On the Geology of the Florida Keys. 393 
due to the elevation of a vast uneven coral reef whose prominent | 
points rising above the water, form the foundation of the Keys, 
the sands driven up by the waves having done the rest. 
The water between the reef and main land is in many places 
shallow ; the bottom is covered with white calcareous sand and 
soft mud, and is intersected by tortuous channels. The first si- 
liceous sand that I saw, was. dredged up off Soldier Key; from 
this point it was traced, on the bottom, to Cape Florida, and be- 
yond this towards the north it covers the Atlantic shore. 
e adaptation of the Keys and adjoining coast to the produe- 
tion of tropical fruits, is well known in the streets of Key West ; 
the cocoa-nut tree ripens its fruit, yet the absence of soil is quite 
remarkable ; I did not see two acres of land having a depth of 
soil of three inches, on any of the islands that I visited. 
Besides Key West, very few of the Keys are inhabited, so 
that it was not a little cheering to come upon the stations and 
signals of the Coast Survey, although operations were suspended 
at that season. ; ; 
ter examining the limestone of Tampa bay, which I think 
“Mr. Conrad has correctly referred to a tertiary formation older 
than the miocene, I did not expect to find the main land at the 
mouth of the Miami made up of beds of limestone of the same 
age as those I examined at Key West and elsewhere inside the 
reef. The fossils are all identical with the shells living in the sur- 
rounding water.* On both sides of the mouth of the river near 
Fort Dallas, I found fossils showing on the weathered surface of 
the rocks; and among them Lucina Jamaicensis, and Pyrula per- 
versa. Like that of Key West, the rock has the appearance of 
sented at the falls in every respec Py i i 
of the river, and it is known that the ‘Everglades’ rest on @ 
vast basin of this limestone. Judging by the eye alone, | 
ridge through which the Miami flows must be twenty or thi 
_ feet in height; while the Glades are said to be but SIX OF | ig 
tog 
Smith, Esq., in a report to 
fter these observations were 
Be Se 
* This fact liad been previously pointed out by B. 
retary of the oats in 1848, which I saw a 
i ee 
mits, Vol. XT, No. 88—May, 1851. 0 
