196 THE OCEAN 



all depths and in all stages of decomposition. 

 The materials of submarine eruptions are 

 doubtless likewise present, but when of small 

 size they are difficult to distinguish from those 

 borne from land-surfaces. With the excep- 

 tion of certain secondary products formed in 

 situ, these are the chief sources of the mineral 

 constituents of marine deposits. 



Next in importance to the above-mentioned 

 mineral particles are the remains of organisms, 

 and chiefly the shells and skeletons of those 

 which secrete calcium carbonate. In shallow 

 water the remains of calcareous algae, of 

 foraminifera, of corals, of molluscs, and of 

 other marine invertebrates, form immense 

 deposits, as for instance off coral reefs, where 

 the percentage of calcium carbonate in the 

 deposit often exceeds 90. These calcareous 

 deposits are especially characteristic of the 

 tropical regions, but the calcium carbonate 

 shells are present in the deposits of all lati- 

 tudes. 



In deposits laid down on the floor of the 

 ocean far from land it is not the shells of bot- 

 tom-living (benthonic) organisms that pre- 

 dominate, but the shells of pelagic (planktonic) 

 organisms, such as coccospheres, rhabdo- 

 spheres, pelagic foraminifera, pteropods and 

 other molluscs. In all but the very greatest 

 depths these shells and skeletons accumulate. 



