PECULIARITIES OF BOUDINAGE 659 



material by shear zones. Similarly, a giant bondin may come to an end 

 at considerable depth in contact with the deep zone of flow. 



If the central part of the boiidins is not compressed as much as the 

 outer arched beds, it is difficult to understand a great stratigraphic series 

 of boudin beds, such as Lohest postulates at Bastogne. Such a series 

 would represent alternating shortening and non-shortening of the in- 

 competent and competent beds respectively — an almost incredible per- 

 formance. 



Furthermore, a great boudin could have smaller boudins only in one 

 row, along its central axis, which is not the case at Bastogne. And every 

 boudin band would represent two great rotational strains swinging out 

 from one another on opposite sides of the boudin axis, where both would 

 be zero. 



Inversely, a series of parallel folds which die out above and below rep- 

 resent two great rotational strains which swing in toward one another to 

 a maximum at their union, starting from zero at distant points on oppo- 

 site sides of the plane of their combination. 



Although the possibility of ordinary parallel folds, so persistent that 

 they die out according to theory, is widely recognized, the writer knows 

 of no case where the phenomenon can be proved in the field. It is almost 

 certain that the most promising series of parallel folds grades into sim- 

 ilar folds at relatively shallow depth. In like manner it is probable that 

 the Bastogne uplift will ultimately be found to be much more compli- 

 cated than a simple giant boudin, although the stratigraphic series of 

 boudin beds almost proves that the quartzitic bands were shortened by 

 compression along their axes to a degree equal to their shortening by 

 arching, and the boudinage is not really the structural opposite of parallel 

 folding, but merely a very unusual type of shortening of a composite 

 member, constituent parts of which adjust themselves to the compression 

 by different mechanical methods. Nevertheless, the recognition of 

 boudinage as a possible type of major folding, as well as a proved type 

 of minor structure, may lead to happy results, and it may be helpful in 

 explaining how deeply buried folds of large curvature may come to an 

 end almost abruptly in contact with an incomplete zone of flow. 



The boudinage of Bastogne and vicinity provides a case of a rare geo- 

 logical phenomenon. There is no mention of this type of deformation 

 in any works in English, so far as the writer is aware, nor does it seem 

 to have had consideration from any but the geologists of Belgium. There 

 are so many geologists in America who are carrying on or planning to 

 undertake experiments which may throw light on the cause and mechan- 



