332 



GEOLOGY. 



If erosion has cut off a fold down to the proper level, there should 

 be, on either limb of the anticline involving the Ordovician strata, 

 a belt where that system appears at the surface (Figs. 148 and 149); 

 but faults have in some cases had the effect of concealing the beds 

 which might otherwise have come to the surface on one or both sides 

 of a truncated fold. Thus the outcrops shown in Fig. 150 would result 



C DS € OSDC 



H H H f- h h H 



Fig. 150. Fig. 151. 



Fig. 150. — Ground-plan diagram, illustrating the failure of one "system of rocks, 0, 



to appear on one side of the anticline. 

 Fig. 151. — Cross-section showing the structure illustrated by the preceding figure. 



On one side of the axis of the anticline, €, O, S, D, and C appear in succession. On 



the other side, O does not appear. 



from the faulting shown in Fig. 151. Faulting may likewise affect the 

 outcrop in regions where the strata are not deformed, but faulting 

 is less common in such regions. The principle involved is illustrated 

 by Fig. 152. 



Close of the Ordovician Period. 



The close of the Ordovician period was marked by geographic 

 changes of more importance than those which took place at its begin- 

 ning or during its progress. These changes were the result of deform- 

 ative movements whose cause is not fully demonstrated, but which 

 may perhaps have been due to the shrinking of the earth, involving 



