BOTTOM SAMPLES, MURRAY ISLAND, THE BAHAMAS, AND FLORIDA. 249 



SUMMARY ON THE MURRAY ISLAND SAMPLES. 



(i) The mechanical analyses show two classes, or grades, of deposits: 

 (a) beach deposits, subject to wave-action at or above high-tide level. Fine 

 gravel (i to 2 mm. in diameter) constitutes from 50 to 80 per cent of the 

 deposit; fine gravel and coarse sand combined range from a little more than 

 80 per cent up to nearly 99 per cent of the material; (b) sands between the reef 

 and the shore, in which coarse gravel is from 35 to about 66 per cent of the 

 material. Class (b) shows increase in percentage of medium and fine sand 

 as the shore is approached until 600 feet offshore; at 200 feet from shore there 

 is increase in the amount of fine gravel. In this class there is a larger per- 

 centage of coarse sand than of any other size. The small percentage of silt 

 and clay is attributed to outwash by currents. 



(2) The MgCOs' content averages 5.745 per cent in the samples behind 

 the reef. The relative importance of organisms 1,600 feet from shore is as 

 follows: (a) corals, 41.9 per cent; (b) calcareous algae, 32.6 per cent; (c) fora- 

 minifera, 12.4 per cent; moUusca, 10.2 per cent. At 200 feet from shore the 

 order is: calcareous algae, 42.5; corals, 34.6 per cent; mollusca, 15.2 percent; 

 foraminifera, 4.1 per cent. The investigation of Dr. Goldman shows the 

 possibility of correlating the chemical composition of an entire sample with 

 the chemical composition of its different constituents. 



(3) The higher percentage of magnesia in the specimen washed up on 

 the reef off the north end of the island may be due to secondary concentra- 

 tion. This subject needs further investigation. 



COMPOSITION OF TWO MURRAY ISLAND BOTTOM SAMPLES ACCORDING 



TO SOURCE OF MATERIAL.^ 



The following are the results of an attempt to determine the group 

 of organisms to which each grain in two different samples of " coral" sand be- 

 longed and to explain the chemical composition of the sands as determined by 

 direct chemical analysis; especially to account for the relative amounts of 

 calcium carbonate and magnesium carbonate. As far as possible, each of a 

 lot of grains was identified either by its external characters or by its internal 

 structure as revealed under the microscope. In this process, as many of the 

 grains had to be destroyed, the weight of grains of each kind had to be calcu- 

 lated from the weight of those preserved. This proved more difficult than 

 had been anticipated. From these weights the proportion of the leading 

 chemical constituents present was calculated by using figures for the composi- 

 tion of each organism, obtained (by the courtesy of Dr. F. W. Clarke) from 

 the manuscript of a paper by Clarke and Wheeler on the composition of the 

 inorganic constituents of marine invertebrates and calcareous algae. From 

 the composition thus computed for each group of organisms the composition 

 of the entire sand was calculated. 



'Hypothetical combination. ^he discussion of this topic is contributed by Dr. Marcus Isaac Goldman. 



