28 vaughan: geologic history of coral reefs 



by virtue of its content of free carbon dioxide; (4) the determina- 

 tions by Dole of the saUnity of the water within the Tortugas 

 lagoon and at the southern end of Biscayne Bay show a higher 

 concentration than that in the open sea-water on the outside, 

 indicating that tidal inflow and outflow are not sufficient com- 

 pletely to mix the water in the lagoons with the water of the sur- 

 rounding sea and that concentration by evaporation is taking 

 place. As the results of these lines of inquiry are so positive, 

 the formation of lagoons by submarine solution may be definitely 

 eliminated from consideration. 



Of the constructional agencies to which the rims of atolls 

 might be attributed attention was directed to the effects of winds 

 and currents. It is a matter of common knowledge that if a 

 current impinges against an obstruction lying across its course it 

 divides, a part swerves to each side and incurves on the back of 

 the obstruction, with the result that crescentic accumulations 

 are built whenever the moving medium deposits material. It is 

 also well known that the form of a deposit from a current moving 

 in a straight line is correspondingly rectilinear. Current-shaped 

 crescentic keys and crescentic sand dunes are familiar phenom- 

 ena; as also are linear ridges formed either of water-borne sedi- 

 ment or of wind-driven sand. Good instances of crescentic keys 

 and a mud atoll (Breton Island) are found off the mouth of Mis- 

 sissippi River. 



As regards coral reefs, Hedley and Taylor have pointed out 

 that, for the Great Barrier Reef of Australia : 



The growth of an individual reef is shown to proceed in a regular 

 cycle. If the reef reaches the surface with its axis along the wind, then 

 its shape endures; but if across the wind, then its extremities are pro- 

 duced backward, forming first a crescent, later a horseshoe, and lastly 

 an oval, thus inclosing a lagoon. 



There is a striking similarity in the configuration of the Mar- 

 quesas and the Tortugas. There is an entrance to the lagoon of 

 each in the southeast, southwest, and northeast quadrants; and 

 the principal arc of the rim of each is from the southeast to the 

 northwest entrance. The bow of this arc in each instance is 

 against the prevailing direction of the wind, which is predomi- 

 nantly from the east, and against the direction of movement of 



