Wealden District and the Bas Boulonnais. 



37 



our assumed conditions, when the boundary of the elevated mass is oval, will be 

 generally intermediate to the straight line and circle, in the sense in which the 

 oval is so. In the particular case in which the fissure should be central, it would 

 be a straight line along the axis ; \i lateral, it would deviate more from the straight 

 line and approximate more to the form of the external oval boundary, according 

 as the position of the fissure should be more remote from the axis, as represented 

 in the following diagram (No. 10). The conditions of symmetry are sufficient to 



establish these results, without entering into the mechanical reasoning on which 

 they really depend. 



A system of transverse fissures (a /3, 7 I, &c.) might also be formed, the law of 

 which, as theoretically determined, would be, that every such fissure should be 

 perpendicular to each longitudinal fissure at the point of intersection. 



Let us now take the case represented in No. 11, in which the extremity, A, of 

 the disturbed district has the same form as in the previous case, while the portions 

 B C and B'C of the boundary are continued indefinitely and parallel to each other. 



B 



11. 



a — 



Towards A, the longitudinal lines of fissure will evidently be curved, as in fig. 4 ; 

 but between B C and B' C, they will be parallel to those lines. The transverse 

 fissures, if formed, will always follow the same general law with reference to the 

 longitudinal fissures as in the previous case. 



Again, let us take the case formed by a combination of the two preceding cases, 



