372 BULLETIN OF THE 



oblithes of various sizes, from fine powder to coarse sand, mingled with 

 broken shells, among which a few living specimens are occasionally 

 found. The margin of the bank is encircled on several points by rocky 

 ridges of the most diversified appearance, and at others edged by sand- 

 dunes. A close examination and comparison of the different keys show 

 that these different formations are in fact linked together, and represent 

 various stages of the accumulation, consolidation, and cementation of the 

 same materials. On the flat top of the bank the loose materials are 

 pounded down to fine sand ; in course of time this sand is thrown up 

 upon the shoalest portions of the bank, and it is curious to notice that 

 these shoalest parts are its very edge, along which corals have formed 

 reefs which have become the basis of the dry banks. The foundation 

 rock, as far as tide, wind, and wave may carry the coarser materials, 

 consists of a conglomeration of coarser oblithes, rounded fragments of 

 corals, or broken shells, and even larger pieces of a variety of corals 

 and conchs, all the species being those now found living upon the 

 bank, among which Strombus gigas is the most common ; beside that, 

 Astrcea. annularis, Siderastrcea siderea, and Mcandrina mammosa pre- 

 vail. The shells of Strombus are so common that they give great 

 solidity and hardness to the rock. The stratification is somewhat 

 irregular, the beds slanting towards the sea at an angle of about seven 

 degrees. Upon this foundation rock immense masses of Strombus, 

 dead shells, and corals have been thrown in banks, evidently the 

 beginning of deposits similar to those already consolidated below; 

 but there is this difference in their formation, namely, that while the 

 foundation rock is slightly inclined, and never rises above the level 

 of high water, the accumulation of loose materials above water-level 

 forms steeper banks, varying from fifteen to twenty and thirty degrees. 

 In some localities broken shells prevail ; in other, coarse and fine 

 sand; and the ridges finis formed, evidently by the action of high 

 waves, rise fo about twelve and fifteen feet. This is evidently the 

 foundation for the accumulation of liner sand driven by the wind 

 over these ridges and forming high sand-dunes, held together by a 

 variety of plant-, among which a trailing vine (Batatas Uttoralis), 

 various grasses, and shrubs are the mosl conspicuous. These dunes 

 rise to about twenty feet ; on their lea side and almost to their sum- 

 mit grows a little palmetto. The sand of the dunes is still loose, but 

 here and there shows a tendency to incrustation at the surface. The 



