DISRUPTION OF STRATA. 



83. In other cases the superior strata, but not the inferior, are 

 broken through, as shown in fig. 28. The edges of the disrupted 



Fig. 28. 



strata are then exposed upon the flanks of the elevation, and are 

 ranged as above described ; while the inferior strata, not disrupted 

 retain their natural order upon the summit. 

 In other cases (fig. 29) the disruption is complete, and the 



Fig. 29. 



broken strata are ranged on each of the opposite declivities in the 

 same order as already described. 



84. It happens often that after the cessation of the disturbing 

 force, by which the strata have been uplifted, the land having 

 been again submerged, new depositions take place, the strata of 

 which are of course horizontal and superposed upon those ren- 

 dered oblique by the previous disturbance. This sort of super- 

 position of strata is called by geologists discordant or unconform- 

 able stratification, and wherever it occurs it affords evidence of 

 the action of a disturbing force from below, the geological date of 

 which can be determined with more or less precision by a due 

 examination and comparison of the superposed strata. 



Cases of this kind of discordant stratification are shown in fig. 

 28 and fig. 29, in both of which horizontal strata deposited upon 

 the oblique strata, are disposed along the slopes of the elevation. 



85. It is evident that the epoch of the action of the disturbing 

 force must, in all cases, have been posterior to that of the deposition 

 of the inclined, that is, of the disturbed strata. It is equally 

 apparent that the disturbing action must have ceased before the 



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