AGASSIZ: THE FLORIDA ELEVATED EEEF. 47 



at the mouth of Tavernier Creek, off the eastern end of Long Island. 

 (See Plate XII.) 



On sounding off Caesar's Creek, in ten feet of water, we obtained very- 

 fine sticky marl-like stuff. This comes from the region of flats behind 

 Key Largo and Elliott Key, and is composed of the disintegrated mate- 

 rial which once constituted the extension of the southern part of the 

 mainland of Florida, and of which only the main line of keys and the 

 isolated and scattered keys remain. These extend westward, becoming 

 smaller and more widely separated as we proceed in that direction, and 

 at the same time the southern shore line of Florida recedes more and 

 more from the main line of keys. 



A glance at the charts (Plate XII., also U. S. Coast Survey Charts, 

 No. 10, and Nos. 167 and 168) will show this gradation in the disin- 

 tegration of the southern extremity of Florida. To the northeast we 

 have large and wide keys, like Elliott Key and Key Largo, forming the 

 southern edge of Key Biscayne Bay, and still more or less connected with 

 the mainland ; next come the long narrow keys to the south of Barnes 

 Sound, with here and there a spur running north ; and finally we come 

 upon a huge stretch of mud flats, with only an occasional islet or man- 

 grove island, the northernmost boundary of which is Cape Sable, and 

 of which the southwestern extremity is formed by the irregular spit of 

 keys reaching from Long Key to the keys and banks to the westward 

 of Key West and towards the Marquesas. 



As we passed the mouth of the channel separating Grass and Duck 

 Keys, a strong northwest wind blowing, we struck a huge fan-shaped 

 stream of discolored water pouring out of the passage, clearly showing 

 the mass of silt which, under cei'tain conditions of disturbance, may 

 rush out from the flats to the north of the keys, and when concen- 

 trated, as at Csesar's Creek or off the line of keys and shoals south of 

 Key Biscayne Bay, may form permanent deposits. 



At Boca Chica (Plate XV.) we examined the quarry from which is 

 obtained the material for building the jetties of the northwest channel 

 out of Key West Bay. It was found to be seolian rock. About five feet 

 above high-water mark it is riddled with tubes full of yellow sand (or 

 often empty) in marked contrast with the surrounding white seolian 

 sand. These tubes are often full of stalagmitic matter, filling the spaces 

 formerly occupied by mangrove roots and slender stems which have 

 become decomposed. Is this not the origin of many of the yellowish 

 and irregularly shaped amygdules which characterize the hai'd ringing 

 limestone into which seolian rock is so often transformed ] The man- 



