THICKNESS OF THE ALLEGHANY COAL MEASURES.
There are several interesting geological problems connected with the coal measures of this great field, which if satisfactorily settled would go far to determine a scientific question of some importance. As before stated, all coal-fields are basin-shaped, and the interior is always deeper than the exterior portions,—in other words, the centre of the basin is always deeper than its outcrops ; and this fact holds good in the Alleghany field as elsewhere. But here, as in most other fields, the coal is deposited in nume rous basins, lying parallel with each other and conforming to the general geological strike of the strata. That is, all our mountain-ranges have a general southwest and northeast course, and all our great valleys, and con sequently basins, have the same general strike of axis, while their trans verse is, of course, the contrary. But these basins are successively deeper as they range from the west towards the east, or from the centre of the Great Appalachian (Mississippi) Valley. This depression is not only from the west to the east, but also from the north to the south ; and, though there is now no external evidence of a southern margin to the great basin, nothing is more certain than that a southern margin must have existed, as high, perhaps, as the boundaries on the east, north, and west. But these boundaries of the great basin, or the ancient Appalachian sea, as described in Chapter III., are not the boundaries of the Alleghany coal-field ; for besides this great field there are two or three others, perhaps not less exten sive, and several of smaller dimensions, but all within the great basin. As before noticed, the eastern margin of the Alleghany field is the eastern margin of the great basin as it now exists ; that is, this field lies on its eastern side, while the Rocky Mountains and their unexplored coal-fields bound it on the west. But the western margin of the Alleghany field lies towards the deeper portions of the great basin, and, as we have said, in the eastern portion of Ohio. Its extreme southern limit lies far in Alabama, and on the summits of the mountains which tower over the vast plains of the Gulf. The abrupt termination of the field in this direction,
and the broken crags which form the terminal points of many of those great mountain-ranges which sweep down from the north along the eastern margin of the great basin, and which form its Atlantic boundary, indicate a violent change in the topography of the South. The continua tion of the coal-field was evidently far beyond the " Lookout" ranges, and its present southern area bears no comparison to its ancient extent when first from the hand of Nature.
Yet, while we consider the southern margin of the ancient sea to have been along the shores of Florida and the mountains of Cuba and Yucatan, we have no reason to suppose that the coal formations originally existed universally along their interior slopes. It seems evident that the subsi dence of the crust was greatest in this direction, and that the interior basin must have been always too deep to admit of the formation of coal. But around the entire basin we find coal wherever there is evidence of a com paratively shallow sea. The depression of the ancient granitic or igneous crust of the earth, and the elevation of the Palaeozoic or sedimentary strata, have been general along the southern and eastern margins of the ancient sea. The change was natural, and, we think, has been clearly set forth in Chapter III.
It will be necessary here to devote a few words to the form and character of the great basin in which not only the Alleghany coal-field but the other great Appalachian fields, which will follow in this description, exist. The accompanying illustration, figure 117, represents the general form of the great basin from east to west, on a line with the Great Kanawha, Ohio, Missouri, Kansas, and Arkansas Rivers, from the Alleghany to the Rocky Mountains. The section is necessarily approximate, and merely gives the general positions of the great coal-fields, and the order of the geological formations and their peculiarities.