Common parallel folds with horizontal axes die out upward and downward at a distance which is theoretically infinite. However, the curvature becomes almost zero at a distance about twenty times the radius of perfect curves, at the center of the structure. Thus a semicircular parallel fold 200 feet in diameter would die out at a depth of 2,000 feet. A fold of the same type, to persist for a depth of 10 miles, would require a diameter of one mile at the surface. This theoretical structure involves enormous shortening of the central line of the folds and a diminishing shortening of the flanks, until at the place where it dies out the shortening becomes zero (figure 1). Such a feature would be a remarkable case of rotational strain of a type in nature unknown to the writer.
Certain folds which die out in depth are described by Van . . .