THE SEA BOTTOM 55 



sea, but this is not so because, as a result of the mixing of 

 the fresh water with the sea, the fine particles become 

 attracted electrically to one another and so form large 

 masses which readily sink just at the region of mixing of 

 the fresh water and the salt. 



At about 100 fathoms the action of waves and of trans- 

 porting currents ceases to be effective, and below this line, 

 which is called the mud-line and corresponds roughly to the 

 continental edge, the particles come to rest permanently 

 and from this region outwards the bed of the ocean is covered 

 everywhere with mud. Terrigenous deposits are found 

 both above and below this line, depending on its nearness 

 to land, and their nature naturally corresponds to the nature 

 of the land from which they are derived. Between tide 

 marks and in shallow waters they consist of sand, gravel, 

 shingle and boulders with mud in sheltered areas, rather 

 further from the shore are gravel, sands, banks of living 

 and dead shells, and, especially opposite estuaries, banks 

 of mud. If the land is volcanic this is reflected in the 

 volcanic nature of the deposits, if of coral origin the deposits 

 are formed of finely divided coral while, off continents, the 

 deposits are usually quartz gravels and sands together with 

 marls. The terrigenous deposits beyond the mud-line 

 were divided by Murray into five categories — blue, red, 

 green, volcanic and coral muds, the first three varying 

 somewhat in their chemical content but having much in 

 common while the nature of the last two is clear. 



In areas far from ]and the deposits on the sea bottom 

 consist exclusively of material which has dropped down 

 from the surface waters. This second type of marine 

 deposit has been called Pelagic by Murray (Plate 19), and 

 is made up of volcanic dust which has fallen on to the surface 

 of the water from the air, or of dead animals or plants 

 which, as they die, rain down upon the bottom far below. 



