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gical period, but* in one brief interval of time, alters its direction to 

 coincide successively with sundry of the different assumed systems of 

 crust elevation. 



Gradations in Flexures. 



Every broad belt of undulated strata exhibits certain gradations 

 in the form of its flexures starting from the side of maximum 

 igneous action, as this is displayed in plutonic eruptions, or in dislo- 

 cations and metamorphism. Crossing the zone, the flexures first 

 met with are invariably of the closely plicated class, their axis planes 

 dipping often at a low angle towards the igneous border. To these 

 succeed more and more open waves, until, from being perpendicular, 

 the steep far sides of the undulations become flatter and flatter in 

 their dips, till at last they assume a slope equal and symmetrical with 

 those of the gentler flanks. Parallel with this gradation is a 

 progressive widening of the waves themselves, and a corresponding 

 sinking or flattening down of the summits, until they finally disappear 

 in imperceptible undulations. All these phenomena of gradation 

 may be clearly discerned in every section across the Appalachian 

 chain, traced from the S.E. towards the N.W., and a perfectly iden- 

 tical structure will be found to exist in the great plicated belt ranging 

 through the Rhenish Provinces and Belgium. In truth, there is no 

 great corrugated zone that does not display a similar law of grada- 

 tion in its flexures, when these are properly traced and generalized. 



Fractures in Undulated Zones. 



Two classes of dislocations abound in all belts of the crust where 

 the strata are greatly undulated. The least conspicuous, but most 

 numerous are comparatively short faults, transverse more or less 

 perpendicularly to the strike of the anticlinal and synclinal axes. 

 These abound in the Appalachians and other corrugated mountain 

 chains, and are a principal cause of the deep transvere ravines and 

 mountain notches which intersect their ridges, and give passage to 

 their streams. The more obvious dislocations are the great longi- 

 tudinal ones coincident either with the anticlinal and synclinal 

 axis planes, or with the steep or inverted sides of the anticlinals. 

 A distinctive character of these great fractures is their parallelism to 



