In all probability, the lower beds occupy three-fourths of the entire area of this field, where the upper beds do not exist: that is, the upper beds, or those lying over the Mahoning sandstone, do not occupy more than one fourth the area of the coal-field. The upper beds, in fact, are confined to a limited area, as the red-ash beds are in the anthracite fields. They occupy a portion of this field in the southern part of Pennsylvania, Maryland, Virginia, and Ohio, and may exist to a limited extent in the deepest portions of the measures in Kentucky. But in the Western coal fields the upper beds do not exist.
The numerous water-courses that traverse all parts of this great field have cut deep valleys through the coal measures, in many places far below their base ; but generally the valleys exist in the coal measures and at the expense of the coal. Not less than one-third the entire amount of coal has been thus denuded by erosion. It will, therefore, appear evident that we can only estimate approximately the total thickness of coal ; since none but the lower beds approach the margin of the field, and the upper beds occupy but a small area, while all portions of the field have suffered much from erosion. The total thickness of the seams ranges from 50 to 75 feet :
perhaps 50 feet total workable thickness would be an average, where the upper and lower groups exist. But we cannot estimate more than half the aggregate thickness as productive throughout the entire field. We have given 30 feet as the total workable thickness of the coals of Pennsylvania, and 20 feet as the aggregate of the United States ; and both are the maximum limits. The amount of available—or what we now call work able—coal is much less; but, under a system of economy that might and should be applied to our coal-beds, perhaps the estimate is reasonable, and it is certainly within the bounds of all such calculations. Under our pre sent wasteful style of mining, however, and our rejection of all seams under three feet as unworkable, the estimate is one-half too much; that is, the pro ductive yield of this great coal-field would be fifteen feet total thickness, instead of 30 feet,-22,500 tons per acre, instead of 45,000 tons. But here, as in the English coal-fields, we have estimated all seams over 12 inches in thickness.