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 Avhich 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. Oft' 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 
bluff; 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 fathoms. 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 coital 
heads are seen. Some of the Gorgonias gi'ow 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 algaj, 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- 
