IV DEPTHS AND DEPOSITS OF THE OCEAN i8i 



This enrichment in magnesium is a famihar phenomenon at 

 shallow depths, notably in and about coral reefs. It has also been 

 shown on the basis of the "Challenger" analyses that bottom- 

 deposits contain more MgCOg in proportion to CaCO,, the less 

 calcareous they are. Granted that accumulation of magnesium 

 does take place, there are two explanations which have been 

 offered, viz. (i) that deposited lime is dissolved away in prefer- 

 ence to magnesia, and (2) that a kind of pseudomorphosis by 

 the interaction of calcium carbonate and dissolved magnesium 

 salts sets in. Both assume MgCOg to be less soluble than 

 CaCOg, and both may well hold good. Even if MgCOg were 

 precipitated as trihydrate, it would sooner or later change into 

 the anhydrous form, or rather into dolomite, that being the most 

 stable and final form. Perhaps this transformation has already 

 been effected in the shell. But dolomite is well known to be 

 less soluble in carbonated water than calcite. As regards 

 enrichment by accession of magnesia, this could only take place 

 if sea-water were nearly saturated for MgC03, a matter which 

 has not hitherto been put to the test ; sea-water is certainly not 

 saturated for the trihydrate, but it is conceivable that anhydrous 

 calcium carbonate would determine the deposition of magnesium 

 carbonate in the anhydrous form, which is relatively very 

 insoluble. Now when calcium carbonate goes into solution, 

 the concentration of CO3" ions in its neighbourhood is increased, 

 whereby the solubility of any other carbonate is lowered ; thus 

 a precipitation of MgCOg might ensue. However, if this action 

 were capable of taking place generally, we should expect a far 

 larger percentage of magnesia in the purer calcareous oozes. 

 On the whole, therefore, the enrichment in magnesia in deep- 

 sea deposits proper is rather to be sought in preferential 

 dissolution of lime. 



The total magnesium carbonate at the bottom of the sea 

 only amounts to a small percentage of the total calcium carbonate. 

 Since the proportion of Mg to Ca, primarily in rocks and 

 secondarily in river-waters, is much larger than this, it is clear 

 that dissolved magnesium is accumulating in the ocean. 



Another of the more important constituents of sea-water, Sulphur. 

 sulphur, suffers transference, on a modest scale, from the sea to 

 the bottom. Nowhere in the deposits of the open ocean has 

 sulphur been found to occur as sulphate, but in those very 

 extensive landward areas where Blue muds form the deposit 

 there is always a small percentage of ferrous sulphide and 

 of free sulphur, which are directly or indirectly derived from 



