MUREAT— DEPTH AND MARINE DEPOSITS. 369 



larger around Cocos-Keeling and Christmas Islands between lat. 7° and 22° S. and 

 long. 88° and 108° E., the smaller to the north-west of the Seychelles in lat. 2° to 5° S. 

 and long. 49° to 62° E. 



Diatom Ooze is distinguished by the prominence of Diatom frustules, and is therefore 

 characteristic of those regions in which these pelagic algae swarm in the surface waters. 

 It occurs in the Southern Indian Ocean as part of the circumpolar band bordering the 

 zone of Bine Mud that surrounds the Antarctic Continent, between approximately 

 lat. 45° and 65° S., with an isolated small area to tlie north of the 40tli parallel in 

 long. 58° E. 



Coral 3Iud and Sand. — These deposits occur off coral-islands and coral-reefs, and are 

 chiefly made up of the fragments of organisms living in the shallow waters and on the 

 reefs, the Coral Sand occurring in the shallower waters nearer the reefs, where the 

 calcareous fragments are larger and the more finely divided calcareous matter less 

 abundant than in the deeper water occupied by the Coral Mud. This type of deposit is 

 found in the Indian Ocean around the different groups of islands, such as the Laccadive, 

 Maldive, Seychelles, Amirante, Chagos, and Comoro groups of islands, off the north 

 coast of Madagascar, round Mauritius and Reunion in the west, and round the Andaman 

 and Nicobar Islands and off Sumatra in the east. 



Other Terrigenous Deposits. — The terrigenous deposits (excluding Coral Mud and 

 Sand) are found everywhere around the continental shores and in all enclosed and 

 partially enclosed seas. The principal type is Blue Mud, composed mainly of materials 

 derived from the disintegration of the land-masses, quartz being the characteristic 

 mineral species, replaced in certain localities, like the Agulhas Bank, by Green Muds 

 and Sands, in which the secondary mineral glauconite is the characteristic species. In 

 the Indian Ocean the area covered by these terrigenous deposits is now estimated to be 

 smaller than it was in 1889 ; they do not, as a rule, extend so far from land as was then 

 supposed. 



Percentage of Calcium Carbonate in the Deposits. 



Plate 24 accompanying this paper is the first attempt to show the amount of calcium 

 carbonate in the deposits of the Indian Ocean ; a similar map for the Pacific Ocean was 

 published last year *. Four shades of blue-green colour are used to indicate the 

 percentage of calcium carbonate in the deposits, the palest shade for those regions 

 where the deposits contain less than 25 per cent, of calcium carbonate, progressively 

 darker shades for those regions where the deposits contain 25 to 50 per cent, and 50 to 

 75 per cent., and the darkest shade for those regions where the deposits contain more 

 than 75 per cent, of calcium carbonate. 



The areas marked out by these four shades have been measured with the planimeter, 

 giving the following approximate results: — 



* See Murray and Lee, " The Depth and Marine Deposits of the Pacific Ocean," Mem. Mus. Comp. Zool. 

 (Cambridge, Mass.), vol. xxxviii. no. 1 (June, 1909). 



