AGASSIZ: THE FLORIDA ELEVATED REEF. 51 



bluffs between the patches and stretches of the old coral reef; or it has 

 accumulated upon the top of patches and stretches of reef to form the 

 higher keys, and this seolian rock has, little by little, either remained 

 more or less mobile, or been solidified by rain or the action of sea spray, to 

 foi'rn the hard ringing limestone of the main line of the Florida Keys. 



From what we now know of the thickness of the reef from the borings 

 of the Artesian well at Key West, the now elevated reef must have 

 grown at no great depth upon the shallow shores of the Postpliocene 

 coast of Southern Florida. The greatest depth upon which it began to 

 grow was probably considerably less than the greatest depth at which 

 reef corals are known to thrive. It was upon bars and flats at 

 modei'ate depths, less than twenty fathoms, or upon their flanks, that 

 the Postpliocene coral reef of Florida originated, — bars and flats ex- 

 tending from the edge of the outer reef of to-day probably far beyond 

 the northern limit reached by Mr. Griswold in his exploration of the 

 Everglades. Over the whole of this southern tract of the peninsula, 

 wherever a coral sand beach was formed, seolian hills and dunes were 

 blown consisting of coral sand oolite, or of coral sand more or less mixed 

 with quartz sand, as we proceed northward and inland from Key Bis- 

 cayne. These aBoliau hills and dunes have, little by little, been solidi- 

 fied into hard rock by the same agencies as those which changed into 

 pitted and honeycombed limestones the stretches of seolian rock covering 

 the land surfaces and sinks of the Bermudas, and the extensive areas of 

 similar formation so characteristic of the Bahamas. 1 The territory 

 covered in Florida by modified seolian rock within the range of the 

 elevated reef and beyond it (see Plate XVII.) is not as extensive as 

 the similar area of the Bahamas. In fact, it is of about the size of the 

 Little Bahama Bank. But there is a marked difference between the 

 two sides of the Florida Strait. In the Bahamas there has been a very 

 marked subsidence since the formation of the seolian hills of that group, 

 — a subsidence amounting perhaps to three hundred feet, while the 

 elevated reef forming the substratum of the Florida Keys indicates 

 a Blight elevation since the formation of the seolian rock of Southern 

 Florida. 



Finally, it is upon the remnants of the old elevated reef that the 

 present growing reef flourishes, forming, as it does in the Bahamas and 

 Bermudas, a comparatively thin crust upon the underlying foundation 

 rocks, which are now known to be Pliocene, and wdiich occur at a depth 

 considerably less than that at which reef corals are known to grow. 

 1 A. Agassiz, " The Bermudas and Bahamas," Bull. Mus. Comp. Zool., Vol. XXVI. 



