=The overthrust fold.=—Whenever a stratum is bent, there is a tendency for its particles to be separated upon the convex side of the bend, at the same time that those upon the concave side are crowded closer together—there is tension in the former case and compression in the latter (Fig. 26). Within an unsymmetrical or an overturned fold, the peculiar distortions in the different sections of the stratum are less simple and are best illustrated by pl. 2 C. This apparatus shows two similar piles of paper sheets, upon the edges of each of which a series of circles has been drawn. When now one of the piles is bent into an unsymmetrical fold, it is seen that through an accommodation by the paper sheets sliding each over its neighbor large distortions of the circles have occurred. In that steeper limb which with closer folding will be overturned the circles have been drawn out into long and narrow ellipses, and this indicates that those rock particles which before the bending were included in the circle have been moved past each other in the manner of the blades of a pair of shears. Such extreme “shearing” action is thus localized in the underturned limb of the fold, and a time must come with continuation of the compression when the fold will rupture at this critical place along a plane parallel to the longest axis of the ellipses or nearly parallel to the axial plane of the anticline. Such structures probably occur in the zone of combined fracture and flow, up into which the beds are forced in cases of close compression. Relief thus being found upon this plane of fracture, the upper portion of the fold will now ride over the lower, and the displacement is described as a _thrust_ or _overthrust_. [Illustration: FIG. 26.—A bent stratum to illustrate tension upon the convex and compression upon the concave side (after Van Hise).] In the long series of experiments conducted by Mr. Bailey Willis of the United States Geological Survey, all the stages between the overturned fold and the overthrust fold were reproduced. Where a series of folds was closely compressed, a parallel series of thrusts developed (pl. 2 B), so that a series of slices cutting across neighboring strata was slid in succession, each over the other, like the scales upon a fish or the shingles upon a roof. Quite remarkable structures of this kind have been discovered in rocks of such closely folded districts as the Northwest Highlands of Scotland, where the overriding is measured in miles. Near the thrust planes the rocks show a crushing of the grains, and the planes themselves are sometimes corrugated and polished by the movement.