PECULIARITIES OF BOUDINAGE 657 



rise to something like them, if cross-veins should separate them similarly. 

 But drag folds and boudins differ essentially in their axial parts. A 

 drag fold is folded from top to bottom; a boudin is unfolded along its 

 axis. Drag folds affect incompetent layers between competent beds; 

 boudins occupy competent beds between incompetent layers. Drag folds 

 are due to shear; boudins appear to be due to extreme rotational strain 

 symmetrical about a horizontal axial plane (figures 7 and 8). 



The boudins look somewhat like the structures called phacoidal struc- 

 tures by the British geologists. They report the case of an epidiorite 

 dike wrenched into a series of isolated lenticles or "phacoiclaF' masses 

 imbedded in a zone of reconstructed granulitic gneiss. The case is cited 



Figure 7. — BoiKlin-lUce Features due to Drag-folding and Cross-fractures 



of a thin basic dike, disrupted and severed into seven detached lenticles 

 of hornblende-schist, from 30 to 130 yards in length, all arranged parallel 

 to the thrust and to the foliation planes of the reconstructed micaceous 

 gneiss by which they are surrounded.^ Similarly pegmatites intruding 

 hornblende-schists and biotite-gneisses have been broken and sheared into 

 the same sort of forms.® Very small lenticles of from 3 to 6 inches long 

 have resulted from the thrust-plane passing above a limestone formation 

 there brecciated for a depth of 15 inches beneath the fault-plane,'^ and 

 Leith^ cites the case of a porph3Ty of the Vermilion district of Minne- 

 sota which, having been sheared into rhombs, was further metamorphosed 

 into a pseudo-conglomerate containing elongated lenses like pebbles. 



All such structures are devoid of definite bedding which has any 

 general relationship to the shape of the mass, whereas each boudin is 



5 B. N. Beach, John Horne, and others : Geological Structure of the Northwest High- 

 lands of Scotland, 1907. Memoirs of the Geol. Survej^ of Great Britain, p. 67. 

 «Idem, p. 263. 

 ^ Idem, p. 504. 

 « C. K. Leith : Structural Geologj', 1913, p. 66. 



