36 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



For let YV be parallel to the direction of one system, XX' (meeting 

 the fissure A'B') to that of the other. The opening of A'B' will relax 



the tension along XX', while that along VV will not be affected. 

 Consequently the ratio of the tensions at Q will not be the same as 

 originally, when AB, A'B' began to be formed. The direction of pro- 

 pagation of the former will evidently deviate towards perpendicularity 

 with YV, and that of the latter in the same manner more nearly to 

 perpendicularity with XX'. They will not therefore in such case pre- 

 serve their parallelism. 



A finite time, however, will be necessary to produce the relaxation 

 at Q, after the opening of A'B", and therefore if the distance between 

 the fissures be not too small, and the velocity of propagation very great, 

 as we have shewn it may be (Art. 13) AB may be propagated through 

 Q before the relaxation is produced there, and the fissures might under 

 such circumstances preserve, at least approximately, their parallelism. 



32. It is evident, however, that in whatever manner a system of 

 parallel fissures may be produced, that, after their formation, the only 

 tension of the mass between them must be in a direction parallel to 

 them. Consequently, should any other system he suhsequeiiUy' formed, 

 if must necessarily be in a direction perpendicular to that of the first 

 system. No two systems of parallel fissures, not perpendicular to each 

 other, could be formed by causes similar to those of which we have been 

 investigating the effects. It will appear also, as in Art. 30, that this 

 second system must be of simultaneous origin. 



