CONTORTIONS OF THE STRATA 45 



pi. 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 FlG - 2Q '~ A bent stratum to illustrate tension 

 , , , , .. . , upon the convex and compression upon the 



other in the manner of the concave side (after Van Hise)> 

 blades of a pair of shears. 



Such extreme " shearing " action is thus localized in the under- 

 turned 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 prob- 

 ably 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. 



In the long series of experiments conducted by Mr. Bailey 

 Willis of the United States Geological Survey, all the stages be- 

 tween the overturned fold and the overthrust fold were reproduced. 

 Where a series of folds was closely compressed, a parallel series of 

 thrusts developed (pi. 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 Scot- 

 land, where the overriding is measured in miles. Near the thrust 

 planes the rocks show a crushing of the grains, and the planes them- 

 selves are sometimes corrugated and polished by the movement. 



Restoration of mutilated folds. Since flexuring of the rocks 

 takes place within the zone of flow at a distance of several miles 



