658 



T. T. QUIRKE BOUDIXAGE- AX UXUSUAL PHEXOMEXOX 



composed of symmetrical arches, grading from those of a high cun-atiire 

 at the periphery to those of no curvature along the axial plane. 



Furthermore, the j^hacoidal structures are commonly surrounded by 

 the envelojDing layers of schistose rock wrapped about them. The boudins 

 of a bed are side by side, unseparated except by quartz veins ; never sepa- 



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Cor>^pQ,-1*er5t". 



Figure 8. — Boudins 



rated by the schistose rock above and below them (figure 9). "We must 

 conclude that boudins differ from all other folds and structural features. 

 Boudins must come as a result of some unusual type of deformation. 

 Considering a series of boudin beds, we seem to recognize great shorten- 

 ing in the incompetent layers and apparently no shortening along the 

 axes of the boudin beds. This shortening of the incompetent beds must 



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^ch (5"l~os.e. 



Figure 9. — Phacoidal Structure (on left) and Boudins hounded hy schistose Rock 



(on right) 



The phacoidal structure is due to rupture of competent layers and abrasion of pieces 

 by flow of schistose matrix. The boudins bounded by schistose rock are not broken nor 

 rounded by movement of the matrix. 



result in thickening and uplift, and this uplift may approximate in shape 

 that of a great boudin. Lohest maintains that the whole uplifted area 

 of Bastogne is a giant boudin. If so, boudinage may offer an explanation 

 of how some folds die out in depth, and how folds which increase in size 

 with depth may pass out of existence at greater depth. In every case the 

 boudins are separated from the overlying and underlying incompetent 



