BOTTOM SAMPLES, MURRAY ISLAND, THE BAHAMAS, AND FLORIDA. 277 



ELEVATED BAHAMAN OOLITES. 



No mechanical analyses of these were attempted, as they would have 

 been impracticable because of induration of the rocks. 



Marine-bedded oolite: Sharp Rock Point, Andros Island, Chemical analysis, page 



269. MgCOj, trace; CaCOs, 99.56 per cent. 

 Wind-blown oolite: (a) Queen's Stairway, Nassau. Partial chemical analysis. 



MgCOs, 0.77 per cent; CaCOj, 99.19 per cent, (b) North ridge at Seven Hills, 



New Providence Island. Partial chemical analysis, page 270. MgCOs, trace; 



CaCOs, 99.98 per cent. 



These samples differ from all others, except the oolite sand from Great 

 Bahama Bank, specimen No. 71, in the low percentage of MgCOg, and the 

 high percentage, over 99, of CaCOs. Specimen No. 71 has 0.38 per cent 

 of MgCOs and 99.49 per cent of CaCOs. Therefore in chemical composi- 

 tion these oolites are more nearly pure calcium carbonate than any other 

 samples at present known from Florida and the Bahamas. This means 

 that although certain leading facts in the formation of oolites have been 

 discovered, conditions absolutely similar to those under which the Bahaman 

 and Floridian oolites formed have not yet been discovered among processes 

 now in operation. The conditions for the formation of such rocks require 

 a more complete suppression of organisms which can contribute MgCOa 

 to the sediments than those which now prevail in Florida and the Bahamas. 

 The evaluation of the difference between the oolitic muds now forming in 

 Floridian and Bahaman waters and the oolites of greater geologic age exposed 

 in the same areas must be left for future investigation, but that they are 

 not precisely the same is apparent, and the significance of the difference 

 should be investigated. 



Here it should also be pointed out that although a certain amount of 

 zonal structure is sometimes found in the oolites of the oolitic muds, there 

 is never so large a number of well-defined concentric shells (or "skins," to 

 use a word suggested by Dr. Merwin) in the mud grains as are usual in the 

 grains of the older rock. That the material has accumulated zonally is 

 obvious, but the precise processes which have determined the zonal arrange- 

 ment have not been discovered. 



A plausible hypothesis is illustrated by the formation of pisolites in 

 hot springs, in which CaCOs is deposited as more or less spherical bodies 

 from a supersaturated solution. The concretions are kept in motion by 

 ebullition, thereby permitting the formation of successive concentric shells 

 of CaCOs-^ The principle involved is that the movement of a nucleus or a 

 concretion already initiated within a solution in which CaCOa is being pre- 

 cipitated furnishes an opportunity for concentric enlargement. Occasional 

 desiccation will emphasize the zonal structure. Of course there is no evidence 

 of hot-spring action in areas underlain by marine oolites, but agencies 

 favoring periodic precipitation combined with the occasional shifting of the 

 position of oolites once started apparently will account for the phenomena. 



'Hayes, C. W., Science, n. s., vol. 33, p. 550, 191 1. 



