RIVER DEPOSITS IN THE SEA. 163 



lakes, which are agitated to the bottom, the materials must neces- 

 sarily be distributed in planes very nearly horizontal, in consequence 

 of the influence of fluctuations at the surface. 



Lacustrine Limestones. Before we dismiss the subject of lakes, 

 it will be proper to take notice of another process tending also to fill 

 them with new deposits. Many streams which enter lakes carry 

 along, dissolved in their waters, a quantity of carbonate of lime, which 

 may afterwards, by the loss of carbonic acid from the water, fall in 

 calcareous sediment, and constitute beds of marl, or by the slow 

 absorption by mollusca, be converted into shells. In the latter case, 

 beds of LimnseaB, Paludinae, &c., are formed, and as the light argillace- 

 ous sediment entering such lakes is generally pretty equally diffused 

 through the waters, the result is a bed of marly clay full of fresh- 

 water shells. This process is daily going on, and in the course of a 

 few years canals and river courses, as well as ditches and ponds, are 

 choked by the abundant accumulation. In this manner, aided by 

 occasional inundations bringing layers of vegetable matter, or 

 detritus of the neighbouring country, many old lakes have become 

 entirely filled up ; and when the deposits are cut open for any pur- 

 pose, they present layers of peat, clay, shell-marl, and sand, a faith- 

 ful image, on a small scale, of those great fresh- water deposits which 

 mark the force and extent of ancient currents on the surface of the 

 earth. 



Deltas. The delivery of the sediment of rivers into quiet, tide- 

 less, land-locked seas is almost perfectly analogous to what happens 

 in a large lake ; but according to variation of circumstances, as the 

 river flows into the open ocean, and contends with strong tides and 

 sweeping currents, or discharges into a gulf, enters deep or shallow 

 water, the disposition of its sediment is different. The most remark- 

 able deltas at the mouths of rivers are formed round such as empty 

 themselves into tideless seas, as the Mediterranean, Black Sea, Cas- 

 pian, Baltic, &c., or into comparatively quiet bays of the ocean, as the 

 Bay of Bengal, the Gulf of Mexico ; and the least effects of this 

 nature are accumulated on coasts which are subject to be raked by 

 lateral currents of the sea. But it is probable that most deltas 

 originate in the materials scoured off the surface of the land by 

 estuarine waters, when the land last emerged from the ocean, which 

 are swept out from the river valley and remain at its junction with 

 the sea ; so that the existence or absence of a delta may depend upon 

 the rate at which the land is raised, and the angle of upheaval. 



Most of the great rivers which enter the Mediterranean are daily 

 increasing their deposits along the coasts, and spreading a quantity of 

 sediment over the general bed of the sea. The Mediterranean has 

 been proved by a line of soundings on the Skerki Shoal from the 

 African to the Sicilian coast, varying unequally from 7 to 91 fathoms, 

 to be divided into two basins. In the western portion, near Gib- 

 raltar, the bottom, consisting of sand and shells, has been reached at 

 5880 feet, and in the straits at 4200 feet, though between Capes 

 Trafalgar and Spartel the depth is nowhere 1200 feet. Almost under 



