24 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



gressive formation, arrive at a point at which Bt will be very nearly 

 equal to the cohesive power, | since that force by hypothesis increases 

 rapidly with /, (Art. 12)1, and where, consequently, the direction of the 

 fissure must necessarily be very approximately that determined by 

 equation (2), as explained in Art. 12. Hence then we may conclude, 

 that under the hypotheses we are taking, whatever may be the direc- 

 tion first given to the fissure by any local cause, its subsequent direc- 

 tion will soon become independent of that cause. 



17. If the fissure, instead of beginning at some point in a line 

 of less resistance meet it, in its progressive formation, it will pass 

 along it, or will cross it, according as a condition exactly similar to 

 that given above (Art. 15), be satisfied or not. At the termination of 

 this line, the fissure will soon resume the direction given to it by the 

 o-eneral systems of tensions to which the lamina is subjected, as just 

 explained. Such also will be the case at the point at which the line 

 of less resistance, should it be a curved or broken line, may assume 

 a direction in which the condition just referred to is no longer 

 satisfied. 



18. The condition given in Art. 15 gives us 



Bl rr 



The first of these ratios will in each particular case be a function 

 of the angle RPR' or EPB, the angle between the line of less re- 

 sistance and the direction AB, (perpendicular to PR) in which the 



general tensions tend to form the fissure, the value of the function 

 decreasing as RPR' or EPB increases from zero to a right angle. 



