Mh HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 39 



VI. If in addition to a system of horizontal tensions, there be 

 also a force acting on the opposite sides of the fissure, perpendicularly 

 to its direction, and tending to increase its width*, the permanence of 

 direction in the progressive formation of the fissure will be diminished, 

 but the permanent direction will remain the same as if there were no 

 other force than the system of horizontal tensions, ?". e. if the direction 

 in which the propagation of the fissure is taking place be disturbed by 

 any partial cause, it will still constantly tend again to perpendicularity 

 with the directions of the system of tensions; but this tendency will 

 be less than if the force always acting perpendicularly to the fissure did 

 not exist. (Art. 20.) Consequently, deviations from the permanent di- 

 rection of cleavage will, in the case we have supposed, be greater 

 than if the sides of the fissure were not subjected to the action of 

 this last-mentioned force. 



VII. If there be no tension acting on the mass, and a fissure be 

 formed solely by this force, acting perpendicularly to its sides, the fis- 

 sure will be propagated in the plane in which it begins to be formed, 

 if the cohesive power of the mass vary according to any continuous law. 

 There will be, however, but little permanence in its du-ection, so that 

 if it be turned from its original direction by planes of less resistance, 

 there will be little tendency to resume that direction, and the fissure 

 may thus assume any form of irregular cui'vature. (Art. 20.) 



VIII. If a fissure commence at, or in the course of its progressive 

 formation meet, a partial plane of less resistance at an acute angle, 

 it will, under certain conditions, be propagated along it; but when from 

 any cause this ceases to be the case, the fissure will almost immediately 

 resume a direction parallel to its original one, supposing it produced 

 by tensions, which, independently of the existence of planes of less le- 

 sistance, would produce rectilinear fissures. (Arts. 17, 18.) 



IX. If the mass be subjected to two systems of parallel tension.s, 

 of which the directions are perpendicular to each other, two systems of 



* This will be the case if the fissure be filled with any kind of fluid subjected to a 

 great pressure from some external cause. 



