STRUCTURE AND POSITION. 



179 



Fig. 154. 



an anticline and syncline, and then the whole tilted and finally eroded 

 down to a nearly level surface. The map of the outcrop of such a 

 series is shown in Fig. 155, 

 in which A A' is an anti- 

 cline and B B' a syncline, 

 and the arrows show the 

 dip on either side of the 

 axes and also the general 

 dip of the whole northward. 

 Fig. 153 is a section of the same. The sinuosity of the outcrop is, of 

 course, the necessary result of the folding and tilting. We have sup- 

 posed the strata folded and 

 tilted, and then showed 

 what the outcrop would be. 

 The field geologist, of 

 course, follows the reverse 

 method : he works out the 

 outcrop, and infers the 

 structure and position of 

 strata. If the strata had 

 been simply folded but not 

 tilted, then the outcrop of 

 the same section (Fig. 153) 

 would be much simpler, i. e., in parallel lines as in Fig. 156. 



The cases represented by these figures are comparatively simple, and 

 we have supposed the soil 

 removed, so that the out- 

 crop is easily traced. But 

 when we remember — 1. 

 The great complexity of 

 the outcrop often produced 

 by folding and tilting; 2. 

 That it is very much in- 

 creased by inequalities of erosion ; 3. That it is still further increased 

 by Assuring and displacement ; and, worst of all, 4. That the rocks are 

 largely covered by soil — we easily see the difficulty of the task of mak- 

 ing a good geological map and section of any region. 



Conformity and Unconformity. — We have just seen that all land- 

 surfaces are deeply eroded and the strata are left with their edges ex- 

 posed. We have also seen (pp. 133-137) that the crust of the earth is 

 everywhere in a state of slow movement : in some places sea-bottoms 

 are rising and becoming land-surfaces, in others land-surfaces are sink- 

 ing to become sea-bottoms. Now, the same thing has taken place in 

 earlier geological times. Suppose, then, an eroded land-surface with the 



Fig. 155.— Plan of Undulating Strata. 



I5 ||S| ,. 



" 1 ill " 





1 " 





|l 



1 , 





,, 



„J .£, 



'« 





'i 



-— ^ „ 



. ;' 



11 



11 III II 





„ 



il 



J " 



i': ll ;:i 



„ 



It III „■* 





(1 





H 



\* H 1 „ 



'■:j'.ii; : i 



11 



" 





ll 





" 





it 





II 



' " 1111 " 





n 



: 



li 





11 





II 



Fig. 156. 



