FLORIDA REEFS. 31 
higher and higher points on the stem, all of which throw out again fresh 
shoots. As these mangrove seeds are stranded in great numbers and often 
close together, such a plantation soon becomes the nucleus around which 
sand and mud accumulate. The most extensive and shoalest mud flats 
occur, therefore, wherever the mangrove islands rise. Here the IIolo- 
thuria are most abundant, while the Gorgonia thrive better on a harder 
bottom, where the flats are covered with four or five feet of water. The flats 
and mangrove islands to the north of the Pine Islands and Boca Chica 
are less mudd}^ consisting rather of a coral sand resembling very minute 
oolites. The bottom becomes rather softer, however, to the seaward and 
north of Key West and Boca Chica. Sea-fans (Gorgonias) frequently occur 
in this region upon the harder bottom, especially where the flats slope 
northward into two or three fathoms of water. The sponges are generally 
found upon hard, smooth sand, or upon coral rock in five or six feet of 
water. They occur, however, even at a depth of three to five fiithoms. We 
cannot explain the greater prevalence of the oolitic sand in the Pine Islands 
and Boca Chica group of keys. It may be owing to the greater width of 
this range of islands, or to the greater width of the reef which gave rise to 
their formation. Nowhere else do the keys cover so wide an area. 
The. Main-land. 
A careful survey of all the varieties of rock occurring at Key West, as 
well as their peculiar superposition, had prepared us for a minute compari- 
son between the keys and the main-land ; but, nevertheless, we were no 
less surprised than delighted to find that the solid foundation of the main- 
land consisted of the same identical modifications of coral rocks which 
form the keys. Along all that part of the shore which was examined, as 
well as upon the shores of the Miami, we found everywhere the same 
coarse, oolitic rock, with cross-stratification, consisting of thin beds, dipping 
at various angles in different directions, precisely as we find it at the 
western extremity of Key West, excepting, perhaps, that the cross-stratifi- 
cation is here more prominent, the strata dipping more frequently in several 
directions within the same extent. 
Attention has been called to the resemblance of the main-land to the keys, 
by Buckingham Smith, Esq., in his Report to the House of Representatives 
respecting the drainage of the Everglades. He refers the formation, however, 
