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



east, or towards the region of dislocation and metamorphism, present a much 

 less regular incurvation along their anticlinals and synclinals, and a far greater 

 amount of interference and of dislocation. These appear indeed to be the sec- 

 tions of the chain, where the greatest amount of tangential wrenching, rupturing, 

 and warping of the crust has taken place, and where the greatest amount of 

 transverse hitching and fracturing has happened to all the strata. The causes of 

 this difference will, I think, be seen, when I shall have developed our theory of 

 the mechanical forces which undulated the Appalachian strata, and set in mo- 

 tion the stupendous billows of the crust, which resulted in the elevation of 

 these mountains. An inspection of the best maps and sections of the more dis- 

 turbed European zones, leads me to believe, that a similar contrast prevails be- 

 tween the curvilinear waves which are convex to the districts of disruption, 

 whence I suppose them to have proceeded, and those which are concave to the 

 same quarters ; but before this law in all its generality can be established, geolo- 

 gists must institute far more critical researches into the physical structure of those 

 undulated and plicated districts than they have hitherto conducted. 



Gradations in Flexures. 



Succession from the Folded to the Symmetrical Waves. 



Several phenomena of gradation will be found to display themselves when we 

 cross any broad belt of plicated and undulated strata. Starting from the side of 

 maximum disturbance and contortion, invariably the quarter of maximum igneous 

 action, — displayed either in Plutonic eruptions through the crust, in crust disloca- 

 tions, or in metamorphism of the sedimentary rocks, — the flexures first met with 

 are of the obliquely plicated form. Advancing towards the middle of the zone, 

 the folds become obviously less close, and proceeding still farther, they gradually 

 open out, displaying more conspicuously their anticlinal and synclinal curves, 

 until the inverted side of each wave becomes only perpendicular. This perpen- 

 dicular altitude of the steep side soon becomes a clip towards the opposite quarter 

 from that previously observed by both sides, and as we proceed, the steepness of 

 the slope of the wave now rectified in position, grows progressively less and 

 less, until on the far side of the zone, both slopes approximate to equality. 



Expansion of the Waves as they pass from the Folded to the Symmetrical Form. 

 Concurrently with this gradation, there is a progressive opening out of the 

 spaces between the crests of the successive waves, such indeed as to amount in 

 the Appalachians, and sundry other broad regions of crust-undulation, to an en- 

 largement by many times of the amplitude of the more compressed class of 

 flexures. 



