SIMULTANEOUS JOINTS 273 



If permanent deformation of notable amount preceded rupture, 

 the single angle would be greater than 70° 32'. 



In the foregoing, it has been supposed that the joints are 

 mere cracks and that no measurable amount of motion occurs 

 on any of them. This is often approximately true in nature. 

 The throw of the faults produced on the joints is sometimes so 

 small as to be microscopic, and I have measured great numbers 

 of such dislocations which were expressible only in hundredths 

 of an inch. Nevertheless, it remains true that a joint does not 

 form except in obedience to a tendency to faulting. When a 

 block of any material is squeezed between a plunger and an 

 anvil, it does not crack until it can yield no further without 

 cracking. In other words, rupture takes place in order to 

 permit of a closer approach between plunger and anvil than is 

 consistent with the continuity of the block subjected to experi- 

 ment. These cracks undergo a certain throw in the very act 

 of forming. In order to perceive the nature of the dislocation 

 it is best to assume that it reaches a considerable amount. I 

 will suppose for example that the shortening of a ruptured 

 block is 10 per cent. Then the dislocation must be of the t3'pe 

 represented in Fig. 9, though a certain variety in the disposi- 

 tion of the residual fragments is evidently possible. Now, Fig. 

 9 shows several large faults, and the shortening evidently could 

 not have been achieved without these or equivalent dislocations. 



It is often assumed that when one fissure faults another the 

 latter is the older, but this inference is not justifiable and they 

 must often be of exactly the same age. Very frequently inter- 

 lacing quartz veins may be studied in which the quartz is con- 

 tinuous from one system of ruptures to the other, and in which 

 there is every indication that the ore was deposited at a single 

 epoch. Such instances show no slickensides within the veins, 

 but even when there are slickensides these may possibly be due 

 to fresh movements on the old surfaces after ore deposition is 

 finished. Of course I do not mean to deny that cases occur in 

 which some veins are younger than others with which they are 

 associated. I merely mean to w^arn colleagues against hasty 

 inferences in regard to the relative age of veins. 



If such a system of dislocations as is shown in Fig. 9 were to 



