44 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



The value of r, according to the same rough approximation, will be 

 nearly = -,y^ , which will always be very large; but as h also is pro- 

 bably large, p may bear a very considerable ratio to T. 



Here then we have the case which has been anticipated in the in- 

 vestigation of Art. 20; and it appears that the action of this force p 

 will greatly tend to increase the effect of any local causes in producing 

 partial deviations in the plane of the fissure from a vertical plane, but 

 that it will not alter generally its position when considered with refer- 

 ence to its whole extent. 



38. Again, with respect to the comparative width of the fissure at 

 different depths, it is manifest, taking the case of the Fig. p. 41, where 

 the extension of each lamina is the same, that if the mass, when relieved 

 from its tension by the rupture, return to its original horizontal length, 

 the width of the fissure will be the same throughout its whole depth ; 

 and in the case of the Fig. p. 42, the same conclusion might be con- 

 sidered as very approximately true under the same hypothesis. If, how- 

 ever, the different lamina, which I have supposed to have different 

 powers of cohesion, have also different degrees of elasticity, this dif- 

 ference may materially affect any approximation to this uniformity of 

 width. It seems probable, however, that the mean width (at least within 

 certain limits) will rather increase than decrease with the depth. 



39. Any number of these fissures might thus be formed simul- 

 taneously, (Art. 30.); and this simultaneous formation would be very 

 much facilitated by the action of the pressure p in the interior of the 

 fissure. If it be supposed, however, that partial causes prevent the com- 

 mencement of the formation of each fissure at the same instant, exactly 

 equal forces will not be exerted in the production of each, and con- 

 sequently they wiU not be propagated with the same velocity. Some 

 therefore will reach the exterior surface sooner than others; and when 

 a certain number have thus been formed from the lower to the upper 



