UNCONFORMITY 



825 



may often become complicated by further folding and erosion, when 

 the complex relationship shown in Fig. 191 is produced. 



In all cases of structural unconformity a considerable time in- 



FiG. 203. Cross-section of the Aletschhorn, showing inverted unconformity 

 of schists upon the laccoHthic crystallines. (After Baltzer.) 



terval remains unrecorded. This is measured by the amount of 

 folding and erosion which the strata have undergone. In this cir- 

 cumstance it is often the case that the last deposited strata prior to 

 the time that folding and erosion commenced were folded down to 



Griinscliiefer 



Fig. 204. 



'. Grauit 



Detail of the peak of the Aletschhorn. i, dragging of greenslate 

 at granite contact ; 2, infolding of greenslate into the granite. 



such an extent that they were preserved from complete removal by 

 erosion. Especially is this the case when the folding has been so 

 intensive as to place the strata in parallel positions, i. e., isoclinal 

 folding. In this case the actual time interval, during which no 

 deposition went on in the region in question, may be determined by 

 a comparison of the ages of the youngest stratum in the folded 



