12 FLOKIDA 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 

 regular 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 aAvay 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 Avhich 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 which 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. 



