Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 27 



of less resistance; then if we suppose the force p not to act effectively 

 in propagating the fissure, except near its extremity*, its action will 

 not extend beyond the portion P^P^^ of the fissure, and consequently 



Its tendency will be to propagate it in the direction of PP produced 

 after it has reached the termination of the line of less resistance There 

 will be no tendency, as in the former cases, to resume any particular 

 direction. 



f Modification of the Tensions in the vicitiifi/ of a Fissure. 



22. Let us now suppose a fissure to have been formed in the 

 manner above described, and extending between two points in the 

 lamina, where we may conceive its propagation to have been arrested 

 either by an increased cohesive power, or by a diminution of inten- 

 sity in the tensions. It is manifest that the state of tension in the 

 vicinity of this fissure, will become entirely different from that which 

 existed previously to its formation ; and that the subsequent formation 

 of any other fissure not very remote from the first, must therefore be 

 influenced by the modification of the original tensions thus produced. 

 It will now therefore be our object to examine this consequence of 

 the existence of a fissure. For the greater simplicity, we may suppose 

 it to be rectilinear. It will also suffice for our immediate pui-pose, to 

 suppose the lamina subjected to two sets of tensions acting perpendi- 

 cularly to each other, the direction of the fissure being perpendicular 

 to that of the system of the greater intensity. 



This will be true in the actual case to which it is intended to apply this part 

 of the investigation. 



S3 



