374 BULLETIN OF THE 



there are many such upon these keys, — some large ones many yards in 

 diameter and others quite small, — evidently formed by the wearing ac- 

 tion of loose pieces of harder coral rocks thrown upon the key by great 

 waves, and only occasionally set in motion by the waters dashing over 

 the key during heavy storms. The pot-holes nearest the water-edge 

 are the most recent, and are mostly clean excavations, either entirely 

 empty or containing sand and limestone pebbles lying loose at the 

 bottom of the holes. Some of these excavations are circular, others 

 oblong, still others have the form of winding caves opening towards 

 the sea or upon the surface of the key. Beyond the reach of ordi- 

 nary tides and of the waves raised by moderate winds, the pot-holes 

 are generally lined with coatings of solid, compact, and hard lime- 

 stone, varying from a thin layer to a deposit of several inches in 

 thickness, and following all the sinuosities of the cavities in which they 

 are accumulating. It is plain from their structure that these coatings 

 are a subaerial formation, increasing by the successive accumulation of 

 limestone particles left upon the older rock by the evaporation of water 

 thrown upon the key when the ocean is so violently agitated as to 

 dash over the whole key. Frequently the hollow of these coated 

 pot-holes is further filled with consolidated oolithes ; or thin layers of 

 minute oolithes alternate with a coat of compact limestone, throughout 

 the excavation, which often has been filled again in this way up to the 

 general level of the surrounding surface. Occasionally these regen- 

 erated surfaces are again hollowed out by the action of storms, and the 

 result is a dismantled pot-hole, in which their structure and the mode 

 of their filling is distinctly exhibited. 



The stratification of the main mass of these keys is very peculiar. 

 Though evidently the result of an accumulation of oolithes thrown up 

 by high waves, the beds are pretty regular in themselves, but slant in 

 every direction towards the sea, showing that they were deposited 

 under the action of winds blowing at different times from every 

 quarter. It is further noteworthy, that, while the thicker layers consist 

 of oolithes readily distinguishable to the naked eye, there are at inter- 

 vals thin layers of very hard, compact limestone, alternating with the 

 oolithic strata, which have no doubt been formed in the same manner 

 as the coating of the pot-holes. 



As in their general aspect the coral formations of the Cuban side of 

 the Gulf Stream differ from those of the American side, so do also the 



