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 Witi1am 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 
