58 Papers from the Marine Biological Laboratory at Tortugas. 



to the northwest entrance, or is on the east, northeast, and north sides. 

 The keys and banks between the southeast and southwest passages and 

 between the southwest and northwest passages are minor as compared with 

 the larger one mentioned. It may also be pointed out that the Marquesas 

 occur just west of Boca Grande Channel, and thac the Tortugas lie just 

 west of Rebecca Channel. 



That the Marquesas were formed through the agency of waves and 

 currents operating under proper conditions has been evident for some time, 

 but the processes by which the Tortugas were built were less clear. It was 

 thought possible that submarine solution had been efficacious in the latter, 

 while it was apparent that it v/as ineffective in the former. In order to 

 answer the question for the Tortugas, two lines of inquiry were undertaken. 

 One of these was the examination of the bottom of the Tortugas Lagoon, 

 to discover whether or not sediment is there accumulating; the other was to 

 ascertain by chemical examination whether or not there was any excess of 

 carbon dioxide in the water flowing into the lagoon and whether there was 

 any difference in the quantity of carbonates in the outflowing and inflowing 

 water. The latter investigation was undertaken by Mr. R. B. Dole. 

 The bottom samples collected at numerous localities in the Tortugas 

 established that the bottom in the channel between Garden Key and Bush 

 Key, in Bird Key Harbor, and in the lagoon channels west and east of White 

 Shoal and north of that shoal is formed of soft calcareous mud, largely of 

 the kind Drew showed to be precipitated by denitrifying bacteria. The 

 examination showed conclusively that sedimentation predominates over 

 the removal of material, perhaps except where currents may corrade the 

 sides or bottoms of the channels along which they flow. The conditions 

 in the Tortugas lagoon are similar to those in the bays and sounds behind 

 the Florida keys and to those in Marquesas Lagoon, in all of which filling 

 by deposition is progressing. 



The specimens examined by Mr. Dole were collected twice a day on the 

 principal flowing and ebbing tides for nearly a lunar period, from May 20 

 to June 15, near buoy No. C3, in Southwest Channel, ofT the southern end 

 of White Shoal. Mr. Dole's results are summarized on page 74 of this 

 publication. They show that none of the water, neither that flowing into 

 nor that flowing out of the lagoon, contains any dissolved free carbon 

 dioxide, that all the carbon dioxide in the water is present in the carbonate 

 or bicarbonate form, and that the inflowing water possesses no power to 

 dissolve calcium carbonate by virtue of its content of carbon dioxide, as 

 its capacity for the solution of lime has been exhausted before flowing into 

 the lagoon. The chemical investigations of the waters and the study of 

 the bottom deposits gave mutually confirmatory results, that the water 

 flowing through the Tortugas Lagoon does not dissolve calcium carbonate 

 and that the lagoon has not been formed by solution. 



As two kinds of phenomena observed in the Tortugas might be con- 

 sidered to afford evidence in favor of solution, both will be briefly discussed. 



