ELASTIC RECOVERY OF FOLDED ROCKS 655 



per yard, or 6.11 per cent. This is about equivalent to the size of the 

 quartz veins between the boudins. 



Elastic Recovery of folded Rocks 



If we suppose, however, that the Coblencian rocks were so strong at 

 the time of their deformation that the lateral forces were incompetent 

 to cause the compression postulated above, we must discard the idea of 

 change of volume of the axial material under compression. This does 

 not eliminate, however, the possible effectiveness of the elasticity of the 

 bent quartzite layers above and below the axial part of the boudins. On 

 release of lateral pressure these arched layers would tend to spring back 

 into their former shapes unless their deformation passed the elastic limit. 

 The elastic limit of rocks is different for the tensional and for compres- 

 sional strains, but the tensional elastic limit is the lower; consequently 

 passing of the elastic limit will result in part failure of the arches by 

 tensional cracks near the periphery. These cracks would be wide at the 

 periphery and would decrease to zero at the axis. Veins of this type do 

 not appear to prevail in the Carriere de la Citadelle, where the best 

 observations are possible, although Lohest describes them as of that 

 character. 



Surely, some of the veins are not wider near the periphery than at the 

 axis. Most of them appear to be widest at the axis and to pinch out to 

 nothing, both above and below, at the periphery. This latter type ap- 

 pears to represent the cases of quartzite beds which were so deeply buried 

 that under lateral compression they bent without rupture and without 

 passing their elastic limit. On relief of lateral compression they tended 

 to recover their former shape, thus increasing their geographical length 

 by straightening their curves. 



Lengthening of the outer beds would make openings necessary in the 

 unfolded material within. Thus, the width of the opening is inversely 

 proportional to the longitudinal recovery. That which was bent the 

 most could recover the most, and that which was unbent could rebound 

 not at all. By the elastic straightening of the bowed beds, the unbent 

 axial beds were pulled apart. The location of these planes of rupture 

 would naturally fall between the boudins, where the opposing spring of 

 neighboring arches would be concentrated. Large boudins would cause 

 wide veins ; little boudins would cause narrow veins. Such is the case. 



Non-elastic Recovery of folded Rocks 



In cases where the arches passed the elastic limit another explanation 

 is necessary. The arches were upheld against the weight of overlying 



