BRANNER: THE STONE REEFS OF BRAZIL. 265 
The chief interest in these analyses lies in the difference in magnesium 
contents between the rock of the living coral and the old coral rock : 
excepting the Millepora none of tho 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 ago 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 per cent 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carbonate had been replaced by magnesia.”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 which 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 В. Silliman, Jr. 
Amer. Journ. Sci., ser. 2, 1846, Vol. I., p. 189-199. 
2 James D. Dana. On coral reefs and islands. Amer. Journ. Sci., ser, 2, 1852, 
XIV, p. 82. 
