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



the curves become, in many instances, broad, depressed, and al- 

 most symmetrical in form. 



From the descriptions here given of the structure of the 

 Appalachian chain and other disturbed districts, it is obviously 

 a general law, that the axis planes of the flexures are not only 

 inclined all in one prevailing direction, though at different angles, 

 but that they dip invariably towards the quarter or zone of maxi- 

 mum disturbance and rupture of the crust. ^ « 



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Fractures or Faults in Tracts of Undulated Strata. 



Two classes of dislocations abound in all zones of plicated and 

 undulated strata, where the crust waves exhibit much steepness, 

 and especially where they have the inverted or folded form. By 

 far the most numerous, though the shortest and least conspicuous 

 class, are the breaks or faults which run approximately trans- 

 verse to the strike of the anticlinal and synclinal axes. These 

 may be extensively recognized in the Appalachians, where they 

 are a primary cause of the deep ravines, or breaches through the 

 ridges, which furnish passage to nearly all the rivers, and even 

 lesser streams which drain this chain. Such ravines are especially - 

 frequent near the extremities of the large anticlinal waves, par- i 

 ticularly where they have been cut through along their crests 

 by denuding waters, and have given rise to valleys of elevation 

 and erosion, inclosed by monoclinal, outward-dipping, sandstone 

 ridges. It would seem as if the elliptical folding round of the 

 strata towards the ends of the great denuded waves had caused 

 the horizontal wrenching which resulted in these fractures. Mr 

 William Hopkins, of Cambridge, has, in an able paper on the 

 subject of dislocations affecting dome-shaped elevations of the 

 earth's crust, indicated the true source, I conceive, of the double 

 system of fractures to be met with in all elliptical anticlinal belts. 

 An elongated anticlinal wave is, in truth, only a greatly lengthened 

 elliptical dome, in which the radial cracks caused by a maximum 

 tension in the strata transmitted from the more central portion 

 of the crust-wave, are distributed, some of them longitudinally, 

 others transversely, as respects the anticlinal axis, the transverse 

 ones multiplying themselves where the elliptical strain has been 

 greatest, towards the two extremities of the waves. 



The other far more conspicuous class of dislocations connected 

 with these crust undulations, are the great longitudinal ones. 

 These are of frequent occurrence in the more contorted portions of the Appala- 



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