BRANNER: THE STONE REEFS OF BRAZIL. 265 



The chief interest in these analyses hes in the difference in magnesium 

 contents between the rock of the living coral and tlie old coral rock : 

 excepting the Millepora none of the living corals contain more than a 

 half of one per cent of magnesium carbonate, while the old rock con- 

 tains nearly 13 per cent. 



Evidently coral polyps secrete pure lime carbonate skeletons, and when 

 a coral reef stands for a long period saturated with sea water, some of the 

 lime of the coral mass is replaced by magnesium from the sea water, and 

 a dolomite, or dolomitic limestone, is thus eventually produced. 



It seems evident also that this process of dolomitization could only 

 take place beneath the sea where magnesium water is available, and in 

 material sufficiently porous to permit some circulation of sea water. 



Since these results were obtained I have found that as long ao-o as 

 1846 Dana had analyses made of the skeletons of living corals, and that 

 very little magnesia was found in them.^ 



In 1852 he reported an analysis by Silliman of "the coral limestone 

 of the elevated coral island Matea" in which 38.07 percent of carbonate 

 of magnesia was found. These facts led Dana to infer that the lime 

 carbonate had been replaced by magnesia.^ 



The results obtained from the Brazilian reefs agree very closely with 

 those obtained by Dana for the Pacific corals except that the analysis of 

 the older Brazilian coral rock was made of a rock still within the reach 

 of the sea water, and in which dolomitization was apparently in process, 

 while the analysis of the Pacific coral rock was made from materials far 

 beyond the reach of the sea and in Avhich dolomitization had evidently 

 proceeded much further. 



Another matter of interest in connection with the old reef rock is that 

 the structure of the mass seems to be disappearing in proportion as the 

 dolomitization takes place. In the fresh materials there is no difficulty 

 in determining the forms of the organisms that have produced the rock, 

 while the structure of the old reef rock is much obscured, and most of 

 the organisms quite impossible of identification. 



1 On the chemical composition of the calcareous corals. By B. Silliman, Jr. 

 Amer. Journ. Sci., ser. 2, 1846, Vol. I., p. 189-100. 



'^ James D. Dana. On coral reefs and islands. Amer. Journ. Sci., ser. 2, 1852, 

 XIV, p 82. 



