THE FLORIDA REEFS. 55 
so-called coral breccia (Fig. 36), and the different grades of 
oólitie (Fig. 97) or compact limestone characteristic of the recent 
reef formation. 
The coral boulders are the remnants of huge masses of as- 
treans and mandrinas, and similar species of corals. The 
broken fragments of corals come from the different species of 
branching madrepores and porites or the smaller masses, such as 
Manicina, Agaricia, and the like ; while the fragments of shells, 
ete., are derived from the limestone carcasses of the hosts of in- 
vertebrates which once lived on the active coral reef. The bor- 
ing mollusks, annelids, and sponges have little by little riddled 
masses of corals with their burrows, weakening them to such an 
extent that the breakers pounding upon the exposed sea-faces 
of the reef break off larger or smaller masses; these are then 
heaped up and ground together to form either the top of the 
reef proper or the inner keys, as just described. 
Of course it can hardly be expected that, with all this pound- 
ing and grinding and constant rehandling of the material which 
goes to make the limestone rock of the keys, many animal re- 
mains should be preserved intact. In fact, with the exception 
of fragments of the stouter shells of some of the mollusks or 
tubes of annelids and the like, there are but few organic remains 
to be found. 
Corallines, or limestone algæ, also play a most important part 
in the formation of keys. They grow in great abundance upon 
flats and upon the dead fields of corals which have reached the 
surface of the sea; their joints are easily recognized in the com- 
ponents of the coral sand of many a Florida key. Nowhere else 
along the reef or the line of the keys do we find indications that 
the highest elevation of the land is due to any causes except 
those now acting in the formation of the keys. There is not 
a single point so high that it cannot be reached by the waves in 
severe storms, or the elevation of which cannot be traced to the 
action of the tides and winds upon the material of the shore lines. 
All naturalists who have visited the Florida reefs have felt the 
difficulty of applying Darwin’s theory of reef formation to the 
peculiar conditions existing along the Straits of Florida. Agas- 
siz, Le Conte, and E. B. Hunt have each in succession attempted 
