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



respects, by the continually varying resistances consequent on the contortions of 

 the beds. 



5. A further objection lies against the pressure theory, in the contradiction it 

 otters between the direction which it assumes the compression to have come from, 

 and the direction in which we can demonstrate the strata to have been actually 

 pressed and moved. In every district of plicated and undulated strata, it can be 

 shown, from the shape of the waves, from the declension in their curvature and 

 height, from their mutual recession, from abatement in all the metamorphic 

 signs of igneous action, and, finally, from the direction of the great planes of 

 fracture in the crust, that the movement and pressure were upward and forward 

 from the quarter of chief crust disturbance. Now it is nearly at right angles to this 

 established direction of the forces, that the hypothesis I am reviewing assumes a 

 pressure to have been applied to produce the cleavage. The planes of fissuration 

 dipping inward towards the igneous side of the belt, any cleavage-producing pres- 

 sure to be perpendicular to these planes, as the theory alleges it was, must have 

 come either from a point or line elevated at least 45° above the earth's surface, or 

 else from a point or region far below the earth's crust on the opposite side, or in the 

 quarter where the cleavage is absent, or is invariably the least distinct, and where 

 the flexures of the strata, and all other evidences of crust movement, are vanish- 

 ing. This is, I conceive, a dynamic dilemma in which the compression theory 

 finds itself, — either to make the force emanate from a quarter external to the 

 crust entirely, or from just that quarter where we have the fullest evidence of the 

 absence of any force at all. Thus, if the theory is applied to explain the south- 

 dipping cleavage of the northern flank of the Alps, it implies either that the pres- 

 sure came, not from within the crust below the crest of the chain, but from some 

 point in the air high over the summits of the mountains, or else from some deep- 

 seated subterranean region far to the north of the Alps, under the undisturbed 

 plains of Northern Switzerland or Germany. In the case of the Appalachians, it re- 

 quires that the pressure should have come, not from under the convulsed and rup- 

 tured region of the Atlantic slope, but from some high aerial point above this, or 

 else from a spot diametric to it, deep under the plains of the Western States, where 

 neither cleavage, metamorphism of any kind, nor undulations of the strata exist, 

 to indicate the former presence there of any compressing force at all. {See Sec- 

 tions of the Appalachians and Alps. Figs. 1 and 3.) 



6. Besides this general difficulty, I have a special one to offer connected with 

 the laws of cleavage dip. This applies not only to the theoretical generalization 

 of Mr Daniel Sharpe respecting the relations of the cleavage planes to each other 

 in different parts of a zone of slaty cleavage, but to the observations upon which 

 his generalization has been built. His sections of the cleavage in North Wales and 

 elsewhere, represent it as perpendicular or steepest in the belts of maximum ig- 

 neous action, and flattest in the regions most remote from these, where he places 



