36 Mr. Hopkins on the Structure of the 



boundary, with the exception of the western extremity, would be distinctly indi- 

 cated by the resulting mountain range. 



6. To define the boundary of that portion of the district which is conceived to 

 have been subjected to a more intense action of the elevatory force than the other 

 portions, we must suppose the boundary line to turn southward at Farnham, and 

 to preserve the same relation to the chalk escarpment from thence by Petersfield 

 to Beachy Head, as that above described with reference to the other portions of 

 that escarpment, and to pass across the Channel from that point to the southern 

 boundary of the Bas Boulonnais. The boundary of the portion of the district now 

 referred to is indicated in the map by a discontinuous line, which will be easily 

 recognised. 



Having established these preliminary points respecting the action of the elevatory 

 force and the boundaries of the disturbed district, we must determine the general 

 directions of lines of dislocation which would be formed in the elevated mass, 

 assuming its approximate homogeneity. If it be not homogeneous, the lines of 

 fracture may be modified as previously explained (page 31), but the results will 

 still be approximately true. 



7. It has been shown in the memoir already referred to, in the Transactions of 

 the Cambridge Philosophical Society, that if the elevatory force be uniform, and 

 the boundary of the elevated area be circular, a system of fissures might be formed, 

 concentric about the centre, or diverging as radii from it. If the force acted with 

 much greater intensity at the centre than elsewhere, the latter system would gene- 

 rally be formed, and not the former. It would then become a case of what I 

 have termed conical elevation. If the circle be of large extent, and the force ap- 

 proximately uniform and of sufficient intensity, both these systems might be formed 

 in a mass constituted like that we have to consider. If, on the contrary, the 

 elevated area were a parallelogram of finite breadth, but. of indefinite length, one 

 system of fissures only could be formed. Its direction would be parallel to the 

 axis of the parallelogram. 



Now the circle, and the parallelogram of indefinite length, may be regarded as 

 the two extreme or limiting cases of the ellipse, or, more generally, of any regular 

 oval. For conceive an oval always preserving its shorter axis the same in magni- 

 tude and position, to change its form by a change in the magnitude of its greater 

 axis. When this variable axis is equal to the constant one, the oval will become 

 circular ; and when the variable axis becomes indefinitely great, the oval will ap- 

 proximate more and more nearly to two straight lines parallel to the greater axis 

 and passing through the extremities of the minor one. And as the general form 

 of the oval is intermediate to these two extreme or limiting cases, it is not difficult 

 to see, that the curve along which a longitudinal fissure would be formed under 



