SIMULTANEOUS JOINTS 269 



act, that is to say, on the top and the bottom of the cube, the 

 traces of the joints will be parallel straight lines perpendicular 

 to A. By these means the cube will be divided into a number 

 of square prisms so placed that the diagonals of the squares are 

 either horizontal or vertical. 



It has come to be pretty generally recognized that two sys- 

 tems of joints such as those described may be produced by a 

 single force acting at an angle of about 45° to each system. If 

 the deformation antecedent to rupture were of sensible amount, 

 the joints would make angles of somewhat more than 45° with 

 the line of force. 



It is not so generally understood that 4 or even more than 4 

 systems of joints may be due to a single force. This case is less 

 common than that of a smaller number of partings, and is usually 

 confined to limited areas, but it is not infrequent, particularly in 

 the disturbed regions which ores so much affect. 



Imagine a second cube, shown in Fig. 2, similar in all re- 

 spects to the first excepting that the faces J5 and its opposite are 

 supported instead of faces A and that opposite to it ; then of 

 course the result will be the formation of prisms whose square 

 cross sections will be visible on B instead of on A. If on the 

 other hand the cube is not supported on any side, or if the resist- 

 ance perpendicularly to the line of force is uniform, then these 

 2 systems of rupture will take place simultaneously, so that on 

 both A and B there will be systems of cracks at 45° to the line 

 of force intersecting one another at 90°, while each of these 

 faces will also show horizontal cracks. By these means, the 

 cube will be divided into octahedral and tetrahedral blocks as 

 indicated in Figs. 3 and 3«. Such rupturing can be and has 

 been experimentally verified, for instance by Daubree ; but I 

 know of no experiments so perfect as instances which may be 

 observed occasionally in rock exposures. 



In experiments on cylinders, the lines of rupture are often 

 found to be conically disposed, and this mode of rupture requires 

 explanation, especially as corresponding phenomena are so rare 

 in nature that I have never met with them. When a cylinder is 

 linearly compressed (say vertically) between masses of much 

 more rigid material, the cylindrical form is not preserved, the 



