366 THEORY OF FRACTURE AND UPHEAVAL. |Ch. XIX. 



Lines of Fracture. — Mr. Martin, in his work on the geology of West- 

 ern Sussex, published in 1828, threw much light on the structure of 

 the Wealden by tracing out continuously for miles the direction of 

 many anticlinal lines and cross fractures ; and the same course of 

 investigation has since been followed out in greater detail by Mr. 

 Hopkins. The geologist and mathematician last mentioned has shown 

 that the observed direction of the lines of flexure and dislocation in 

 the Weald district coincide with those which inio-ht have been antici- 

 pated theoretically on mechanical principles, if we assume certain sim- 

 ple conditions under which the strata were lifted up by an expansive 

 subterranean force.* 



His opinion, that both the longitudinal and transverse lines of frac- 

 ture may have been produced simultaneously, accords well with that 

 expressed by M. Thurmann, in his work on the anticlinal ridges and 

 valleys of elevation of the Bernese Jura.f For the accuracy of the 

 map and sections of the Swiss geologist I can vouch, from personal 

 examination, in 1835, of part of the region surveyed by him. Among 

 other results at which he arrived, it appears that the breadth of the 

 anticlinal ridges and dome-shaped masses in the Jura is invariably 

 great in proportion to the number of the formations exposed to view ; 

 or, in other words, to the depth to which the superimposed groups of 

 secondary strata have been laid open. (See fig. 71, p. 55, for structure 

 of Jura.) He also remarks, that the anticlinal lines are occasionally 

 oblique and cross each other, in which case the greatest dislocation of 

 the beds takes place. Some of the cross fractures are imagined by him 

 to have been contemporaneous with others subsequent to the longi- 

 tudinal ones. 



I have assumed, in the former part of this chapter, that the rise of 

 the Weald was gradual, whereas many geologists have attributed its 

 elevation to a single effort of subterranean violence. There appears 

 to them such a unity of effect in this and other lines of deranged 

 strata in the southeast of England, such as that of the Isle of Wight, 

 as is inconsistent with the supposition of a great number of separate 

 movements recurring after long intervals of time. But we know that 

 earthquakes are repeated throughout a long series of ages in the same 

 spots, like volcanic eruptions. The oldest lavas of Etna were poured 

 out many thousands, perhaps myriads of years before the newest, and 

 yet they, and the movements accompanying their emission, have pro- 

 duced a symmetrical mountain ; and if rivers of melted matter thus 

 continue to flow upwards in the same direction, and towards the same 

 point, for an indefinite lapse of ages, what difficulty is there in con- 

 ceiving that the subterranean volcanic force, occasioning the rise or fall 

 of certain parts of the earth's crust, may, by reiterated movements, 

 produce the most perfect unity of result ? 



* Geol. Soc. Proceed. No. 14, p. 363, 1841, and G. S. Trans., 2d Series, vol. vii. 

 f Soulevemens Jurassiques. 1832. 



