72 



LYELL'S ELEMENTS OF GEOLOGY. 



Anticlinal and Synclinal Lines. 



Fig. 67. 



Fig. 68. 



north and south, and the 

 dip east and west. A line 

 2 drawn along the summit of 

 the ridges A, B would be 

 an anticlinal line, and one 

 following the bottom of the 

 adjoining valleys a syn- 

 clinal line. It will be ob- 



Ground plan of the denuded ridge C, fig- Served that SOmO of thcsC 



ridges, A, B, are unbroken on the summit, whereas one of them, 

 C, has been fractured along the line of strike, and a portion of 

 it carried away by denudation, so that the edges 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 may be expressed by the diagram Fig. 67., 

 and the cross section of the same by Fig. 68. The line D E, 

 Fig. 67., is the anticlinal line, on each side of which the dip is 

 in opposite directions, as expressed by the arrows. The emer- 

 gence of strata at the surface is called by miners their outcrop 

 or basset. 



If, instead of being folded into parallel ridges, the beds form a 

 boss or dome-shaped protuberance, and if we suppose the sum- 

 mit of the dome carried off, the ground plan would exhibit the 

 edges of the strata forming a succession of circles, or ellipses, 

 round a common centre. These circles are the lines of strike, 

 and the dip being always at right angles is inclined in the course 

 of the circuit to every point of the compass, constituting what is 

 termed a qua-qua-versal dip — that is, turning each way. 



In the majority of cases, an anticlinal axis forms a ridge, and 

 a synclinal axis a valley, as in A, B, Fig. 58. p. 65. but there 

 are exceptions to this rule, the beds 

 ig"- sometimes sloping inwards from either 



side of a mountain, as in Fig. 69. 



On following the anticlinal line of 

 the ridges of the Jura, before men- 

 tioned, A, B, C, Fig. 66., we often 

 discover longitudinal fissures along the 

 line where the flexure was greatest. 

 At the eastern extremity of the Pyrenees a curious illustration 

 of an analogous phenomenon may be seen on a small scale (Fig. 

 70.) The strata there laid open, in the sea-cliffs, consist of 

 marl, grit, and chert, belonging to a formation of the age of the 

 green-sand of English geologists. Some of the bendings are so 

 sharp, that fragments of the slaty chert — a hard flinty rock — 

 taken from the points where they form an angle at a, might be 



