i8o DEPTHS OF THE OCEAN 



of sea-water surcharged with Hme. This Hme, though in a 

 metastable condition, finds no nuclei to deposit upon and 

 remains in solution, being carried about until it reaches an area 

 impoverished of lime by precipitation, when its condition 

 becomes stable, or until it is itself reprecipitated by coming 

 into the sphere of action of an ammonia-producing organism. 

 Thus the ocean as a whole remains just about saturated for 

 calcium carbonate. 



Oceanic calcium undergoes extensive circulation between 

 the dissolved and undissolved states. When calcareous frag- 

 ments fall on a clayey or muddy bottom, they fall into water 

 which can take up lime, and are dissolved as the water passes 

 over them, while on falling on distinctively calcareous deposits 

 like Pteropod ooze they fall into water-layers, immediately above 

 the bottom, which can dissolve no more lime. In either case 

 the lime depends for its redistribution on the slow processes of 

 diffusion by convection and other currents. In those areas 

 covered by Globigerina and Pteropod oozes lime is being 

 steadily withdrawn from the ocean. Over Red clay areas, on 

 the other hand, lime is being returned to the ocean. From 

 the state of saturation of sea-water we may infer that the 

 aggregate accessions of lime to the bottom exactly balance the 

 aggregate supply from land and from the direct decomposition 

 of submarine rocks. On the whole, lime at the present time 

 appears to be accumulating towards the equator. 



Another element present in the sea, magnesium, shares the 

 vicissitudes of calcium, but in a very minor degree. Magnesium, 

 in contrast with calcium, is very prone to form hydrated and 

 basic carbonates, ahd when the carbonate is precipitated from ' 

 solutions of magnesium salts, it comes down not in the anhydrous 

 crystalline form, but mainly as a trihydrate. Now solubility- 

 determinations in pure water and in salt-solutions indicate that 

 MgCO., as bicarbonate, in equilibrium with trihydrate, is of the 

 order of ten times more soluble than CaCOo. Hence the former 

 is far less likely to be precipitated than the latter, even though 

 there is about three times as much magnesium in the sea as 

 calcium. Moreover, it is well known that magnesium carbonate 

 is not readily brought down in presence of ammonia. Thus we 

 find that in living shells, corals, and algse the proportion of 

 MgCOg to CaCOg is usually below i per cent. It is observed, 

 however, that in dead carbonates, e.g. Coral sands and muds 

 and calcareous oozes which have been for a long time at the 

 bottom, there are markedly greater admixtures of magnesium. 



