190 



STRUCTURAL GEOLOGX. 



FIG 107. Plan of undulating strata. 



We have spoken of folded strata and the way in which 

 they outcrop ; but in a survey the process is reversed, i.e., 

 it is the outcrop which is observed, and from this we con- 

 struct the section. Now, when we remember the complex 

 folding, then the tilting after folding, then the displace- 

 ment by fractures, and then, worst of all, the covering of 

 the whole deeply with soil, leaving exposed only patches 

 here and there, we can easily see how difficult a problem 

 it often is to construct a section of the stratified rocks of 

 a country. If the strata be exposed on a cliff or a canon- 

 side, there is little difficulty, but, in the absence of such, 

 the geologist takes advantage of every exposed patch, 

 examines every gulch or stream-bed, every quarry or 

 railroad-cutting, and thus constructs an ideal section. 



Conformity and Unconformity. We have just seen 

 that the strata composing the country rock of a land- 

 surface are usually tilted and crumpled and always eroded, 

 so that their edges are exposed (see Figs. 106, 107). But 

 we have also seen (pages 164-170) that in some places 

 land-surfaces are now sinking beneath the sea, and in 

 others sea-bottoms are rising to become land-surfaces. 

 The same is true for all geological epochs. Now, suppose 

 at any time an eroded land-surface sank below sea-level 

 so that sediments were deposited on the eroded edges and 

 filling the erosion-hollows of the strata, and finally the 



