Building of the Marquesas and Tortugas Atolls, etc. 59 



One is the frequent occurrence of submerged coral heads, the interiors of 

 which have been removed, leaving a partially inclosing skeletal shell covered 

 by living polyps. Some of these heads have a suggestive resemblance to 

 atolls, but the resemblance is only superficial. The excavation of the 

 interior is to be explained by the upper surface of the envelope of living 

 tissue having been damaged in some way, thus giving an opportunity for 

 boring organisms of great variety to begin and prosecute their work of 

 disintegrating the skeleton. Waves and currents wash out the broken-up 

 material, forming a cavity which is bounded by a shell of substance whose 

 outer surface is covered by living polyps. The other phenomena that 

 might be attributed to solution are the overhanging projections along the 

 steep, submarine cliffs or rock pinnacles which in places occur on the sides 

 of channels. Since these stand against the sides of the currents it is probable 

 that submarine scour, corrasion, has been a factor in producing them; 

 and subaerial modification deserves consideration, as will later appear. 

 As the dynamic agencies have been more and more thoroughly studied, the 

 necessity for referring any phenomena observed in the Tortugas to sub- 

 marine solution has been eliminated, and the conclusion definitely reached 

 that such solution produces no geologic effects of importance. 



As the production of the Marquesas and Tortugas atolls with their 

 inclosed lagoons by solution is eliminated, other factors, especially waves 

 and currents, will be considered. There are two current-shaped physio- 

 graphic forms to which it is desired particularly to call attention— these 

 are current-shaped crescents and current-shaped linear ridges. With 

 reference to crescents it may be said that if an obstruction lies across the 

 line of direction of a constant current, the current shears to each side 

 and will drift detrital material in such directions as to form a crescentic 

 accumulation, the bow of the crescent facing the current, while the horns 

 of the crescent will curve before the current. An eddy may be produced 

 in the space between the horns of the crescent, and a deposit may be formed 



there.^ 



Crescentic sand-dunes and crescentic keys are both well known. Should 

 there be no cross obstruction, currents both aerial and aqueous will form 

 linear ridges, under conditions which induce dropping material. Both 

 linear sand-dunes and linear submarine banks are well known. Of current- 

 transported detritus, crescentic accumulations lie across the main direction 

 of the current, while linear ridges lie along its direction. 



The currents of the Florida keys are of three kinds : wind-formed currents, 

 which are accompanied by waves; the Florida counter current; and tidal 

 currents. The winds at Key West, where accurate records have been 

 kept, prevailingly range in direction from northeast to southeast, those 

 from the east predominating over those from any other direction. Both 

 the Marquesas and the Tortugas lie within the tract of the Florida counter 

 current, which moves in general toward the west. The directions of the 



I Dr. A. G. Mayer called my attention to the occurrence of this eddy. 



