OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 457 



or the adjoining- strata, some alternate upward and downward force must undu- 

 late them, or they must contain alternate weak and strong belts, and even then 

 these must be somewhat undulated ; none of which conditions the hypothesis of 

 subsidence is prepared to supply. 



Hypothesis of a simple Horizontal Compression. 



A somewhat favourite and familiar mode of accounting for the undulation and 

 plication of strata, is that which assumes them to have been corrugated by a purely 

 horizontal or tangential pressure, without elevation and without pulsation ; and 

 this imagined mode of folding has been ingeniously illustrated by Sir James Hall, 

 Sir H. De la Beche, and other geologists, by their placing flexible layers of clay, or 

 cloth, or other substances, horizontally under a weight in a trough, and forcing 

 one or both ends towards the centre, so as to contract the length of the strata, and 

 thereby produce a series of miniature plications. It has been alleged that this 

 folding of the clay or cloth is an exact imitation of the flexures of strata seen in 

 nature ; but I must deny the assumed analogy. The plications thus produced 

 are merely irregular contortions ; they exhibit no definite form of curvature, no 

 constancy in the direction of their gentler and steeper slopes, and no law of 

 regular gradation. Their anticlinal and synclinal axis planes, if they can be said 

 to have any, lean some one way and some another ; and the flexures, when the 

 crowding is great, have a tendency to the horse-shoe form, and not to that of 

 waves. 



This hypothesis of corrugation, while erroneous in thus failing to present 

 a true representation of the waves of the crust, is also defective in its me- 

 chanical principles, for it assigns no cause for the origination of the wave struc- 

 ture. A purely lateral or horizontal force should, as already intimated, simply 

 bulge out to a feeble extent the whole compressed arch, but ought not of itself to 

 wave it; some independent agency, producing alternate upward and downward 

 flexure is indispensable to give even the most powerful tangential pressure the 

 ability to plicate the flexible mass. This hypothesis is furthermore imperfect in 

 not suggesting any cause in nature for the assumed horizontal pressure. It has 

 been already shown, when discussing the hypothesis of simple elevation, and of 

 simple subsidence of areas of the earth's crust, that neither of those movements, 

 unaccompanied by an actual pulsation of the strata, would be competent to cor- 

 rugate the crust at all ; the pure elevation of an igneous axis having the tendency 

 to stretch rather than compress the adjoining strata; and the simple sinking of 

 an area, by retreat of support beneath, having only the effect to irregularly warp 

 the surface, but in nowise to undulate it. 



