90 
Proceedings of Royal Society of Edinburgh. [sess. 
arising from the decomposition of animal products, in presence of 
the sulphate of lime of sea water, becomes carbonate of lime and 
sulphate of ammonia.* Thus the whole of the lime salts in sea 
water may, in these circumstances, be changed by this reaction into 
carbonate, and may in this way be presented to the coral and shell 
builders in a form suitable for their requirements. The temperature 
of the water is of great importance in this reaction. In cold water, 
of which the great bulk of the ocean consists, the decomposition of 
nitrogenous organic matter is greatly retarded, whereas, in tropical 
surface waters, it proceeds with great rapidity. Here, then, we 
have probably the explanation of the great development of the 
massive structures formed by lime-secreting organisms in the coral 
reef regions, which, as has been pointed out, are also the regions of 
highest and most uniform temperature in the ocean. In the same 
way we may account for the great extension of lime-secreting pelagic 
organisms in the tropical surface currents that flow north and south 
from the equator. Thus coral reef-builders and pelagic organisms 
may not only benefit by the decomposition products arising from 
their own effete matters, but also from the undecomposed nitrogenous 
matter carried to equatorial regions from the cold water of the deep 
sea or from polar regions 
The quantity of carbonate of lime normally in sea water is ex- 
ceedingly small, and the opinion hitherto held seems to have been 
that lime-secreting organisms had to pump enormous quantities of 
sea water through their bodies in order to be able to separate out a 
sufficient quantity to form their shells and skeletons, f It seems 
* The sulphate of ammonia is in turn absorbed by the marine flora which 
forms the food of the marine fauna, and is in part resolved into nitrates and 
free nitrogen. 
f Bischof, Ghem. and Tiny s. Geol ., vol. i. p. 180, says: — “In order to form 
a conception of what testacea are capable of effecting by organic agency, I 
determined the weight of ten oysters and their shells. After they had been 
opened the enclosed sea water was, as far as possible, removed. The weight of 
the shells varied from 278 to 7 '57 that of the oysters 
“No one can doubt that it was the carbonate of lime dissolved in the sea 
water which alone furnished the material for the formation of these shells. 
“If now we assume that the sea water contains of carbonate of lime, 
and that the oysters are capable of deriving all their calcareous substance from 
the water by organic agency, it follows that the above number of oysters re- 
quired for the formation of their shells 345 to 587 lbs., or 5 '2 to 8 '9 cubic feet 
of sea water. 
“This quantity is from 27,760 to 75,714 times the weight of their shells. 
