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Ch. XIX.] 



CAUSES OF STRATIFICATION IN DELTAS. 



489 



to 



mil 



Thus, for example, 



may 



at one period of the year, drift wood 



and, at another, mud, as was before stated to be the case in 



the delta of the Mississippi ; or at one time, when the volume 



earn 



may be spread over a certain area, over which, when the 



waters are low, fine matter or chem 



precipitates are 



formed. During inundations, the turbid current of fresh 

 water often repels the sea for many miles ; but when the 

 river is low, salt water again occupies the same space. 

 When two deltas are converging, the intermediate space is 

 often, for reasons before explained, alternately the receptacle 

 of different sediments derived from the converging streams 

 (see p. 457). The one is, perhaps, charged with calcareous, 

 the other with argillaceous matter; or one sweeps down sand 

 and pebbles, the other impalpable mud. These differences 

 may be repeated, with considerable regularity, until a thick- 

 ness of hundreds of feet of alternating beds is accumulated. 

 The multiplication, also, of shells and corals in particular 

 spots, and for limited periods, gives rise occasionally to lines 

 of separation, and divides a mass which might otherwise be 

 homogeneous into distinct strata. 



An examination of the shell marl now forming: in the Scotch 



lakes, or the sediment termed ' warp/ which subsides from 

 the muddy water of the Humber and other rivers, shows that 

 recent deposits are often composed of a great number of ex- 

 tremely thin layers, either even or slightly undulating, and 

 preserving a general parallelism to the planes of stratification. 

 Sometimes, however, the laminse in modern strata are disposed 

 diagonally at a considerable angle, which appears to take 

 place where there are conflicting movements in the waters. 

 In January, 1829, I visited, in company with Professor L. A. 

 Necker, of Geneva, the confluence of the Ehone and Arve, 

 when those rivers were very low, and were cutting channels 

 through the vast heaps of debris thrown down from the waters 

 of the Arve in the preceding spring. One of the sand-banks 

 which had formed, in the spring of 1828, where the opposing 

 currents of the two rivers neutralised each other, and caused 

 a retardation in the motion, had been undermined ; and the 





