Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGV. 



23 



consider the effect of the existence of lines of les. resi.tcaice in the 

 lamina, in which case the continuity above assumed will no longer 

 exist along these lines. *' 



Let DB be a line of this description, along which the cohesive 

 power estimated in a direction perpendicular to it = IT, that of the 

 lamina near to DE being = n. Also let i2„ acting in the direction 



PR, be as before, the resultant at the time t and at the point P of 

 the general systems of tensions impressed upon the lamina; and 'let 

 Rl denote the tension along PR perpendicular to BE at the time t 

 Then if 



Rl . ^ R> 



it is manifest that the fissure will begin to be formed along the line 

 nE rather than ,n a direction perpendicular to R„ in which it would 

 be formed m the absence of a line of less resistance*. 



16. Let us now suppose this line to terminate at JJ and E 

 When the fissure has been propagated to those points, its progress will 

 be arrested till the tension i?, and that superimposed just beyond the 

 extremities of the fissure, and before denoted by <t>, (Art. 11), produce a 

 resultant tension greater than the cohesive power n. The direction in 

 which the fissure will be then immediately continued, will not be 

 known, 4., being unknown ; but without staying to enquire what this 

 may be, we may observe, that the fissure must very soon in its pro- 



• It is assumed in the above condition, that if the fissure be formed along DE, the pn- 

 ucles on oppos.te s.des of the fissure in sepa^-ating would move in lines jferpendicula! to 

 JJi.. Ihis would be only appro.\uuately true. 



