262 PLANTS, SEEDS, AND CURRENTS 



are a sub-aerial formation, increasing by the successive accumula- 

 tions of limestone particles left upon the older rock by the evapora- 

 tion 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 oolite; or 

 thin layers of fine-grained oolite alternate with a coat of compact 

 limestone, throughout the excavation, which has often been filled 

 in this way up to the general level of the surrounding surface. . . ." 

 The structure and the mode of filling of these ancient pot-holes are, 

 as Agassiz goes on to say, distinctly exhibited in the cases of those 

 that have been partially cleared out by the action of storms. 



The account of the keys of the Salt Key Bank is given in full by 

 Dana in his Corals and Coral Islands (1872 p. 213,). The elder 

 Agassiz supplies an illuminating description of the history of the 

 formation of these aeolian rocks on these islands. The bank, covered 

 by from four to six fathoms of water, is formed of the " oolitic " 

 grains of coral materials mingled with broken shells. Its margin 

 is encircled in some places by rocky ridges and in 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." That the aeolian rocks have 

 been formed by the consolidation of the sand-dunes, he has no 

 doubt. The sand composing them " must have been blown up by 

 the wind, and accumulated in the form of high dunes before it 

 became consolidated." This dune-sand is still loose, but, as on 

 Salt Key, it shows " here and there a tendency to incrustation at 

 the surface." Then in the case of another key, where the aeolian 

 sandstone prevails, he says : " It is evident that what is beginning 

 on Salt Key has here been completed." All this, it should be noted, 

 has been accomplished with the sea at its present level, a matter 

 that is dealt with again below. Different localities have their 

 individual lessons, and I may here remark that though sand-dunes 

 occur in places near the coasts of the larger islands of the Turks 

 Group, they very rarely exhibit a tendency to surface-consolidation. 



The Basement of the jEolian Sandstone. — For reasons before 

 explained, underlying rocks of a character different from that of the 

 aeolian rock are rarely exposed. It has already been observed that 

 the claim made by Professors Heilprin and Rice for the existence of a 

 basement of old reef-rock in the Bermudas is disallowed by A. Agassiz, 

 who regards it as the effect of the action of the sea in cementing the 

 strata together and destroying their aeolian structure. But the 

 weightiest evidence against him is supplied by his father in the case 

 of the islands of the Salt Key Bank. Their foundation is not aeolian 

 sandstone, but " a conglomeration of coarser oolitic grains, 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 on the Bank." Stratification is displayed, the beds dipping 

 towards the sea at an angle of about seven degrees. This founda- 

 tion never rises above the level of high- water. It is, as the present 

 writer will show, but a coarse kind of the " beach-rock " that is 



