BOTTOM SAMPLES, MURRAY ISLAND, THE BAHAMAS, AND FLORIDA. 259 



chemical precipitates, also [unidentified] material such as described for [the 

 clay of] sample 27353a. "^ In regard to the calcite grains it should be noted 

 that they considerably exceeded in diameter the maximum diameter of 

 the "clay" portion. It must be concluded, therefore, that they were not 

 originally present in the sample, but were precipitated in evaporating down 

 the water in which the "clay" was suspended. The "clays" of two other 

 samples from the same region were examined and like sample No. 2735313 

 were not found to contain any precipitated calcite. 



Before comparing the results of the final summary with the direct 

 analysis of the sample, some allowance must be made for the contents of 

 the silt and clay. Trustworthy indication as to the probable composition 

 of these portions is lacking, but their amount is so small that unless some great 

 divergence from the composition of the other portions were indicated, the 

 effect in changing the ratio of Ca to Mg in the total sample would not be 

 appreciable in any case. 



Analyses of sample No. 27S53a, line I, i, 600 feet from shore. 



'Ca organisms = Corals and moUusks; Mg organisms = Algse and foraminifera. 

 'Includes coccolithophoridae {^Pontosphtsra). 



The foregoing table indicates a tendency for the proportion of magnesia 

 to increase in the finer portions, and this may continue into the silt; but 

 whether it also continues into the clay is very uncertain. The assumption 

 that it is carried into the silt is supported by the apparent abundance of 

 foraminifera in this portion, which is in conformity with their relative increase 

 in the very fine sand; but what effect the increasing abundance of coccolitho- 

 phoridae has on the chemical composition is not known, since these forms 

 have not been segregated and analyzed. It has merely been assumed that 

 they have a composition similar to the algse; but, in any case, the amount of 

 silt is so small that all of it must undoubtedly be thrown to the CaCOa. 

 In the "clay" the proportion of the coccolithophoridae appears even greater; 

 then, too, the undetermined mineral ingredient and probably other indeter- 

 minate factors enter, so that the factors controlling the composition of this 

 portion are of an entirely new sort. Furthermore, it seems proper to place 

 nearly all the SiOa and (Al, Fe)203 into this portion, though a small part is 

 undoubtedly present in the other portions. Charge say 0.5 per cent out of 



'Letter of May 1916, from Dr. H. E. Merwin. 



