656 



T. T. QUIRKE BOUDINAGE^ AN UNUSUAL PHENOMENON 



sediments by lateral compression. After decrease of this lateral support 

 the arches tried to flatten down again. However, this can only come 

 about by lengthening of the boudin bed. This lengthening can be ac- 

 commodated easily by the arched beds, but the unbent beds must fail by 

 jointing in the necessity of keeping up with the lateral movement of the 

 arched layers above and below them. The result is just the same as if 

 the beds sprang back into their former shape by elastic recovery, to wit : 

 cross-fractures which are wide at the axis and closed at the peripheries 

 (figures 5 and 6). 



Formation of Boudinage 



In the formation of boudinage we suppose something like the follow- 

 ing course of events : first, the presence of very competent thin beds 



i i i 



ooooocoo 

 t t t 



i 



I I 



T t t 



Figure 5. — -Foiination of a Boudin 'by lat- 

 eral PrcftfiHie — a rotational Strain 



Figure 6. — Recovery, clastic or otherwise, 

 of a Boudin, after Release of lateral 

 Pressure 



inclosed in incompetent material; second, the concentration on these 

 beds of compression great enough to thicken notably the incompetent 

 members and to telescope the competent beds into boudins. Unless the 

 overburden were large and highlv competent, the boudin member would 

 simply arch upward. However, it arched l)oth upward aiul downward to 

 an equal degree. 



Peculiarities of Boudinage 



There are no ordinary structures or portions in ordinary folds where 

 these boudins can be formed. They are symmetrical about a vertical 

 axis and about a horizontal axis ; therefore they can not be formed on 

 the flanks of folds. The axis of a fold might give place to one boudin 

 in a bed, but not to a string of them. A series of drag folds might give 



