188 



STRUCTURAL GEOLOGY. 



and the perpendicular the dip (Fig. 105). The perpen- 

 dicular is made shorter, as the dip is at a higher angle. 



Anticline and Syncline. When a series of strata dip 

 in one direction in one place, the same series will usually 

 be found to dip in a contrary direction in another place. 

 In other words, strata are usually disturbed by lateral 

 pressure, which throws them into folds, sometimes wide 

 and gentle, like undulations, sometimes closely appressed. 

 Thus strata usually occur in alternate saddles and troughs 

 (Figs. 102, 103). The saddles are called anticlines, the 

 troughs synclines. An anticlinal axis, then, may be de- 



FIG* 102.. 



fined as a line on either side of which the strata repeat 

 one another, dipping in opposite directions, away from 

 the axis. A synclinal axis is a line on either side of 

 which the strata repeat each other, dipping in opposite 

 directions, but toward the axis. In Figs. 102 and 104, a 

 is an anticline, and s a syncline. 



In anticlines the strata lie in saddles and in synclines 

 in troughs, but the surface configuration of the ground 

 may or may not correspond. Sometimes the ground is 

 comparatively level, though the foldings are strongly 

 marked (Fig. 102). Sometimes the anticlines are ridges, 

 and the synclines valleys (Fig. 103), and sometimes the 



FIG. 103. 



