AGASSIZ: THE FLORIDA ELEVATED REEF. 4T 
at the mouth of Tavernier Creek, off the eastern end of Long Island. 
(See Plate XII.) 
On sounding off Casar’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 XIL, 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 certain conditions of disturbance, may 
rush out from the flats to the north of the keys, and when concen- 
trated, as at Caosar’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 Day. It was found to be eolian 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 molian 
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 hard ringing 
limestone into which æolian rock is so often transformed? The man- 
