MUEEAT — DEPTH AND MAEINE DEPOSITS. 367 



high precipitation (the heavy rain of the summer monsoon), little evaporation, and large 

 additions of fi-esli water from the land-masses. 



Low salinities are likewise found in the Indian Ocean south of lat. 40° S. Between 

 lat. 40° and 50° S. the salinity is greater than 34 per thousand in spite of the heavy 

 rainfall. South of lat. 50° S. in the region of extensive ice-melting it is less than 34 per 

 thousand. 



With the exception of the work done in the Bay of Bengal, Arabian Sea, Red Sea, 

 Persian Gulf, and in the southerly part of the Indian Ocean, very few observations have 

 been made at different depths and at the bottom. The analysis of the bottom samples of 

 water taken by the ' Valdivia ' expedition throughout the Indian Ocean showed that they 

 contained 34*5 to 35'1 per thousand of solids in solution; north of Amsterdam Island in 

 lat. 33° S. at a depth of 3548 metres (1940 fathoms) 3474 per thousand ; between Ceylon 

 and the Maldives at a depth of 4133 metres (2260 fathoms) 3478 per thousand ; and 

 between the Seychelles and East Africa 34'66 to 35-02 per thousand. 



Deposits. 



With the increase in the number of soundings taken in recent years in the Indian 

 Ocean our knowledge of the deposits on its floor has become much more definite, and 

 this has led to the modification of the maps published in 1889 and 1891 * in several 

 particulars. The distribution of the various types of deposits over the floor of the 

 Indian Ocean, according to the present state of our knowledge, is shown by different 

 colours on the accompanying map (Plate 23), yellow being used for Coral Muds and 

 Sands, blue for the remaining types of terrigenous deposits, pink for Globigerina Ooze, 

 red for Pteropod Ooze, pale brown for Hed Clay, dark brown for Eadiolarian Ooze, and 

 green for Diatom Ooze. The apjjroximate areas covered by the different kinds of 

 deposits as measured by the planimeter, and the percentages to the total area of the 

 Indian Ocean as already defined, are as follows : — 



Square miles. 



Globigerina Ooze 14,711,000 



Red Clay 4,448,000 



Diatom Ooze 4,696,000 



Radiolarian Ooze 623,000 



Pteropod Ooze 23,000 



Coral Mud and Sand 291,000 



Other Terrigenous Deposits . . . 2,808,000 



27,600,000 



Percentage. 

 53-3 

 16-1 

 17-0 



2-3 



01 



1-0 

 10-2 



100-0 



This table shows that the pelagic or oceanic deposits cover about 89 per cent, of the 

 total area of the Indian Ocean and the terrigenous deposits about 11 per cent. More 

 than one-half of the entire sea-floor is covered by Globigerina Ooze, which is thus the 

 characteristic deposit of the Indian Ocean as Bed Clay is the characteristic deposit of 



* Scott. Geogr. Mag., vol. v. p. 405 (1889) ; 'Challenger' Keport on Deep-Sea Deposits, 1891, Chart I. 



