JOINT AGE AND THE STRAIN ELLIPSOID 673 



inches. A small subsidiary block on the extreme right, having 

 evidently acted somewhat as a unit, is seen to be in turn crossed 

 by short joints of the two inclined sets. 



RELATIVE AGE OF THE JOINTS 



It is evident from the facts related above that the fractures of 

 the three systems were not all developed at the same time. Those 

 of the steejD set are clearly the oldest. They constituted favorable 

 planes for the injection of the aplitic magma. Still later they were 

 broken across by the development of the joints of the inclined sets, 

 and they were displaced upward and westward along the planes 

 of the east-dipping set, a set which became a series of minute thrusts, 

 outward from the batholithic mass. 



APPLICATION OF THE STRAIN ELLIPSOID 



There would appear to be good reason to accept the two inter- 

 secting sets of inclined joints as an illustration of failure by fracture 

 under lateral compression. It is obvious that, as the result of the 

 summation of the minute thrusts, the rock mass has been elongated 

 upward, in the direction of easiest relief, with shortening in approxi- 

 mately the horizontal direction. 



Assuming that the two inclined sets of joints are closely related 

 in origin we may apply the strain ellipse in the customary manner 

 to the plane of the outcrops of the photographs. It is obvious that 

 the major axis, representing the direction of elongation of the rock 

 mass, should be in approximately the vertical direction. The 

 planes of maximum shear are evidently represented by the two 

 inclined sets of joint planes, as in the adjoining sketch (Fig. 4). 

 Here, then, the major axis of the ellipse is the bisectrix of the 

 obtuse angle made by the traces of the shear joints in the plane of 

 the outcrop. 



The fact that we are here left in no doubt as to the cor- 

 rect orientation of the elHpse makes the analysis of the joints of 

 these outcrops especially desirable. Exactly similar jointage is 

 often encountered in folded sedimentary rocks. In the interpreta- 

 tion of concealed structure from the study of such joints it is often 

 difficult to determine what is the correct position of the ellipse. 



