1895. RESULTS OF " CHALLENGER " EXPEDITION. 21 



actual presence of carbonate of lime for the formation of hard 

 calcareous egg-shells and tests respectively ; and it was found that, pro- 

 vided lime was present in their food, in such forms as phosphate, sili- 

 cate, sulphate, or nitrate, the animals found no difficulty in forming a 

 sufficiency of carbonate of lime for their shells and tests. No shells 

 were formed when salts of strontium or magnesium were substituted 

 for lime salts. The formation of carbonate of lime is presumably due 

 to the influence of ammonium carbonate, one of the ultimate products 

 of the decomposition of excreta. This was so far confirmed in that 

 the addition of ammonium carbonate to sea-water resulted in a pre- 

 cipitate having the same composition as that of corals and shells. 



The separation of such a precipitate took place much more 

 rapidly at a high temperature (80° Fahr.) than at a low (as 35° to 

 40"^ Fahr.). This may explain why in tropical latitudes the 

 secretion of carbonate of lime by organisms is much more abundant 

 than in polar waters, as, for instance, by corals, molluscs, foraminifera 

 and algae. 



Diatoms were experimentally shown to be capable of forming 

 siliceous tests when grown in water in which fine clayey matter was 

 suspended ; and it is interesting to note in this connection that such 

 siliceous organisms occur in greatest abundance in those regions of 

 the ocean in which there is a large admixture of fresh water holding 

 in suspension detrital matter from the land ; for instance, near river 

 mouths, the Polar regions, and the North-west Pacific, where the 

 power of the water of holding matter in suspension is, from its low 

 specific gravity, relatively great. It is thus probable that marine 

 siliceous organisms do not depend, wholly at least, upon the silica in 

 solution in the sea for their skeletal parts, as has been generally held. 



The blue muds laid down in the proximity of the land are com- 

 paratively rich in organic matters, the decomposition of which 

 initiates, in the circumambient sea-water, the reduction of the 

 sulphates to sulphides and carbonates, thus increasing the alkalinity 

 of the sea-water associated with the muds. This increased alkalinity 

 lends to the water an additional power of decomposing the sedimentary 

 material on the bottom, and in this way brings about numerous 

 chemical changes, such as the formation of zeolites, phosphatic and 

 manganese nodules, and glauconite. As iron is a constant con- 

 stituent of these deposits, the ultimate result of the chemical 

 changes is the formation of sulphide of iron, giving to these muds 

 their characteristic bluish-black colour. In the same way sulphide of 

 manganese is formed in these muds ; but, unlike the sulphide of iron, 

 it is decomposed by carbonic acid, forming soluble manganese 

 bicarbonate, which remains in solution until it meets with an excess 

 of oxygen, as in the water overlying the mud or on the surface of 

 current-swept ridges. It is then precipitated as a higher oxide of 

 manganese, which may again be reduced, go into solution, and be 

 re-precipitated elsewhere. There will obviously tend to be an 



