AGASSIZ: BAHAMAS. 169 



The amount of CaO in sea water from the Bahamas appears to be quite 

 constant, as far as shown by the analyses kindly made for me by Dr. 

 Richards. Water taken from the Tongue of the Ocean, No. 1, or from 

 the central part of the Great Bahama Bank half-way between Green Cay 

 and Exuma Sound, No. 2, or close to Exuma Sound on the eastern edge 

 of the bank from the west side of Flamingo Cay, No. 3, or near the 

 western edge of the bank south of New Providence, No. 4, showed only 

 slight variations in the composition of the sea water. The mean of two 

 measurements of 1,000 c.c. of sea water at 4° were as follows : No. 1 

 contained 0.585 gram CaO; -No. 2, 0.585 gram; No. 3, 0.580 gram; 

 No. 4, 0.588 gram. 



The percentage of carbonate of lime present in the bottoms surround- 

 ing coral reefs depends in part upon the presence of magnesia or upon 

 the admixture of decomposed vegetable or animal matter, and in part 

 upon the amount of moisture. In districts where the coral reefs occur 

 on shores consisting of stratified or volcanic rocks, the coral sand is also 

 more or less mixed with particles derived from the rocks composing the 

 basement or abutments of the reefs. In volcanic regions the limestones 

 are often greatly modified, either by heat or by their mixture with vol- 

 canic ash. The same action was also effective in modifying the reef 

 limestones of the dolomitic region of Tyrol. We also find in volcanic 

 regions a proportionally larger percentage of magnesia. 



A number of specimens of coral, of coral rock, and of coral beach sand, 

 from different localities, have been kindly analyzed for me by Prof. F. W. 

 Clark. The greatest variations in the percentage of carbonate of lime 

 are found in the specimens from the volcanic region of the Sandwich 

 Islands, and in the deposits of Sombrero (West Indies). The coralline 

 bottoms and coral sands from the Sandwich Islands also contained a 

 larger percentage of MgO than any similar bottoms from the Florida 

 Reefs or Bahama Banks. The elevated limestones of Cuba from the 

 Yunque are not dolomitic, and show no trace of magnesia. In the coral 

 sand mixed with volcanic ash the amount of magnesia in one case rose 

 to over eleven per cent. 



Piece of Siderastrea 



Very coarse sand, coral fragments, Tortugas 

 Finer sand, " " " . 



Tortugas, fairly coarse sand 



" finer sand 

 Key West, very fine beach coral sand 

 Loggerhead Key (stratified shore coral rock) 



