more or less rich in carbon and hydrogen—have in recent years attracted much notice as sources of oil for illuminating purposes. Two manufacturing chemists, Butler in 1833 and Du Buisson in 1845, worked patents for the extraction of paraffin from coal-tar. The process introduced by the latter in France of distilling certain bituminous shales at a comparatively low temperature was afterwards tried in England, being used for a time in distilling a Dorsetshire bituminous shale, sometimes called 'Kimmeridge coal.' From this mineral a burning oil, a lubricating oil, and a naphtha for dissolving caoutchouc were obtained. But neither in France nor in England did the attempt to make a profitable manufacture succeed: in the former country the poverty of the shales was the chief drawback; in the latter the disagreeable smell of the oil, which could not be effectually removed, prevented it from obtaining favour in the market.
On account of these failures the process fell into abeyance, until it was revived again by the success of the well-known patent of Mr James Young (see PARAFFIN), secured in 1850 for the production of paraffin and paraffin-oil from coal. With the exception of the solid paraffin, which Mr Young was the first to obtain on the large scale, and the employment of cannel coal instead of shale, the processes of Du Buisson and Young are essentially the same. This process has created a new and rapidly- increasing branch of industry, paraffin-oil and paraffin being economically obtained by it from either cannel coal or shale of certain kinds. Some of these shales yield as much as 30 to 50 gallons of crude oil per ton. Their exploitation has called into existence many large works in the Carboniferous tracts of Scotland, as well as at various localities in England and Wales.
Owing partly to the comparative cheapness of shale, and partly also to the fact that these products are obtained from it in a state more easily purified than when they are got from coal, the use of the latter as a source of them is now almost entirely given up. In Scotland, where the manufacture of paraffin-oil is chiefly carried on, the shales used are called 'oil shales,' and there are now between 1,000,000 and 2,000,000 tons of this material annually distilled. The yield of crude oil, paraffin or burning oils, lubricating oil, paraffin scale or wax, and sulphate of ammonia in various periods of years will be found at the article PARAFFIN. In the refining process the crude oil is reduced to about one-half of its bulk before it is fit for burning. Besides the above, there is also a considerable quantity of 'coal gas' unavoidably produced, and partly wasted. But for the distance of the oil-works, this would be consumed in some of the larger Scottish towns. Shales found in the Lias and some other formations likewise yield mineral oil. See also CLAY, ARGILLACEOUS ROCKS.