JOINTAGE AND THE STRAIN ELLIPSOID 675 



The danger of thus misinterpreting joints is obviated only if 

 the student disregards the customary sketches and remembers that 

 the acute bisectrix may or may not be the major axis of the elhpse; 

 that the angle between bedding joints and inclined compression 

 joints facing the direction of movement may be acute, or obtuse, 

 or 90°, depending upon the brittleness of the rock at the time of 

 the development of the joints. In the case under study it is the ob- 

 tuse angle that faces the direction of movement. 



The importance of these considerations has recently been re- 

 emphasized by Bucher, calling attention to the well-known experi- 

 mental results embodied in Hartmann's Law that "in brittle 

 materials, the acute angle formed by the shearing planes is bisected 

 by the axis of maximum compression, and the obtuse angle by the 

 axis of minimum compression which is generally negative, represent- 

 ing tension" in agreement with the mathematical results of Mohr.' 

 It is also shown, from the consideration of Karman's experiments, 

 that the angle between the shearing planes becomes less and less 

 acute with decrease in brittleness, the usual models and diagrams 

 giving expression "only to that case where the planes of shearing 

 form an obtuse angle in the direction of compressive stress."^ 



The exposures here discussed are, then, a field illustration of the 

 case where the acute angle lies in the direction of compressive stress, 

 which, as suggested by Bucher, is the characteristic angle for brittle 

 materials. The occurrence represents Bucher's third case of the 

 application of Hartmann's Law, where the greatest compression is 

 approximately horizontal. ^ It is evidently similar in many respects 

 to the case of the symmetrical shearing plane faults in the limestone 

 of the Hamilton Shale, referred to by Bucher and photographed and 

 described by Sheldon.*^ Here likewise the acute bisectrix is the 

 axis of maximum compression. 



'Walter H. Bucher. "The Mechanical Interpretation of Joints," Jour. Geol., 

 Vol. XXVIII (1920), p. 712. 



= Ibid., Vol. XXIX, pp. 13-14. 



3 Ibid., Vol. XXVIII, p. 727. 



^Ibid., p. 728. Also Pearl Sheldon, "Some Observations and Experiments on 

 Joint Planes," Jour. Geo!., Vol. XX (1912), pp. 60-61. 



