« AnteriorContinuar »
already given a general history of its progress, which we now proceed to illustrate more in detail.
We would, however, first remark, that the extraordinary varieties which exist in the natural condition of different parts of the same reef, or of different reefs, when compared with each other, fully explain the discrepancies between the reports which have been obtained, respecting the reefs of Florida, prior to our investigations.
It had been stated that the reefs consisted solely of living corals; and, indeed, this report is true of the outer reef, which is called by all the inhabitants of Florida “the reef” par excellence, and is unfounded only with regard to those few islands which rise above the surface of the sea at Sand key and the Sambos. Others, who had noticed only the larger accumulations of coral fragments which occur on the shores of some of the islands forming part of the Florida reet, had reported the islands to be formed of coral rocks; while some who had, perhaps, observed the extensive excavations made around Key West, have told us only of the existence of oolitic and compact rocks, almost destitute of corals or other remains of animal life; and from still other localities comes the opinion, that the rocks consist of nothing but more or less disintegrated shells, cemented together. *
This fullness and variety of animal life is particularly obvious within the boundaries of coral fields, the natural limits assigned to the growth of these animals being those in which animals of other classes range in greater profusion, and the coral reefs themselves also affording very favorable circumstances for the display of numerous living forms. Hence the extraordinary assemblage of all classes of animals upon the reef, where, besides those particular kinds of corals which contribute largely to its formation, we find upon it, or on the foundation from which it rises, a great variety of other corals, which, though too insignificant in size to take a conspicuous part in building up these extensive accumulations of organic lime-rock, add none the less their small share in the work, contributing especially to fill up the vacant spaces left by the more rapid and durable growth of the larger kinds. They are to the giants of the reef what the more slender parts are to the lords of the forest, adding the elegance and delicacy of slighter forms to the strength, power, and durability of their loftier companions.
But besides the stony corals, we find in the reef a great variety of soft polyps, either attached to the surface of dead corals, dead shells, or of the naked rock, or boring into the coral sand and mud.
Such are different species of Arca, the date-fish among the Mollusca, and many worms, especially Serpula among articulates, the agency of which in the formation of the keys will be described hereafter. All these animals and plants contribute, more or less, to augment the mass of solid materials which is accumulating upon the reef, and increase its size. Not only are the hard parts of shells, echinoderms, worms, or their
broken fragments, heaped among the detritus of the corals, but occasionally even the bones of fishes and turtles, which are very numerous along the reef, may be found in the coral formations.
The decaying soft parts of all these animals undoubtedly have their influence upon the chemical process, by which the limestone particles of their solid frame are cemented together, in the formation of compact rocks. Upon this point we may expect further information from Professor Horsford, who is now submitting to chemical analysis all the variety of rocks and the solid stems of the different corals obtained in Florida. Respecting the relations of the solid and soft parts of the living coral, and their mode of growth, we would refer to a paper of ours now in press, to appear in the next volume of the Smithsonian Contributions to Knowledge. *
. We see everywhere that the larger boulders and the coarser fragments have been the first to find a resting-place upon the dead reet; the minuter particles and coral sand, which are periodically washed away from its crest during heavy gales, never accumulating upon it till large boulders and more solid materials have collected to such an extent as to form sufficient protection for the more movable, looser fragments. This fact is beautifully illustrated by an accurate survey of Sand key, where a wide field of large boulders is partially laid bare at low water, presenting the appearance of an extensive key, with a low hill of minute materials, the product of some heavy gale, heaped upon the summit, against which the sea plays without disturbing it materially, even at high water, when it leaves in sight only a nucleus, as it were, for a greater accumulation of such loose materials which may in time cover the whole surface of the larger boulders. We have here in reality the same phenomenon which is observed upon all beaches, where larger materials have first accumulated on a sboal shore, being followed, in the course of time, by more minute fragments which have found a resting-place upon levels where the sea was powerless to increase the collection of coarser matter. In attempting to understand these formations, it must be remembered that the accumulation of the larger materials, collected at a certain level, may modify the action of the water at a subsequent period, thus producing a combination of substances, heaped unconformably upon each other. This is, in reality, the case throughout the whole main range of keys, which have been raised to their present level by the action of the tides and gales for ages past, the fragments of which they are composed baving been thrown up at different periods, and overlying each other in such a manner as to present the same irregularity which is found in all drift stratification. Layers upon layers are seen resting unconformably, dipping in different directions so as to present all the modificatious which may be observed in torrential stratification, each layer following, with more or less regularity, the course of the flood under which it has been accumulated.
By a process, not yet fully understood, but to which we shall return hereafter, these loose collections are gradually cemented into solid rock, presenting the most diversified appearance, according to the substances of which it is composed. Then we find a coarse breccia, consisting of larger fragments of corals and shells, enclosing sometimes coral boulders; and this is the sort of rock which generally overlies the immediate surface of that portion of the keys which has been formed by the progress of the reef, growing in situ. Such rock was seen among the foundations of the new light-house at Sand key, where the large boulders are very numerous, and seem almost as fresh as if they had been lying on the spot but for a few years. It may be, indeed, that during the hurricane of 1846, the whole cap of the reef was renewed at that spot. + + + o + + * +
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 waterlevel. 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 waterlevel, and been subsequently raised to their present height. The evidence of this statement rests upon certain facts obtained from observa
tion of the reef itself, at Sand key and the Sambos. + :* + *
Let us now return from this digression to the consideration of the keys themselves, under the different aspects which they present. We find, then, that some have more abrupt shores, being, as it were, narrow shelves with ragged edges, rising without a beach from deep water; these are undoubtedly such as were formed upon the narrowest part of the old reef. Others spread more uniformly, having an extensive beach, and dip gradually under the sea, presenting a gentle, submarine slope, covered with coral sand and mud; these were, no doubt, formed upon the broader parts of the reef, where it descends gently on both sides. Again, we find those which, though resembling the last in general appearance, may have more abrupt shores, owing to the denudation of parts of .*.*. deposites. Occasionally we see that more recent layers have filled again such worn places, thus presenting, on a miniature scale, among the latest formations among layers which belong altogether to the present geological age, all the diversity of unconformable deposites which occur in former geological periods.
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 confusionflung pell-mell among the fragments of niore 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 date-fish, Lithodomus, several Saxicava, Petricola, Arca, and many worms, of which the 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-fishsome 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.
The broad channel extending the whole range of the reef, between the main keys and the outer reef, is rather uniform, having the same width throughout, with the exceptions of those few places where the reef widens, or the mud flats from the keys encroach upon it. Its narrowest passages are between Looe key and the Pine islands, between Pickle and French reefs, and between Key Rodrigues and Tavernier. It is also somewhat narrowed between Alligator reef and Indian key, and is widest off Key West. Its depth varies also slightly, being shoaler in its eastern range than to the west. The shallowest part is between Pickle reef and Key Rodriguez, and between Looe key and Pine islands.
etween Formes three fathoms beter between Carysfor
But if we do not take into account those spots where the depth is reduced from local circumstances, we may say, that, as a whole, the ship channel begins to the east, with a depth of about two fathoms between Fowey rocks and Soldier key, increasing gradually thence, until it reaches three fathoms between Pacific reef and Old Rhodes, then becomes again slightly reduced between Carysfort reef and Key Largo; after which, with the exception of the shoals between Pickle reef and Key Rodriguez, it deepens again to three, four, five, or even six fathoms, until, between Looe key and Pine islands, it shoals once more to fourteen feet. Farther on, it increases again to five, sis, and seven fathoms, the average depth between Key West and the reef being five or six fathoms; and still beyond, more towards the west, sinks to eight, nine, and ten fathoms between the western extremity of the Marquesas and the western end of the reef, where it spreads into the great depression separating the Tortugas from the Marquesas. The character of the bottom varies in different parts, as do also the living beings which it supports. Where it is the most shoal, as between Fowey rocks, Triumph reef, and Long reef, on one side, and Soldier key and the Ragged keys on the other, the bottom consists of coral saud, overgrown with what is called the country grass ; that is to say, a variety of the limestone algæ, mingled with Gorgonia, among which rise a number of coral heads.
. . . . . To the west of Long reef, especially between Carysfort and Key Largo, the coral sand rises here and there, in the form of shoal sandhanks, intermixed with coral headsman arrangement which is probably owing to the more rapid currents flowing in that part of the channel, which is precisely 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 which 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. Off Bahia Honda, again, it is rocky—that is, studded with large heads, surmounted with soft, muddy sand. This change in the character 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 connexion 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 the keys.