THE COAL MEASURES.
In this outline sketch we wish to present clearly and in a practical manner, not only the place of Coal among the rocks, but to illustrate briefly the extent of our great Appalachian formations, or the immense area of the American coal-fields within its wide embrace, which reaches from the Laurentian,basins to the cane-brakes of Alabama, and from the anthracite fields of Pennsylvania to the uncertain deposits of the Black Hills on the eastern slope of the Rocky Mountains.
The greatest development of the coal measures or the coal which they contain is on its eastern borders, in the anthracite basins of Pennsylvania, and the great Alleghany coalfield, which occupies so large a part of its vast area; and this seems to be in conformity with the subordinate stratas, which also have their greatest development along their Atlantic margin. The average vertical thickness of the workable Pennsylvania anthracites being sixty feet, while its total or maximum thickness will reach over a hundred feet; the workable coals of the Cumberland basins, in Maryland, is thirty-five feet, while the maximum is about fifty feet. The average
workable thickness of the Alleghany coal-field is twenty-five feet, While its maximum is fifty feet. The average workable thickness of most of the developed Western fields is ten feet, while the maximum may be computed at twenty-five feet; and thus we find a gradual thinning, of not only the subordinate strata, but also the overlying coals which they support, from the cast to the west. But we may here state the fact, our extreme western margin of this great coal area is to the present generation a terra incognita as far as its geology and minerals are concerned, and we cannot say to what extent the common depreciation has been carried beyond the central basins.