470 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 



raoval of volcanic matter beneath it, supplies no explanation of the origin of 

 zones of regular undulations. The sinking of any weak segment of the earth's 

 crust might produce a trivial general warping, but not a belt of waves. 

 /. The hypothesis of a simple lateral or horizontal compression, illustrated by the 

 folding of layers of any flexible material squeezed edgewise, and kept down by a 

 weight, is open to the objection that this mode of folding offers no true analogy 

 to the great symmetrical parallel flexures met with in nature. Flexures thus 

 artificially produced, show neither the forms nor the gradations characteristic 

 of the crust waves. A purely tangential force would cause the district within 

 its influence to bulge slightly upward, but not to corrugate into regular undu- 

 lations, and it fails to find an origin for a pressure in the direction assumed. 



10. Theories of cleavage and foliation. 



a. The prevailing notions of geologists respecting the origin of cleavage and foli- 

 ation are, on the one hand, that they have been produced by different intensities 

 of molecular crystallizing polarities, excited by heat operating in a definite 

 direction, on the other hand, that they have been caused by mechanical com- 

 pression of the strata applied perpendicularly to the cleavage and foliation 

 planes. 



b. One main objection to the pressure hypothesis is, that it does not account for 

 the existence of planes of alternately stronger and weaker cohesion. 



c. Another difficulty is, that it fails to explain the dependence of cleavage upon 

 the texture of the rock, especially its chemical nature, and particularly where 

 cleavable and non-cleavable strata alternate in close contact. 



d. A third important objection exists in the dynamic difficulty, that the cleavage 

 is nearly parallel and constant in its dip, despite the inequalities which a 

 lateral pressure should undergo in its transmission through all the contortions 

 and various postures prevailing in the strata. 



e. A fourth difficulty, analogous to the last, is presented by the constancy in the 

 amount and direction of the elongation or creep of the cleavage rocks in the 

 direction of their cleavage dip ; a constancy not compatible with the ever- 

 varying tension which the flexures and bendings of the strata would occasion. 



/. An additional objection presents itself, in the direction of the pressure implied 

 by the theory, which, assuming the force to have been perpendicular to the cleav- 

 age planes,i mplies it to have come either from a source above the earth's sur- 

 face, on the side towards which the cleavage planes are dipping, or from a source 

 far beneath the crust, in that quarter where invariably the cleavage and all 

 other symptoms of metamorphism are least abundant, or entirely wanting. 



g. Still another important objection arises, in the contradiction exhibited be- 

 tween the law of gradation in the steepening of the cleavage dip demanded 

 by the theory, or one form of it, and the actual law of the gradation of this dip 



