THE QUARTZ VEINS 



653 



the dipping beds, which indicates that this is on the flank of the anti- 

 cline, if we suppose the veins to follow ordinary fracture cleavage. Fur- 

 thermore, in this place the veins are larger near the top than near the 

 bottom of the boudins, and the arguments in regard to the central thick- 

 ness of quartz veins do not hold. 



Elastic and non-elastic Compression 



In general, the boudins appear to have been formed by lateral com- 

 pression under enormous loading, so that the terrane has been neither 

 arched nor crumpled, but the competent beds have been compressed 

 longitudinally into symmetrical folds of relatively small size. During 

 this deformation the schistose rock conformed to the more competent 



Shea.1 



y^ 









Figure 4. — Boudinage, Bastogne, Belgium 

 Traced from photograph used In figure 8. Transverse lines are quartz veins. 



bands of quartzite, which arched both upward and downward into the 

 less resistant ^^hyllites. The natural conclusion is that after compression 

 the quartzite yielded to tension, giving rise to cracks which were dis- 

 tributed equally throughout. The yielding to tension was more closely 

 spaced in the thin beds than in the thick beds. 



The actual openings appear to have been wider near the axes of the 

 beds than near the edges. The edges of the boudins were folded the 

 most. Apparently, the folded parts were straightened enough to take up 

 the slack, but the central parts, being unshortened originally, had to 

 supply means of expansion, and therefore cracked open instead. The 

 central part might have been compressed during the arching of the other 

 beds, and therefore had a certain amount of elastic recovery. 



