218 THE OCEAN 



fragile of the foraminifera shells. In still 

 greater depths — say between 2000 and 3000 

 fathoms — only the heavier and more massive 

 foraminifera shells are present in the deposits, 

 and many of these are corroded and in process 

 of decomposition. In the greatest depths — 

 between 3000 and 5000 fathoms — it is often 

 difficult to find even a trace of these pelagic 

 shells. 



The shells must be removed by solution, 

 either while falling through the water to the 

 greater depths or shortly after reaching the 

 bottom. As soon as a lime-secreting organism 

 dies in the surface waters, it commences to 

 fall towards the bottom, and its shell is exposed 

 to solution from the solvent action of the sea- 

 water and of the carbonic acid present in the 

 sea-water, produced perhaps by the decom- 

 position of its own body. The great majority 

 of the shells are, however, only partially 

 removed during the first few hundred fathoms, 

 and therefore reach the bottom at the lesser 

 depths and accumulate there. More of the 

 shells may be dissolved as they lie at the 

 bottom, but in depths of a few hundreds of 

 fathoms they are soon covered up by the fall of 

 other shells, and thus protected to some extent 

 from further solution. Another point is that, 

 since sea-water can take up only a relatively 

 small quantity of calcium carbonate before 



