PALEOZOIC CORAL REEFS 241 



Decrease in temperature with increase of depth in the ocean is well 

 known. 



TEMPERATURE 



Dana long ago showed the minimum temperature of the year to be a 

 critical factor in determining the possibility of reef corals living in a 

 locality where conditions were otherwise favorable for their growth, and 

 designated 68 degrees Fahrenheit as the lowest temperature these organ- 

 isms could stand. The mean annual temperature of the water must be 

 above 70 degrees Fahrenheit. Therefore a high temperature is neces- 

 sary for the vigorous growth of reef -building corals. 



The importance of high temperature for the secretion of carbonate of 

 lime in quantity by marine organisms can best be presented by giving 

 the words of Sir John Murray:^ 



"During the past year or two I have carefully collected all the available 

 temperatures of the surface waters of the ocean, and. from these have con- 

 structed a map showing the annual range of temperature in different regions 

 of the ocean. This map shows that the surface of the sea may be grouped 

 into five great zones, namely: (1) A nearly continuous equatorial zone, where 

 the temperature is high and the range throughout the year does nqt exceed 

 10 degrees Fahrenheit. This zone includes all the principal coral-reef regions. 

 (2 and 3) Two polar zones, where the temperature is low and the annual 

 range likewise does not exceed 10 degrees Fahrenheit. In these zones there 

 are relatively few lime-secreting organisms. (4 and 5) Two regions lying 

 between the equatorial zone and the two polar zones, where a wide range ot 

 temperature occurs between the different seasons (the annual range amount- 

 ing to as much as 52 degrees Fahrenheit in some places). In these temperate 

 regions the secretion of carbonate of lime appears to be much more active m 

 the warmer than in the colder months. It thus appears that the most favor- 

 able conditions for lime-secreting organisms are met with in the warm, 

 equable tropical waters of the ocean, and here, as a matter of fact, we find 

 the greatest development of corals and the largest number of lime-secreting 

 pelagic organisms. In the polar areas and in the cold water of the deep sea 

 there is, as is well known, a feeble development of all carbonate of lime 

 structures in marine organisms. 



"From experiments which have been carried out by Mr. Irvine and myself 

 at the Granton Marine Station we have reason to believe that this distribution 

 is dependent primarily on the physical or temperature conditions of the 

 oceanic waters. When carbonate of lime is precipitated by alkaline solutions, 

 such as carbonate of soda, carbonate of ammonia, or carbonate of methyl- 

 amine, the effect of temperature is very marked, and it appears to be the case 

 that the secretion of carbonate of lime by organisms is of the nature of a fine 

 precipitation in the interior of the soft structures.^ If we add sufficient car- 

 bonate of ammonia to sea water at different temperatures to convert all the 



° Natural Science, vol. ii, 1897, pp. 25-27. 



« Murray and Irvine : Proc. Roy. Soc. Edin., vol. xvii, 1890, pp. 79-109. 



