468 PROF. H. D. EOGERS ON THE LAWS OF STRUCTURE 



5. Gradations in flexures. 



a. In all undulated zones, the succession, starting from the most disturbed side, 

 is invariably from the folded waves to the unequally sloping or normal ones, 

 and from these to the equally sloping or symmetrical. 



b. The waves grow progressively wider apart, or increase their amplitude, as 

 they pass from the folded to the equally sloping form. 



c. The waves progressively flatten down as they recede from the folded side of 

 the belt. 



d. The axis planes of the flexures of any great undulated zone all incline towards 

 the same quarter, that of maximum disturbance, the angle of inclination being 

 less the nearer the wave or plication is to that side. 



6. Fractures or faults. 



In undulated districts, the dislocations are of two kinds : (1.) Numerous short 

 ones, transverse to the strike of the axes, and shifting the strata to but a tri- 

 vial extent; (2.) longitudinal ones, fewer in number, of great length, and pro- 

 ducing often great displacement. 



The longitudinal faults very generally dip towards the same quarter as the 

 axis planes ; indeed they are either ruptures in the axis planes of the flexures, 

 or in the steep or inverted sides of the waves. 



This slanting of the plane of dislocation parallel with the leaning of the wave, 

 causes the newer or upper formations, on the inverted side, to dip under the 

 older or lower on the uninverted side of the flexure or the fault, for almost 

 always the uninverted side has been shoved forward and upward across the 

 inverted. 



Some undulated belts are single, or have all the axis planes of the flexures 

 dipping to one quarter, as the Appalachians and the zone of Southern Belgium. 

 Others are double, or consist of two such zones, both dipping inwards towards 

 one central line of chief igneous disturbance, aud these latter present in this 

 inward general leaning a fan-like structure, as in the Alps. 



7. Phenomena of slaty cleavage. 



a. The cleavage dip is independent of the dip of the strata ; and still more re- 

 markably, the cleavage planes of a district are generally parallel to the axis 

 planes of its flexures. 



b. Immediately within the anticlinal and synclinal axes, the cleavage planes de- 

 part from their parallelism to the axis planes to dip inwards towards them in 

 a kind of fan-like arrangement. 



c. The cleavage is only present where the rocks consist of certain materials, 

 abounding most where they are most argillaceous and of finest texture, disap- 

 pearing and reappearing with changes in the composition of the strata, even 

 where this closely alternates. 



