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FOLDED STRUCTURES. An upfold, in which the strata dip away from a line drawn along the crest and called the axis of the fold, is known as an ANTICLINE. A downfold, where the strata dip from either side toward the axis of the trough, is called a SYNCLINE. There is sometimes seen a downward bend in horizontal or gently inclined strata, by which they descend to a lower level.

Passing across this older ridge of denuded Silurian and other rocks, we reach the famous Illinois and Indiana coal-field, whose coal-measures lie in a broad trough, bounded on the west by the uprising of the carboniferous limestone of the upper Mississippi. This limestone formation appears here for the first time, having been absent on the eastern side of the Ohio anticline.

This area subsided during the Carboniferous period to a depth of nearly ten thousand feet. THE CENTRAL REGION lay west of the peninsula of the Cincinnati anticline, and extended from Indiana west into eastern Nebraska, and from central Iowa and Illinois southward about the ancient island in Missouri and Arkansas into Oklahoma and Texas.

THE APPALACHIAN REGION skirts the Appalachian oldland on the west from the southern boundary of New York to northern Alabama, extending west into eastern Ohio. The Cincinnati anticline was now a peninsula, and the broad gulf which had lain between it and Appalachia was transformed at the beginning of the Pennsylvanian into wide marshy plains, now sinking beneath the sea and now emerging from it.

THE CINCINNATI ANTICLINE. Over an oval area in Ohio, Indiana, and Kentucky, whose longer axis extends from north to south through Cincinnati, the Ordovician strata rise in a very low, broad swell, called the Cincinnati anticline. The Silurian and Devonian strata thin out as they approach this area and seem never to have deposited upon it.

On the west of the chain of mountains the foldings become gentler, and the coal assumes an almost horizontal position. In passing through Ohio we find a saddle-back ridge or anticline of more ancient strata than the coal, and in consequence of this, we have a physical boundary placed upon the coal-fields on each side.

Hence, in order to hold them from escaping to the surface, the reservoir must have the shape of an ANTICLINE, DOME, or LENS. It must also have an IMPERVIOUS COVER, usually a shale. In these reservoirs gas is under a pressure which is often enormous, reaching in extreme cases as high as a thousand five hundred pounds to the square inch.