AGASSIZ: THE FLORIDA ELEVATED REEF. 45 



separating the patches of the elevated reef we have no means of meas- 

 uring accurately now. 1 



The belt probably occupied by eeolian rocks and reef patches I have 

 indicated by cross ruling on Plate XVII. The lines of rivers indicated 

 on the chart are only approximate lines of drainage, as the Everglade 

 district has not been surveyed accurately. 



The persistence of dunes composed mainly of siliceous sand along the 

 east coast of Florida is natural, while it is equally natural to find that 

 the dunes composed of calcareous sand from the parallel of Cape Florida 

 have in great measure disappeared. 



Professor Shaler rightly attributes to the southward movement of the 

 siliceous sands along the east face of Florida and the adjoining keys 

 the overwhelming of the northern extension of the reef, as well as the 

 absence of flats, to the steepness of the shore formations. He thinks 

 that a great part of the material between Lake Okeechobe and Cape 

 Sable is probably made up of organic waste accumulated behind the 

 coral reef. It seems to me more probable, from Mr. Griswold's obser- 

 vations, that the sand hills and the accumulation behind the reef are 

 in great part of seolian nature, for the prevalence of easterly winds 

 over the southernmost extremity of Florida is in marked contrast with 

 the more variable winds of the northern regions of Florida. The 

 existence of this sand does not, it seems to me, prove the submergence 

 of the peninsula subsequently to the elevation of the coral reef. That 

 appears to be the last disturbance in the general topography of South- 

 ern Florida. 



The southern extremity of Key Biscayne marks the limit of the ex- 

 tension of the northern siliceous sand. Its limit can readily be traced 

 on the charts by the nature of the soundings, and for a considerable 

 distance south from the channel leading past Cape Florida into Key 

 Biscayne Bay we find the bottom a mixture of siliceous and calcareous 

 sand ; the former driven by the prevailing winds into the channel south 

 of Cape Florida, where it becomes more or less masked by the calcareous 

 sand driven shoreward from the outer reef, and by the calcareous sand 

 brought seaward over the reef separating Key Biscayne Bay from the 

 waters of the Straits of Florida. (Coast Survey Chart, No. 166.) 



Professor Shaler, in an interesting article on the " Topography of 

 Florida," 2 has given a very clear account of the immense disintegration 



i Judging from the depth found within the main channel which separates the 

 keys and the outer reef, these channels were prohably comparatively shallow. 

 2 Bull. Mus. Comp. Zool., Vol. XVI. No. 7, 1890. 



