CENTRAL COAL-FIELD IN WESTERN KENTUCKY.
A singular increase in the thickness of the coal measures takes place in the Western-Kentucky portion of the Central coal-field, which we can only account for by the greater angle of dip and, consequently, deeper basins of this region. A basin of coal is always filled to its own peculiar horizon, which, of course, must conform to the coal-field in which it exists. But in a field where three or four seams may be the general number found within the measures, a local depression having the basin-shape and lying below the base of the surrounding measures will accumulate an additional amount of measures, and, consequently, additional seams, until the general horizon of the field is reached. But, singularly, the lower seams are always identical with each other all through the field, while the local additions are always the upper ones. We said singularly ; but naturally would be the best expression. It presents evidence that all coal-seams are formed in water, and that as soon as the formation reaches the surface the growth of coal and the accumulation of the measures cease.
In Kentucky the basins lie deeper than in any other part of the Central coal-field, and perhaps as deep as the deepest part of the Alleghany coal field. As an evidence of depth, we find the coal-seams to be of a minimum thickness, while they are in maximum numbers. All our investigations prove that the coal-beds are invariably thin when formed in basins of great depth, or in very shallow basins. The thickest seams always exist in basins of a medium depth.
We consider basins of 2000 feet, and of course over that depth, to be deep basins, and those of 1000 feet depth to be medium basins. By shallow basins we mean bogs, swamps, and shallow waters from 0 to 100 feet in depth.
The Alleghanies rise generally about 2500 feet above sea-level, though some points attain an altitude of 3000 feet or more. The highest coal on the eastern margin of the Alleghany field, outcropping on the high plateaus of the Alleghany Mountains, is about 2300 feet above tide. This we must accept as the water-line of the great basin at the commencement of the coal era. The elevation of the Ohio at Cairo is 290 feet above the sea-level : therefore, if the coal-basins of Western Kentucky are not deeper than the waters of the Ohio, the depth of the water in those basins must have been 2000 feet deep at the commencement of the coal era. We know that this does not conform with the theories of coal formation generally entertained, and that a great depression must have taken place in the Mississippi Valley ; but we know, too, that this depression was greater in the East than in the West, and greater beneath the Alleghanies than beneath the basins of West ern Kentucky. If the centre of the great basin was depressed during the formation of coal, the depression of the Alleghany range was greater : there fore the depth which we have cited will hold good under any argument. We
do not, however, set up the foregoing depth of coal-basins as an arbitrary rule. We have simply come to this conclusion from the examination of a number of basins; but we are bound to state that the process of contraction has undoubtedly increased the depth of most of our coal-basins, though the relative proportions have been retained; and, under such circumstances, we might assume 1000 feet as a maximum, 500 feet as a medium, and 100 as a minimum depth to coal-basins as they originally existed at the commence ment of the Carboniferous era. But the facts contradict this; and all the great coal-fields in Europe and America, having their original or normal forms unaltered, prove the greater depth to be the nearest correct.
The following vertical section is from Dr. D. D. Owen's survey of Western Kentucky, which we copy from a pamphlet on the identification of the coal-seams, kindly furnished us by Professor Lesquereux, of Columbus, Ohio.
We must here state that our identification of the Lower seams does not agree fully with that of this eminent Palontologist, simply from the fact that he has accepted the Wilkesbarre section, as laid down in the geological survey of Pennsylvania, as correct, and has identified the Mammoth with B in consequence. But, notwithstanding this grave error,—which must have occasioned him much difficulty and uncertainty at times, for which, however, he is not responsible,—it is wonderful how nearly the identifica tion, which we have worked out from the actual facts presented, and which he has scientifically arrived at by evidence, agree with each other.
Until this moment we have been working without the least knowledge of each other's labor; and though we may differ widely in points of mere opinion, the facts elicited, though obtained by different processes and from different points of investigation, are corroborative and in evidence of the correctness of the modes by which the result is obtained.
We obtained the foregoing data too late for the purpose of illustrating them with an engraving, uniformly with the other principal sections in this work.
The celebrated Breckinridge cannel coal we presume to be in position over B, and identified with the lower cannel on the Great Kanawha, and is, therefore, the third seam from the millstone grit. The cannel-coal seams of Western Kentucky appear to be more numerous than elsewhere in the Western bituminous fields, except in the Great Kanawha Valley. In the Pennsylvania section, figure 118, though an equal depth of measures exists, there is less cannel, and not as many coal-seams.
We have been unable to locate the Kentucky cannel-beds correctly, for the want of the proper data. We will endeavor to do so in the Appendix.
The amount of coal mined in Western Kentucky may be stated at 250,000 tons per annum, and the whole production of the Great Central coal-field thus:—