On the Geology of the Key of Sombrero, W. 1. 277 



mation of stalactitic deposits and of crystals of various phos- 

 phates of lime.* Subsidence to sea-level and denudation of 

 surface. 



Eighth elevation, to over 160 feet. Formation again shat- 

 tered. 



Ninth subsidence, of about 40 feet. Erosion of joints and 

 cavities. Formation of a third guano-deposit. Lateral erosion 

 of island nearly to its present dimensions. Formation of a sand- 

 bank to the north-west. Change in the ocean-currents by an 

 elevation of the neighboring banks (?). 



Sudden fall of about 80 feet, beginning the modern period. 

 Denudation of surface. Probable repeated formation and remo- 

 val of superficial deposits of guano and sand. Formation of 

 boulders and spray-pools. 



The deposits formed above the sea-level thus comprise three 

 layers of guano at successive periods, the greater bulk of each 

 of which has been swept from the surface of the Key, though 

 not until its most valuable portion was disseminated in veins 

 through the subjacent limestone. Of these three, the first is 

 the only one present in considerable quantity ; the second is, in 

 part, unfit for agricultural purposes ; and the third is the easiest 

 to obtain. Regarding the beds now beneath the sea-level, it is 

 likely that their corresponding oscillations were entirely con- 

 fined below that limit. But if the bottom of the lagoon some- 

 times emerged, as it is possible, to a considerable height above 

 the sea, they may now contain ancient guano deposits in inter- 

 laminated beds or in veins. 



§ 21. Curve of Oscillations. — By the simplest (though not 

 necessarily the most correct) interpretation of the phenomena 

 of the six beds, seventeen alternating sudden elevations and 

 slow depressions seem to have occurred before the modern 

 period. Fig. 3 is a graphic representation of these, for the 

 average section of the southern end of the Key. Little accuracy 

 can be expected and is claimed for the abscissae and ordinates 



* Am. Jour. Sci., Nov. 1865. 

 APRIL 1866. 21 Ann - Ltc - Nat - Hist., Vol. VIII. 



