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drawings of coral

Historical Images from the First Florida Reef Study

NOAA Photo Library

The first scientific study of the florida reefs

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The Keys
 

drawing of coralP. 153 A careful survey of the character of the rocks in the keys affords satisfactory evidence that they have been formed at whatever height they may rise , by the same action which is now going on upon the reef--that is, by the accumulation of loose materials above the water-level. That part of the keys which rises above the level of the water is, therefore, a sub-aerial and not a submarine accumulation of floating matter, thrown above high-water mark by the tempestuous action of the water. We insist upon the fact, that the keys furnish in themselves, by the internal structure of their rock, the fullest evidence that they have been formed above high-water mark by the action of gales and hurricanes, instead of having grown as a reef up to the water-level, and been subsequently raised to their present height. The evidence of this statement rests upon certain facts obtained from observations of the reef itself, at Sand key and the Sambos.

Coral Reefs

P. 153-154 After examining a growing coral reef, so full of life, so fresh in appearance, so free from heterogeneous materials, in which the corals adhere so firmly to the ground, or if they rise near the surface seem to defy the violence of the ocean, standing uninjured amid the heaviest breakers, an observer cannot but wonder why, in the next reef, the summit of which begins to rise above the level of the water, the scene is so completely changed. Huge fragments of corals, large stems, broken at their base, gigantic boulders, like hemispheres of Porites, and Macandrina, lie scattered about in the greatest confusion--flung pell-mell among the fragments of more delicate forms, and heaped upon those vigorous madrepores which reach the surface of the sea.

The question at once arises, how is it that even the stoutest corals, resting with broad base upon the ground, and doubly secure from their spreading proportions, become so easily a prey to the action of the same sea which they met shortly before with such effectual resistance? The solution of this enigma is to be found in the mode of growth of the corals themselves. Living in communities, death begins first at the base or centre of the group, while the surface or tips still continue to grow, so that it resembles a dying centennial tree, rotten at the heart, but still apparently green and flourishing without, till the first heavy gale of wind snaps the hollow trunk, and betrays its decay. Again, innumerable boring animals establish themselves in the lifeless stem, piercing holes in all directions into its interior, like so many augers, dissolving its solid connexion with the ground, and even penetrating far into the living portion of these compact communities. The number of these boring animals is quite incredible, and they belong to different families of the animal kingdom: among the most active and powerful we would mention the datefish, Lithodomus, several Saxivaca, Petricola, Arca, and many worms, of which Serpula is the largest and most destructive, inasmuch as it extends constantly through the living part of the coral stems, especially in Macandrina.

On the loose basis of a Macandrina, measuring less than two feet in diameter, we have counted not less than fifty holes of the date-fish--some large enough to admit a finger--besides hundreds of small holes made by worms.

But however efficient these boring animals may be in preparing the coral stems for decay, there is yet another agent, perhaps still more destructive . We allude to the minute boring-sponges, which penetrate them in all directions, until they appear at last completely rotten throughout.


Coast Survey

But it may be asked, what is the practical use of such detailed descriptions of the coral reefs for the coast survey? We need only allude to the universal impression of the dangers arising to navigation, from the growth of such reefs, to satisfy the most skeptical that a minute knowledge of the extent and mode of formation of those belonging to our own shores must be of paramount importance, were it only with reference to the position of light-houses.

Changes in ages to come

Among the questions contained in your instructions, you ask whether the growth of coral reefs can be prevented, or the results remedied, which are so unfavorable to the safety of navigation. I may say that here, as in most cases where the operations of nature interfere with the designs of man, it is not by a direct intervention on our part that we may remedy the difficulties, but rather by a precise knowledge of their causes, which may enable us, if not to check, at least to avoid the evil consequences. I do not see the possibility of limiting in any way the extraordinary increase of corals, beyond the bounds which nature itself has assigned to their growth.
 

......we may therefore rest assured that the changes which are going on will chiefly consist in bringing up the reef, for its whole extent, to the surface of the water, with occasional intervening channels kept open by the currents, such as exist now between the keys; that this reef once matured, will be covered by coral debris, becoming transformed into a range of keys, similar to that which exists now inside of it; that the depth of the ship channel between the reef and main range of keys will gradually lessen, and the channel itself be changed into mud flats, similar to those stretching now between the keys and the main land. In still more remote ages the present mud flats may become swamps, elevated above the reach of the tide-waters, like the everglades; and this process may perhaps be extended to the present ship channel. But unless some great revolution in nature modifies the present relative level between land and sea, it may safely be maintained that the present outer reef is the final southern boundary of the North American continent, and that the sooner a system of light-houses and signals is established along the whole reef, the better; for this is, after all, the shore which is to be lighted, and not the range of keys which is within the reef.

These practical results--for so we venture to call the general conclusions last presented-- although they are purely scientific deductions from general principles, may satisfy the most obstinate supporters of the matter-of-fact side of all questions, of the advantages of scientific illustrations in the daily walks of life, and also justify the course which has been followed with so much success by the Coast Survey, in combining the strictest scientific methods with its practical operation.

Respectfully submitted:

L. Agassiz


Publication of the National Oceanic & Atmospheric Administration (NOAA), NOAA Central Library.

Last Updated: June 8, 2006 9:24 AM

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