FRACTURES FORMED BY UPHEAVAL. 



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north-and-south fractures, parallel to the great Pennine fault, and the 

 east-and-west fractures, at right angles to these, compose a system in 

 accordance with the mechanical theory. 



Again, if the force under a given district be determined by any 

 peculiarity of the rocks to a conical elevation, there will be radiating 

 primary fissures and secondary concentric ones. Such a case, per- 

 haps, occurs in the volcanic elevation of Mont Dore. An elliptical 

 elevation would have characters intermediate between the two, and 

 the same district may show traces of one of these superadded to 

 the narrow rectangular elevation first noticed. 1 Such a case occurs 

 in the Weald of Sussex. By cautiously employing this ingenious 

 mode of interpretation, we shall be able to determine, in any given 

 region where the fractures of the strata are well traced, the whole 

 area of the ground subject to any movement at a given time. 



It is unnecessary to quote examples in which Mr. Hopkins' views 

 find a useful application. We will only state a single case of the 

 parallelism of trap dykes, which has been furnished by Archdeacon 

 Verschoyle, in the north-west part of Mayo and Sligo. 2 He describes 

 no less than eleven basaltic and amygdaloidal dykes, which, in a 

 space of n^ miles in breadth, traverse the northern part of the dis- 

 trict in a nearly east-and-west direction, and cut through all forma- 

 tions from gneiss to the Carboniferous limestone. One of these dykes 

 he traced between sixty and seventy miles, and believed it might be 

 followed much farther to the eastward. Two of the dykes are crossed 

 by others having a north-and-south direction. 



Hopkins on the Elevation of the Weald. In the large ellip- 

 tically elevated area of the Weald, 150 miles long from east to west, 

 and 40 miles broad from north to south, between the chalk escarp- 

 ments, Mr. Hopkins recognises, besides the general broad anticlinal 

 slopes which determine the main features of the district, several lines 

 of flexure and fracture, and anticlinal axes ; and he also defines some 

 of those transverse lines of movement, depending on the main axis 

 and boundaries of the district, which are directly deducible from his 

 theory. He combines with the elliptical elevation of the Weald the 

 more elongated system of parallel movements of the Isles of Wight 

 and Purbeck. The remarkable breaks in the bounding chalk ranges 

 which give passage to the rivers flowing from the Wealden northward 

 and southward were supposed to correspond in situation with cross 

 fractures, indicated by theory, and occasionally proved by observa- 

 tion. One considerable decisive and simultaneous movement is ap- 

 pealed to for the dislocations of the elevated mass, and for the pro- 

 duction of its main physical features ; but there is still a necessity of 

 admitting a slow and gradual continental elevation to account for the 

 denudation of the district. 3 And we shall do well, before accepting 

 the origin of these river valleys in faults, to consider the history of 

 river valleys in the South of Ireland advanced by the late Pro- 

 fessor Jukes, F.R.S., 4 and the application of similar views to the 



1 Camb. Phil. Trans., 1837. 2 Proc. Geol. Soc., 1833. 



3 Geol. Trans., vol. vii. " 4 J. B. Jukes, Q. J. G/S., vol. xviii. p. 378. 



