SIMULTANEOUS JOINTS 275 



volume nearly a third greater than before rupture. It follows 

 then that in a rock mass which has been jointed there must 

 be an increase of volume which cannot be without geological 

 importance. There are various ways in which this volume 

 might make itself manifest. If the material surrounding the 

 crushed mass does not yield, and if one surface of the crushed 

 mass coincides with the surface of the earth, then the area 

 affected by joints must rise regularly or irregularly above this 

 level. If on the other hand, the space subjected to crushing is 

 so placed that a vertical swelling is impossible, the masses sur- 

 rounding the crushed volume must be driven back and thus 

 either deformed or crushed. Now nearly all rocks are jointed, 

 and the total increment of volume over a large area affected 

 b}' joints is likely to find expression in technically important 

 faults or even in tectonic movements. 



Of course rock masses are usually supported on all sides, or 

 on all sides but one, by masses presenting great resistance 

 either to deformation or to rupture. This would not obviate 

 the tendency to the formation of joints on systems like those 

 represented in Figs, i, 2 and 3, but it might prevent, or par- 

 tially prevent, the realization of these ruptures. In such a case 

 w'hat would be the internal changes affecting the rock masses? 

 Evidently there would be relative movements along the lines on 

 which joints tended to form, but since no opportunity or an in- 

 sufficient opportunity was afforded for an increase of volume, 

 few joints or none at all would result. In the extreme case, the 

 result would be deformation w'ithout rupture. In so far, how- 

 ever, as the actual relative motion of the particles exceeds the 

 limit of elastic recovery, it is natural to suppose that cohesion 

 along these surfaces would be diminished and that the mass 

 would manifest this alteration of structure by splitting more 

 readily in these directions than in others bearing no relation to 

 them. It is to such a weakening along the surface of relative 

 motion that I have ascribed the origin of schistosity and slaty 

 cleavage, subjects which I have discussed at quite sufficient 

 length elsewhere. 



