36 Remarlcs on the Formation of Alluvial Deposites. 



All the deltas, as they are called, at the mouths of the great- 

 est rivers, whether flowing into seas or into the ocean, are repe- 

 titions of the same appearances upon a greater scale. The ap- 

 pellation Delta, though strikingly applicable in ancient times to 

 the alluvium included between the branches of the Nile, is less 

 descriptive of those sectors, or irregularly lozenge-shaped areas, 

 which we observe at the mouths of the Volga, the Danube, the 

 Po, and other great rivers. All of these have their types in the 

 diagrams^ Figs. 24 and 26, though various natural and artificial 

 agencies produce modifications of them, which, on so great a 

 scale, may strike the observer as very important deviations. The 

 tendency of the apex of a delta (i. e. of the point b in Fig. 26,) 

 to move downward, as remarked by Rennell, and exemplified 

 in the ancient and modern state of the Nile below Memphis, is 

 easily explained on the principles which have been illustrated. 



M. De la Beche states *, that the pebbles carried into the 

 Mediterranean by the Var and Paglion, are immediately depo- 

 sited in deep water, where they remain undisturbed, extending 

 but a short distance seaward ; and, on the other hand. Sir T. 

 Lauder observes f , that in sailing down Loch Linnhe, which is 

 an arm of the sea, he found, on its south side, the same kind of 

 shelf, which occurs along the border of fresh- water lakes, but 

 on a larger and ruder scale. 



In the ocean, tides and currents modify both the regular co- 

 lloidal forms of fluvial sediment, and the taluses produced by 

 atmospheric agencies along the shores ; but whenever the sedi- 

 ment is conveyed to sufficiently deep and still water, it obeys 

 the same laws which are observed to prevail in lakes and tide- 

 less seas. The soundings on the eastern coast of South Ame- 

 rica prove, that the ground shelves gradually from St Mary's 

 Point at the mouth of the La Plata to the distance of about 

 100 miles out at sea, and then in about 75 fathoms water, sud- 

 denly passes into a steep declivity. This abrupt passage, from 

 comparatively shallow to very deep water, is common along 

 sea-coasts, especially off the mouths of the great rivers. 



Independently of '..he discharge of rivers into the ocean, its 

 own waves, tides, and currents, distribute the alluvium after the 

 same manner, not only in estuaries, but upon all shores, and es- 



• Geolo^cal IVotes, p. 10. f Tr. of R. S. Ed. vol. ix. p. 17- 



