In 1909 the United States Geological Survey reported workable beds of coal aggregating 13,130 sq.
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.
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.
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.
They consist chiefly of sandstone and conglomerate, but include workable seams of coal.
This is actually the case; the Carboniferous, Cretaceous and Jurassic systems (qq.v.) contain coal-bearing strata though in unequal degrees,- the first being known as the Coal Measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe.
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.
If the workable areas are poor, and appear only at long intervals along the outcrop, the chances of discovering richer areas by a shaft are very small.
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.