8o6 



PRINCIPLES OF STRATIGRAPHY 



the plane of the paper, either up or down, until the three sides a d, 

 a e and a i coincide with the apices of all tiiree (d c i), meeting in a 

 common point. The triangles a d f and a c g would then represent 

 the walls of which the original dip measurements were made, the 

 angles in each case heing represented by the angles a f d and age, 

 respectively. A plane resting on the three hypothenuses would rep- 

 resent the inclined stratum. 



Strike as affected by pitching axis of folds. 



As long as the axis of an anticline or syncline continues hori- 

 zontal, the outcrops of the beds exposed by planing ofif the summit 

 of the fold in a horizontal surface will be in the form of parallel 

 bands, the lowest appearing at the center and the repetition of the 



a b 



Fig. 190. a. Eroded anticline with horizontal axis. b. Eroded anticline with 

 pitching axis, showing resulting outcrops of strata. \ 



beds being in the same order from the center outward in both direc- 

 tions. (Fig. 190, a.) 



When the axis of the fold is inclined the strike of the strata on 

 opposite sides of the axial plane will conv'erge and finally meet. 

 (Fig. 190, h.) In an anticlinal fold the inner strata are the older; 

 in a synclinal fold the inner strata are the younger. 



Folding as indication of unconformity. 



In a complexly folded region an unconformity may sometimes 

 be detected between two formations not actually seen in contact by 

 the fact that the lower formation is folded much more strongly 

 than the upper one. In this case it is apparent that the lower forma- 

 tion was folded and truncated before the upper one was deposited, 

 after which both were again folded. (See Fig. 191.) 



