Mb HOPKINS, OM RESEARCHES IN PHYSICAL GEOLOGY. 19 



their intensity to increase till the resultant tension becomes greater than 

 the cohesive power at any proposed point P. A fissure will then begin 

 to be formed in the direction determined by equation (2), in which the 

 values of /.„ ^„ &c. express the ratios of the different tensions at P 

 at the instant the fissure begins to be formed there. Let us suppose 

 the fissure AB to have been thus formed, and that the cohesive power 



of the lamma beyond A and B is suflficient to prevent its further 

 propagation, and let us then consider whether any modification of the 

 tensions will be produced immediately beyond A and B, which may 

 possibly influence the direction in which there will be the greatest 

 tendency to continue the fissure. 



10. Let GK be any physical line broken by the fissure. It is ob- 

 vious that if it pass near the extremity of the fissure, its extension 

 and therefore its tension, will not be very much diminished ; but since' 

 this tension is no longer counteracted at g and k by an equal and 

 opposite tension, as in its unbroken state, it is manifest that the force 

 exerted by each portion Gff, Kh, must produce an increased stress upon 

 the portions of the lamina, immediately contiguous to and beyond the 

 extremity of the fissure; and since a similar effect, differing only in 

 degree, will be produced by each physical line broken by the fissure 

 It IS possible that the intensity of the whole additional tension, thus 

 partially superimposed upon the lamina, may be very considerable in 

 comparison with the general tensions impressed upon it. 



Now it is manifest, that the direction in which there is the 

 greatest tendency to continue the fissure from A or B, under the cir- 

 cumstances we are supposing, will be determined by the whole tension 



C 2 



