266 



EDWARD STEIDTMAN 



The massive quartzite beds and the massive igneous rocks were 

 deformed largely under conditions of fracture, excepting at the con- 

 tact of the two formations where some of the deformation was accom- 

 plished by flow. Those rocks which were deformed under conditions 

 of fracture ruptured as soon as the stresses exceeded the ultimate 

 strength of the rocks. According to the best mechanical analysis, 

 rupture which results from compression takes place along the planes 

 of no distortion or constant area, since these are the planes of maxi- 

 mum tangential stress. In Figs. 3 and 4 they are represented by XF 

 and LM. It is evident that LM is equivalent to the bedding joints, 



X f 



North 



Fig. 3. — abed represents A of Fig. 2; e/g/i .represents the parallelogram of strain 

 resulting from deformation; the circle inscribed in A has been deformed into an ellipse. 

 Jf F and LM are the planes of no distortion or the planes of maximum tangential stress, 

 and of rupture. 



and XF is the vertical, or north-dipping strike fracture, diagonal to 

 the bedding of the South Range, or the nearly horizontal strike frac- 

 ture of the North Range, where the beds are steeply inclined. The 

 strike fractures constitute a series of overthrust faults of minute throw, 

 which approximately bisect the angle between the longest axis of 

 the strain ellipsoid and a normal to the bed. The initial angle between 

 the longest axis of the strain ellipsoid and a normal to the bed is 45 

 degrees. The rotation of the longest axis of the strain ellipsoid 

 beyond the initial 45-degree position depends upon the amount of 

 internal flow coincident in direction with the movement between the 

 beds. Since more internal flow takes place in weak beds than in strong 

 beds other conditions being the same, the strain ellipsoids are rotated 



