Workable coal beds are distributed through 3000 ft.
They consist chiefly of sandstone and conglomerate, but include workable seams of coal.
In metal mining, when the workable portions of the deposit are small and separated by unworkable areas, the levels serve also the purpose of exploration, and in such cases must not be so far apart as to risk missing valuable mineral.
As the minimum workable thickness, and after making all necessary deductions, estimated the available quantity of coal in the proved coalfields of the United Kingdom as 100,914,668,167 tons.
From the experience of such workings it is considered that 1 soo metres would be a possible workable depth, the rock temperature being 132°, and those of the intake and return galleries, 92° and 108° respectively.
The Nahuatl lapidaries had at hand many varieties of workable and beautiful stone - onyx, marble, limestone, quartz and quartz crystal, granite, syenite, basalt, trachyte, rhyolite, diorite and obsidian, the best of material prepared for them by nature; while the Mayas had only limestone, and hard, tenacious rock with which to work it, and timber for burning lime.
Veins of cinnabar are known elsewhere in the Rocky Mountain and Sierra Nevada regions but not in workable quantities.
The district is poor in minerals; the yield of silver and copper has almost ceased, but there are workable coal seams near Offenburg, where the Kinzig debouches on the plain.
Upper: Red and grey sandstones, marls and clays with occasional breccias, thin coals and limestones with Spirorbis, workable coals in the South Wales, Coal Bristol, Somerset and Forest of Dean coalfields.
The so-called " unproductive " or barren strata, that is, those without workable coals, are not always limestones; quite as often they are shales, red sandstones and red marls.
The Molteno beds contain several workable seams of coal.
In the latter place they contain workable coal-seams. The Carboniferous Limestone often contains black flint (chert), and at some horizons conglomerates occur, the pebbles being derived from the unconformable ridges of the " Caledonian " land.
It must be remembered that the line between a workable deposit and one that cannot be profitably worked is often very narrow and that the majority of mineral deposits are not workable.
As soon as it appears reasonably certain that the property is workable the mine will be opened by one or more shafts, drifts or tunnels, and the underground passages for active mining operations will be started.
The thin seams of the Calciferous Sandstone are not workable, but the bituminous shales in the Firth of Forth basin are largely worked for the manufacture of mineral oil.
The theoretical conclusion has been confirmed by the discovery of Coal Measures, with workable coal seams, at Dover at a depth of 2000 ft.