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



those districts where the prevailing inclinations are steep, and where they are 

 directed to opposite points, it will be found invariably that the inclined masses 

 are but the parts of successive arches, or rather waves, the denuded or broken 

 crests of which approach each other the closer as the dips are steeper. 



Parallelism of the Crust Undulations. 



It is, therefore, another general fact regarding disturbed zones of the crust, 

 that where the displacement from horizontality has been great, the strata are 

 arranged in longitudinal tracts, or great belts of parallel waves. These, where their 

 symmetry of structure is not marred by dislocations of the crust, or hid by over- 

 lapping superficial deposits, exhibit a remarkable and beautiful resemblance 

 to those great and continuous billows which are called by seamen rollers, and by 

 mechanicians waves of translation. Far more continuous in their crests, more 

 strictly parallel, and more symmetrical in form than the wind-produced waves 

 upon the waters of the globe, such great swells or rolling billows, engendered by 

 wholly different forces, are, I conceive, the true archetypes of the undulations 

 visible in the more corrugated portions of the earth's crust. Perhaps in no 

 uplifted district of the surface are these crust-waves so symmetrically deve- 

 loped, or so readily recognised, as in the Appalachian Mountains of the United 

 States. It was there that William B. Rogers and myself, analyzing their forms, 

 and tracing and connecting their axes, detected those phenomena of shape and 

 gradation which led us to the general laws of crust flexures which we have ven- 

 tured to publish. 



But we believe that all mountain zones, and all corrugated districts gene- 

 rally, which have been elevated, like the Appalachians, at one epoch, and by 

 crust movements observing only one prevailing direction, will be found to pos- 

 sess this wave-like structure, under similar conditions of gradation, and in a like 

 conspicuous manner. It is only those tracts which have been revisited several 

 times by the elevating and undulating forces, and especially those where the 

 successive disturbances have not coincided in direction, but have crossed each 

 other, causing interference and intersection of the waves, as in what is called 

 a chopped sea, — such districts, for example, as the Swiss Alps and the mountains 

 of Cumberland and Wales, — that we fail readily to discern the wave structure of 

 the strata, or, perceiving it in part, are unable, without extreme toil and patience, 

 to connect the originally related outcrops of the rocks, and reconstruct in our 

 minds, and represent to the eye, the undulations that actually exist in a broken 

 and disguised condition. 



Wherever we have been led, either from observations in the field, or from a 

 careful perusal of the descriptions of geologists, to a clear recognition of the dip- 

 structure of any corrugated zone, whether mountain chain or otherwise, not con- 

 fused by different systems of elevatory movements of the crust, we have become 



