I4 OUTI.IXKS (>F III! P > KOLOGY I-ART i 



the minor examples I have spoken of, the line of axis is 

 ;nined from the angles taken among the inclined 

 strata on either side. And here the advantage of one 

 or more definite horizons is made strikingly apparent. 

 If we can recognise the same stratum on both sides of a 

 fold, we have in its position and angle of inclination a 

 datum line from which to measure the extent of the fold. 

 In Fig. 44, for example, if the bed marked a c can be 

 identified on each side of the arch, we can estimate 

 from the angles of inclination between it and the axis 

 how high the arch must have been when this bed formed 

 its crown, and what amount of material has since been 

 removed. On the other hand it is easy in a similar 

 way to calculate the depth of the trough (Fig. 45) from 

 the centre down to the position of the stratum a, c. 



We might expect that these curvatures of the solid 

 rocks should always produce features at the surface ; that 

 the lines of anticlinal axis should correspond with ridges 

 or hills, while those of synclinal folds should define the 



FIG. 46. Anticlines forming valleys ; synclinet forming hills. 



trend of valleys. But it often, perhaps we may even say 

 generally, happens that neither anticlines nor synclines 

 produce any marked influence on the surface (Fig. 47). 

 In walking over the two sections (Figs. 44 and 45), the 

 observer not attending to the angles of inclination, would 

 never suspect that he was crossing a geological ridge in 

 the one case, and a geological valley in the other. When 



