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 Holo- 

 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 muddy, 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 fathoms. 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 

 Avell 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, 



