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 dip 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. 
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