SECONDARY STRUCTURES OF BARABOO QUARTZITE 267 



more in weak beds than in strong beds, and hence the diagonal 

 strike fractures are more nearly parallel to the bedding in weak 

 beds than in strong beds. This principle accounts for they^shaped 

 fractures in the shaly layers at Devil's Lake, previously described. 

 Near the upper and the lower surfaces of these weak beds, where inter- 

 nal adjustment was greatest, these fractures are more nearly parallel 

 to the bedding than in the center of the beds, where internal adjust- 

 ment was least. The stretching of the beds parallel to the longest 



Fig. 4. — abed represents B of Fig. 2 ; efgh represents the parallelogram of strain 

 resulting from the deformation of abed. XY and LM are the intersections of the 

 circle inscribed in abed and the strain ellipse resulting from the deformation of the 

 circle. XY and LM are the directions of maximum tangential stress, and of rupture. 



axis of the strain ellipsoid causes tension fractures to develop normal 

 to the direction of maximum elongation. See diagonal strike frac- 

 tures which intersect the /-shaped strike fractures of Fig. i. 



The development of bedding and diagonal strike shearing joints 

 by the folding on the north limb of the Baraboo synchne is illustrated 

 by Chamberlin and Sahsbury in Geology, Vol. I, p. 445, Fig. 367. 

 The bed on the left of Fig. 367 is a weak, shaly bed stratigraphically 

 beneath the stronger quartzite bed to the right. In the weak bed 

 the diagonal strike fractures are closely spaced, while both bedding 

 and nearly horizontal, diagonal strike fractures, rather widely spaced, 



