508 OF THE PHYSICAL STRUCTURE 



world. It appears to imply a powerful tangential movement, al- 

 ways operating in the same direction for the same region, during 

 the epoch of disturbance. A merely vertical force, exerted either 

 simultaneously or successively, along a system of parallel lines, 

 could only produce the same number of symmetrical anticlinal 

 arches, while again, a horizontal or tangential pressure, uncom- 

 bined with an alternate upward and downward motion, at regu- 

 lar intervals, could not possibly result in a system of parallel 

 folds, or axes, or lead to any change in the position of the strata, 

 beyond an imperceptible bulging of the whole tract, or else a 

 confused rumpling and dislocation, dependent on local inequali- 

 ties in the thickness or resistance of the crust, in different spots. 



That the wave-like flexures of our Appalachian strata are the 

 result of an actual onward^ billowij moveme?it, proceeding from 

 beneath, and not of a folding due simply to some great horizontal 

 or lateral compression, will appear from the following considera- 

 tions. In the first place, it is absolutely impossible to conceive, 

 that any force, of an intensity however vast, exerted in the direction 

 of a tangent to the earth's surface, could by itself shove a thick and 

 imperfectly flexible crust into a system of close alternate folds. 

 Beyond the imperceptible bulging of the whole tract laterally from 

 the line of application of the force, no flexure could arise, other, 

 perhaps, than some diminutive, but irregular plications, caused 

 by inequalities in the strata or crust, and these, it is needless to 

 remark, would be destitute of any law of parallelism and gi-ada- 

 tion, such as that which strikingly characterizes the Appalachian 

 and other regions. No system of narrow waves of the strata, how- 

 ever flat, could originate from the most enormous lateral pressure, 

 if vmaccompanied by some vertical oscillation, producing parallel 

 lines of easy flexure. Precisely such an alternate movement 

 would ensue, if a succession of actual leaves on the surface of the 

 subterranean lluid rock rolled in a given direction beneath the 

 bending crust. 



The inadequacy of the tangential or horizontal force, as a cause 

 of the Appalachian axes, is still further obvious, when we con- 

 sider, that no igneous rocks, of any sort, were thrust to the sur- 

 face, except in the belt of country bordering this broad system of 



