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AGASSIZ: THE FLORIDA ELEVATED REEF. 45 
separating the patches of the elevated reef we have no means of meas- 
uring accurately now.! 
The belt probably occupied by solian rocks and reef patches I have 
indicated by cross ruling on Plate XVII. Тһе 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 tho reef, as well as the 
absence of flats, to the steepness of the shore formations. Не thinks 
that a great part of the material between Lake Okeechobe and Cape 
Sable is probably made up of organie 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 solian nature, for the prevalence of easterly winds 
over the southernmost extremity of Florida is in marked contrast with 
the more variable winds of tho 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 оҒ South- 
ern Florida. 
The southern extremity of Key Biscayne mfhrks 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 
distanee south from the channel leading past Cape Florida into Key 
Biscayne Bay we find the bottom a mixture of siliceous and caleareous 
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 caleareous 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 
1 Judging from the depth found within the main channel which separates the 
keys and the outer reef, these channels were probably comparatively shallow. 
? Bull. Mus. Comp. Zoól., Vol. XVI. No. 7, 1890. 
