THE BOUDINS 



651 



The Coblencian rocks (Lower Devonian) are composed of quartzite 

 and schist, the quartzite being in beds of from about a foot to about eight 

 feet in thickness. The quartzite layers appear to have been compressed 

 in the direction of their bedding, with the result that they have been 

 thickened. The thickening has been distributed in such a way that the 

 beds have been separated into a number of parts distinct from one an- 

 other, the parts in cross-section looking like barrels, the ends of which 

 are separated from neighboring barrels by veins of quartz. 



In places where quarrying operations have removed the schists from 

 the quartzite, one is able to see that the barrel shapes extend into cylin- 

 ders which look like enormous sausages strung out side by side, from 



^Ka.1 



I T X" ■ ¥ " -T ' ■I^'I 1 I IF ? 



^ H ak. 1 e 



a« fe^ 



S h at 



Figure 2. — Icealized Bo idinaaen 



The figure a^ 6^ c, d shows the shape of the rock before it became a boudiu. a h and 

 c d show the maximum shortening. Shortening decreases to nothing at the axes, e e 

 indicate quartz veins. 



which appearance this type of deformation gets its nama of boudinage. 

 The quartz vein separating each boudin from its neighbor is quite char- 

 acteristic, and the width of spacing between veins is evidently a function 

 of the width of the bed. The boudins approximate a circle in cross- 

 section. In small beds the circle is small; in large beds the circle is large. 



The Quartz Veins 



The quartz veins do not penetrate the schist above and below these 

 bands of quartzite. They appear to be confined entirely to tlie beds of 

 quartzite. It is thought by Lohest,^ following Stainier, that these veins 



^ M. M. Lohest : Congres Geologicjue International Livret Guide. Excursion Ai!. p. 8. 



