TEKRANES. 



105 



as in Fig. 103 ; that is, the dip is alike eastward on the east and west sides of the range, 

 and it is not clear whether the overthrust flexure is anticlinal or synclinal. But toward 

 the other end, the dips of the east side change, through the positions in C and B to 

 that in A ; and here they are plainly in opposite directions on the two sides, and indicate 

 thus that the mountain is a synclinal flexure, basin-like at one end, and a careened trough 

 at the other. 



Flexures have ordinarily, if not always, the ridge-line inclined instead of 

 horizontal. The making of horizontal flexures (that is, those with the ridge 

 line horizontal) would require perfectly equable pressure along a region, and 

 also perfectly equable resistance, neither of which conditions could exist, 

 because of the varying texture in the rocks, if for no other reason, and 

 hence horizontality seldom occurs. 



103. 



Synclines of Mount Washington, Mass. D. '87. 



In a single ordinary flexure, therefore, the strike may vary nearly 180°, 

 and the dip as greatly. For the edge of the horizontal plane {feg in Fig. 104) 

 is a horizontal line ; hence it corresponds to the strike for each point it 

 passes over through a circuit of 180°. Supposing the flexure to run north 



and south, the strike may vary from N.-S. through E.-W. to N.-S. again. 

 Further : since the dip of the outer layer at any point is at right angles to 

 the strike, it is at right angles to the line sir. The dip of the beds is least 

 along the axis of the fold. 



Folds derive complexity also from torsion in the upturning movement. 

 The following figure of a mountain scene in Colorado (Fig. 105) shows, 

 besides the effects of erosion, those of a twist or torsion in the strata. The 

 light-shaded stratum has opposite dip in the near and distant parts, and of 

 course the strata either side participated in the torsion. The effect is proba- 

 bly far more common than is believed, for only in a region of bare, uncovered 

 rocks is such a condition likely to be appreciated. 



Besides the above-mentioned irregularities in a region of flexures, others 

 come from variations in the length of parallel flexures, one flexure lapping 



