12 FLORIDA REEFS. 
reef .and down its slope on either side, and we have keys corresponding 
in essential features to those forming the main range. The difference, 
however, is not simply one of greater or less accumulation. There is a 
reo-ular gradation in the character of the materials. All these loose mate- 
rials, in proportion to their size and the violence of the waves, are now 
acted upon by the sea. The smaller fragments, even under the gentle flow 
of the ordinary tides, are constantly moved to and fro, while heavier winds 
and storms break away the larger masses and split them into smaller bits, 
and may in the end grind them to rounded pebbles, and finally to sand or 
to an almost impalpable coral powder. This powder is stirred up from the 
bottom by the waves, and often remains suspended in the water for days, 
giving it a milky hue. Of course a great portion of these debris, both large 
and small, will be thrown upon the reef, and find a resting-place there. Of 
such materials the Florida keys are made. It is worth our while to study 
the mode of formation of these deposits, since it may throw some light on 
the extensive accumulations, always a puzzle to geologists. The nature 
of the larger boulders, and even of the pebbles, cannot be mistaken, since 
the organic structure of the coral stocks is perceptible in them all, and 
yields but slowly to decomposition or attrition. Such changes as they un- 
dergo may be observed in coral stems the summits of which are still alive 
and growing. Meantime the lower parts are gradually dying or filling with 
amorphous limestone. This limestone often has within the very centre of 
the coral stem a crystalline structure as perfect as any crystalline limestone. 
When so acted upon, these corals resemble the corals of the Paleozoic rocks. 
From such well-preserved specimens to the smaller fragments, where only 
some traces of organic characters remain, and from these again to the still 
finer pebbles, particles, or powder from Avhicli all signs of organization have 
disappeared, the transition is so gradual as to leave no doubt that even the 
sand is of coral origin. Here and there the stronger shells, such as Strom- 
bus gigas, Fusus giganticus, Fasciolaria Rhinoceros, and others, may be 
preserved among the coral fragments ; but the more delicate shells, as well 
as the carcasses of crabs and lobsters, the solid parts of sea-urchins and 
starfishes, are very soon ground to powder. This explains the rarity of 
well-preserved organic remains in coral rock. Seeing the profusion of ani- 
mal life on the reef, one is at first surprised at this ; but a little observation 
shows that the hard parts of both animals and plants, about the reef, are 
transformed by long-continued attrition into coral breccia, coral oolites or 
compact limestone. 
