7i8 WALTER H. BUCHER 



the plane normal to Od and 90° from Oi, represents the direction 

 in space of the tensile stress, and the line ml, in the vertical plane 

 Ikdm, gives the horizontal trend of this stress. 



This analysis leads to the following conclusions: 



The direction of Ok, of the tensile stress, differs only 3° from the 

 horizontal, as would be expected at the crest of an anticlinal bulge. 



The pull was slightly inclined downward in the direction 

 N33E. 



The very crest of the anticline, therefore, must be sought on 

 the left side of the exposure, a short distance to the southwest. 

 The differential movement which develops when strata slip past 

 each other in the process of folding was here directed toward the 

 crest and favored the development of the joints of set I which 

 are more numerous, more regular, and stronger than those of 

 set II. 



The direction N 33 E of the greatest tension suggests in a 

 general way the dip. and therewith also the strike, of the strongly 

 cross-bedded sandstone. 



2. Both, compressive and tensile stresses horizontal. — a) When 

 Daubree subjected to torsion narrow strips of glass, measuring 

 about a yard in length, and produced on them the well-known 

 system of intersecting fractures, he gave the science of geology 

 one of its most impressive laboratory experiments and one of its 

 most popular textbook illustrations on the subject of joints. 



Careful analysis, however, reveals the fact that the conjugate 

 systems of fractures which he produced, do not correspond directly 

 to similar joint systems in nature. Figure 5 is a sketch of the 

 fractures forming two prominent "fans" on one of Daubree's 

 plates.^ 



The tendency to form such "fans" is obvious in all torsion 

 experiments made with glass. Duparc and LeRoyer found that it 

 is the more pronounced, the thicker the glass plate is which is used 

 for the experiment.^ 



' The one in the center of the plate reproduced on Plate XII of Haug, Geologic, 

 Vol. I (Paris, 191 1) (opp. p. 228). 



2 L. Duparc and A. LeRoyer, Contributions a I'etude experimentale des diaclases 

 produites par torsion," Archives des Sciences phys. et nat., 3me ser. XXII (Geneve, 

 1889), p. 307. Daubree used plates 7 mm. thick; on plates of 2 mm. thickness or 

 less, "fans" may not form at all. 



