28 FLORIDA REEFS. 



the turning-point of the direction of the reef Such heads occur again about 

 a mile and a half off Vermont Key, half way between Key Tavernier and 

 Indian Key, outside of which Gorgonia and sponges are very abundant, 

 upon a hard, white sand bottom. Similar heads are seen between Long Key 

 and Tennessee Reef, and nearer the reef there are shoals of white coral 

 sand, covered with Gorgonia ; but farther west, off Duck Key, the bottom 

 becomes softer. Off Bahia Honda, again, it is rocky, — that is, studded with 

 large heads, surmounted with soft, muddy sand. This change in the char- 

 acter of the bottom is more obvious westward, where the heads are fewer 

 and the bottom more generally muddy, or covered with finer-grained sand. 

 For instance, hard sand is observed between Loggerhead Key and Saddle 

 bluif ; but nearer the reef, as far as the American Shoals, we have soft mud, 

 with shoals and coral heads. Off Boca Chica, the channel way has also 

 a bottom of soft coral mud, while shoals, with coral heads, may be traced 

 for three fourths of a mile along the shores, as, again, towards the Sambos, 

 in a depth of from three to two fithoms. The softness of the bottom in the 

 vicinity of Key West, considered in connection with the scarcity of coral 

 heads in that region, shows that a soft mud formation is unfavorable for the 

 growth of corals ; and, indeed, this holds also good for the flats north of 

 tke keys. 



Between the main-land and the main range of keys, and north of Pine 

 and Mangrove Islands where the mud flats are most extensive, no coral 

 heads are seen. Some of the Gorgonias grow upon hard sand, others 

 chiefly upon coral heads, others in the more muddy places. Gorgonia fla- 

 bellum, for instance, is most abundant upon hard sand, while Gorgonia 

 anceps is found on the mud flats, and Gorgonia dicotoma, again, upon coral 

 heads. If the mud flats, especially when the mud is very soft and deep, are 

 poor in animal life, they are favorable to the growth of the sea weeds 

 known as limestone algae, the tissues of which are filled with lime. The 

 Nullipores, also of this family, grow either upon the dying corals near the 

 level of low water or upon mud flats that are almost dry. We have been 

 surprised at the general lack of animal remains in this mud, as well as at 

 the singular appearance of such as were found in it. Shells otherwise well 

 preserved were incrusted with Bryozoa or with patches of hardened 

 mud. Neither did the corals show a clean surface ; indeed, all the solid 

 parts of animal remains brought up with this mud reminded us, in their 

 state of preservation, of the fossils found in those beds of the Jura Moun- 



