2. Carboniferous limestone series, embracing three subdivisions, the highest of which contains three or more limestones with thick beds of sandstone and some coals, the middle includes several valuable seams of coal and ironstone, and the lowest is characterized by several beds of marine limestone with sandstone, shales, some coals, and ironstones.
1. Calciferous sandstone series, forming two subdivisions. The upper is known as the oil-shale group, and is over 3,000 feet in thickness, and contains, in its highest part, beds of coal, usually of inferior quality, and, farther down, about six main seams of oil-shale, inter-stratified with beds of sandstone, shale, fire-clay, marl, and estuarine limestones.
Although the calciferous sandstone series is well developed in other parts of Scotland, it has not hitherto yielded any oil-shale of economic importance beyond the limits of West Lothian, Mid Lothian, and Fife. Thin seams of oil-shale do occur in various places in the counties of Haddington and Berwick, but, generally speaking, the quantity is not sufficient to be practically worked.
A word or two as to the oil-shales themselves. The shales, as known in the Lothians, are fine black or brownish clay shales, with certain special features which enable them to be easily distinguished in the field. Miners draw a distinction between “plain” and “curly” shale, the former variety being flat and smooth, and the latter contorted or “curled,” and polished or glossy on the squeezed faces. In internal structure, oil-shale is minutely laminated, which is apparent in the “spent” shale after distillation, when it is thrown out in fragments, composed of extremely thin sheets like the leaves of a book.
Before touching upon the methods employed in mining the shale and the treatment it receives during distillation, it is interesting to note that the industry in Scotland has passed through many vicissitudes since its establishment. At that time, the American oil industry was but in its infancy, and the production in the States was utilized mainly on the American markets. Consequently, there was a great demand for the Scottish oils in this country, and in 1870 there were no fewer than ninety small oil-works in the Lothians, the majority of which were operating the shales. It was about this time that the American illuminating oil came over to this country, and a very sorry blow was dealt the Scottish industry. So disastrous was the resulting competition between the Scottish products on the one hand, and the American and Russian petroleums on the other, that one by one the Scottish companies closed down, and, after less than eight years of competition, the number of operating companies had fallen to twenty-six. The decay continued until the number of active concerns in the Scottish shale-oil industry could be counted on one’s fingers.
The industry exists to-day simply as a result of the great improvements which have been made in the retorting of the shale, by which larger quantities of products are produced—including ammonia. It is thus able to withstand foreign competition.
To-day, it is estimated that nearly 4,000,000 tons of the Scottish shales are treated every twelve months by the several operating oil companies. The most important of these concerns—the Pumpherston Oil Company—has been regularly operating since 1883, and, inasmuch as it deals with by far the largest quantities of shale treated, a brief account of its operations will be of advantage in enabling the reader to understand the methods by which a total of nearly 400,000 tons of oil are produced each year in Scotland.
The operations of the Pumpherston Oil Company are upon a scale of considerable magnitude, for the Company’s works comprise the crude oil plant, the sulphate of ammonia plant, oil and wax refineries, etc. The Seafield and Deans works, 7 and 4 miles distant respectively, possess only crude-oil and sulphate-producing plants, the refining plants being confined to Pumpherston. The Company’s works cover 100 acres, while the shale fields extend over many thousands of acres in and around the district of Pumpherston.
As has already been mentioned, the shale fields so far operated lie, in the main, in the Lothians, and, as one motors by road from Edinburgh to Glasgow, the shale country is passed through. Before the commercial development of a shale field, trial borings are sunk, now more generally by means of a diamond bore, for by its revolving action a solid core is obtained which readily shows the character and inclination of the strata passed through. When a seam of shale has been found by boring operations, and the exact position and depth of outcrop determined, it is necessary, before sinking a mine, to put down a trial shaft for the purpose of making sure as to the true gradient at which the shale is lying, and the thickness as well as the quality of the same.
In the shales in the Pumpherston district there are five distinct seams, dipping from 29 degrees to 38 degrees, and the mine is driven in the middle seam, the other seams being entered by level cross-cut mines driven from one to another. Each of these seams is worked separately, the cross-cut shown in the sketch serving the purposes of communication and transit. In some cases, where the inclination of the shales is at a different angle, it is necessary to sink a vertical shaft, and this method is applied to the series known as the Mid-Calder.