22 NATURAL SCIENCE. July, 



accamulation of manganese in any region which is pecuUarly free 

 from reducing matter, as over the red-clay areas of the great ocean 

 basins. These phenomena have been adduced to explain the forma- 

 tion of manganese nodules and concretions. (PL xv. Figs, i and 2.) 



Attempts have been made towards the explanation of the forma- 

 tion of dolomites. The fact that corals placed in a solution of copper 

 svilphate become superficially converted into pseudo-morphic copper 

 carbonate, hinted at a possible solution, viz., that the dolomites arise 

 by the interchange of magnesium and calcium in lime formations 

 when exposed to such a solution of magnesium salts as occurs in 

 modern seas. To test this a Tridacna (Giant Clam) shell, whose 

 great age was attested by its size, was examined chemically in 

 various parts, with the result that the outside or oldest part was 

 found to be much richer in magnesia than the inside or more recent 



formation. 



Robert Irvine. 



Marine Deposits. 



The researches of the " Challenger " gave the first definite infor- 

 mation regarding the sediment now accumulating in various regions 

 and depths on the ocean floor. A study of the " Challenger " collec- 

 tions, together with those made by subsequent deep-sea expeditions, 

 has enabled Messrs. Murray and Renard to give a comprehensive 

 sketch of the composition and distribution of these deposits, which 

 has proved of the highest interest to geologists and physical 

 geographers. 



The marine accumulations are divided into two great classes — 

 Terrigenous Deposits and Pelagic Deposits. (See Plate iii.) 



The Terrigenous Deposits are those formed in shallow and deep- 

 water close to the shore, and are mostly made up of mineral particles 

 and detrital matters washed down from the dry land by rain, or torn 

 from the coasts by the action of waves and currents. Their peculiari- 

 ties are thus determined by the character of the adjacent land. Blue 

 Muds prevail in enclosed seas and off the mouths of large rivers, 

 while off bold coasts Green Sands and Muds with much glauconite 

 are more abundant ; off volcanic islands are Volcanic Sands and 

 Muds, while off coral islands Coral Sands and Muds are found. The 

 rock-fragments and mineral particles diminish in size and abundance 

 with increasing distance from land, and it may be stated generally 

 that, at an average distance of 200 miles from the shore, particles of 

 quartz and other continental minerals exceeding -i mm. in diameter 

 rarely make up any appreciable part of the deposit, except in those 

 regions affected by floating ice. Thus, at their seaward margin, the 

 terrigenous deposits pass gradually into pelagic deposits. 



The character of the Pelagic Deposits is determined by the con- 

 ditions prevailing at the surface of the ocean, since they are largely 

 made up of the shells and skeletons of pelagic organisms. In tropical 



