MUSEUM OF COMPARATIVE ZOOLOGY. 375 



rooks of the latter differ from the rocks observed upon the banks of 

 Salt Key, Double-headed Shot Key, and Orange Key. We find upon 

 the Florida reefs, as well as between the innumerable keys stretching 

 along the American coast, and upon the coral plateau sloping towards 

 the main trough of the Gulf Stream, extensive beds of regularly strati- 

 fied rocks of various kinds. I have already described the limestone 

 conglomerate of the Pourtales plateau, p. 365. Such a formation exists 

 nowhere else within the range of the Gulf Stream, unless it should be 

 hereafter ascertained that a similar deposit extends along the submarine 

 border of our continent, edging the American wall of the deeper part 

 of the Atlantic trough. But in the shoal waters intervening between 

 the coast of the peninsula of Florida and the keys and reefs there 

 exist various deposits of an entirely different structure, the accumulation 

 and increase of which is constantly going on. The most extensive of 

 these formations is a regularly stratified oolithic rock, the grains of 

 which vary from imperceptible granules to larger and larger oolithes, 

 approaching the dimensions of pisolithes, and cemented together by an 

 amorphous mass of limestone mud. The oolithes themselves are formed 

 in the manner first described by Leopold von Buch. Hard particles of 

 the most heterogeneous materials, reduced to the smallest dimensions, 

 and tossed to and fro in water charged with lime, are gradually coated 

 with a thin film of limestone, and then another and another, until it 

 sinks to the bottom, to be further rolled up and down the sloping shore 

 bottom until it becomes cemented with other similar grains, and forms 

 part of the growing limestone bed. Of course the finer oolithes are seen 

 nearest the shore line, and it is instructive to see at low tide the little 

 ripples of successive larger oolithes left dry as the water subsides. 

 Naturally these materials are frequently thrown up along the beaches 

 in layers of varying thickness, and in course of time become cemented, 

 and are transformed into solid rock, over which crusts of hard, compact 

 limestone are in the end formed by the evaporation of calcareous water 

 dashed upon the dry surfaces. 



In very shallow waters, which are not powerfully affected by tidal 

 movements, and upon the bottom of which no oolithes are forming, we 

 find extensive beds of a dull amorphous limestone, formed of lime-mud, 

 alternating with seams of a more compact, hard limestone, in which a 

 few oolithes may occasionally be seen that were floated over the flats 

 in which such formations are going on. These deposits resemble 



