170 Mr. De la Beche's Notes on the Formation 



V. Discharge of Riveis into Tideless Seas. 



These discharges are more or less modified, according to 

 the open waters and prevalent winds to which they are ex- 

 posed ; and in general they tend to push forward deltas be- 

 fore them (fig. 4.), which more or less protrude according to 

 the depth of water into which the rivers deliver themselves, 

 the greater or less shelter of the coasts, the quantity and na- 

 ture of the detritus held in mechanical suspension, and the 

 force of the current. Those rivers which push forward great 

 deltas, such as the Nile, Rhone, Po, Danube, and Volga, bear 

 mud and silt before them, and of these materials the deltas 

 are almost wholly composed. The rivers which bear down 

 pebbles into tideless seas are short, rapid, and of the torrent 

 kind. Most frequently, from the high and mountainous na- 

 ture of the coasts, the gravel is deposited in deep water, and 

 therefore, being out of the influence of breakers and waves, re- 

 mains quietly at the bottom, unless carried by currents suffi- 

 ciently strong to remove it: of currents so strong we have not 

 any known examples in a tideless sea. Nice will afford a good 

 example of such deposits. The Var and the Paglion bring 

 down pebbles into the Mediterranean, which are almost imme- 

 diately conveyed into deep water and remain undisturbed, ex- 

 tending but a short distance seaward ; for the gravel soundings 

 obtained further from the coast must not be confounded with 

 the river detritus, such soundings being upon the prolongation 

 of the tertiary conglomerates beneath the level of the sea*. 



If tideless seas, such as the Mediterranean, Black and Cas- 

 pian Seas, were to become dry, these deltas and gravel deposits 

 would be very apparent, both more or less presenting the ad- 

 vance noticed in the case of lakes, and we should not have 

 beds of detritus parallel to the coasts, but a series of projections 

 with a stratification peculiar to each, but not common to the 

 whole. 



Upon a review of the phaenomena productive of gravels on 

 sea beaches and in river beds, it will, I think, appear pro- 

 bable that in neither case could pebbles be furnished in such 

 a way as to afford materials for those great deposits of gravel 

 and conglomerate, which we observe in rocks that must have 

 been formed at various epochs; the coasts present lines of 

 shingle or sand, more advanced in cases of the embouchures 

 of rivers into tideless, generally calm, or nearly tideless seas, 

 and the rivers afford mere lines of pebbles. To make these 



_ _ * It should always be recollected tliat in gravel soundings the probabi- 

 lities are as great of finding rounded pebbles beneath the sea as on the sur- 

 face of the land. 



materials 



