2S0 THEORY OE 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 investi- 

 gation 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 might have been anticipated theo- 

 retically on mechanical principles, if we assume certain simple 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 exami- 

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

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

 nal 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 sec- 

 ondary 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 lon- 

 gitudinal 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 iEtna were 

 poured out many thousands, perhaps myriads of years before the 

 newest, and yet they, and the movements accompanying their emis- 

 sion, have produced 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 diffi- 

 culty is there in conceiving 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. 74, p. 363, 1841, and G. S. Trans. 2 Ser. vol. 7. 

 f Soulevemens Jurassiques. 1832. 



