404 B. WILLIS — A THEORY OF CONTINENTAL STRUCTURE 



able eastern section, which sanl<: to the depth of the Atlantic basin, and 

 the strata that had accumulated beyond its western shore in the mediter- 

 ranean sea were folded up to a notable mountain range which stood upon 

 the negative elements. Subsequently the whole eastern portion of North 

 America was reduced to a peneplain, and in relatively modern times has 

 suffered that broad doming with subordinate corrugation which consti- 

 tutes the -existing Appalachian mountains. The early effects that we 

 recognize as the Appalachian revolution and those which have followed 

 may be credited, I think, to the tangential thrust which is exerted by the 

 sub-Atlantic mass on the elements of the Appalachian province, both 

 positive and negative, whereas the conditions which prevailed prior to the 

 Appalachian revolution were peculiar to continental elements that were 

 not under dominant tangential stress. 



The distinction might be explained by the periodicity of tangential 

 movement, if we could assume that no notable thrust had been exerted 

 by the sub-Atlantic mass before the close of the Paleozoic; but, while it 

 is true that there were long intervals of inactivity, there is, for instance, 

 the Taconic disturbance, which affected New England and not Pennsyl- 

 vania. Special developments of mechanical relations appear to afford 

 a better explanation of this and other obscure changes of condition. 

 Thus, in explanation of the supposed effectiveness of a sub-Atlantic 

 thrust upon the eastern United States at the present time, let us consider 

 the larger movements of the Appalachian revolution and their mechanical 

 effects. It is well established that the folding of the Paleozoic strata in 

 the Appalachian zone corresponds to a narrowing of the zone by 35 miles 

 or more — that is, the Blue ridge approached the Cumberland plateau 

 from a distance of 100 miles to within 65 miles. The general effect may 

 best be described as a composite overthrust from southeast toward north- 

 west. Keith's recent investigations* show that overthrusts of equal or 

 greater displacement traverse the gneisses of the Smoky mountains. 

 Hence it is a moderate statement to say that during the Appalachian 

 revolution that portion of the continent southeast of the Cumberland 

 Plateau rim moved northwestward at least 50 miles. If this movement 

 be conceived as affecting a superficial film, the meclianical effects need 

 not be supposed to modify the continental structure, but the phenomena, 

 such as the dip of the thrust planes, for instance, indicate a deeper 

 origin, and it appears reasonable to regard this displacement as one 

 which involves the subcontinental and suboceanic masses down to the 



* Arthur Keith : Folded faults of the southern Appalachians. Compte Rendu de la 

 ix session du Congres Int. a Vienne, vol. 2, 1903, p. 541 ; also Atlas U. S. Geol. Survey, 

 Eoan Mountain and other folios. 



