ih orm 
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 Hatt, 
Sir H. Dexa Becue, 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. 
