Structure of the Wealden District, and the Bas Boidonnais. 323 



line of flexure, with a great dip to the north, but without the cor- 

 responding dip to the south necessary to form an anticlinal arrange- 

 ment, except in one or two localities. Towards the west it runs 

 immediately at the foot of the Hog's Back with a dip, which, near its 

 western extremity, amounts to 70 or 80 degrees. Near Guildford 

 it passes by the foot of the hill on which Margaret's chapel stands. 

 To the east it passes south of Dorking and Reigate to the summit of 

 Tilburstow Hill. It is afterwards continued by Limpsfield to the east 

 of Seven Oaks, as formerly described by Dr. Fitton. At some points 

 between these last-mentioned places, the line assumes a distinct an- 

 ticlinal character. 



Transverse valleys exist in the greensand ridge of this part of the 

 district as well as in that on the southern side. The author also 

 alludes to what he conceives to be incipient valleys of this descrip- 

 tion, and states his reasons for believing them to be indications of 

 transverse fractures. He conceives this opinion to be strongly cor- 

 roborated by the existence of the perennial springs by which these 

 valleys are characterized. Several are pointed out, especially in Leith 

 Hill and the Seven Oaks ridge overlooking the valley of the Weald. 



Transverse river-courses through the Chalk escarpment form one of 

 the most striking features in the geology of this district. The ana- 

 logy which they bear to the transverse valleys across the greensand 

 ridges would seem to leave no doubt of their being referable to the 

 same physical cause ; and as there are in many instances direct evi- 

 dence which renders the origin of these latter valleys in transverse 

 fractures highly probable, the same conclusion appears almost equally 

 probable with respect to the river-courses through the Chalk. In 

 the evidence of dislocation which the Chalk itself affords, there is 

 nothing, however, very conclusive ; but it must be remembered, that 

 the evidence of faults is always difficult to detect in a massive forma- 

 tion like the Chalk, possessing not more than two general divisions 

 which admit of distinct identification. 



The central chalk ridge of the Isle of Wight is traversed in like 

 manner by three transverse valleys, two of which are river-courses. 

 The author has pointed out some direct evidence in support of the 

 conclusion, that the central one (that of the Medina) has originated 

 in transverse dislocation. 



Bas Boulonnais. — With respect to the structure of the Bas Bou- 

 lonnais, it is onlynecessary here to state, that the author has recog- 

 nised three parallel lines of dislocation commencing at the coast and 

 running in a direction coinciding with that of the fines of elevation 

 of the Weald, supposing them produced across the Channel accord- 

 ing to the law which they follow on this side of it. The southern- 

 most of these lines passes immediately to the north of Boulogne. 



II. In the second part of this paper, previously to his comparing the 

 observed phenomena with theoretical deductions, the author recapi- 

 tulates some fundamental points of his theory. It is assumed, that 

 an elevatory force has acted simultaneously at every point of the 

 lower surface of the elevated mass in each district throughout which 

 the phenomena of elevation are observed to follow the same law, 



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