184 CARNEGIE INSTITUTION OF WASHINGTON. 



Bahamas, but there is practically none in the mud along the Florida keys a 

 short distance from the mainland. There is in this region an instance of an 

 extensive deposit of an almost pure limestone forming near the shore of a 

 land-mass which is of low relief and across which no large streams bear 

 detritus. The investigations also show that in areas in which oolitic lime- 

 stone is being formed testaceous as well as most other organisms are scarce, 

 in this respect corresponding to the geologically older oolites. The chemical 

 analyses show the presence of a very little phosphate, a fact which may be 

 correlated with the general paucity of organisms. Another fact of interest 

 is the small percentage of magnesium in both the bottom muds and in the 

 Pleistocene oolites, the range of magnesium oxide (so expressed in the 

 analyses) being from a trace to 2.14 per cent. In one instance (shore mate- 

 rial from the west side of Andros Island) there is 5.82 per cent of magnesium 

 oxide, the highest percentage recorded. 



During the field season of 1913 the study of the bottom deposits of the 

 area and of geologic processes in general for the purpose of obtaining a 

 proper basis for interpreting the geologic history of the area was continued. 

 The bottom deposits of Marquesas and Tortugas lagoons received special 

 attention. Among the geologic processes investigated were the effect of 

 waves and currents in building and shaping banks, the relations of the banks 

 and keys of the Marquesas and Tortugas to waves and currents, cutting by 

 waves and currents, the building of terraces by waves; a special investi- 

 gation of submarine solution by flowing sea-water was undertaken in asso- 

 ciation with Mr. R. B. Dole, of the U. S. Geological Survey. As in each 

 instance positive results were obtained, and as the principal factors in the 

 formation of Marquesas and Tortugas atolls and their inclosed lagoons now 

 seem clear, although some details still require elucidation, a brief account of 

 the investigations and the conclusions based on them has been written and 

 will appear in Publication No. 182 of this Institution. This account is 

 followed by a sketch of the geologic history of the entire Florida reef tract, 

 and comparisons are made with Andros Island, Bahamas, Cuba, and some 

 of the tropical Pacific areas. 



A report prepared by Mr. Dole, giving a summary statement of the results 

 of his examination of the Tortugas water for salinity and carbon dioxide, will 

 appear in PubHcation No. 182 of this Institution, now going through press. 



