480 



GKOUPING OF STEATA IN DELTAS. 



[Ch. XIX. 



The deltas also of the Po and Granges have each, as we 

 have seen (p. 484) , when probed by the Artesian anger, borne 

 testimony to a gradnal snbsidence of land to the extent of 

 several hundred feet — old terrestrial surfaces, turf, peat, 

 forest-land, and ' dirt-beds/ having been pierced at various 

 depths. The changes of level at the mouth of the Indus in 

 f!n+.p.h (we below. Ch. XXVIII.), and those of New Madrid in 



XXVIII 



demonstrating 



in the levels of those areas into which running water is tran- 

 sporting sediment. 



modei 



deltas could be known, it is scarcely probable that we should 

 find any two of them in the world to have coincided in date, 

 or in the time when their earliest deposits originated. 



Grouping of strata in deltas. 



The changes which have 



taken place in deltas, even within the times of history, may 



suggest 



many 



important considerations in regard to 



the 



manner in which subaqueous sediment is distributed. If a 

 lake, for example, be encircled on two sides by lofty moun- 

 tains, receiving from them many rivers and torrents of dif- 

 ferent sizes, and if it is bounded on the other sides, where the 

 surplus waters issue, by a comparatively low country, it is not 

 difficult to define some of the leading geological features 

 which must characterise the lacustrine formation, when this 

 basin shall have been gradually converted into dry land by 

 the influx of sediment. 



The detritus washed down by rivers and torrents from 

 the adjoining heights to the edge of the lake, would sink 

 at once into deep water, all the heavier pebbles and sand 

 subsiding near the shore. The finer mud would be carried 

 somewhat farther out, but not to the distance of many 

 miles, for the greater part, as is seen where the Rhone enters 

 the Lake of Geneva, falls down in clouds to the bottom, 

 not far from the river's mouth. Tims alluvial tracts are 

 formed near the shore, at the mouths of every torrent and 

 iver ; pebbles and sand are then transported farther from 

 the mountains; but in their passage they decrease in s ize 

 by attrition, and are in part converted into mud and sand. 

 At length some of the numerous deltas, which are all di- 



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