Building of the Marquesas and Tortugas Atolls, etc. 6i 



head Key, where both the north and south horns of the ke}^ curve before 

 the prevailing easterly winds. A reversal of a part of the horn of the 

 north end of the key was observed during a swell from the northwest. 

 During this reversal of the direction of the run of the waves, a reversed 

 crescent was formed at the end of the spit, the bow of the crescent being 

 toward the current while the horns curved before it. The more indurated 

 beach rock on each side of Loggerhead Key slopes toward the sea, showing 

 that the key is wave-built, on the surface at least. In this connection it 

 may be said that a current too weak to pick up and carry detritus will, 

 when agitation has lifted particles above the bottom, appreciably move 

 them in the direction of the current. Loggerhead Key stands on the lee 

 side of the lagoon, where waves from the east would be weaker than on the 

 windward side, and where waves from the west would exert their full force. 

 The following is offered as an hypothesis for the formation of the bank on 

 which this key stands: It is largely or mostly a detrital ridge, formed in a 

 partial eddy on the leeward side of the lagoon, mainly along the direction of 

 flow of the stronger tidal currents. Wave agitation from the east and west 

 has facilitated the movements of detritus and has built the sloping beach 

 rock on both the lagoon and the sea sides. The stronger winds from the 

 east have given a slight westward curvature to the extremities of the bank. 



Before leaving the discussion of Loggerhead Bank and Key, attention 

 will be directed to changes that have occurred since October iQio, when the 

 last severe hurricane visited this region. Since that time the waves have 

 acted with only slight cessation on the eastern side of the key and have rut 

 away the eastern side of its northern end, while building has taken place 

 on the western side; thus the outline of the northern end of the key has 

 shifted westward. Accurate measurements of some of these changes have 

 been kept but will not be given here. 



Besides the banks and keys forming the perimeter of the Tortugas, 

 there are also shoals and some small keys in the lagoon. Some of the 

 shoals. White Shoal for example, lie along the direction of flow of the major 

 tidal currents, although with their ends slightly curving toward the we^t; 

 others are isolated rocks. That some of the latter may be coral masses is 

 suggested by an experience on Garden Key, where all large rocks examined 

 proved to be dead coral heads. 



The rocks composing both Marquesas and Tortugas are known only 

 from superficial examination. The foundation of the Marquesas may be 

 oolite, similar to that found on Boca Grande Key, as some of the bottom 

 samples from them were found to contain hard oolite grains. The keys, 

 however, are composed of calcareous detritus, of which Halimeda is an im- 

 portant constituent. There is no important coral growth around the 

 Marquesas. The keys of the Tortugas are mostly composed of the com- 

 minuted remains of a considerable variety of organisms that secrete calcium 

 carbonate, mollusca, corals, nullipores, echinoids, etc. The corals here, in 

 contradistinction to the Marquesas, are important. Coral reefs, patches. 



