EDWARD STEIDTMAN 



are visible in the stronger bed. Tlie movement of the stronger 

 bed has been upward with reference to the weaker bed. 



The preceding analysis seems to account for all the fractures 

 which show by their strike, dip, and other relations that they are 

 related to the folding of the range. It is to be noted that these 

 fractures are the result of compression rather than tension. Theo- 

 retically, it is possible that tension may have caused a set of vertical, 

 gaping strike joints especially on the crest and trough of the fold. 

 It is difficult to determine what part of the strike and dip joints are 

 of this origin, but most of this class have been connected by observa- 

 tion with the compression joints produced by the folding of the 



range, and thus far no 

 definite evidence of tension 

 joints related to the folding 

 has been found. 



Wherever deformation 

 was accompHshed largely by 

 f]ow, cleavage resulted. 

 Cleavage is the capacity of 

 a rock to part along parallel 

 surfaces which are deter- 

 mined by the orientation 

 of the mineral particles 

 parallel to the longer axis 

 of the strain ellipsoid. Fig. 5 shows the strain elhpsoid and the 

 direction of cleavage parallel to the longest axis AB oi the strain 

 ellipsoid as compared with the directions of maximum tangential 

 stress or rupture XY and LM. The degree of rotation of the 

 strain ellipsoid, and hence the inclination of the cleavage with respect 

 to the bedding, depends upon the amount of flow. On the North 

 Range, at the contact of the igneous rocks and the quartzite, the 

 plane of cleavage is nearly parallel to the bedding of the quartzite. 

 Farther to the north in the igneous rocks where flowage was less 

 intense, the dip of the cleavage is diagonal to the inclination of the 

 quartzite beds. The cleavage, in the granite porphyry on the South 

 Range at Devil's Nose, is inclined to the north at a steeper angle 

 than the overlying quartzite beds. 



Fig. 5. — AB, the longest axis of the strain 

 ellipsoid, is the direction of cleavage in a rock 

 deformed by flow. XY and LM are the direc- 

 tions of rupture in a rock deformed under con- 

 ditions of fracture. 



