THE FLORIDA REEFS. 71 
on а flat plateau, the low islands now constituting the western 
edge of the reef were at one time exposed to the full action of 
the sea. As their sea-slope became little by little surrounded 
by the wall now encircling the outer edge of the reef, the 
growth of the corals was checked, the interstices and spaces 
between the coral masses were gradually choked by sand and 
coral fragments, until the low islands now known as Perez, 
Chica Pajaros, and the great sand-bars of the western edge of 
the reef were slowly built up a few feet above the level of the 
sea, — mere strips of sand gradually cemented together by the 
accumulation of the loose materials held in suspension by the 
water. 
The outer eastern edge of the reef or wall is from one to two 
hundred yards in width, and inside of this extends for a max- 
imum distance of seven to eight miles an extensive section which 
is more or less filled with masses of astreans, of mzandrinas, of 
gorgonians, and madrepores in more or less irregular patches, 
with a general westerly trend. They reach nearly to the water’s 
edge, where it is quiet, and are separated by deep gullies of clear 
water varying from one to seven fathoms in depth. Through 
these gullies a regular current sets to the westward, and has de- 
posited on its path the coral sand held in suspension, thus keep- 
ing these gullies clear of coral and gorgonian growth. These 
heads are gradually but steadily approaching the western strip 
of islands, and will in time completely fill the whole space in the 
interior of the reef, or will leave only a narrow channel of vari- 
ous depth between the outer broad bank to the east and the 
narrow islands to the west. 
The whole structure of this reef shows its identity of forma- 
tion with that of the main Florida Reef, and with that of the 
reefs on the northern coast of Cuba, where the line of distinct 
and powerful elevation can still be plainly traced by old coral 
slopes and by the ancient coral reefs in the hills surrounding 
Havana and extending to Matanzas. These hills attain a height 
of over 1,200 feet, and are entirely composed of species of corals 
identical with those now found on the living reefs. Alacran 
Reef thus gives us an easy explanation of atol!s, apparently 
formed in areas of elevation; I look upon the structure of this 
