34 Mb HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



If the fissure be of considerable length, p will be extremely large, 

 and this equation will hold approximately for large values of i/, and 

 if y be less than p, T will be less than T. 



28. Hence then it appears, that if the fissure he such that the curva- 

 ture of its sides is extremely small, the greatest tension at any point 

 ivithin the lines AU a7id BV, aiid not extremely remote from AB, will 

 be in a direction parallel to AB ; and that consequently, if any fissure 

 were propagated through Q, hy the tension there, it must tiecessarily he 

 in a direction perpe?idicular to that line. 



fi. On the Formation of Systems of Fissures. 



29. The result enunciated in the last Article is important, as 

 shewing the impossibility of forming in succession parallel fissures 

 not far distant from each other in a mass subjected to such ten- 

 sions as we have supposed. Let us suppose, for instance, a fissure 

 AB to have been formed in a lamina subjected to two systems of ten- 

 sions, of which the directions are perpendicular to each other. The 



propagation of the fissure beyond A and B, may be conceived to have 

 been prevented by a greater cohesive power of the lamina there, or 

 by a diminished intensity of the tensions perpendicular to AB. I^et 

 us also suppose another fissure to commence at A', subsequently to the 

 formation of AB, and not remote from it, from the increased intensity 

 of the tensions perpendicular to AB. Its direction AE will be parallel 

 to AB, but it cannot be propagated in that direction from E to F; 

 for the tension at Q along EF (as above stated) will be greater than 

 that in a direction perpendicular to it, and therefore if a fissure be 

 formed at all through that point, it must be perpendicular to EF. Nor 

 would the formation of a fissure from E io F he rendered the more 

 possible by the existence of this fissure through Q perpendicular to AB ; 



