274 BECKER 



be produced under any considerable external pressure, it is 

 manifest that the several residual fragments might be pressed 

 against one another with ver}- great force. In such a case the 

 mere grinding action accompanying the dislocation would tend 

 to produce further ruptures in the residual fragments. It is not 

 easy to work out a satisfactor}* theory of the distribution of such 

 secondary fractures. It is fairly evident, however, that in an 

 extensive complex, of which Fig. 9, PI. XII, may represent a 

 small portion, there is likely to be a repetition of identical con- 

 ditions, so that many separate blocks will be similarly situated 

 with reference to their neighbors. If secondary rupture takes 

 place, such blocks will be similarly affected and their fissures 

 will be parallel, but probably not continuous throughout the 

 mass. The more numerous the groups of similarly oriented 

 blocks after the original jointing, the more numerous will be 

 the systems of blind secondary joints. These latter may in- 

 deed be regarded as subsequent to the original joints, yet the 

 difference in age may be only a second or two and the brevity 

 of the interval should be taken into account in reading the his- 

 tory of the district. 



To me it appears questionable whether in a region once 

 jointed by a system of forces, the application of a new system 

 of forces could produce a fresh set of joints systematically ar- 

 ranged. The resistance of a jointed rock mass is so extremely 

 unequal in different directions, and so small in manv of them, 

 that fresh movements on the old joints or the reduction of the 

 formation to a chaotic rubble seems more probable than any- 

 thing comparable with renewed systematic jointing. Thus 

 forces acting on a brick wall usually produce cracks which fol- 

 low the joints between bricks, and if bricks were not designedly 

 laid so as to "break joints," and carefully cemented besides, 

 cracked bricks in dama<red walls would be still rarer than 

 they are. 



Fig. 9 shows that a cube fractured by pressure must occupy 

 a larger volume than it did before fracture. Following the in- 

 dications of Fig. 9 the lateral expansion would amount to over 

 two-tenths when the vertical diminution of height is one-tenth, 

 so that the crushed cube under these conditions would occup}- a 



