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Ch. XIX.] 



GROUPING OF STRATA IN DELTAS. 



487 



reeted towards a common centre, approach, near to each 

 other; those of adjoining torrents become united, and each 

 is merged, in its turn, in the delta of the largest river, which 

 advances most rapidly into the lake, and renders all the minor 

 streams, one after the other, its tributaries. The various 

 mineral ingredients of all are thus blended together into one 

 homogeneous mixture, and the sediment is poured out from 

 a common channel into the lake. 



As the average size of the transported particles decreases, 

 while the force and volume of the main river augments, the 

 newer deposits are diffused continually over a wider area, and 

 are consequently more horizontal than the older. When at 

 first there were many independent deltas near the borders of 

 the basin, their separate deposits differed entirely from each 

 other ; one may have been charged, like the Arve where it 

 joins the Rhone, with, white sand and sediment derived from 

 granite — another may have been black, like many streams in 

 the Tyrol, flowing from the waste of decomposing rocks of 

 dark slate — a third may have been coloured by ochreous se- 

 diment, like the Red River in Louisiana — a fourth, like the 



have held much carbonate of lime in 

 solution. At first they would each form distinct deposits of 

 sand, gravel, limestone, marl, or other materials ; but, after 

 their junction, new chemical combinations and a distinct 

 colour would be the result, and the particles, having been 

 conveyed ten, twenty, or a greater number of miles over al- 

 luvial plains, would become finer. The more ancient sys- 

 tem of strata would be composed for the most part of coarser 

 materials, and would sometimes dip at a considerable angle, 



may 



especially if consisting of beds of pebbles. 



The beds 



in 



the newer group would, on the whole, be finer-grained, 

 and more homogeneous in colour and mineral composition 

 throughout large areas. But, although the law or arrange- 

 ment here alluded to would cause the older or more littoral 



to be characterised in great part by coarser materials than 



the newer group, this latter, as it advanced to a great dis- 

 tance from the shore, would be thrown down, not imme- 

 diately on older rocks, but on strata made up of the finest 

 mud which had been carried out to great distances when the 





