Ch. V.] 



DIP AND STRIKE. 



55 



line, it will be seen that the amount of inclination may still be measured 

 by the hands with equal facility. 



It has been already seen, in describing the curved strata on the east 

 coast of Scotland, in Forfarshire and Berwickshire, that a series of con- 

 cave and convex bendings are occasionally repeated several times. These 

 usually form part of a series of parallel waves of strata, which are pro- 

 longed in the same direction throughout a considerable extent of country. 

 Thus, for example, in the Swiss Jura, that lofty chain of mountains has 

 been proved to consist of many parallel ridges, with intervening longi- 

 tudinal valleys, as in fig. 71, the ridges being formed by curved fossilif- 

 erous strata, of which the nature and dip are occasionally displayed in 

 deep transverse gorges, called " cluses," caused by fractures at right angles 

 to the direction of the chain.* Now let us suppose these ridges and 

 parallel valleys to run north and south, we should then say that the 

 strike of the beds is north and south, and the dip east and west. Lines 

 drawn along the summits of the ridges, A, B, would be anticlinal lines, 

 and one following the bottom of the adjoining valleys a synclinal line. 



Fig. 71. 



Fig. 72. 



Fig. 73. 



Section illustrating the structure of the Swiss Jura. 



It will be observed that some of these ridges, A, B, are unbroken on the 

 summit, whereas one of them, C, has been fractured along the line of 

 strike, and a portioi of it carried away by denudation, so that the ridges 

 of the beds in the formations a, b, c, come out to the day, or, as the 



miners say, crop out, on the sides 

 of a valley. The ground plan of 

 such a denuded ridge as C, as given 

 in a geological map, may be ex- 

 pressed by the diagram fig. 72, and 

 the cross section of the same by 

 fig. 73. The line D E, fig. 72, is 

 the anticlinal line, on each side of 

 which the dip is in opposite direc- 



Ground plan of the denuded ridge C, fi ; 



* See M. Thurmann's work, " Essai sur les Soulevemens Jurassiques du Por- 

 rentruy, Paris, 1832," with whom I examined part of these mountains in 1835. 



