336 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



mountain-building which were rather severely strained by the previous 

 much smaller figures. Consideration of such matters will be taken up 

 on later pages. 



Inferences 

 direction of the deformixg force 



From the foregoing analysis of the Appalachians it is clear, as has 

 always been understood, that the narrowing of the folded belt was in the 

 northwest-southeast direction. While it has been generally accepted that 

 the portion of the crust which moved actually went in a northwesterly 

 direction, there is by no means agreement on this point. It is, of course, 

 agreed that the mass southeast of the Appalachian Valley moved north- 

 westward relatively to the little folded area of the plateaus. It is urged, 

 however, by some geologists that while this is true in a relative way it is 

 because the great mass of the crust underlying the plateaus moved south- 

 eastward and under the closely folded area, pushing up the folds. 



From the standpoint of physics, there is no question but that when 

 there is shortening under strain the part which moves rides outward and 

 upward on the part which is passive. In other words, relief from 

 pressure is not inward toward the center of the mass but outward toward 

 its exterior. It is for this very reason that folded mountain ranges stand 

 high. That this is not only theoretically true but actually so, all experi- 

 ments w^hich have been performed to simulate mountain folding give 

 perfectly clear evidence. As early as 1860 the English geologists Hall 

 and Lyell made experiments wdiich showed this. In a more elaborate 

 way Willis in 1887 experimented with models intended to copy Appa- 

 lachian conditions rather closely. In each of Willis' models anticlines 

 rose, lifted a heavy load, then overturned and produced faults, but in no 

 case were synclines deepened. The experiments performed by Adams 

 in 1905 produced similar results. The cylinders of marble which he 

 compressed endwise within steel jackets expanded outward on planes of 

 shear, forming around each cylinder a bulge which was in reality an 

 anticline. 



The relations which were deduced from theory and corroborated by 

 experiments are readily to be seen in the finished facts of nature. The 

 principle is, as has already been noted, that the folds and thrusts are 

 largely supported and controlled by competent or rigid beds. Thousands 

 of instances of this are observable on every possible scale. It can not be 

 gainsaid that a rigid bed will thrust itself into weak, pliable strata ; but 

 how can the reverse be done? One instance may be given which makes 

 the point clear. In northeastern Tennessee and adjoining parts of North 



