674 



ROY R. MORSE 



Unless some good indication of the direction of movement of the 

 beds in folding is found one may be left in doubt as to the 

 interpretation. 



In the present case slipping has obviously occurred on one of 

 the sets, up and to the left. Let us suppose that, instead of grano- 

 diorite, these outcrops consisted of massive jointed sandstone or 

 quartzite, in the vicinity of an axis of folding, and that the east- 

 dipping joints were bedding joints. Since the movement on these 

 has been upward and to the left the customary inference would be 

 that the outcrops belong to the east (right) limb of a normal anti- 



FiG. 4. — Position of the strain ellipse when applied to the outcrop shown in Figure 

 2. The acute angle faces the direction of shortening. This does not illustrate a simple 

 distortion, as there is an increase in volume. 



cline. In this supposition the second set of joints crosses the bed- 

 ding joints obliquely, as a set of inclined compression strike joints 

 facing the direction of movement with an obtuse angle. 



Now, if the direction of movement on the bedding joints were 

 not known, the student might be tempted to prophesy the position 

 of the anticlinal axis upon the erroneous assumption that the acute 

 angle between inclined joints of this character points in the direc- 

 tion of movement, for this is indeed the inference one gets from the 

 customary diagrams of texts showing the strain ellipse applied to 

 joints. If such an application be made to the case here considered 

 we reach the surprisingly erroneous conclusion that the outcrops 

 belong to the right limb of an overturned syncline. 



