490 



CAUSES OF STRATIFICATION IN DELTAS. 



[Ch. XIX. 



following is an exact representation of the arrangement of 

 laminee exposed in a vertical section. The length of the por- 

 tion here seen is about twelve feet, and the height five. The 

 strata a a consist of irregular alternations of pebbles and sand 

 in undulating beds : below these are seams of very fine sand 

 b b, some as thin as paper, others about a quarter of an inch 



thick. 



com 



reenish- 



grey sand as thin as paper. Some of the inclined beds will 

 be seen to be thicker at their upper, others at their lower 

 extremity, the inclination of some being very considerable. 



Fig. 38 



River 



A rue 



Section of a sand-bank in the bed of the Arve at its confluence with the 

 Ehone, showing the stratification of deposits where currents meet. 



These layers must have accumulated one on the other by 

 lateral apposition, probably when one of the rivers was very 

 gradually increasing or diminishing in velocity, so that the 

 point of greatest retardation caused by their conflicting cur- 

 rents shifted slowly, allowing the sediment to be thrown down 

 in successive layers on a sloping bank. The same phenomenon 

 is exhibited in older strata of all ages."* 



If the bed of a lake or of the sea be sinking, whether at a 

 uniform or an unequal rate, or oscillating in level during the 



deposition of sediment, these movements will give rise to a 



different class of phenomena, as, for example, to repeated 

 alternations of shallow-water and deep-water deposits, each 

 with peculiar organic remains, or to frequent repetitions 01 

 similar beds, formed at a uniform depth, and enclosing the 



* Seo Manual of Geology by the Author. 



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