Conglomerate and Gravel Deposits. 401 



The fiver in this case escapes through a gap which it has formed 

 in the bank itself. 



It is obvious that in these cases the sea rejects the detritus it 

 receives from the rivers, and forces it back, with the cliff detri- 

 tus, upon the land. 



The great flats on the western coast of South America are 

 excellent examples of mud and sandy detritus forced back upon 

 the land. 



V. Discharge of Rivers into Tideless Seas. 



These discharges are more or less modified, according to the 

 open waters and prevalent winds to which they are exposed ; 

 and in general they tend to push forward deltas before them, 

 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 nature 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 com- 

 posed. The rivers which bear down pebbles into tideless seas 

 are short, rapid, and of the torrent kind. Most frequently, 

 from the high mountainous nature of the coasts, the gravel is 

 deposited in deep water, and therefore, being out of the in- 

 fluence of breakers and waves, remains quietly at the bottom, 

 unless carried by currents sufficiently strong to remove it. of 

 currents so strong we have not any known examples in a tideless 

 sea. Nice will afford a good examples of such deposits. The 

 Var and the Paglion bringdown pebbles into the Mediterranean, 

 which are almost immediately conveyed into deep water and 

 remain undisturbed, extending 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 

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

 whole. 



Upon a review of the phcenomena productive of gravels on 

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



• It should always be recollected that in gravel soundings the probabilities are 

 as great of finding rtfan&d pebbles beneath the sea as on the surface of the land. 



3 K 



