370 PERCY SLADEN TRUST EXPEDITION. 



These figures show that the area covered by deposits containing less than 25 per cent, 

 of calcium carbonate is the largest (equal to 37 per cent, of the total area of the Indian 

 Ocean), though the area covered by deposits containing 50 to 75 per cent, of calcium 

 carbonate is almost equally extensive (35 per cent.) ; while the area covered by deposits 

 containing over 75 per cent, of calcium carbonate is about 15 per cent., and that covered 

 by deposits containing 25 to 50 per cent, of calcium carbonate about 13 per cent. Thus 

 one-half of the sea-floor of the Indian Ocean is covered by deposits containing over 

 50 per cent, of calcium carbonate, and the other half by deposits containing less than 

 50 per cent, of calcium carbonate. 



Deposits with a low percentage of calcium carbonate (under 50 per cent.) occur 

 (1) in the far south (approximately south of lat. 50° S.), (2) in the deep water to the 

 south of Australia, (3) in the deep water in the eastern half of the tropical Indian 

 Ocean as far west as the longitude of the peninsula of India and as far south, as 

 lat. 30° S., (4.) in the deep water to the east of Southern Madagascar, and (5) in the 

 shallow^er waters along the coasts of the continents and continental islands. Deposits 

 with a high percentage of calcium carbonate (over 50 per cent.) occur (1) throughout 

 almost the entire western half of the Indian Ocean north of 50° S., (2) in the eastern 

 half of the ocean between 25° and 50° S. and between the Equator and 15° N., and 

 (3) off the coasts of the East Indian islands bordered by coral-reefs. The deposits of the 

 Arabian Sea have on the whole a higher percentage of calcium carbonate than those of 

 the Bay of Bengal, and the deposits of the Bed Sea than those of the Persian Gulf. 



On comparing this calcium carbonate map with the deposit map (Plate 23), it will be 

 observed that the pale shades of colour denoting a low percentage of calcium carbonate 

 correspond approximately with the areas occupied by terrigenous deposits other than 

 Coral Muds and Sands, and with the areas occupied by Hcl Clays, Pi.adiolarian Oozes, 

 and Diatom Oozes; while the dark shades of colour denoting a high percentage of 

 calcium carbonate correspond with the areas occupied by Coral Muds and Sands, 

 Globigerina Oozes, and Pteropod Oozes. 



"When compared with the depth-map (Plate 22), this calcium carbonate map shows 

 that the pale shades of colour denoting a low percentage of calcium carbonate cori'espond 

 generally with the shallower waters around the continental shores in low latitudes, and 

 with both deep and shallow waters in the far south, where calcareous organisms are not 

 abundant at the surface, and with the very deep areas in the tropical and subtropical 

 regions, where the calcium carbonate is apparently removed in solution ; \^hile the dark 

 shades of colour correspond generally with the m;)derate depths in the open ocean and 

 with the shallower waters around coral-reefs and islands. 



