62 Papers from the Marine Biological Laboratory at Tortugas. 



or heads occur along the west side of Loggerhead bank throughout its 

 entire length and around both its southern and northern ends. Bush Key 

 Reef, southwest of Southeast Passage, is luxuriant, and outer reef patches 

 occur northward of Southeast Passage. Reef corals also occur at many 

 places along the sides of channels within the lagoon. Other species of corals, 

 not so large and massive as those on the reefs proper, live on the shallow 

 flats, where not overwhelmed with detritus. Although it has not been 

 practicable to make for the Tortugas a quantitative estimate of the pro- 

 portions of material contributed by the different kinds of organisms, corals 

 are rather surely the most important. The Marquesas, therefore, are 

 composed of calcareous detritus mostly of other than coral origin; while 

 the Tortugas are composed of calcareous material, of complex origin but 

 largely due to the activity of corals. 



The foregoing remarks may be summarized as follows: (i) Submarine 

 solution has not been instrumental in the formation of the Marquesas or 

 the Tortugas lagoon; (2) the atoll rings are constructional phenomena and 

 were shaped by prevailing currents, those caused by winds, the Florida 

 counter current, and tidal currents. The detrital material on which theses 

 agencies have worked in the Marquesas is mostly not of coral origin; while 

 in the Tortugas, although of complex composition,^ it is largely composed of 

 coral debris. 



The relative ages of the Marquesas and Tortugas atolls and their 

 relations to oscillation of water-level will now be briefly considered, and in 

 this connection further attention will be given to the probable relations of 

 corals to the formation of the latter atoll. The Marquesas rim is composed 

 of unconsolidated detrital material and evidently requires no oscillation 

 of water-level for its formation. No indication of change of level during or 

 since the building of the rim around the lagoon was observed. 



Such a structure as the Tortugas would scarceh' be built in the Florida 

 region through material derived from reef corals without change in depth, 

 as the depth of water in the lagoon, 13 fathoms, and outside the atoll rim, 

 13 fathoms or more, is greater than that in which the Florida reef species 

 are known to flourish .^ Other evidence, to be adduced in giving an account 

 of the coastal oscillacion, renders it reasonably certain that the Tortugas 

 were initially outlined during subsidence.' Certain facts indicate that the 

 history of the Tortugas is more complicated than that of the Marquesas. 



• The following literature is cited in this connection: 



Vaughan Cornish. On the formation of sand dunes. Geog. Jour. vol. 9, pp. 278-309. 1897. 



C. Hedley and T. Griffith-Taylor. Corals of the Great Barrier, Queensland, a study of their structure, 

 life-distribution, and relation to mainland physiography. Austral. Assoc. Adv. Sci. Adelaide meeting, 17 pp., 

 3 pis. 1907. 



F. Wood-Jones. Coral and atolls. London. Lovell Reeve & Co. Ltd. (especially chapters xii and xiii, 

 pp. 253-277). 1910. 



James Bryce. South America observations and impressions. New York, Macmillan Co. 1912. [Pp. 

 S8, 59 give a good description of crescentic sand-dunes, "the convex of the crescent always facing the wind," 

 on the great inner plateau of Peru.] 



2 In the following discussion of oscillation the terms "uplift," "depression," and "subsidence" are used 

 with reference to land, but it is recognized that oscillation may be due to negative or positive movement of 

 sea-level independent of crustal movement. 



' The wells bored on Key Vaca by the Florida East Coast Railway, under the supervision of Samuel 

 Sanford, showed the thickness of the elevated coral reef rock to be 105 feet ( = I7i fathoms), indicating that 

 the Pleistocene barrier reef was formed under conditions of subsidence, similar to those postulated for the 

 initial building of the Tortugas rim. 



