Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 3.5 



for it will be immediately seen, that this latter fissure together with 

 AB, would destroy all tension at Q, and would of course prevent the 

 possibility of the formation of any other fissure through that point*. 



30. Hence it follows, that in any system of parallel fissures which 

 are not remote from each other, the fissures could not he formed in succes- 

 sion. It will be easy however to understand how, in the case above as- 

 sumed, of two systems of tension perpendicular to each other, any number 

 of parallel fissures may be formed simultaneously. Let AB, A'B' be 

 two such fissures, and let GH be parallel to and equidistant from them. 



Now if the two fissures begin simultaneously at A and A', (the line 

 AA' being perpendicular to the direction of propagation,) and be pro- 

 pagated with equal velocity, it is obvious that no point in the physical 

 line GH will have any motion communicated to it by the relaxation 

 of the portion of the lamina between the fissures. Hence, if the line 

 GH were to become absolutely fixed, the formation of the fissures would 

 not be affected; but in this case the portions of the lamina on opposite 

 sides of GH might be regarded as two absolutely distinct laminje, 

 having that line for a common fixed boundary. Consequently it is as 

 easy to understand the simultaneous formation of any number of parallel 

 fissures, under the circumstances supposed, as that of a single fissure. 



31. Let us assume the two systems of tension not to be perpen- 

 dicular to each other, and suppose AB, A'B', two parallel fissures of 

 which the directions are perpendicular to the maximum resultant tension. 

 These fissures would not necessarily be continued parallel to each other. 



• It must be recollected that the imposisibility here spoken of assumes the tensions not 

 to be produced by impulsive forces acting on the mass, the intensity of these tensions being 

 always supposed to increase continuously, till sufficient to produce the fissure, and not to acquire 

 that requisite intensity instantaneously, as previously stated in the Introduction, p. H. 



e2 



