112 Papers from the Department of Marine Biology. 



action were responsible. To determine the first, a chemical research 

 was conducted into the dissolving power of sea-water due to the CO2 it 

 contained, and the material from the bottom of the lagoon was exam- 

 ined. This latter examination showed the bottom to be fine calcareous 

 mud, precipitated by denitrifying bacteria, as discovered by Drew.^ 

 The former chemical research was made by Dole^ on daily observations 

 of the amount of CO2 in the sea-water flowing into the lagoon and the 

 amount of carbonates in the water flowing out. The result of this 

 work showed that the dissolving action of CO2 in the water was negU- 

 gible and from the bottom samples it was ascertained that deposition 

 was going on at a rate far exceeding any solubility. 



The above study eUminated the first theory of action by solubility, 

 but the theory of formation by waves and currents remained. As 

 observed by various writers, sand dunes and water-carried detritus 

 are aUke molded into two shapes by certain wind and water currents. 

 The latter are formed by an obstacle or a counter-current shearing a 

 constant current, which drifts material to both sides and a crescent is 

 formed with the bow of the arc facing the current. Vaughan further 

 says that there are three kinds of currents working in this region: 

 first, wind-formed currents, accompanied by waves; second, the Florida 

 counter-current; third, tidal currents. The prevailing wind in the 

 Tortugas is from northeast to southwest, and the islands also Ue in the 

 sweep of the Florida counter-current, which moves west, so that these 

 two sorts of currents cooperate; on the other hand, the general tidal 

 current is from north to south — i. e., flowing transversely to the two 

 before mentioned ; thus the arc of the Tortugas Atoll is bowed toward 

 the east, against the prevailing wind and the counter-current. The 

 southwest part of the perimeter trails along the direction of these cur- 

 rents. The southwest and southeast passages in the atoll are attrib- 

 uted to the influence of the counter tidal current. White Shoal and 

 BriUiant Shoal indicate the direction of this current, while Loggerhead, 

 the largest of the group, shows the elongated " Unear-ridge " shape due 

 to the deposition of material by currents traifing along from the tails 

 of the crescent, and the indurated beach rock on this island shows it to 

 be wave-built, by its seaward slope. 



On Garden Key, as seen by borings made by Vaughan, the larger 

 rock-masses were found to be massive dead coral-heads. All of the 

 Tortugas Keys are composed of calcareous detritus, the remains 

 of various CaCOs-secreting organisms, mollusks, corals, nulHpores, 

 echinoderms, and the calcareous algse. The coral fauna of the Tor- 



'Drew, G. H., On Precipitation of CaCOa in the Sea by Marine Bacteria and an Account of 

 Denitrifying Bacteria in Tropical and Temperate Seas. Carnegie Inst. Wash. Pub. No. 182, 

 pp. 7-45. 



^Dole, R. B., Some Chemical Characteristics of Sea-Water at Tortugas, Florida. Carnegie 

 Inst. Wash. Pub. No. 182, pp. 69-78. 



