SHALE, or SLATE-CLAY, an indurated day, which often forms beds in the coal meas ures. It is chiefly composed of silica and alumina, in variable proportions, but also fre quently contains a considerable amount of carbonate of lime and of oxide of iron. It is of a gray or grayish-black color, or brownish-red when containing much iron. Its struc ture is more or less slaty. It is soft, and easily reduced to powder. It is use d for making slate-pencils. When free from lime and iron, it is reduced to powder, and used for tire-bricks, for which it alibi ds an excellent material. Slide very often.con tains a quantity of bitumen, and when this is so much the case that the mineral has a shining resinous streak; and crackles and blazes in the fire, a black smoke and a bituminous odor, it is known as bitillni7108 Male. This variety sometimes passes on the one hand into common shale. and on the other into coal. Impressions of ferns and other plants arc very frequently found in shale.
Slate, Schist, and Shale are names employed to denote those kinds of rock which are laminatud or fissile—that is, which possess a structure readily splitting into thin layers. Shale and schist are almost synonymous, although the latter should be restricted to rocks with their layers irregular or foliated. True slate differs from them in not having its lamination produced by bedding. See SLATE. Nevertheless, all three names are often applied to the sane substance.
Shale varies much in its composition. Clay, sand. lime, bitumen, and other bodies, either singly or any mixture of them, are included under the name, if they form rocks which split into layers in the direction of their bedding; 'clay, however, being an ingre dient in most shales. Strange as it may seem, the line between even coal and sonic kinds of 'shale is not well defined: and in the case of the Torbanehill mineral, found near Bath gate, the question by which of the two names it should be called led to a lengthened and costly litigat ion.
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 Has 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—that is, shales more or less rich in carbon and hydrogen—form another class of these bodies which have. in recent years, attracted much notice as sources of oil for illuminating purposes. It is now more than thirty years since a French man, named Du Buisson, introduced a method of distilling certain bituminous shales in France. at a comparatively low temperature, so as to obtain burning-oil and other prod ucts. The process was afterward tried in England, being used for a time in distilling Dorsetshire bituminous shale, sometimes called " Kimmeridge coal." From this min
eral 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 favor 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. JallICS 1 onng (see NAPHTHA), secured in 1850 the production of paraffine and par:aline oil from coal. With the exception of the soli 1 paraffiic. which Mr. Young was the first to obtain on the large scale, and the employment of coal instead of shale. the processes of Du Buisson and Young are essen tially the same. This process has created a new and rapidly-increasing branch of indus try, paraffiue oil and paraffine being economically obtained by it from either coal or shale of certain kinds. Those who have paid any attention to the various beds of minerals which go to form what is geologically called the coal measures are aware that it is only the seams of coal, ironstone, fire-clay, sandstone, and limestone which until very lately have been looked upon as of any industrial importance. Intel stratified between these and the other minerals of the series are numerous beds of carbonaceous or bituminous shale, until recently considered useless. .31any of these shales were found upon trial to yiethl from 30 to 50 gallons of crude oil per ton; and works—several of them of great size—have accordingly been started in many places over the entire area of the coal for mation in Scotland, and also at various localities in England and Wales, for the mum:• facture of mineral oil, paraffine, etc., from this material.
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 it is estimated that there are now 800,000 tons of this material annually distilled, Such a quantity yields the following products: Crude oil 25,000 gallons.
Paraffine 5,800 tuns.
Lubricating oil 9,800 " Sulphate of ammonia 2,950 " 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 sonic of the larger Scottish towns. Shales found in the Lies and some other formations, likewise yield mineral oil.